Chapter 14 Characteristics, Inspection and Evaluation of Culverts

Topic 14.1 Culvert Characteristics

14.1.1 Introduction

A culvert is a structure designed hydraulically to take advantage of submergence to increase water carrying capacity. Culverts, as distinguished from bridges, are usually covered with embankment and are composed of structural material around the entire perimeter. Some culverts are supported on spread footings with the streambed serving as the bottom of the culvert. If culverts satisfy NBIS bridge length requirements of 20 feet or greater, they may be classified as bridges in the National Bridge Inventory (NBI).

Over the years, culverts have traditionally received less attention than bridges. Since culverts are less visible it is easy to put them out of mind, particularly when they are performing adequately. Additionally, a culvert usually represents a significantly smaller investment than a bridge.

Since 1967 there has been an increased emphasis on bridge safety and on bridge rehabilitation and replacement programs. In many cases small bridges have been replaced with multiple barrel culverts, box culverts, or long span culverts (see Figure 14.1.1). There have also been recent advances in culvert design and analysis techniques. Long span corrugated metal culverts with spans in excess of 40 feet were introduced in the late 1960's.

Photograph of a culvert

Figure 14.1.1 Culvert Structure

As a result of these developments, the number, size, complexity, and cost of culvert installations have increased. The failure of a culvert may be more than a mere driving inconvenience. Failure of a major culvert may be both costly and hazardous.

Bridge-size culverts are inspected regularly to identify potential safety problems and maintenance needs. Culverts smaller than bridges may or may not be inspected, depending on the state. Preserving the investment in the structure and minimizing property damage due to improper hydraulic functioning are also key reasons for regular inspections and other maintenance actions.

Purpose of Culvert Inspection

The National Bridge Inspection Program (NBIP) was designed to insure the safe passage of vehicles and other traffic. The inspection program provides a uniform database from which nationwide statistics on the structural and functional safety of bridges and large culvert-type structures are derived. Although these bridge inspections are essentially for safety purposes, the data collected is also used to develop rehabilitation and replacement priorities.

Bridges with spans over 20 feet in length are inspected on a two-year cycle in accordance with the National Bridge Inspection Standards (NBIS). According to the American Association of State Highway and Transportation Officials (AASHTO) the definition of bridges includes culverts with openings measuring more than 20 feet along the centerline of the road and also includes multiple pipes where the distance between openings is less than or equal to half of the pipe opening. Multiple barrel culvert installations with relatively small pipes can therefore meet the definition of a bridge.

Structures included in the NBIS are evaluated by utilizing a standardized inventory appraisal process that is based on rating certain structural and functional features. The data obtained is recorded on standardized inspection forms. The minimum data required for bridge length culverts is shown on the Structure Inventory and Appraisal Sheet (SI&A). Procedures for coding these items are provided in the FHWA Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Bridges (Coding Guide)

While the importance of the NBIS inspection program cannot be overemphasized, the SI&A data sheets are oriented toward bridges rather than culverts; thus, they do not allow an inspector to collect either detailed condition data or maintenance data for culverts. Additionally, the NBIS program does not specifically address structures where the total opening length is less than 20 feet. However, some type of formal inventory and inspection is needed for culverts that are not bridge length. In many cases, the failure of a culvert or other structure with openings less than 20 feet long can present a life threatening hazard. Although the primary purpose of this and other sections relating to culverts is to provide inspection guidelines for culverts included in the NBIS program, the guidelines are also generally applicable to culverts with openings which are less than 20 feet long. For culverts (and span-type structures) less than 20 feet in length, the state in which the structure is located will incorporate it into their inventory and inspection program. In this case, the state defines the criteria whereby culverts are to be included in the their inventory and inspection program.

Ideally, all culverts are inventoried and periodically inspected. Some limitations may be necessary because a considerable effort is required to establish a current and complete culvert inventory. Small culverts may not warrant the same rigorous level of inspection as large culverts. Each agency defines its culvert inspection program in terms of inspection frequency, size, and type of culverts to be inventoried and inspected, and the information to be collected. Culverts larger than 20 feet are inspected every two years under the NBIS program. If possible, all culverts are inventoried and inspected to establish a structural adequacy and to evaluate the potential for roadway overtopping or flooding.

The types and amount of condition information to be collected is based on the purpose for which the information will be used. For example, if small pipes are not repaired but are replaced after failures occur, then the periodic collection of detailed condition data may not be warranted. Documentation of failures as well as the causes of failures may be all the condition data that is needed. However, the inventory is updated whenever a replacement is accomplished.

Safety

Safety is the most important reason why culverts as well as bridges are inspected. To ensure that a culvert is functioning safely, the inspector evaluates the structural integrity, hydraulic performance, and roadside compatibility of the culvert.

Maintenance Needs

Lack of maintenance is a prime cause of improper functioning of culverts and other drainage structures. Regular periodic inspections allow minor problems to be spotted and corrected before they become serious.

Outcomes

The primary outcome of this topic as well as Topics 2.1, 2.2, 3.1, 4.2, 4.3, 7.6, 13.2, 14.2, and 14.3 is to provide information that will enable bridge inspectors to perform the following tasks:

To meet the primary outcome, the topics in this reference manual provide general procedures for conducting, reporting, and documenting a culvert inspection, and guidelines for evaluating specific hydraulic and structural culvert components.

A second outcome of these sections is to provide inspectors with the information necessary to understand and evaluate the significance of defects and their effect on hydraulic and structural performance. Topics 14.2 and 14.3 present information on rigid and flexible culverts. Durability concepts are also reviewed in these topics.

14.1.2 Differentiation Between Culverts and Bridges

Traditional definitions of culverts are based on the span length rather than function or structure type. For example, the NBIS bridge length definition included in the FHWA Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation’s Bridges states:

“A structure including supports erected over a depression or a obstruction, such as water, highway, or railway, and having a track or passageway for carrying traffic or other moving loads, and having an opening measured along the center of the roadway of more than 20 feet between undercopings of abutments or spring lines of arches, or extreme ends of openings for multiple boxes.”

Therefore, structures that are less than 20 feet may be known as culverts.

Many structures that measure more than 20 feet along the centerline of the roadway have been designed hydraulically and structurally as culverts. The structural and hydraulic design of culverts is substantially different from bridges, as are construction methods, maintenance requirements, and inspection procedures. A few of the more significant differences between bridges and culverts are:

Hydraulic

Culverts are usually designed to operate at peak flows with a submerged inlet to improve hydraulic efficiency. The culvert constricts the flow of the stream to cause ponding at the upstream or inlet end. The resulting rise in elevation of the water surface produces a head at the inlet that increases the hydraulic capacity of the culvert. Bridges may constrict flow to increase hydraulic efficiency or be designed to permit water to flow over the bridge or approach roadways during peak flows. However, bridges are generally not designed to take advantage of inlet submergence to the degree that is commonly used for culverts. The effects of localized flooding on appurtenant structures, embankments, and abutting properties are important considerations in the design and inspection of culverts.

Structural

Culverts are usually covered by embankment material. Culverts are designed to support the permanent load of the soil over the culvert as well as transient loads including vehicular traffic. Either transient loads or permanent loads may be the most significant load element depending on the type of culvert, type and depth of cover, and amount of live load. However, transient live loads on culverts are generally not as significant as the permanent loads unless the cover is shallow. Box culverts with shallow cover are examples of the type of installation where transient live loads may be significant. Permanent and transient loading is presented in detail in Topic 14.1.3.

Photograph of a box culvert with a shallow cover

Figure 14.1.2 Box Culvert with Shallow Cover

In most culvert designs the soil or embankment material surrounding the culvert plays an important structural role. Lateral soil pressures enhance the culverts ability to support vertical loads. The stability of the surrounding soil is important to the structural performance of most culverts.

Maintenance

Because culverts usually constrict flow, there is an increased potential for waterway blockage by debris and sediment, especially for culverts subject to seasonal flow. Multiple barrel culverts may also be particularly susceptible to debris accumulation. Scour caused by high outlet velocity and turbulence at inlet end is a concern. As a result of these factors, routine maintenance for culverts primarily involves the removal of obstructions and the repair of scour and undermining. Prevention of joint leakage may be critical in culverts bedded in pipeable soils to prevent undermining and loss of support.

Traffic Safety

A significant safety advantage of many culverts is the elimination of bridge parapets and railings. Culverts can usually be extended so that the standard roadway cross section can be carried over the culvert to provide a vehicle recovery area. However, when culvert ends are located near travel lanes or adjacent to shoulders, guardrails may be used to protect the traffic. Another safety advantage of culverts is that less differential icing occurs. Differential icing is the tendency of water on the bridge deck to freeze prior to water on the approaching roadway. Since culverts are under fill material and do not have a bridge deck, the temperature of the roadway over the culvert is at or near the temperature of the roadway approaching the culvert.

Construction

Careful attention to construction details such as bedding, compaction, and trench width during installation is important to the structural integrity of the culvert. Poor compaction or poor quality backfill around culverts may result in uneven or differential settlement over the culvert and possibly structural distress of the culvert.

Durability

Durability of material is a significant problem in culverts and other drainage structures. In very hostile environments such as acid mine drainage and chemical discharge, corrosion and abrasion can cause deterioration of all commonly available culvert materials.

Inspection

The inspection and assessment of the structural condition of culverts requires an evaluation of not only actual distress but circumstantial evidence such as roadway settlement, pavement patches, and embankment condition.

14.1.3 Structural Characteristics of Culverts

Loads on Culverts

In addition to their hydraulic functions, culverts also support the weight of the embankment or fill covering the culvert and any load on the embankment. There are two general types of loads that are carried by culverts: permanent loads and transient loads.

Permanent Loads

Permanent loads include the earth load or weight of the soil over the culvert and any added surcharge loads such as buildings or additional earth fill placed over an existing culvert. If the actual weight of earth is not known, 120 pounds per cubic foot is generally assumed.

Transient Loads

The vehicular live loads and live load surcharge on a culvert include the loads and forces, which act upon the culvert due to vehicular or pedestrian traffic. The highway wheel loads (as part of the AASHTO HL-93 design load) used for design and analysis are shown in Figure 14.1.3. The effect of live loads decreases as the height of cover over the culvert increases. When the cover is less than two feet, concentrated loads may be considered as being spread uniformly over a rectangle with sides 1.15 times the depth of cover plus the initial footprint. This concept is illustrated in Figures 14.1.4 and 14.1.5. In addition to the truck load, the HL-93 is also comprised of a 640 pound lane load. This load converts into an additional 64 pounds per square foot, but may be ignored if the depth of the cover is greater than 8 feet.

Schematic of the AASHTO wheel loads and wheel spacings

Figure 14.1.3 AASHTO Wheel Loads and Wheel Spacings

(Source:Concrete Pipe Design Manual, American Concrete Pipe Association, April 2007)

Schematic of the AASHTO wheel load surface contact area (also known as foot print)

Figure 14.1.4 AASHTO Wheel Load Surface Contact Area (Foot Print)

(Source: Concrete Pipe Design Manual, American Concrete Pipe Association, April 2007)

Schematic of the spread load area of a single dual wheel

Figure 14.1.5 Spread Load Area (Single Dual Wheel)

Soil Type H, ft P, lbs Spread a, ft Spread b, ft

Select Granular Soil Fill

H < 2.03

16,000

a + 1.15H

b + 1.15H

Other Soils

H < 2.33

16,000

a + 1.00H

b + 1.00H

(Source: Concrete Pipe Design Manual, American Concrete Pipe Association, April 2007)

Categories of Structural Materials

Based upon material type, culverts are divided into two broad structural categories: rigid and flexible.

Rigid Culverts

Culverts made from materials such as reinforced concrete or stone masonry are very stiff and do not deflect appreciably. The culvert material itself provides the needed stiffness to resist loads. In doing this, zones of tension and compression are created. The culvert material is designed to resist the corresponding stresses.

Rigid Culverts are presented in detail in Topic 14.2.

Flexible Culverts

Flexible culverts are commonly made from steel or aluminum. In some states composite materials are used. Flexible culverts rely on the surrounding backfill material to maintain their structural shape. Since they are flexible, they can be deformed significantly with no cracks occurring.

As vertical loads are applied, a flexible culvert will deflect if the surrounding fill material is loose. The vertical diameter decreases while the horizontal diameter increases. Soil pressures resist the increase in horizontal diameter.

For flexible culverts with large openings, sometimes longitudinal and/or circumferential stiffeners are used to prevent excessive deflection. Circumferential stiffeners are usually metal ribs bolted around the circumference of the culvert. Longitudinal stiffeners may be metal or reinforced concrete. This type of stiffener is sometimes called a thrust beam.

Flexible culverts are presented in detail in Topic 14.3.

Construction and Installation Requirements

The structural behavior of flexible and rigid culverts is often dependent on construction practices during installation (see Figure 14.1.6). Items, which require particular attention during construction, are discussed briefly in the following text. This information is provided so that the bridge inspector may gain insight on why certain structural defects are found when inspecting a culvert.

Schematic of culvert construction and installation requirements

Figure 14.1.6 Culvert Construction and Installation Requirements

14.1.4 Culvert Shapes

A wide variety of standard shapes and sizes are available for most culvert materials. Since equivalent openings can be provided by a number of standard shapes, the selection of shape may not be critical in terms of hydraulic performance. Shape selection is often governed by factors such as depth of cover or limited headwater elevation. In such cases a low profile shape may be needed. Other factors such as the potential for clogging by debris, the need for a natural stream bottom, or structural and hydraulic requirements may influence the selection of culvert shape. Each of the common culvert shapes are discussed in the following paragraphs.

Circular

The circular shape is the most common shape manufactured for pipe culverts (see Figure 14.1.7). It is hydraulically and structurally efficient under most conditions. Possible hydraulic drawbacks are that circular pipe generally causes some reduction in stream width during low flows. It may also be more prone to clogging than some other shapes due to the diminishing free surface as the pipe fills beyond the midpoint. With very large diameter corrugated metal pipes, the flexibility of the sidewalls dictates that special care be taken during backfill construction to maintain uniform curvature.

Photograph of a circular culvert

Figure 14.1.7 Circular Culvert Structure

Pipe Arch and Elliptical Shapes

Pipe arch and elliptical shapes are often used instead of circular pipe when the distance from channel invert to pavement surface is limited or when a wider section is desirable for low flow levels (see Figure 14.1.8). These shapes may also be prone to clogging as the depth of flow increases and the free surface diminishes. Pipe arch and elliptical shapes are not as structurally efficient as a circular shape.

Photograph of a pipe arch culvert

Figure 14.1.8 Pipe Arch Culvert

Arches

Arch culverts offer less of an obstruction to the waterway than pipe arches and can be used to provide a natural stream bottom where the stream bottom is naturally erosion resistant (see Figure 14.1.9). Foundation conditions must be adequate to support the footings. Riprap is frequently used for scour protection.

Photograph of a concrete arch culvert

Figure 14.1.9 Arch Culvert

Box Sections

Rectangular cross-section culverts are easily adaptable to a wide range of site conditions including sites that require low profile structures (see Figure 14.1.10). Due to the flat sides and top, rectangular shapes are not as structurally efficient as other culvert shapes. In addition, box sections have an integral floor.

Photograph of a concrete box culvert

Figure 14.1.10 Concrete Box Culvert

Multiple Barrels

Multiple barrels are used to obtain adequate hydraulic capacity under low embankments or for wide waterways (see Figure 14.1.11). In some locations they may be prone to clogging as the area between the barrels tends to catch debris and sediment. When a channel is artificially widened or when a culvert is constructed, excessive sedimentation is more likely to occur in any or all of the barrels based upon the conditions. The span or opening length of multiple barrel culverts includes the distance between barrels as long as that distance is less than half the opening length of the adjacent barrels.

Photograph of a concrete multiple cell concrete culvert

Figure 14.1.11 Multiple Cell Concrete Culvert

Frame Culverts

Frame culverts are constructed of cast-in-place (see Figure 14.1.12) or precast reinforced concrete. This type of culvert has no floor (concrete bottom) and fill material is placed over the structure.

Photograph of a concrete frame culvert

Figure 14.1.12 Frame Culvert

14.1.5 Culvert Materials

Precast Concrete

Precast concrete culverts are manufactured in six standard shapes:

With the exception of box culverts, concrete culvert pipe is manufactured in up to five standard strength classifications. The higher the classification number, the higher the strength. Box culverts are designed for various depths of cover and live loads. All of the standard shapes are manufactured in a wide range of sizes. Circular and elliptical pipes are available with standard sizes as large as 180 inches in diameter, with larger sizes available as special designs. Standard box sections are also available with spans as large as 144 inches. Precast concrete arches on cast-in-place footings are available with spans up to 41 feet. A listing of standard sizes is provided in Topic 14.2. Refer to Topic 14.2 for a detailed discussion of precast concrete culverts.

Cast-in-Place Concrete

Culverts that are reinforced cast-in-place concrete are typically either rectangular or arch-shaped. The rectangular shape is more common and is usually constructed with multiple cells (barrels) to accommodate longer spans. One advantage of cast-in-place construction is that the culvert can be designed to meet the specific requirements of a site. Due to the long construction time of cast-in-place culverts, precast concrete or corrugated metal culverts are sometimes selected. However, in many areas, cast-in-place culverts are more practical and represent a significant number of installations. Refer to Topic 14.2 for a detailed discussion of cast-in-place concrete culverts.

Metal Culverts

Flexible culverts are typically either steel or aluminum and are constructed from factory-made corrugated metal pipe or field assembled from structural plates. Structural plate products are available as plate pipes, box culverts, or long span structures (see Figures 14.1.13 and 14.1.14). Several factors such as span length, vertical and horizontal clearance, peak stream flow and terrain determine which flexible culvert shape is used. Refer to Topic 14.3 for a detailed discussion of metal culverts.

Photograph of a large structural plate pipe arch culvert

Figure 14.1.13 Large Structural Plate Pipe Arch Culvert

Photograph of a large structural plate box culvert

Figure 14.1.14 Large Structural Plate Box Culvert

Masonry

Stone and brick are durable, low maintenance materials. Prior to the 1920's, both stone and brick were used frequently in railroad and road construction projects because they were readily available from rock cuts or local brickyards. Currently stone and brick are seldom used for constructing culvert barrels. Stone is used occasionally for this purpose in locations which have very acidic runoff, but the most common use of stone is for headwalls where a rustic or scenic appearance is desired. A stone masonry arch culvert is shown in Figure 14.1.13. Refer to Topic 14.2 for a detailed discussion of stone masonry.

Photograph of a stone masonry arch culvert

Figure 14.1.15 Stone Masonry Arch Culvert

Timber

There are a limited amount of timber culverts throughout the nation.

Timber culverts are generally box culverts and are constructed from individual timbers similar to railroad ties. Timber culverts are also analogous to a short span timber bridge on timber abutments (see Figure 14.1.14). Refer to Topic 14.2 for a detailed discussion of timber culverts.

Photograph of a timber box culvert

Figure 14.1.16 Timber Box Culvert

Plastic

Plastic culverts are relatively new and are not as common. They are round in shape, similar to corrugated metal culverts (see Figure 14.1.17). Refer to Topic 14.3 for a detailed description of plastic culverts.

Schematic of a single walled plastic culvert

Figure 14.1.17 Schematic of a Single Walled Plastic Culvert

Other Materials

Aluminum, steel, concrete, and stone masonry are the most commonly found materials for existing culverts. There are several other materials which may be encountered during culvert inspections, including cast iron, stainless steel, terra cotta, and asbestos cement. These materials are not commonly found because they are either labor intensive (terra cotta) or used for specialized situations (stainless steel and cast iron).

14.1.6 Culvert End Treatments

Culverts may have end treatments or end structures. End structures are used to control scour, support backfill, retain the embankment, improve hydraulic efficiency, protect the culvert barrel, and provide additional stability to the culvert ends.

The most common types of end treatments are:

Photograph of a culvert end projection

Figure 14.1.18 Culvert End Projection

Photograph of a culvert mitered end

Figure 14.1.19 Culvert Mitered End

Photograph of a culvert skewed end

Figure 14.1.20 Culvert Skewed End

Photograph of culvert headwall and wingwalls

Figure 14.1.21 Culvert Headwall and Wingwalls

Miscellaneous Appurtenance Structures may also be used with end treatments to improve hydraulic efficiency and reduce scour. Typical appurtenances include:

Photograph of a culvert apron

Figure 14.1.22 Apron

Photograph of a riprap basin

Figure 14.1.23 Riprap Basin

14.1.7 Hydraulics of Culverts

Culverts are primarily constructed to convey water under a highway, railroad, or other embankment. A culvert which does not perform this function properly may jeopardize the throughway, cause excessive property damage, or even loss of life. The hydraulic requirements of a culvert usually determine the size, shape, slope, and inlet and outlet treatments. Culvert hydraulics can be divided into two general design elements:

A hydrologic analysis is the evaluation of the watershed area for a stream and is used to determine the design discharges or the amount of runoff the culvert is designed to convey.

A hydraulic analysis is used to select a culvert, or evaluate whether an existing culvert is capable of adequately conveying the design discharge. To recognize whether a culvert is performing adequately, it is important for the inspector to understand the factors that influence the amount of runoff to be handled by the culvert as well as the factors which influence the culvert's hydraulic capacity.

Hydrologic Analysis

Most culverts are designed to carry the surface runoff from a specific drainage area. While the selection and use of appropriate methods of estimating runoff requires a person experienced in hydrologic analysis and would usually not be performed by the inspector, it is helpful to understand how changes in the topography of the drainage area can cause major changes in runoff. Climatic and topographic factors are briefly presented:

Climatic Factors

Climatic factors that may influence the amount of runoff include:

Topographic Factors

Topographic factors that may influence runoff include:

Land use is the most likely characteristic to change significantly during the service life of a culvert. Changes in land use may have a considerable effect on the amount and type of runoff. Some surface types will permit more infiltration than other surface types. Practically all of the rain falling on paved surfaces will drain off while much less runoff will result from undeveloped land. If changes in land use were not planned during the design of a culvert, increased runoff may exceed the capacity of an existing culvert when the land use does change.

The size, shape, and slope of a culvert's drainage area influence the amount of runoff that may be collected and the speed with which it will reach the culvert. The amount of time required for water to flow to the culvert from the most remote part of a drainage area is referred to as the time of concentration. Changes within the drainage area may influence the time of concentration.

Straightening or enclosing streams and eliminating temporary storage by replacing undersized upstream pipes are examples of changes which may decrease time of concentration. Land use changes may also decrease time of concentration since water will flow more quickly over paved surfaces. Since higher rainfall intensities occur for shorter storm durations, changes in time of concentration can have a significant impact on runoff. Drainage areas are sometimes altered and flow diverted from one watershed to another.

Hydraulic Analysis

The factors within a hydraulic analysis affecting a culvert's capacity may include headwater depth (see Figure 14.1.24), tailwater depth, inlet geometry, the slope of the culvert barrel, barrel area, barrel length, and the roughness of the culvert barrel. The various combinations of the factors affecting flow can be grouped into two types of conditions in culverts:

Inlet Control

Under inlet control the discharge from the culvert is controlled at the entrance of the culvert by headwater depth and inlet geometry (see Figure 14.1.24). Inlet geometry includes the cross-sectional area, shape, and type of inlet edge. Inlet control governs the discharge as long as water can flow out of the culvert faster than it can enter the culvert.

Schematic showing the factors that affect the culvert discharge

Figure 14.1.24 Factors Affecting Culvert Discharge (Source: Concrete Pipe Design Manual, American Concrete Pipe Association, April 2007

D = Inside diameter for a circular pipe

HW = Headwater depth at culvert entrance

L = Length of culvert

N = Surface roughness of the pipe wall, usually expressed in terms of Manning's n

So = Slope of the culvert pipe

TW = Tailwater depth at culvert outlet

Most culverts, except those in flat terrain, operate under inlet control during peak flows. Since the entrance characteristics govern, minor modifications at the culvert inlet can significantly affect hydraulic capacity. For example, change in the approach alignment of the stream may reduce capacity, while the improvement of the inlet edge condition, or addition of properly designed headwalls and wingwalls, may increase the capacity.

Outlet Control

Under outlet control water can enter the culvert faster than water can flow through the culvert. The discharge is influenced by the same factors as inlet control plus the tailwater depth and barrel characteristics (slope, length, and roughness). Culverts operating with outlet control usually lie on flat slopes or have high tailwater.

When culverts are operating with outlet control, changes in barrel characteristics or tailwater depth may affect capacity. For example, increased tailwater depth or debris in the culvert barrel may reduce the capacity.

Special Hydraulic Considerations

Inlet and Outlet Protection

The inlets and outlets of culverts may require protection to withstand the hydraulic forces exerted during peak flows. Inlet ends of flexible pipe culverts, which are not adequately protected or anchored, may be subject to entrance failures due to buoyant forces. The outlet may require energy dissipators to control erosion and scour and to protect downstream properties. High outlet velocities may cause scour which undermines the headwall, wingwalls, and culvert barrel. This erosion can cause end-section drop-off in rigid sectional pipe culverts.

Protection Against Piping

Seepage along the outside of the culvert barrel may remove supporting material. This process is referred to as “piping”, since a hollow cavity similar to a pipe is often formed. Piping can also occur through open joints. Piping is controlled by reducing the amount and velocity of water seeping along the outside of the culvert barrel. This may require watertight joints and in some cases anti-seep collars. Good backfill material and adequate compaction of that material are also important.

14.1.8 Factors Affecting Culvert Performance

Some of the common factors that can affect the performance of a culvert include the following:

14.1.9 Types and Locations of Culvert Distress

Types of Distress

The combination of high earth loads, long pipe-like structures and running water tends to produce the following types of distress:

Photograph of the bending or shear failure of a steel culvert

Figure 14.1.25 Bending or Shear Failure

Photograph of the cracking of a culvert end treatment due to the settlement of the foundation

Figure 14.1.26 Cracking of Culvert End Treatment Due to Foundation Settlement

Photograph of scour and undermining at the culvert inlet

Figure 14.1.27 Scour and Undermining at Culvert Inlet

 

Photograph of debris and settlement buildup at the culvert inlet

Figure 14.1.28 Debris and Sediment Buildup

Inspection Locations

A logical sequence for inspecting culverts helps ensure that a thorough and complete inspection will be conducted. In addition to the culvert components, look for high water marks, changes in the drainage area, and other indications of potential problems. In this regard, the inspection of culverts is similar to the inspection of bridges.

For typical installations, it is usually convenient to begin the field inspection with general observations of the overall condition of the structure and inspection of the approach roadway. Select one end of the culvert and inspect the embankment, waterway, headwalls, wingwalls, and culvert barrel. Progress to the other end of the culvert. The following sequence is applicable to all culvert inspections:

Overall Condition

General observations of the condition of the culvert are made while approaching the culvert area. The purpose of these initial observations is to familiarize the inspector with the structure. They may also point out a need to modify the inspection sequence or indicate areas requiring special attention. Remain observant for changes in the drainage area that might affect runoff characteristics and hydraulic analyses.

Approach Roadway and Embankment

Inspection of the approach roadway and embankment includes an evaluation of the functional adequacy (see Figure 14.1.29).

Inspect the approach roadway and embankment for the following functional requirements:

Photograph showing the approach roadway at a culvert site

Figure 14.1.29 Approach Roadway at a Culvert Site

Defects in the approach roadway and embankment may be indicators of possible structural or hydraulic problems in the culvert. Inspect the approach roadway and embankment for the following conditions:

Examine approach roadways for sudden dips, cracks, and sags in the pavement. These usually indicate excessive deflection of the culvert or inadequate compaction of the backfill material.

New pavement can temporarily hide approach problems. It is advisable for the inspector to have previous inspection reports that may indicate the age of the present overlay (see Figure 14.1.30).

Photograph of a repaired roadway over a culvert

Figure 14.1.30 Repaired Roadway Over a Culvert

It is important to note that not all defects in the approach roadways have an adverse affect on the culvert. Deterioration of the pavement may be due to excessive traffic and no other reason.

Embankment

Inspect the embankment around the culvert entrance and exit for slide failures in the fill around the box (see Figure 14.1.31). Check for debris at the inlet and outlet and within the culvert. Also note if vegetation is obstructing the ends of the culvert.

Photograph of a slide failure

Figure 14.1.31 Slide Failure

End Treatments

The SI&A Inspection Sheet does not specifically address end treatments in terms of inventory data or condition. The condition rating of end treatments is part of SI&A Item 62, Culvert Condition, and can have an impact on SI&A Item 67, Structural Evaluation.

Inspections of end treatments primarily involve visual inspection, although hand tools such as a plumb bobs, hammers, and probing rods are used to check for misalignment, sound for defects, and check for scour and undermining. In general, inspect headwalls for movement or settlement, cracks, deterioration, and traffic hazards (see Figure 14.1.32). Check culvert ends for undermining, scour, and evidence of piping.

Photograph of a headwall and wingwall end treatment on a concrete box culvert

Figure 14.1.32 Headwall and Wingwall End Treatment on Box Culvert

The most common types of box culvert end treatments are:

Both end treatment types use wingwalls to retain the embankment around the opening.

Inspect wingwalls to ensure they are in proper vertical alignment (see Figures 14.1.32 and 14.1.33). Wingwalls may be tilted due to settlement, slides or scour. See Topic 12.1 for a detailed description of defects and inspection procedures of wingwalls.

Photograph that shows the potential for tilted wingwalls

Figure 14.1.33 Potential for Tilted Wingwalls

Skewed Ends - Skewing the end of a culvert has nearly the same effect on structural capacity as does mitering (see Figure 14.1.34). Stresses increase because a full box shape is not present at the end.

Photograph of the skewed end of a culvert

Figure 14.1.34 Skewed End

Headwalls — Inspect headwalls and wingwalls for undermining and settlement. Cracking, tipping or separation of culvert barrel from the headwall and wingwalls is usually evidence of undermining (see Figure 14.1.35).

Photograph of the culvert headwall and wingwall end treatment

Figure 14.1.35 Culvert Headwall and Wingwall End Treatment

Appurtenance Structures

Typical appurtenance structures are:

Aprons — Check aprons for any undermining or settlement. Also inspect the joints between the apron and headwalls to see if they are watertight (see Figure 14.1.36). Piping may occur is water is allowed contact with the culvert outer surfaces.

Photograph of a culvert apron

Figure 14.1.36 Apron

Energy Dissipaters — Energy dissipaters may include stilling basins, riprap or other devices. Inspect energy dissipaters for material defects, settlement, undermining, and overall effectiveness (see Figure 14.1.37).

Photograph of an energy dissipater

Figure 14.1.37 Energy Dissipater

Culvert Barrel

Inspect the full length of the culvert from the inside. Visually examine all components of the culvert barrel including walls, floor, top slab, and joints. It is important to time the inspection so that water levels are low. Culverts with small diameters can be inspected by looking through the culvert from both ends or by using a small movable camera. The condition of the culvert barrel is rated under SI&A Item 62, which covers all structural components of a culvert.

Inspect the culvert barrels for defects such as misalignment, joint defects, cracking, spalling, section loss, and other material defects. For a detailed description of culvert inspection, refer to Topic 14.2 for rigid culverts or Topic 14.3 for flexible culverts.

14.1.10 Durability

Although the structural condition is a very important element in the performance of culverts, durability problems are probably the most frequent cause of replacement. Culverts are more likely to "wear away" than fail structurally. Durability is affected by two mechanisms: corrosion and abrasion. See Topics 14.2 and 14.3 for detailed explanations on how abrasion and corrosion affects the durability of rigid and flexible culverts.

14.1.11 Soil and Water Conditions that Affect Culverts

Certain soil and water conditions have been found to have a strong relationship to accelerated culvert deterioration. These conditions are referred to as "aggressive" or "hostile." The most significant conditions of this type are:

pH Extremes

pH is a measure of the relative acidity or alkalinity of water. A pH of 7.0 is neutral; values of less than 7.0 are acid, and values of more than 7.0 are alkaline. For culvert purposes, soils or water having a pH of 5.5 or less are strongly acid and those of 8.5 or more are strongly alkaline.

Acid water stems from two sources, mineral and organic. Mineral acidity comes from sulfurous wells and springs, and drainage from coal mines. These sources contain dissolved sulfur and iron sulfide which may form sulfurous and sulfuric acids. Mineral acidity as strong as pH 2.3 has been encountered. Organic acidity usually found in swampy land and barnyards rarely produce a pH of less than 4.0. Alkalinity in water is caused by strong alkali-forming minerals and from limed and fertilized fields. Acid water (low pH) is more common to wet climates and alkaline water (high pH) is more common to dry climates. As the pH of water in contact with culvert materials, either internally or externally, deviates from neutral, 7.0, it generally becomes more hostile.

Electrical Resistivity

This measurement depends largely on the nature and amount of dissolved salts in the soil. The greater the resistance the less the flow of electrical current associated with corrosion. High moisture content and temperature lower the resistivity and increase the potential for corrosion. Soil resistivity generally decreases as the depth increases. The use of granular backfill around the entire pipe will increase electrical resistivity and will reduce the potential for galvanic corrosion.

Several states rely on soil and water resistivity measurements as an important index of corrosion potential. Some states and the FHWA have published guidelines that use a combination of the pH and electrical resistivity of soil and water to indicate the corrosion potential at proposed culvert sites. The collection of pH and electrical resistivity data during culvert inspections can provide valuable information for developing local guidelines.

Soil Characteristics

The chemical and physical characteristics of the soil, which will come into contact with a culvert, can be analyzed to determine the potential for corrosion. The presence of base-farming and acid-forming chemicals is important. Chlorides and other dissolved salts increase electrical conductivity and promote the flow of corrosion currents. Sulfate soils and water can be erosive to metals and harmful to concrete. The permeability of soil to water and to oxygen is another variable in the corrosion process.

14.1.12 Culvert Protective Systems

There are several protective measures that can be taken to increase the durability of culverts. The more commonly used measures are:

Extra Thickness

For some aggressive environments, it may be economical to provide extra thickness of concrete or metal.

Bituminous Coating

This is the most common protective measure used on corrugated steel pipe. This procedure can increase the resistance of metal pipe to acidic conditions if the coating is properly applied and remains in place. Careful handling during transportation, storage, and placement is required to avoid damage to the coating. Bituminous coatings can also be damaged by abrasion. Make field repairs when bare metal has been exposed. Fiber binding is sometimes used to improve the adherence of bituminous material to the metallic-coated pipe.

Bituminous Paved Inverts

Paving the inverts of corrugated metal culverts to provide a smooth flow and to protect the metal has sometimes been an effective protection from particularly abrasive and corrosive environments. Bituminous paving is usually at least 1/8 inch thick over the inner crest of the corrugations. Generally only the lower quadrant of the pipe interior is paved. Fiber binding is sometimes used to improve the adherence of bituminous material to the metallic-coated pipe.

Other Coatings

There are several other coating materials that are being used to some degree throughout the country. Polymeric, epoxy, fiberglass, clay, and reinforced concrete field paving, have all been used as protection against corrosion. Galvanizing is the most common of the metallic coatings used for steel. It involves the application of a thin layer of zinc on the metal culvert. Other metallic coatings used to protect steel culverts are aluminum and aluminum-zinc.

Topic 14.2 Rigid Culverts

14.2.1 Introduction

Culverts are classified as rigid culverts when the load-carrying capacity of the culvert is primarily provided by the structural strength of the culvert, with little strength developed from the surrounding soil. By this definition, rigid culverts do not bend or deflect appreciably when loaded.

Unlike bridges, culverts have no distinction between substructure and superstructure. Culverts also have no "deck", since earth backfill separates the culvert structure from the riding surface (see Figure 14.2.1).

Photograph of a rigid culvert

Figure 14.2.1 Rigid Culvert

14.2.2 Design Characteristics

Concrete Culverts

Concrete culverts are the most common type of rigid culverts used today. Types of concrete culverts include:

See Figures 14.2.25a through 14.2.25c at the end of this topic for standard sizes of concrete pipes and Figure 14.2.26 at the end of this topic for standard concrete pipe shapes.

Concrete Box Culverts

One of the most common rigid culverts used today is the concrete box culvert (see Figure 14.2.2). A box culvert has an integral bottom slab that supports the side walls and provides a lined channel for the water to flow. The dimensions of the box culvert are determined by hydraulic, structural and geotechnical design criteria, as well as site constraints, which include channel dimensions and the amount of available cover. Box culverts are used in a variety of circumstances for both small and large channel openings and are easily adaptable to a wide range of site conditions, including sites that require low profile structures. In situations where the required size of the opening is very large, a multi-cell box culvert can be used (see Figure 14.2.3). It is important to note that although a box culvert may have multiple barrels, it is still a single structure. The internal walls are provided to reduce the unsupported length of the top slab.

Photograph of a concrete box culvert

Figure 14.2.2 Concrete Box Culvert

Photograph of a multi-cell concrete box culvert

Figure 14.2.3 Multi-Cell Concrete Box Culvert

There are two basic types of concrete box culverts: cast-in-place and precast. Precast concrete box culverts are generally the preferred type of concrete box culvert. For situations with complex site geometries or other special applications, cast-in-place concrete box culverts may be the preferred choice.

Cast-in-Place

Reinforced cast-in-place (CIP) concrete box culverts are typically constructed with multiple cells (barrels) to accommodate longer spans. The major advantage of cast-in-place construction is that the culvert can be designed to meet the specific geometric requirements of the site. Cast-in-place box culverts are also generally preferred for special applications, such as side- or slope-tapered inlets, aquatic organism passage, or customized fit with other infrastructure including additional culverts, stormdrains and drop inlets.

Precast

Precast concrete box culverts are designed for various depths of cover and various live loads and are manufactured in a wide range of sizes. One of the major advantages of precast concrete box culverts is the increased speed of construction. Standard box sections are available with spans as large as 12 feet (see Figure 14.2.4). Some box sections may have spans of up to 20 feet if a special design is used.

ASTM C 1433 Precast Reinforced Concrete Box Sections for Culverts, Storm Drains and Sewers is an industry recognized reference. These specifications cover single-cell precast reinforced concrete box sections intended to be used for the construction of culverts for the conveyance of storm water, industrial wastes, and sewage.

Photograph of a precast concrete box culvert

Figure 14.2.4 Precast Concrete Box Culvert

Concrete Pipe Culverts

Precast concrete pipe culverts are manufactured in three standard shapes:

Circular pipe culverts are very common (see Figure 14.2.5). In situations where the required size of the opening is very large, two or more concrete pipe culverts may be used (see Figure 14.2.6).

Photograph of a concrete pipe culvert

Figure 14.2.5 Concrete Pipe Culvert

Photograph of twin concrete pipe culverts

Figure 14.2.6 Twin Concrete Pipe Culvert

The size of the opening is primarily determined by the following factors: a) magnitude of the peak design flow; b) allowable headwater (pooled water surface) at the inlet for the peak design flow; c) permissible barrel and outlet flow velocities; and d) aquatic organism passage design considerations. The circular shape is the most common shape manufactured for pipe culverts. It is hydraulically and structurally efficient under most conditions. Elliptical shapes are used in situations where horizontal or vertical clearance is limited. The oblong shape allows the pipe to fit where a circular pipe may not, but still allows for the necessary size opening. Elliptical shaped pipe culverts may also be used when a wider section is desirable for low flow levels. No matter the shape, a pipe culvert tends to reduce the flow area of the design discharge, and possibly lesser flows, thereby increasing the flow velocity. An increased flow velocity has greater potential to scour the streambed at the outlet of the pipe.

Concrete culvert pipe is manufactured in up to five standard strength classifications. Higher classification numbers indicate higher strength. All of these standard shapes are manufactured in a wide range of sizes. Circular and elliptical pipes are available with standard sizes as large as 12 feet in diameter, with larger sizes available for special designs. Several factors such as span length, vertical and horizontal clearance, peak stream flow and terrain determine which shape of pipe culvert is used.

Concrete Arch Culverts

An arch culvert is a curved-shape culvert that works primarily in compression and does not have a bottom, or floor (see Figure 14.2.7). This type of culvert, as well as embedded culverts (i.e., culverts having buried inverts), are commonly and effectively used at stream crossings required to provide aquatic organism passage.

A variation of the arch culvert is the tied arch culvert. It is basically the same as the arch culvert, but it has an integral floor serving as a tie between the ends of the arch. Concrete arch culverts are either cast-in-place or precast.

Photograph of a concrete arch culvert

Figure 14.2.7 Concrete Arch Culvert

Concrete Frame Culverts

Concrete frame culverts are either cast-in-place or precast reinforced concrete, which is generally shaped similar to a box culvert. It differs from a box culvert, however, since there is no floor in a frame culvert (see Figure 14.2.8). Rigid culverts with a natural bottom (by way of embedment or having an open bottom) are commonly used to provide for aquatic organism passage.

Photograph of a concrete frame culvert

Figure 14.2.8 Concrete Frame Culvert

Masonry Culverts

Stone Masonry Arch Culverts

Stone and brick are durable, low maintenance materials. Currently stone and brick are seldom used for constructing new culvert barrels. Stone masonry culverts, when constructed, were usually in the shape of an arch (see Figure 14.2.9).

Photograph of a stone masonry arch culvert

Figure 14.2.9 Stone Masonry Arch Culvert

Timber Culverts

Timber Box Culverts

There are a limited amount of timber culverts throughout the nation. Timber culverts are generally box culverts and are constructed from individual timbers similar to railroad ties (see Figure 14.2.10). These culverts are normally utilized in areas of seasonal flows, such as heavy flow in the spring and little to no flow during the summer months.

Photograph of a timber box culvert

Figure 14.2.10 Timber Box Culvert

Loads on Culverts

There are several basic loads applied in the design of a culvert and include:

Box Culverts

Box culverts face similar types of loads on each slab and wall of the culvert (see Figure 14.2.11).

Schematic showing the loads on concrete and timber box culverts

Figure 14.2.11 Loads on a Concrete and Timber Box Culvert

Pipe Culverts

Pipe culverts are subject to the same types of forces that are placed upon the box culverts which are dead loads, vertical earth pressure, horizontal earth pressure, and live loads

Arch and Frame Culverts

Arch and frame culverts have the same types of loads as box culverts.

For a detailed description of loads on pipe, arch and frame culverts, see Topics 14.1.3.

Primary and Secondary Members

Primary members for culverts may vary based upon the type of culvert. Primary members for the various types of culverts are:

There are no secondary members for the culvert barrels. Wingwalls and headwalls are discussed in Topic 14.2.4 inspection locations.

Steel Reinforcement for Concrete Culverts

Steel reinforcement for culverts is in the form of either primary or secondary reinforcement. Depending upon the potential for corrosion, chemical attack or other steel reinforcement deficiencies, states may use epoxy-coated reinforcing bars. Some states have also incorporated stainless steel reinforcement into concrete culverts.

Primary Reinforcement

The primary reinforcing steel for box culverts resists tension and shear forces. Tension reinforcement is placed transversely in the box culvert slabs and vertically in the walls. Shear reinforcement may be placed diagonally in each of the box culvert corners (see Figure 14.2.12). Single cell precast concrete box culverts may use steel welded wire for tension and shear reinforcement.

Primary reinforcement for arch (see Figure 14.2.14) and pipe culverts (see Figure 14.2.15) also resists tension and shear. Arch and pipe culvert primary reinforcement is placed transversely in the walls of the culverts.

Secondary Reinforcement

Longitudinal temperature and shrinkage reinforcement is placed in the slabs and the walls of box culverts (see Figure 14.2.12).

Ducts may be provided in the precast box sections for optional longitudinal post-tensioning of the boxes with high strength steel strands or bars ( see Figure 14.2.13).

Secondary reinforcement for arch ( see Figure 14.2.14) and pipe culverts ( see Figure 14.2.15) follow the shape of the culvert itself from support to support

Schematic of the steel reinforcement in a concrete box culvert

Figure 14.2.12 Steel Reinforcement in a Concrete Box Culvert

Schematic of the post-tensioning steel ducts located in a precast concrete box culvert section

Figure 14.2.13 Precast Box Section with Post-tensioning Steel Ducts

Schematic of steel reinforcement in a concrete arch culvert

Figure 14.2.14 Steel Reinforcement in a Concrete Arch Culvert

Schematic of steel reinforcement in a concrete pipe culvert

Figure 14.2.15 Steel Reinforcement in a Concrete Pipe Culvert

14.2.3 Overview of Common Deficiencies

Common deficiencies that occur in concrete rigid culverts include:

Refer to Topic 6.2 for a detailed explanation of the properties of concrete, types and causes of concrete deterioration, and the examination of concrete.

Common deficiencies that occur in masonry rigid culverts include:

Refer to Topic 6.5.4 for a detailed explanation of the properties of masonry, types and causes of masonry deterioration, and the examination of masonry.

Common deficiencies that occur in timber rigid culverts include:

Refer to Topic 6.1.5 for a detailed explanation of the properties of timber, types and causes of timber deterioration, and the examination of timber.

14.2.4 Inspection Methods and Locations

Previous inspection reports and as-built plans, when available, are reviewed prior to, and during, the field inspection. Review of previous reports familiarizes the inspector with the structure and makes detection of changed conditions easier. Reviewing the previous inspection reports also indicate critical areas that need special attention and the possible need for special equipment.

A logical sequence for inspecting culverts helps ensure that a thorough and complete inspection is conducted. In addition to the culvert components, the inspector looks for high-water marks, changes in the drainage area, settlement of the roadway, and other indications of potential problems. In this regard, the inspection of culverts is similar to the inspection of bridges.

Methods

Inspection methods for various rigid culvert materials include timber Topic 6.1.7, concrete Topic 6.2.8, and masonry Topic 6.5.6.

Visual
Concrete

The inspection of concrete culverts for cracks, spalls, and other deficiencies is primarily a visual activity.

Masonry

The inspection of masonry culverts for cracks, loose or missing mortar, vegetation, water seepage, crushing, missing stones, bulging, and misalignment is primarily a visual activity.

Timber

The inspection of timber culverts for checks, splits, shakes, fungus decay, deflection, and loose fasteners is primarily a visual activity.

Physical
Concrete

Hammer sounding of the exposed concrete is performed to determine areas of delamination. A delaminated area has a distinctive hollow “clacking” sound when tapped with a hammer. A hammer hitting sound concrete results in a solid “pinging” type sound.

Masonry

Physical inspection of a masonry culvert is similar to that of concrete.

Timber

Hammer sounding of the exposed timber is performed to determine areas of internal decay. If the area has internal decay, there is a hollow sound when the hammer is tapped.

Advanced Inspection Methods
Concrete/Masonry

Several advanced methods are available for concrete and masonry inspection. Nondestructive methods, described in Topic 15.2.2, include:

Other methods, described in Topic 15.2.3, include:

Timber

Several advanced methods are available for timber inspection. Nondestructive methods, described in Topic 15.1.2, include:

Other methods, described in Topic 15.1.3, include:

Locations

Areas Subjected to Movement and Misalignment
Vertical Movement

Vertical movement can occur in the form of uniform settlement or differential settlement. Uniform settlement has little effect on the culvert. However, differential settlement can produce severe distress which varies in magnitude based upon the span length. This may cause cracking of the culvert. See Topic 6.2 for a detailed presentation of concrete deficiencies including cracking. Common causes of vertical movement are soil bearing failure, consolidation of soil, scour, undermining, and subsidence from mining or solution cavities. Locations to inspect for vertical movement include the following:

Lateral Movement

Lateral movement occurs when the horizontal earth pressure acting on the walls exceeds the friction forces that hold the structure in place. Common causes of lateral movement are slope failure, seepage, changes in soil characteristics (i.e. frost and ice), and time consolidation of the original soil. Locations to inspect for lateral movement include the following:

Rotational Movement

Rotational movement, or tipping, of the culvert is generally the result of unsymmetrical settlements or lateral movements due to horizontal earth pressure. Common causes are undermining, scour, saturation of backfill, and improper design. Locations to inspect for rotational movement include the following:

Vertical and horizontal misalignment is checked by visual observation. Look for culvert sagging, cracking or separation of joints in precast culverts. Sags can best be detected during low flows by looking for areas where the water is deeper or where sediment has been deposited. Sags may also trap water which may further aggravate settlement problems by saturating the soil.

When excessive accumulations of sediment are present, it may be necessary to have the sediment removed before checking for sags. An alternate method is to take profile elevations of the top slab. Check horizontal alignment or bulging for straightness or smooth curvature for those culverts that were constructed with a curved alignment. It can be checked by sighting along the walls and by examining joints for differential movement (see Figure 14.2.16).

Alignment problems may be caused by improper installation, undermining or uneven settlement of the fill. It is important to determine which of these problems may be causing the settlement. If it is determined that undermining is the cause, notify maintenance forces since the damage will continue until the problem is corrected. Also, try to determine whether the undermining is due to piping (loss of fill from underneath the culvert), water exfiltration or infiltration of backfill material. Look for holes in the downstream side embankment. If the misalignment is due to improper installation or uneven settlement, repeat inspections may be necessary to determine if the settlement is progressing or if it has stabilized.

Photograph of an inspector using sighting along a culvert sidewall used to check for horizontal alignment

Figure 14.2.16 Sighting Along Culvert Sidewall to Check Horizontal Alignment

Bearing Areas

Bearing zones for rigid culverts will be located where the footing is supported by the earth. For concrete and masonry culverts, look for cracking and spalling. In timber culverts, look for crushing.

Shear Zones

Horizontal and vertical forces can cause high shear zones in culvert walls or slabs. For concrete and masonry culverts, look for diagonal cracking. In timber culverts, look for splitting.

Flexural Zones

High flexural moments are caused by horizontal and vertical forces which occur at the slabs and culvert walls. These moments cause compression and tension depending on the load type and location of the neutral axis. Look for deficiencies caused by overstress due to compression or tension caused by flexural moments. Check compression areas for splitting, crushing or buckling. Check tension areas for cracking or distortion.

Areas Exposed to Drainage

Examine areas that are exposed to drainage for decay on timber culverts. For concrete culverts, examine for spalling, delamination and exposed rebar (see Figure 14.2.17). Also inspect concrete culvert headwalls and wingwalls, since these areas are often exposed to surface drainage carrying road salts, which chemically attack and destroy the walls. In masonry culverts, look for spalling, delamination, and seepage which can result in stone and mortar deterioration with the eventual loosening and/or the loss of stones (see Figure 14.2.18).

Photograph showing spalls and delaminations on the top slab of a concrete box culvert

Figure 14.2.17 Spalls and Delaminations on Top Slab of Concrete Box Culvert

Photograph showing missing stones in a masonry culvert

Figure 14.2.18 Missing Stones in Masonry Culvert

Areas Exposed to Traffic

Check for collision damage from vehicles passing adjacent to the culvert.

Damage to concrete culverts may include spalls and exposed reinforcement and possibly steel reinforcement section loss. Damage to timber culverts includes split or broken members.

Scour and Undermining

Scour is the removal of material from a streambed as a result of the erosive action of running water. Scour can cause undermining or the removal of supporting foundation material from beneath the culvert. Refer to Topic 13.2 for a more detailed description of scour and undermining.

Inspection for scour includes probing around the culvert inlet and outlet for signs of undermining. Sometimes silt loosely fills in a scour hole and offers no protection or bearing capacity for the culvert inlet and outlet. Also check timber culverts frames (no floors) for these conditions.

Joints

Expansion joints are carefully inspected to verify that the filler material or joint sealant is in place and that the joint is not filled with incompressible material that would prohibit expansion (see Figure 14.2.19). When inspecting a joint in a rigid culvert, be sure to check for the following deficiencies:

Photograph of a precast concrete box culvert joint

Figure 14.2.19 Precast Concrete Box Culvert Joint

Cracks
Longitudinal Cracks

Concrete is strong in compression but weak in tension. Reinforcing steel is provided to accommodate the tensile stresses. Hairline longitudinal cracks in the crown or invert indicate that the steel has accepted part of the load. Cracks less than 0.01 inches in width are minor and only need to be noted in the inspection report. Document cracks greater than 0.01 inches in width but less than 0.1 inches, in the inspection report and noted as possible candidates for maintenance. Longitudinal cracking in excess of 0.1 inches in width may indicate overloading or poor bedding. If the pipe is placed on hard material and backfill is not adequately compacted around the pipe or under the haunches of the pipe, loads will be concentrated along the bottom of the pipe and may result in flexure or shear cracking (see Figure 14.2.20).

Schematic showing flexure longitudinal cracking due to poor side support, shear cracking due to poor haunch support, and no cracking due to good side support

Figure 14.2.20 Longitudinal Cracks in Pipe Culvert

Also note other signs of distress such as differential movement, efflorescence, spalling, or rust stains. When cracks are wider than 0.1 inches, take measurements of the fill height and the diameter of the pipe both horizontally and vertically to permit analysis of the original design. Crack measurements and photographs are useful for monitoring conditions during subsequent inspections.

Transverse Cracks

Transverse cracks may also be caused by poor bedding (see Figure 14.2.21). Cracks can occur across the bottom of the pipe (broken belly) when the pipe is only supported at the ends of each section. This is generally the result of poor installation practices such as not providing indentions (bell holes) in hard foundation material for the ends of bell and spigot-type pipe or not providing a sufficient depth of suitable bedding material. Cracks may occur across the top of pipe (broken back) when settlement occurs and rocks or other areas of hard foundation material near the midpoint of a pipe section are not adequately covered with suitable bedding material.

Schematic showing how a properly prepared bedding evenly distributes loads
Schematic showing how an improperly prepared bedding may result in stress concentrations, leading to transverse cracks in a pipe culvert

Figure 14.2.21 Transverse Cracks in Pipe Culvert

Spalls

A spall is a depression in the concrete resulting from the separation and removal of a portion of the surface concrete, revealing a fracture roughly parallel to the surface of the concrete. In precast concrete culverts, spalls often occur along the edges of either longitudinal or transverse cracks when the crack is due to overloading or poor support rather than simple tension cracking. Spalling may also be caused by the corrosion of the steel reinforcing when water is able to reach the steel through cracks or shallow cover. As the steel corrodes, the oxidized steel expands, causing the concrete covering the steel to spall. Spalling may be detected by visual examination of the concrete along the edges of cracks. Perform tapping with a hammer along cracks to check for areas that have fractured but are not visibly separated. These areas will produce a hollow sound when tapped. These areas may be referred to as delaminations.

Slabbing

Slabbing, also known as shear-slabbing or slab shear, refers to a radial failure of the concrete which occurs from straightening of the reinforcement cage due to excessive deflection. This is characterized by large slabs of concrete "peeling" away from the sides of the pipe and a straightening of the reinforcing steel (see Figure 14.2.22). Slabbing may be a severe problem that can occur under high fills.

Photograph showing shear slabbing

Figure 14.2.22 Shear Slabbing (Source: FHWA Culvert Inspection Manual)

Durability

Durability is a measure of a culvert's ability to withstand chemical attack and abrasion. Rigid culvers are subject to chemical attack in strongly acidic environments such as drainage from mines and may also be damaged by abrasion. Note any mild deterioration or abrasion that is less than 1/4 inch deep in the inspection report. Document severe surface deterioration greater than 1/4 inch deep as a potential candidate for maintenance. When the invert is completely deteriorated, it may be considered a critical finding. Note in the report when linings are used to protect against chemical attack or abrasion. Also document the condition of the lining, if present.

End Section Drop-off

This type of distress is usually due to outlet erosion as discussed earlier in the sections on end treatments and waterways. It is caused by the erosion of the material supporting the pipe sections on the outlet end of the culvert barrel.

Wingwalls and Headwalls

Wingwalls and headwalls are provided to support the embankment around the openings of the culvert (see Figure 14.2.23). Inspect wingwalls for differential settlement and proper vertical alignment. See Topic 14.1 for general culvert characteristics including wingwalls and Topic 12.1 for a detailed description of deficiencies and inspection methods of wingwalls.

Photograph of a cast-in-place concrete headwall and wingwall

Figure 14.2.23 Cast-in-Place Concrete Headwall and Wingwall

14.2.5 Evaluation

State and Federal rating guideline systems have been developed to aid in the inspection of rigid culverts. The two major rating guideline systems currently in use are the FHWA's Recording and Coding Guide for the Structural Inventory and Appraisal of the Nation's Bridges used for the National Bridge Inventory (NBI) component condition rating method and the AASHTO Manual for Bridge Element Inspectionfor element level condition state assessment.

NBI Component Condition Rating Guidelines

Using NBI component condition rating guidelines, a one-digit code on the Federal Structure Inventory and Appraisal (SI&A) sheet indicates the condition of the culvert (Item 62). This item evaluates the alignment, settlement, joints, structural condition, scour, and other items associated with culverts. Component condition rating codes range from 9 to 0, where 9 is the best rating possible. See Topic 4.2 (Item 62) for additional details about NBI component condition rating guidelines. Item 62 component condition rating guidelines are included in Figure 14.2.24. It is also important to note that Items 58-Deck, 59-Superstructure, and 60-Substructure are coded “N” for culvert structures.

For rigid culverts, the NBI component condition rating guidelines yield a one-digit code on the Federal (SI&A) sheet that indicates the overall condition of the culvert. The culvert item not only evaluates the structural condition of the culvert, but also encompasses the alignment, settlement in the approach roadway and embankment, joints, scour, headwalls and wingwalls. Integral wingwalls are included in the evaluation up to the first construction or expansion joint. The one-digit code that best describes the culvert’s overall condition is chosen, and the component condition rating codes range from 9 to 0, where 9 is the highest possible component condition rating.

Consider previous inspection data along with current inspection findings to determine the correct component condition rating.

Element Level Condition State Assessment

In an element level condition state assessment of a rigid culvert, possible AASHTO National Bridge Elements (NBEs) and Bridge Management Elements (BMEs) are:

NBE No. Description

Substructure

241

Reinforced Concrete Culvert

242

Timber Culvert

243

Other Culvert

244

Masonry Culvert

245

Prestressed Concrete Culvert

BME No. Description

Wearing Surfaces and Protection Systems

520

Concrete Reinforcing Steel Protective System

521

Concrete Protective Coating

The unit quantity for culverts is feet and represents the culvert length along the barrel multiplied by the number of barrels (for multiple barrel culverts). The inspector visually evaluates each 1-foot slice of the culvert barrel(s) and assigns the appropriate condition state description. The total length is distributed among the four available condition states depending on the extent and severity of the deficiency. The unit quantity for protective coatings and protective systems is square feet, with the total area distributed among the four condition states depending on the extent and severity of the deficiency. The sum of all condition states equals the total quantity of the National Bridge Element or Bridge Management Element. Condition State 1 is the best possible rating. See the AASHTO Manual for Bridge Element Inspection for condition state descriptions.

 

The culvert item evaluates the alignment, settlement, joints, structural condition, scour, and other items associated with culverts. The rating code is intended to be an overall condition evaluation of the culvert. Integral wingwalls to the first construction or expansion joint shall be included in the evaluation.

Code Description

N

Not applicable. Use if structure is not a culvert.

9

No deficiencies.

8

No noticeable or noteworthy deficiencies which affect the condition of the culvert. Insignificant scrape marks caused by drift.

7

Shrinkage cracks, light scaling, and insignificant spalling which does not expose reinforcing steel. Insignificant damage caused by drift with no misalignment and not requiring corrective action. Some minor scouring has occurred near curtain walls, wingwalls, or pipes. Metal culverts have a smooth symmetrical curvature with superficial corrosion and no pitting.

6

Deterioration or initial disintegration, minor chloride contamination, cracking with some leaching, or spalls on concrete or masonry walls and slabs. Local minor scouring at curtain walls, wingwalls, or pipes. Metal culverts have a smooth curvature, non-symmetrical shape, significant corrosion, or moderate pitting.

5

Moderate to major deterioration or disintegration, extensive cracking and leaching, or spalls on concrete or masonry walls and slabs. Minor settlement or misalignment. Noticeable scouring or erosion at curtain walls, wingwalls, or pipes. Metal culverts have significant distortion and deflection in one section, significant corrosion or deep pitting.

4

Large spalls, heavy scaling, wide cracks, considerable efflorescence, or opened construction joint permitting loss of backfill. Considerable settlement or misalignment. Considerable scouring or erosion at curtain walls, wingwalls, or pipes. Metal culverts have significant distortion and deflection throughout, extensive corrosion or deep pitting.

3

Any condition described in Code 4 but which is excessive in scope. Severe movement or differential settlement of the segments, or loss of fill. Holes may exist in walls or slabs. Integral wingwalls nearly severed from culvert. Severe scour or erosion at curtain walls, wingwalls, or pipes. Metal culverts have extreme distortion and deflection in one section, extensive corrosion, or deep pitting with scattered perforations.

2

Integral wingwalls collapsed, severe settlement of roadway due to loss of fill. Section of culvert may have failed and can no longer support embankment. Complete undermining at curtain walls and pipes. Corrective action required to maintain traffic. Metal culverts have extreme distortion and deflection throughout with extensive perforations due to corrosion.

1

Bridge closed. Corrective action may put bridge back in light service.

0

Bridge closed. Replacement necessary.

Figure 14.2.24 NBI Component Condition Rating Guidelines for Culverts

Dimensions and Approximate Weights of Concrete Pipe

*ASTM C 76 — Reinforced Concrete Culvert, Storm Drain and Sewer Pipe, Tongue and Groove Joints

WALL A WALL B WALL C
Internal Diameter inches Minimum Wall Thickness, inches Approximate Weight, pounds per foot Minimum Wall Thickness, inches Approximate Weight, pounds per foot Minimum Wall Thickness, inches Approximate Weight, pounds per foot

96

8

2710

9

3090

3355

102

3078

3480

10¼

3760

108

9

3446

10

3865

10¾

4160

Large Sizes of Pipe Tongue and Groove Joint

Internal Diameter Inches Internal Diameter Feet Wall Thickness Inches Approximate Weight, pounds per foot

114

3840

120

10

10

4263

126

10½

10½

4690

132

11

11

5148

138

11½

11½

5627

144

12

12

6126

150

12½

12½

6647

156

13

13

7190

162

13½

13½

7754

168

14

14

8339

174

14½

14½

8942

180

15

15

9572

* For description of ASTM C 76 see page 14.2.30

Figure 14.2.25a Standard Sizes for Concrete Pipe (Source: American Concrete Pipe Association)

Typical Cross Section of Arch Pipe

Schematic of a typical cross section of horizontal and vertical ellipse pipes

Horizontal and Vertical Ellipse Pipe

Dimensions and Approximate Weights of Elliptical Concrete Pipe

*ASTM C 507 — Reinforced Concrete Elliptical Culvert, Storm Drain and Sewer Pipe

Equivalent Round Size, inches Minor Axis, inches Major Axis, inches Minimum Wall Thickness, inches Water-Way Area, square feet Approximate Weight, pounds per foot

96

77

121

52.4

3420

102

82

128

59.2

3725

108

87

136

10

66.4

4050

114

92

143

10½

74.0

4470

120

97

151

11

82.0

4930

132

106

166

12

99.2

5900

144

116

180

13

118.6

7000

* For description of ASTM C 507 see page 14.2.30

Figure 14.2.25b Standard Sizes for Concrete Pipe (Source: American Concrete Pipe Association)

Typical Cross Section of Arch Pipe

Schematic of a typical cross section of an concrete arch pipe

Dimensions and Approximate Weights of Concrete Arch Pipe

*ASTM C 506 — Reinforced Concrete Arch Culvert, Storm Drain and Sewer Pipe

Equivalent Round Size, inches Minimum Rise, inches Minimum Span, inches Minimum Wall Thickness, inches Water-Way Area, square feet Approximate Weight, pounds per foot

96

77 ¼

122

9

51.7

3110

108

87 ⅛

138

10

66.0

3850

120

96 ⅞

154

11

81.8

5040

132

106 ½

168 ¾

10

99.1

5220

* For description of ASTM C 506 see page 14.2.30

Figure 14.2.25c Standard Sizes for Concrete Pipe (Source: American Concrete Pipe Association)

American Society for Testing and Materials (ASTM) Descriptions for Select Rigid Pipe Culverts
ASTM C 76

Reinforced Concrete Culvert, Storm Drain, and Sewer Pipe: Covers reinforced concrete pipe intended to be used for the conveyance of sewage, industrial wastes, and storm waters, and for the construction of culverts. Class I — 60 inches through 144 inches in diameter; Class II, III, IV and V — 12 inches through 144 inches in diameter. Larger sizes and higher classes are available as special designs.

ASTM C 506

Reinforced Concrete Arch Culvert, Storm Drain, and Sewer Pipe: Covers pipe to be used for the conveyance of sewage, industrial waste, and storm water and for the construction of culverts in sizes from 15 inch through 132 inch equivalent circular diameter. Larger sizes are available as special designs.

ASTM C 507

Reinforced Concrete Elliptical Culvert, Storm Drain, and Sewer Pipe: Covers reinforced elliptically shaped concrete pipe to be used for the conveyance of sewage, industrial waste and storm water, and for the construction of culverts. Five standard classes of horizontal elliptical, 18 inches through 144 inches in equivalent circular diameter and five standard classes of vertical elliptical, 36 inches through 144 inches in equivalent circular diameter are included. Larger sizes are available as special designs.

Table showing the standard concrete pipe shapes, their range of sizes, and their common uses

Figure 14.2.26 Standard Concrete Pipe Shapes

(Source: FHWA Culvert Inspection Manual, Supplement to the BIRM, July 1986)

Topic 14.3 Flexible Culverts

14.3.1 Introduction

Like all culverts, flexible culverts are designed for full flow. Unlike bridges, culverts have no distinction between substructure and superstructure and because earth backfill separates the culvert structure from the riding surface, culverts have no "deck." Most flexible culverts have a circular or elliptical configuration (see Figure 14.3.1). Some flexible box and arch culverts are in use today (see Figure 14.3.2). From their design nature, flexible culverts have little structural bending strength without proper backfill. The material from which they are made, such as corrugated steel or aluminum can be flexed or bent and can be distorted significantly without cracking. Consequently, flexible culverts depend on the backfill support to resist bending. In flexible culvert designs, proper interaction between the soil and structure is critical.

Photograph of a pipe arch flexible culvert

Figure 14.3.1 Pipe Arch Flexible Culvert

Photograph of a flexible box culvert

Figure 14.3.2 Flexible Box Culvert

14.3.2 Design Characteristics

Structural Behavior

A flexible culvert is a composite structure made up of the culvert barrel and the surrounding soil. The barrel and the soil are both vital elements to the structural performance of the culvert.

Flexible pipe has relatively little bending stiffness or bending strength on its own. Flexible culvert materials include steel, aluminum, and plastic. As loads are applied to the culvert, it attempts to deflect. In the case of a round pipe, the vertical diameter decreases and the horizontal diameter increases (see Figure 14.3.3). When good embankment material is well-compacted around the culvert, the increase in horizontal diameter of the culvert is resisted by the lateral soil pressure. With round pipe the result is a relatively uniform radial pressure around the pipe which creates a compressive thrust in the pipe walls. As illustrated in Figure 14.3.4, the compressive thrust is approximately equal to vertical pressure times one-half the span length.

Schematic of how loads affect a flexible culvert, the load versus the shape

Figure 14.3.3 Flexible Culvert: Load vs. Shape

Schematic showing the formula for ring comprssion

Figure 14.3.4 Formula for Ring Compression

An arc of a flexible round pipe, or other shape will be stable as long as adequate soil pressures are achieved, and as long as the soil pressure is resisted by the compressive force C on each end of the arc. Good quality backfill material and proper installation are critical in obtaining a stable soil envelope around a flexible culvert.

In long span culverts the radius (R) is usually large. To prevent excessive deflection due to permanent dead and/or transient live loads, longitudinal or circumferential stiffeners are sometimes added. The circumferential stiffeners are usually metal ribs bolted to the outside of the culvert. Longitudinal stiffeners may be metal or reinforced concrete. Concrete thrust beams provide some circumferential stiffening as well as longitudinal stiffening. The thrust beams are added to the structure prior to backfill. They also provide a solid vertical surface for soil pressures to act on and a surface which is easier to backfill against. The use of concrete stress relieving slabs is another method used to achieve longer spans or reduce minimum cover. A stress-relieving slab is cast over the top of the backfill above the structure to distribute transient live loads to the adjacent soil.

14.3.3 Types and Shapes of Flexible Culverts

Flexible culverts are constructed from corrugated steel or aluminum pipe or field assembled structural plate products. Structural plate steel products are available as structural plate pipes, box culverts, or long span structures. See Figure 14.3.5 for standard shapes for corrugated flexible culverts.

Table of the standard corrugated steel culvert shapes, the range of sizes, and their common uses

Figure 14.3.5 (Exhibit 11 Culvert Inspection Manual Report No. FHWA-IP-86-2) Standard Corrugated Steel Culvert Shapes (Source: Handbook of Steel Drainage and Highway Construction Products, American Iron and Steel Institute)

Corrugated Pipe

Factory-made pipe is produced in two basic shapes: round and pipe arch. Both shapes are produced in several wall thicknesses, several corrugation sizes, and with annular (circumferential) or helical (spiral) corrugations. Pipes with helical corrugations have continuously welded seams or lock seams. Both round and arch steel pipe shapes are available in a wide range of standard sizes.

Structural Plate

Structural plate steel pipes are field assembled from standard corrugated galvanized steel plates. Standard plates have corrugations with a 6-inch pitch and a depth of 2 inches. Plates are manufactured in a variety of thicknesses and are pre-curved for the size and shape of structure to be erected.

Structural steel plate pipes are available in four basic shapes:

Structural plate aluminum pipes are field assembled with a 9 inch pitch and a depth of 2.5 inches.

Structural plate aluminum pipes are produced in five basic shapes:

Long Span Culverts

Long span steel culverts are assembled using conventional 6 by 2 inch corrugated galvanized steel plates and longitudinal and circumferential stiffening members. There are five standard shapes for long span steel structures:

Each long span installation represents, to a certain extent, a custom design. The inspector reviews the design or as-built plans when checking dimensions of existing long span structures.

Long span aluminum structures are assembled using conventional 9 by 2 1/2 inch corrugated aluminum plates and aluminum rib stiffeners. Long span aluminum structures are essentially the same size and available in the same five basic shapes as steel long spans.

See the end of this Topic for the different standard sizes for each flexible culvert shape (pg 164-193 Culvert Inspection Manual Report No. FHWA-IP-86-2)

Box Culverts

Corrugated steel box sections use standard corrugated galvanized steel plates with special reinforcing elements applied to the areas of maximum moments. Steel box culverts are available with spans that range from 10 feet to 21 feet.

The aluminum box culvert utilizes standard aluminum structural plates with aluminum rib reinforcing added in the areas of maximum bending stresses. Ribs are bolted to the exterior of the aluminum shell during installation. Aluminum box culverts are suitable for shallow depths of fill.

Plastic Culverts

Plastic culverts are most commonly made using high density polyethylene (HDPE). These round sections utilize one or more "walls" and are available up to 60 inches in diameter. Single-walled culverts are often corrugated on the inner and outer surfaces (see Figure 14.3.6), while dual-walled culverts have a smooth inner surface and either a smooth or corrugated outer surface (see Figure 14.3.7). Heavy-duty plastic culverts are also available in sizes up to 36 inches.

Schematic of a single walled plastic culvert

Figure 14.3.6 Schematic of a Single Walled Culvert

Schematics of two different dual walled plastic culverts: corrugated outer wall with a smooth inner wall (left culvert) and with a smooth inner and an outer wall (right schematic)

Figure 14.3.7 Schematics of Dual Walled Culverts

Plastic culverts offer several advantages over traditional corrugated metal pipe (CMP) sections:

Applications utilizing plastic culverts include:

14.3.4 Overview of Common Deficiencies

Common deficiencies that can occur to flexible culvert materials include the following:

Refer to Topic 6.3 for a more detailed presentation of the properties of steel, types and causes of steel deterioration, and the examination of steel.

14.3.5 Inspection Methods and Locations

Refer to Topic 14.1 for a more detailed presentation of methods and locations of culvert distress.

A logical sequence for inspecting culverts helps ensure that a thorough and complete inspection will be conducted. In addition to the culvert components, look for highwater marks, changes in the drainage area, settlement of the roadway, and other indications of potential problems. In this regard, the inspection of culverts is similar to the inspection of bridges.

For typical installations, it is usually convenient to begin the field inspection with general observations of the overall condition of the structure and inspection of the approach roadway. Select one end of the culvert and inspect the embankment, waterway, headwalls, wingwalls, and culvert barrel. Progress toward the other end of the culvert. The following sequence is applicable to all culvert inspections:

Method

Visual

Most defects in flexible culverts are first detected by visual inspection. In order for this to occur, a hands-on inspection, or inspection where the inspector is close enough to touch the area being inspected, is required. The types of defects to look for when inspecting the culvert barrel will depend upon the type of culvert being inspected. In general, inspect corrugated metal culvert barrels for cross-sectional shape and barrel defects such as joint defects (exhiltration or infiltration through joints or joint misalignment), seam defects (exhiltration or infiltration through seams or seam misalignment), plate buckling, lateral shifting, missing or loose bolts, corrosion, excessive abrasion, material deficiencies, and localized construction damage. A critical area for the inspection of long span metal culverts is at the 2 o'clock and 10 o'clock locations. An inward bulge at these locations may indicate potential failure of the structure.

It is becoming more common that flexible culverts are being repaired or rehabilitated with structural plate sections or with structural invert paving by using reinforced concrete. Inspect the concrete for deficiencies such as surface cracks, spalls, wear, and other deficiencies is primarily a visual activity. Structural plates can be visually inspected for deficiencies to those discussed previously for steel.

Physical

A geologist's pick hammer can be used to scrape off heavy deposits of rust and scale and to check the longitudinal seams by tapping the nuts. The hammer can then be used to locate areas of corrosion by striking the culvert walls. The walls will deform or the hammer will break through the culvert wall if significant section loss exists.

For aluminum structural plate, the bolts are checked with a torque wrench.

Sometimes surveying the culvert is necessary to determine if there is any shape distortion, and if there is distortion how much exists.

It is important to check the repairs for deficiencies as well. For concrete repairs, be sure to check for delaminations by using a hammer to “sound” the concrete. A delaminated area will have a distinctive hollow "clacking" sound when tapped with a hammer or revealed with a chain drag. A hammer hitting sound concrete will result in a solid "pinging" type sound.

For the structural plates, inspect for section loss. This is achieved by using a wire brush, grinder, or a hammer to remove loose or flaked steel and then measure the remaining section and compare to a similar section with no loss.

It may be necessary to get a permit to work in culverts due to the confined spaces which have the potential for hazardous conditions for the inspector.

Advanced Inspection Methods

In metal culverts, visual inspections can only point out surface defects. Therefore, advanced inspection methods may be used to achieve a more rigorous and thorough inspection of the flexible culvert, including:

Several advanced methods are available for steel inspection. Nondestructive methods, described in Topic 15.3.2, include:

Other inspection methods or tests for material properties, described in Topic 15.3.3, include:

Locations

End Treatments

End treatments are inspected like any other structural component. Their effectiveness can directly affect the performance of the culvert.

The most common types of end treatments for flexible culverts are:

Projections

Indicate the location and extent of any scour or undermining around the culvert ends. The depth of any scouring is measured with a probing rod. In low flow conditions scour holes have a tendency to fill up with debris or sediment. If no probing rod is used, the scour could be mistakenly reported as less than has taken place.

Inspect end treatments for evidence of water leaking around the end treatment and into the embankment. Water flowing along the outside of a culvert can remove supporting material. This is referred to as piping and it can lead to the culvert end being unsupported. If not repaired in time, piping can cause cantilevered end portions of the culvert to bend down and restrict the stream flow.

Mitered Ends

Inspection items for mitered ends are the same as for projecting ends. Take additional care to measure any deformation of the end. Mitering the end of corrugated pipe culvert reduces its structural capacity.

Pipe End Sections

Pipe end sections are typically used on relatively smaller culverts. For inspection purposes, treat the pipe end section similar to a projection.

Excerpts from a reproduction of the out-of-print Culvert Inspection Manual Report No.-IP-86-2 are located on page 14.3.12 of this topic.

14.3.6 Evaluation

State and Federal rating guideline systems have been developed to aid in the inspection of flexible culverts. The two major rating guideline systems currently in use are the FHWA's Recording and Coding Guide for the Structural Inventory and Appraisal of the Nation's Bridges used for the National Bridge Inventory (NBI) component condition rating method and the AASHTO Manual for Bridge Element Inspection for element level condition state assessment.

NBI Component Condition Rating Guidelines

Using NBI component condition rating guidelines, a one-digit code on the Federal Structure Inventory and Appraisal (SI&A) sheet indicates the condition of the culvert (Item 62). This item evaluates the alignment, settlement, joints, structural condition, scour, and other items associated with culverts. Component condition rating codes range from 9 to 0 where 9 is the best rating possible. See Topic 4.2 (Item 62) for additional details about NBI component condition rating guidelines. The component condition rating code is intended to be an overall evaluation of the culvert. Integral wingwalls to the first construction or expansion joint shall be included in the evaluation. It is also important to note that Items 58-Deck, 59-Superstructure, and 60-Substructure shall be coded "N" for all culverts.

Consider previous inspection data along with current inspection findings to determine the correct component condition rating.

Element Level Condition State Assessment

In an element level condition state assessment of a flexible culvert, possible AASHTO National Bridge Elements (NBEs) and Bridge Management Elements (BMEs) are:

NBE No. Description

Substructure

240

Steel Culvert

243

Other Culvert

BME No. Description

Wearing Surfaces and Protection Systems

515

Steel Protective Coating

The unit quantity for culverts is feet and represents the culvert length along the barrel multiplied by the number of barrels (for multiple barrel culverts). The inspector visually evaluates each 1-foot slice of the culvert barrel(s) and assigns the appropriate condition state description. The total length is distributed among the four available condition states depending on the extent and severity of the deficiency. The unit quantity for protective coatings is square feet, with the total area distributed among the four condition states depending on the extent and severity of the deficiency. The sum of all condition states equals the total quantity of the National Bridge Element or Bridge Management Element. Condition state 1 is the best possible rating. See the AASHTO Manual for Bridge Element Inspection for condition state descriptions.

The following excerpts are from a reproduction of the Culvert Inspection Manual Report No.-IP-86-2 — Chapter 5, Section 4 which can be found at the following website: http://www.fhwa.dot.gov/

Section 4 - CORRUGATED METAL CULVERTS
5-4.0 General

Corrugated aluminum and corrugated steel culverts are classified as flexible structures because they respond to and depend upon the soil backfill to provide structural stability and support to the culvert. The flexible corrugated metal acts essentially as a liner. The liner acts mainly in compression and can carry large ring compression thrust, but very little bending or moment force. (Rib reinforced box culverts are exceptions.) Inspection of the culvert determines whether the soil envelope provides adequate structural stability for the culvert and verifies that the "liner" is capable of carrying the compressive forces and protecting the soil backfill from water flowing through the culvert. Verification of the stability of the soil envelope is accomplished by checking culvert shape. Verification of the integrity of the "liner" is accomplished by checking for pipe and plate culvert barrel defects.

This section contains discussions on inspecting corrugated metal structures for shape and barrel defects. Because shape inspection requirements do vary somewhat for different shapes, separate sections with detailed guidelines are provided for corrugated metal pipe culvert shapes and long-span culvert shapes. Section 5 of this chapter addresses corrugated metal pipe culverts, and section 6 covers long-span corrugated metal culverts.

5-4.1 Shape Inspections

The single most important feature to observe and measure when inspecting corrugated metal culverts is the cross-sectional shape of the culvert barrel. The corrugated metal culvert barrel depends on the backfill or embankment to maintain its proper shape and stability. When the backfill does not provide the required support, the culvert will deflect, settle, or distort. Shape changes in the culvert therefore provide a direct indication of the adequacy and stability of the supporting soil envelope. By periodic observation and measurement of the culvert's shape, it is possible to verify the adequacy of the backfill. The design or theoretical cross-section of the culvert should be the standard against which field measurements and visual observations are compared. If the design cross section is unknown, a comparison can be made between the unloaded culvert ends and the loaded sections beneath the roadway or deep fills. This can often provide an indication of structure deflection or settlement. Symmetrical shape and uniform curvature around the perimeter are generally the critical factors. If the curvature around the structure becomes too flat, and/or the soil continues to yield under load, the culvert wall may not be able to carry the ring thrust without either buckling inward or deflecting excessively to the point of reverse curvature. Either of these events leads to partial or total failure.

As explained earlier in this Topic, an arc of a circular pipe or other shape structure will be stable and perform as long as the soil pressure on the outside of the pipe is resisted by the compression force in the pipe at each end of the arc.

Corrugated metal pipes can change shape safely within reasonable limits as long as there is adequate exterior soil pressure to balance the ring compression. Therefore, size and shape measurements taken at any one time do not provide conclusive data on backfill instability even when there is significant deviation from the design shape. Current backfill stability cannot be reliably determined unless changes in shape are measured over time. It is therefore necessary to identify current or recent shape changes to reliably check backfill stability. If there is instability of the backfill, the pipe will continue to change shape.

In general, the inspection process for checking shape will include visual observations for symmetrical shape and uniform curvature as well as measurements of important dimensions. The specific measurements to be obtained depend upon factors such as the size, shape, and condition of the structure. If shape changes are observed, more measurements may be necessary. For small structures in good condition, one or two simple measurements may be sufficient, for example, measuring the horizontal diameter on round pipe. For larger structures such as long span culverts, key measurements may be difficult to obtain. Horizontal diameters may be both high and large. The inspection process for long span culverts generally requires that elevations be established for key points on the structure. Although some direct measurements may also be required for long-span structures, elevations are needed to check for settlement and for calculating vertical distances such as the middle ordinate of the top arc. For structures with shallow cover, observations of the culvert with a few live loads passing over are recommended. Discernible movement in the structure may indicate possible instability and a need for more in-depth investigation.

The number of measurement locations depends upon the size and condition of the structure. Long-span culverts should normally be measured at the end and at 25 foot intervals. Measurements may be required at more frequent intervals if significant shape changes are observed. The smaller pipe culverts can usually be measured at longer intervals than long-span culverts.

Locations in sectional pipe can be referenced by using pipe joints as stations to establish the stationing of specific cross-sections. Stations should start with number 1 at the outlet and increase going upstream to the inlet. The location of points on a circular cross section can be referenced like hours on a clock. The clock should be oriented looking upstream. On structural plate corrugated metal culverts, points can be referenced to bolted circumferential and longitudinal seams.

It is extremely important to tie down exact locations of measurement points. Unless the same point is checked on each inspection, changes cannot be accurately monitored. The inspection report must, therefore, include precise descriptions of reference point locations. It is safest to use the joints, seams, and plates as the reference grid for measurement points. Exact point locations can then be easily described in the report as well as physically marked on the structures. This guards against loss of paint or scribe marks and makes points easy to find or reestablish. All dimensions in structures should be measured to the inside crest of corrugation. When possible, measurement points on structural plate should be located at the center of a longitudinal seam. However, some measurement points are not on a seam.

When distortion or curve flattening is apparent, the extent of the flattened area, in terms of arc length, length of culvert affected, and the location of the flattened area should be described in the inspection report. The length of the chord across the flattened area and the middle ordinate of the chord should be measured and recorded. The chord and middle ordinate measurements can be used to calculate the curvature of the flattened area using the formula shown in Exhibit 66.

Schematic showing how to check the curvature by the curve and the middle ordinate

Figure 14.3.8 (Exhibit 66) Checking Curvature by Curve and Middle Ordinate

5-4.2 Inspecting Barrel Defects

The structural integrity of corrugated metal culverts and long-span structures is dependent upon their ability to perform in ring compression and their interaction with the surrounding soil envelope. Defects in the culvert barrel itself, which can influence the culvert's structural and hydraulic performance, are discussed in the following paragraphs. Rating guidelines are provided in the sections dealing with specific shapes.

  1. Misalignment - The inspector should check the vertical and horizontal alignment of the culvert. The vertical alignment should be checked visually for sags and deflection at joints. Poor vertical alignment may indicate problems with the subgrade beneath the pipe bedding. Sags trap debris and sediment and may impede flow. Since most highway culverts do not have watertight joints, sags which pocket water could saturate the soil beneath and around the culvert, reducing the soil's stability. The horizontal alignment should be checked by sighting along the sides for straightness. Vertical alignment can be checked by sighting along bolt lines. Minor horizontal and vertical misalignment is generally not a significant problem in corrugated metal structures unless it causes shape or joint problems. Occasionally culverts are intentionally installed with a change in gradient.
  2. Joint Defects - Field joints are generally only found with factory manufactured pipe. There are ordinarily no joints in structural plate culverts, only seams. (In a few cases, preassembled lengths of structural plate pipe have been coupled or banded together like factory pipe.)

    Field joints in factory pipe serve to maintain the water conveyance of the culvert from section to section, to keep the pipe sections in alignment, keep the backfill soil from infiltrating, and to help prevent sections from pulling apart. Joint separation may indicate a lack of slope stability as described in section 5-4.2 e., circumferential seams. Key factors to look for in the inspection of joints are indications of backfill infiltration and water exfiltration. Excessive seepage through an open joint can cause soil infiltration or erosion of the surrounding backfill material reducing lateral support. Open joints may be probed with a small rod or flat rule to check for voids. Indications of joint defects include open joints, deflection, seepage at the joints, and surface sinkholes over the culvert as illustrated in Exhibits 67 and 68. Any evidence of joint defects should be recorded. Culverts in good condition should have no open joints, those in fair condition may have a few open joints but no evidence of soil infiltration, and those in marginal to poor condition will show evidence of soil infiltration.
    Schematic showing the surface indications that infiltration has occurred - depicting Permeable Soil and its effect on unpaved and paved areas 

Figure 14.3.9 (Exhibit 67) Surface Indications of Infiltration

Photograph of a surface hole just above an open joint

Figure 14.3.10 (Exhibit 68) Surface Hole Above Open Joint

  1. Seam Defects in Fabricated Pipe - Pipe seams in helical pipe do not carry a significant amount of the ring compression thrust in the pipe. That is the reason that a lock seam is an acceptable seam. Helical seams should be inspected for cracking and separation. An open seam could result in a loss of backfill into the pipe, or exfiltration of water. Either condition could reduce the stability of the surrounding soil.

    In riveted or spot welded pipes, the seams are longitudinal and carry the full ring compression in the pipe. These seams, then, must be sound and capable of handling high compression forces. They should be inspected for the same types of defects as those described in the text for structural plate culverts, Section 12.4.3, Structural Pipe. When inspecting the longitudinal seams of bituminous-coated corrugated metal culverts, cracking in the bituminous coating may indicate seam separation.
  2. Longitudinal Seam Defects in Structural Plate Culverts - Longitudinal seams should be visually inspected for open seams, cracking at bolt holes, plate distortion around the bolts, bolt tipping, cocked seams, cusped seams, and for significant metal loss in the fasteners due to corrosion.

    Culverts in good condition should have only minor joint defects. Those in fair condition may have minor cracking at a few bolt holes or minor opening at seams that could lead to infiltration or exfiltration. Marginal to poor culvert barrel conditions are indicated by significant cracking at bolt holes, or deflection of the structure due to infiltration of backfill through an open seam. Cracks 3 inches long on each side of the bolts indicate very poor to critical conditions.
    1. Loose Fasteners - Seams should be checked for loose or missing fasteners as shown in Exhibit 69. For steel structures the longitudinal seams are bolted together with high-strength bolts in two rows; one row in the crests and one row in the valleys of the corrugations. These are bearing type connections and are not dependent on a minimum clamping force of bolt tension to develop interface friction between the plates. Fasteners in steel structural plate may be checked for tightness by tapping lightly with a hammer and checking for movement.
    2. Photographic close-up of loose and missing bolts at a cusped seam. Loose fasteners are usually detected by tapping the nuts with a hammer.

      Figure 14.3.11 (Exhibit 69) Close-Up of Loose and Missing Bolts at a Cusped Seam; Loose Fasteners are Usually Detected by Tapping the Nuts with a Hammer

      For aluminum structural plate, the longitudinal seams are bolted together with normal strength bolts in two rows with bolts in the crests and valleys of both rows. These seams function as bearing connections, utilizing bearing of the bolts on the edges of holes and friction between the plates. The seams in aluminum structural plate should be checked with a torque wrench (125 ft-lbs minimum to 150 ft-lbs maximum). If a torque wrench is not available fasteners can be checked for tightness with a hammer as described for steel plates.

    3. Cocked and Cusped Seams - The longitudinal seams of structural plate are the principal difference from factory pipe. The shape and curvature of the structure is affected by the lapped, bolted longitudinal seam. Improper erection or fabrication can result in cocked seams or cusped effects in the structure at the seam, as illustrated in Exhibit 70. Slight cases of these conditions are fairly common and frequently not significant. However, severe cases can result in failure of the seam or structure. When a cusped seam is significant the structure's shape appearance and key dimensions will differ significantly from the design shape and dimensions. The cusp effect should cause the structure to receive very low ratings on the shape inspection if it is a serious problem. A cocked seam can result in loss of backfill and may reduce the ultimate ring compression strength of the seam.
      Photograph of a cocked seam with a cusp effect

      Figure 14.3.12 (Exhibit 70) Cocked Seam with Cusp Effect

    4. Seam Cracking - Cracking along the bolt holes of longitudinal seams can be serious if allowed to progress. As cracking progresses, the plate may be completely severed and the ring compression capability of the seam lost. This could result in deformation or possible failure of the structure. Longitudinal cracks are most serious when accompanied by significant deflection, distortion, and other conditions indicative of backfill or soil problems. Longitudinal cracks are caused by excessive bending strain, usually the result of deflection, Exhibit 71. Cracking may occasionally be caused by improper erection practices such as using bolting force to "lay down" a badly cocked seam.
    5. Schematic that is showing cracking due to deflection

      Figure 14.3.13 (Exhibit 71) Cracking Due to Deflection

    6. Bolt Tipping - The bolted seams in structural plate culverts only develop their ultimate strength under compression. Bolt tipping occurs when the plates slip. As the plates begin to slip, the bolts tip, and the bolt holes are plastically elongated by the bolt shank. High compressive stress is required to cause bolt tipping. Structures have rarely been designed with loads high enough to produce a ring compression that will cause bolt tip. However, seams should be examined for bolt tip particularly in structures under higher fills. Excessive compression on a seam could result in plate deformations around the tipped bolts and failure is reached when the bolts are eventually pulled through the plates.
  3. Circumferential Seams - The circumferential seams, like joints in factory pipe, do not carry ring compression. They do make the conduit one continuous structure. Distress in these seams is rare and will ordinarily be a result of a severe differential deflection or distortion problem or some other manifestation of soil failure. For example, a steep sloping structure through an embankment may be pulled apart longitudinally if the embankment moves down as shown in Exhibit 72. Plates should be installed with the upstream plate overlapping the downstream plate to provide a "shingle" effect in the direction of flow.
Schematic of circumferential seam failure due to embankment slippage

Figure 14.3.14 (Exhibit 72) Circumferential Seam Failure Due to Embankment Slippage

The circumferential seam at one or more locations would be distressed by the movement of the fill. Such distress is important to note during inspections since it would indicate a basic problem of stability in the fill. Circumferential seam distress can also be a result of foundation failure, but in such cases should be clearly evident by the vertical alignment.

  1. Dents and Localized Damage - All corrugated metal culverts should be inspected for localized damage. Pipe wall damage such as dents, bulges, creases, cracks, and tears can be serious if the defects are extensive and can impair either the integrity of the barrel in ring compression or permit infiltration of backfill. Small, localized examples are not ordinarily critical. When the deformation type damages are critical, they will usually result in a poorly shaped cross section. The inspector should document the type, extent, and location of all significant wall damage defects. When examining dents in corrugated steel culverts, the opposite side of the plate should be checked, if possible, for cracking or disbonding of the protective coating.
  2. Durability (Wall Deterioration) - Durability refers to the ability of a material to resist corrosion and abrasion. Corrosion is the deterioration of metal due to electrochemical or chemical reactions. Abrasion is the wearing away of culvert materials by the erosive action of bedload carried in the stream.

Abrasion is generally most serious in steep or mountainous areas where high flow rates carry sand and rocks that wear away the culvert invert. Abrasion can also accelerate corrosion by wearing away protective coatings.

Metal culverts are subject to corrosion in certain aggressive environments. For example, steel rapidly corrodes in salt water and in environments with highly acidic (low pH) conditions in the soil and water. Aluminum is fairly resistant to salt water but will corrode rapidly in highly alkaline (high pH) environments, particularly if metals such as iron or copper and their salts are present. The electrical resistivity of soil and water also provide an indication of the likelihood of corrosion. Many agencies have established guidelines in terms of pH and resistivity that are based on local performance. The FHWA has also published guidelines for aluminum and steel culverts including various protective coatings.

Corrosion and abrasion of corrugated metal culverts can be a serious problem with adverse effects on structural performance. Damage due to corrosion and abrasion is the most common cause for culvert replacement. The inspection should include visual observations of metal corrosion and abrasion. As steel corrodes it expands considerably. Relatively shallow corrosion can produce thick deposits of scale. A geologist's pick-hammer can be used to scrape off heavy deposits of rust and scale permitting better observation of the metal. A hammer can also be used to locate unsound areas of exterior corrosion by striking the culvert wall with the pick end of the hammer. When severe corrosion is present, the pick will deform the wall or break through it. Protective coatings should be examined for abrasion damage, tearing, cracking, and removal. The inspector should document the extent and location of surface deterioration problems.

When heavy corrosion is found by observation or sounding, special inspection methods such as pH testing, electrical resistivity measurement, and obtaining cores from the pipe wall are recommended. A routine program for testing pH and electrical resistivity should be considered since it is relatively easy to perform and provides valuable information.

Durability problems are the most common cause for the replacement of pipe culverts. The condition of the metal in corrugated metal culverts and any coatings, if used, should be considered when assigning a rating to the culvert barrel. Suggested rating guidelines for metal culverts with metallic coatings are shown in Exhibit 73. Modification of these guidelines may be required when inspecting culverts with non-metallic coatings. Aluminum culvert barrels may be rated as being in good condition if there is superficial corrosion. Steel culverts rated as in good condition may have superficial rust with no pitting. Perforation of the invert as shown in Exhibit 74 would indicate poor condition. Complete deterioration of the invert in all or part of the culvert barrel would indicate a critical condition as shown in Exhibit 75. Culverts with deteriorated inverts may function as an arch structurally, but are highly susceptible to failure due to erosion of the bedding.

Table giving the suggested rating criteria for the condition of corrugate metal

Figure 14.3.15 (Exhibit 73) Suggested Rating Criteria for Condition of Corrugated Metal

Photograph of the perforation of the invert due to corrosion

Figure 14.3.16 (Exhibit 74) Perforation of the Invert Due to Corrosion

Photograph of invert deterioration

Figure 14.3.17 (Exhibit 75) Invert Deterioration

  1. Concrete Footing Defects - Structural plate arches, long-span arches, and box culverts use concrete footings. Metal footings are occasionally used for the arch and box culvert shapes. The metal "superstructure" is dependent upon the footing to transmit the vertical load into the foundation. The structural plate arch is usually bolted in a base channel which is secured in the footing.

    The most probable structural defect in the footing is differential settlement. One section of a footing settling more than the rest of the footing can cause wrinkling or other distortion in the arch. Flexible corrugated metal culverts can tolerate some differential settlement but will be damaged by excessive differential settlement. Uniform settlement will not ordinarily affect a metal arch but can affect the clearances in a grade separation structure if the footings settle and the road does not. The significance of differential footing settlement increases as the amount of the difference in settlement increases, the length it is spread over decreases, and the height of the arch decreases. This concept is illustrated in Exhibit 76.
Schematic showing differential footing settlement, with no distress in the arch (left schematic) and with distress in the arch (right schematic)

Figure 14.3.18 (Exhibit 76) Differential Footing Settlement

The inspection of footings in structural plate and long-span arches should include a check for differential settlement along the length of a footing. This might show up in severe cracking, spalling, or crushing across the footing at the critical spot. If severe enough, it might be evidenced by compression or stretching of the corrugations in the culvert barrel. Deterioration may occur in concrete and masonry footings which is not related to settlement but is caused by the concrete or mortar. In arches with no invert slab, the inspector should check for erosion and undermining of the footings and look for any indication of rotation of the footing as illustrated in Exhibits 77 and 78.

Schematic showing footing rotation due to undermining

Figure 14.3.19 (Exhibit 77) Footing Rotation due to UnderminingPhotograph of the erosion of the invert, leading to the undermining the footing of the arch

Photograph of the erosion of the invert, leading to the undermining the footing of the arch

Figure 14.3.20 (Exhibit 78) Erosion of Invert Undermining footing of Arch

Culverts rated in good condition may have minor footing damage. Poor to critical condition would be indicated by severe footing undermining, damage, or rotation, or by differential settlement causing distortion and circumferential kinking in the corrugated metal as shown in Exhibit 79.

Photograph of the erosion damage to a concrete invert

Figure 14.3.21 (Exhibit 79) Erosion Damage to Concrete Invert

  1. Defects in Concrete Inverts - Concrete inverts in arches are usually floating slabs used to carry water or traffic. Invert slabs provide protection against erosion and undercutting, and are also used to improve hydraulic efficiency. Concrete inverts are sometimes used in circular, as well as other culvert shapes, to protect the metal from severe abrasive or severe corrosive action. Concrete invert slabs in arches should be checked for undermining and damage such as spalls, open cracks, and missing portions. The significance of damage will depend upon its effect on the footings and corrugated metal.

The following excerpts are from a reproduction of the out-of-print Culvert Inspection Manual (Supplement to Manual 70), July 1986 — Chapter 5, Section 5.

Section 5 - SHAPE INSPECTION OF CORRUGATED METAL CULVERT BARRELS
5-5.0 General

This section deals with shape inspections of common culvert shapes including round and vertical elongated, pipe arches, arches, and box culvert shapes. Specific guidelines for recommended measurements to be taken for each location are provided for each typical culvert shape. Additional measurements are also recommended when field measurements differ from the design dimensions or when significant shape changes are observed. Rating guidelines are also provided for each shape. The guidelines include condition descriptions with shape and barrel defects defined for each rating.

5-5.1 Using the Rating Guidelines

When using the rating guidelines, the inspector should keep the following factors in mind:

  1. The inspector should select the lowest rating which best describes either the shape condition or the barrel condition. Structure shape is the most critical factor in flexible culverts, and this should be kept in mind when selecting the rating.
  2. The shape criteria described for each numerical rating should be considered as a group rather than as separate criteria for each measurement check listed. Good curvature and the rate of change are critical. Significant changes in shape since the last inspection should be carefully evaluated even if the structure is still in fairly good condition.
  3. The guidelines merely offer a starting point for the inspector. The inspector must still use judgment in assigning the appropriate numerical rating. The numerical rating should be related to the actions required. The inspector may wish to refer to Section 4.2 of this manual.
5-5.2 Round and Vertical Elongated Pipe

Round and vertically elongated pipes are expected to deflect vertically during construction resulting in a slightly increased horizontal span. Round pipes are sometimes vertically elongated five percent to compensate for settlement during construction. It is frequently difficult to determine in the field if a pipe was round or elongated when installed. Large round pipes may appear to be elongated if they were subjected to minor flattening of the sides during backfill.

Vehicular underpasses sometimes use 10 percent vertically elongated very large pipe which is susceptible to side flattening during installation. In shallow cover situations, adequate curvature in the sides is the important factor. The soil pressures on the sides may be greater than the weight of the shallow fill over the pipe. The result is a tendency to push the sides inward rather than outward as in deeper buried or round pipes. Side flattening, such as that shown in Exhibit 80, can be caused by unstable backfill. A deteriorated invert may have contributed to the problem by reducing the pipe's ability to transmit compressive forces.

Photograph of excessive side deflection

Figure 14.3.22 (Exhibit 80) Excessive Side Deflection

Flattening of the top arc is an indication of possible distress. Flattening of the invert is not as serious. Pipes not installed on shaped bedding will often exhibit minor flattening of the invert arc. However, severe flattening of the bottom arc would indicate possible distress.

The inspector should note the visual appearance of the culvert's shape and measure the horizontal span as shown in Exhibit 81. Almost all round or vertical elongated pipe can be directly measured and will not require elevations. Exceptions are large vertical elongated grade separation structures. On such structures, elevations should be obtained similar to those recommended for the long-span pear shape.

Schematic showing the shape inspection of circular and vertical elongated pipes

Figure 14.3.23 (Exhibit 81) Shape Inspection Circular and Vertical Elongated Pipe

If the visual appearance or measured horizontal diameter differs significantly from the design specifications, additional measurement, such as vertical diameter, should be taken. Flattened areas should be checked by measuring a chord and the mid ordinate of the chord. The chord length and ordinate measurement should be noted in the report with a description of the location and extent of the flattened area.

Round and vertically elongated pipe with good to fair shape will have a generally good shape appearance. Good shape appearance means that the culvert's shape appears to match the design shape, with smooth, symmetrical curvature and no visible deformations. The horizontal span should be within 10 percent of the design span. Pipe with marginal shape will be indicated by characteristics such as a fair or marginal general shape appearance, distortion in the upper half of the pipe, severe flattening in the lower half of the pipe, or horizontal spans 10 to 15 percent greater than design.

Pipe with poor to critical shape will have a poor shape appearance that does not match the design shape, does not have smooth or symmetrical curvature, and may have obvious deformations. Severe distortion in the upper half of the pipe, a horizontal diameter more than 15 percent to 20 percent greater than the design diameter, or flattening of the crown to an arc with a radius of 20 to 30 feet or more would indicate poor to critical condition. It should be noted that pipes with deflection of less than 15 to 20 percent may be rated as critical based on poor shape appearance. Guidelines for rating round corrugated metal culvert are presented in Exhibit 82.

Table giving the condition rating guidelines for round or vertical elongated corrugated metal pipe barrels

Figure 14.3.24 (Exhibit 82) Condition Rating Guidelines

Pipe Arch

The pipe arch is a completely closed structure but is essentially an arch. The load is transmitted to the foundation principally at the corners. The corners are much like footings of an arch. There is relatively little force or pressure on the large radius bottom plate. The principal type of distress in a pipe arch is a result of inadequate soil support at the corners where the pressure is relatively high. The corner may push down or out into the sail while the bottom stays in place. The effect will appear as if the bottom pushed up. This problem is illustrated in Exhibits 83 and 84.

Schematic showing bottom distortion in pipe arches

Figure 14.3.25 (Exhibit 83) Bottom Distortion in Pipe Arches

Photograph of the bottom and corners of a pipe arch which have settled

Figure 14.3.26 (Exhibit 84) Bottom and Corners of this Pipe Arch have Settled

The bottom arc should be inspected for signs of flattening and the bottom corners for signs of spreading. The extent and location of bottom flattening and corner spreading should be noted in the inspection report.

Complete reversal of the bottom arc can occur without failure if corner movement into the foundation has stabilized. The top arc of the structure is supporting the load above and its curvature is an important factor. However, if the "footing" corner should fail, the top arc would also fail. The spreading of the corners is therefore very important as it affects the curvature of the top arc.

The inspector should record the visual appearance of the shape and measure both the span and the rise. If the span exceeds the design span by more than 3 percent, the span of the top arc, the mid ordinate of the top arc, and the mid ordinate of the bottom arc should also be measured. Recommended measurements are shown in Exhibit 85.

Schematic showing the shape inspection of a structural plate pipe arch

Figure 14.3.27 (Exhibit 85) Shape Inspection Structural Plate Pipe Arch

Pipe arches in fair to good condition will have a symmetrical appearance, smooth curvature in the top of the pipe, and a span less than five percent greater than theoretical. The bottom may be flattened but should still have curvature. Pipe arches in marginal condition will have fair to marginal shape appearance, with distortion in the top half of the pipe, slight reverse curvature in the bottom of the pipe, and a horizontal span five to seven percent greater than theoretical. Pipe in poor to critical condition will have characteristics such as a poor shape appearance, severe deflection or distortion in the top half of the pipe, severe reverse curvature in the bottom of the pipe, flattening of one side, flattening of the crown to an arc with a radius of 20 to 30 feet, or a horizontal span more than seven percent greater than theoretical. Guidelines for rating pipe arches are shown in Exhibit 86.

Table giving the condition rating guidelines for corrugated metal pipe-arch barrels

Figure 14.3.28 (Exhibit 86) Condition Rating Guidelines

5-5.4 Arches

Arches are fixed on concrete footings, usually below or at the springline. The springline is a line connecting the outermost points on the sides of a culvert. This difference between pipes and arches means that an arch tends to deflect differently during backfill. Backfill forces tend to flatten the arch sides and peak its top because the springline cannot move inward like the wall of a round pipe as shown in Exhibit 87. As a result, important shape factors to look for in an arch are flattened sides, peaked crown, and flattened top arc.

Schematic of arch deflection during installation

Figure 14.3.29 (Exhibit 87) Arch Deflection During Installation

Another important shape factor in arches is symmetrical shape. If the arch was erected with the base channels not square to the centerline, it causes a racking of the cross section. A racked cross-section is one that is not symmetrical about the centerline of the culvert. One side tends to flatten while the other side tends to curve more while the crown moves laterally and possibly upward. If these distortions are not corrected before backfilling the arch, they usually get worse during backfill. Exhibit 88 illustrates racked or peaked arches.

Schematic of a racked and peaked arch
Photograph of a racked and peaked arch

Figure 14.3.30 (Exhibit 88) Racked and Peaked Arch

Visual observation of the shape should involve looking for flattening of the sides, peaking or flattening of the crown, or racking to one side. The measurements to be recorded are illustrated in Exhibit 89. Minimum measurements include the vertical distance from the crown to the bottom of the base channels and the horizontal distances from each of the base channels to a vertical line from the highest point on the crown. These horizontal distances should be equal. When they differ by more than 10 inches or 5 percent of the span, whichever is less, racking has occurred and the curvature on the flatter side of the arch should be checked by recording chord and midordinate measurements. Racking can occur when the rise checks with the design rise. When the rise is more than 5 percent less than the design rise, the curvature of the top arc should be checked.

Schematic of the shape inspection of a structural plate arch

Figure 14.3.31 (Exhibit 89) Shape Inspection Structural Plate Arch

Arches in fair to good condition will have the following characteristics: a good shape appearance with smooth and symmetrical curvature, and a rise within three to four percent of theoretical. Marginal condition would be indicated when the arch is significantly non-symmetrical, when arch height is five to seven percent less or greater than theoretical, or when side or top plate flattening has occurred such that the plate radius is 50 to 100 percent greater than theoretical. Arches in poor to critical condition will have a poor shape appearance including significant distortion and deflection, extremely non-symmetrical shape, severe flattening (radius more than 100 percent greater than theoretical) of sides or top plates, or a rise more than eight percent greater or less than the theoretical rise. Guidelines for rating structural plate arches are shown in Exhibit 90.

Table giving the condition rating guidelines for structural plate arch barrels

Figure 14.3.32 (Exhibit 90) Condition Rating Guidelines

5-5.5 Corrugated Metal Box Culverts.

The box culvert is not like the other flexible buried metal structures. It behaves as a combination of ring compression action and conventional structure action. The sides are straight, not curved and the plates are heavily reinforced and have moment or bending strength that is quite significant in relation to the loads carried.

The key shape factor in a box culvert is the top arc. The design geometry is clearly very "flat" to begin with and therefore cannot be allowed to deflect much. The span at the top is also important and cannot be allowed to increase much.

The side plates often deflect slightly inward or outward. Generally an inward deflection would be the more critical as an outward movement would be restrained by soil.

Shape factors to be checked visually include flattening of top arc, outward movement of sides, or inward deflection of the sides. The inspector should note the visual appearance of the shape and should measure and record the rise and the horizontal span at the top of the straight legs as shown in Exhibit 91. If the rise is more or less than 1½ percent of the design rise, the curvature of the large top radius should be checked.

Schematic of the shape inspection of a structural plate box culvert

Figure 14.3.33 (Exhibit 91) Shape Inspection Structural Plate Box Culverts

The radius points are not necessarily located at the longitudinal seams. Many box culverts use double radius plates and the points where the radius changes must be estimated by the inspector or can be determined from the manufacturer's literature. These points can still be referenced to the bolt pattern to describe exactly where they are. Since these are all low structures, the spots should also be marked and painted for convenient repeat inspection.

Box culverts in fair to good condition will appear to be symmetrical with smooth curves, slight or no deflection of the straight legs, a horizontal span length within five percent of the design span and the middle ordinate of the tops are within ten percent of the design. Culverts in marginal condition may appear to be non-symmetrical, have noticeable deflection in the straight legs, have spans that differ from design by five percent, or have a middle ordinate of the top arc that differ from design by 20 to 30 percent. Poor to critical conditions exist when the culvert shape appears poor, the culvert has severe deflections of the straight legs, a horizontal span that differs from design by more than five percent, or a middle ordinate of the top arc that differs from the theoretical by more than 40 to 50 percent. Guidelines for rating structural plate box culverts are shown in Exhibit 92.

Table giving the condition rating guidelines for corrugated metal box culvert barrels

Figure 14.3.34 (Exhibit 92) Condition Rating Guidelines

The following excerpts are from a reproduction of the out-of-print Culvert Inspection Manual (Supplement to Manual 70), July 1986 — Chapter 5, Section 6.

Section 6. CORRUGATED METAL LONG-SPAN CULVERTS
5-6.0 General.

This section describes methods for conducting shape inspections of long-span structures. The long-span structures addressed include four typical shapes: low profile arch, horizontal ellipse, high profile arch, and pear. These shapes are illustrated in Exhibit 93. The evaluation of shape characteristics of long-spans will vary somewhat depending upon the typical­ shape being inspected. However, the top or crown sections of all long-span structures have very similar geometry. The crown sections on all long-span structures can be inspected using the same criteria. This section therefore includes separate discussions on the crown section and on each of the typical long-span shapes. Guidelines are also provided for rating the condition of each shape in terms of shape characteristics and barrel defects. The methods for using the rating guidelines are the same as those described in section 5-5.1.

Schematic of typical long-span shapes for culverts

Figure 14.3.35 (Exhibit 93) Typical Long-Span Shapes

Shape inspections of long-span structures will generally consist of 1) visual observations of shape characteristics such as smooth or distorted curvature and symmetrical or non-symmetrical shape, 2) measurements of key dimensions, and 3) elevations of key points. Additional measurements may be necessary if measurements or observed shape differ significantly from design.

The visual observations are extremely important to evaluate the shape of the total cross section. Simple measurements such as rise and span do not describe curvature, yet adequate curvature is essential, as shown in Exhibit 94. However, measurements and elevations are also needed to document the current shape so that the rate change, if any, can be monitored.

Schematic showing erosion damage to a concrete invert

Figure 14.3.36 (Exhibit 94) Erosion Damage to Concrete Invert

Many long-spans will be too large to allow simple direct measuring. Vertical heights may be as large as 20 to 30 feet and horizontal spans may be large and as high as 12 to 15 feet above inverts. Culverts may have flowing water obscuring the invert and any reference points there. It is, therefore, in general desirable to have instrument survey points, which can be quickly checked for elevation. When direct measuring is practical a 25 foot telescoping extension rod can be used for measuring. Such rods can also serve as level rods for taking elevations.

5-6.1 Long-Span Crown Section - Shape Inspection.

As previously mentioned, the section above the springline is essentially the same for most long-span shapes. With the exception of pear shapes, the standard top geometry uses a large radius top arc of approximately 80 degrees with a radius of 15 to 25 feet. The adjacent corner or side plates are from one-half to one-fifth the top arc radius. The most important part of a long-span shape is the standard top arch geometry. Adequate curvature of the large radius top arc is critical. Inspection of the crown section should consist of a visual inspection of the general shape for smooth curvature (no distortion, flattening, peaks, or cusps) and symmetrical shape (no racking).

An inspection should also include key measurements such as the middle ordinate of the top arc. Recommended measurements and elevations are shown in Exhibit 95.

Schematic of the shape inspection of a crown section of a long span structure

Figure 14.3.37 (Exhibit 95) Shape Inspection Crown Section of Long Span Structures

The initial inspection should establish elevations for the radius points and the top of the crown. From these elevations the middle ordinate for the top arc can be calculated. If the actual middle ordinate is 10 percent more or less than the theoretical design mid-ordinate the horizontal span for the top arc should also be measured. For standard 80 degree arcs the theoretical middle ordinate is equal to 0.234 times the theoretical radius of the top arc. This span is not easy to measure on many long-span structures and need not be measured if the top arc mid-ordinate is within 10 percent of theoretical. Even if it is convenient and practical to direct measure the vertical heights of the points on the top arc from the bottom of the structure, it is wise to also establish their elevations from a reliable benchmark. Bottom reference points can be wiped out by erosion, covered with debris, or covered by water. When direct vertical measuring is practical, the shape may be checked on subsequent inspections with direct measurement. However, it is still important to establish elevations in case bottom reference points are lost or inaccessible.

Crown sections in good condition will have a shape appearance that is good, with smooth and symmetrical curvature. The actual middle ordinate should be within 10 percent of the theoretical, and the horizontal span (if measured) should be within five percent of theoretical. Crown sections in fair condition will have a fair to good shape appearance, smooth curvature but possibly slightly non-symmetrical. Middle ordinates of the top arc may be within 11 to 15 percent of theoretical and the horizontal span may differ by more than 5 percent of theoretical.

Crown sections in marginal condition will have measurements similar to those described for fair shape. However, the shape appearance will be only fair to marginal with noticeable distortion, deflection, or non-symmetrical curvature. When the curvature is noticeably distorted or non-symmetrical, the sides should be checked for flattening by measuring the middle ordinates of the halves of the top arc. Crown sections with marginal shape may have middle ordinates for top half arcs that are 30 to 50 percent less than theoretical.

Crown sections in poor to critical condition will have a poor to critical shape appearance with severe distortion or deflection. The middle ordinate of the top arc may be as much as 20 percent less than theoretical, while middle ordinates of the top arc halves may be 50 to 70 percent less than theoretical.

5-6.2 Low Profile Long-Span Arch - Shape Inspection.

The low profile arch is essentially the same as the crown section except that the sides are carried about 10 degrees below the springline to the footing. These structures are low and can be measured more easily than other long-span shapes. Recommended measurements and elevations are shown in Exhibit 96. Rating guidelines are listed in Exhibit 97.

Schematic of the shape inspection of a low profile long span arch

Figure 14.3.38 (Exhibit 96) Shape Inspection Low Profile Long Span Arch

Table giving the condition rating guidelines for low profile arch long-span culvert barrels

Figure 14.3.39 (Exhibit 97) Condition Rating Guidelines

Because arches are fixed on concrete footings, backfill pressures will try to flatten the sides and peak the top. Another important shape factor is symmetry. If the base channels are not square to the centerline of the structure racking may occur during erection. In racked structures, the crown moves laterally and the curvature in one side becomes flatter while the curvature in the other side increases. Backfill pressures may cause this condition to worsen.

5-6.3 High Profile Long-Span Arch — Shape Inspection.

High profile arches have a standard crown section geometry but have high large radius side walls below the springline. Curvature in these side plates is important. In shallow fills or minimum covers, the lateral soil pressures may approach or exceed the loads over the culvert. Excessive lateral forces could cause the sidewall to flatten or buckle inward.

Inspectors should visually inspect high profile arches for flattening of the side plates. Additionally, high profile arches have the same tendencies as regular arches for peaking and racking, so inspectors must also look for peaked top arcs and non-symmetrical or racked arches.

Recommended measurements and elevations are shown in Exhibit 98. The shape of the crown section is the most important shape factor. It can be measured and evaluated using the same criteria as that described for the standard crown section. If flattening is observed in the high sidewall the curvature of the sides should be checked by measuring the middle ordinate of the side walls. If the sidewall middle ordinate is no more than 50 to 70 percent less than the theoretical middle ordinate and no other shape problems are found the arch's shape may be considered fair. When the middle ordinate approaches 75 to 80 percent less than theoretical, the shape should be considered marginal. If the middle ordinate is more than 80 to 90 percent less than theoretical the shape should be considered poor to critical. Rating guidelines are provided in Exhibit 99.

Schematic of the shape inspection of a high profile long-span arch

Figure 14.3.40 (Exhibit 98) Shape Inspection High Profile Long-Span Arch

Table giving the condition rating guidelines for high profile arch long-span culvert barrels

Figure 14.3.41 (Exhibit 99) Condition Rating Guidelines

5-6.4 Pear Shape Long-Span — Shape Inspection.

The crown section of the pear shape differs from the standard top arch in that smaller radius corner arcs stop short of the horizontal springline. The large radius sides extend above the plane of the horizontal span. In checking curvature of the sides, the entire arc should be checked. Side flattening, particularly in shallow fills, is the most critical shape factor.

The pear shape behaves similarly to the high profile arch. It is essentially a high profile with a metal bottom instead of concrete footings. Pears may be inspected using the criteria for a high profile arch. The recommended measurements and elevations are shown in Exhibit 100. Rating guidelines are provided in Exhibit 101.

Schematic of the shape inspection of a long span pear-shape culvert

Figure 14.3.42 (Exhibit 100) Shape Inspection Long Span Pear-Shape

Table giving the condition rating guidelines for pear shaped long-span culvert barrels

Figure 14.3.43 (Exhibit 101) Condition Rating Guidelines

5-6.5 Horizontal Ellipse — Shape Inspections.

For horizontal ellipses the most important shape factor is adequate curvature in the crown section. The crown section uses the standard long-span crown geometry. The sides and bottom behave similar to the corners and bottom of pipe arches. The invert has relatively minor pressure when compared with the sides, which may have several times the bearing pressure of the invert. As a result the corners and sides have the tendency to push down into the soil while the bottom does not move. The effect is as if the bottom pushed up. Inspectors should look for indications of bottom flattening and differential settlement between the side and bottom sections, as illustrated in Exhibit 102.

Schematic showing the potential for differential settlement in a horizontal ellipse

Figure 14.3.44 (Exhibit 102) Potential for Differential Settlement in Horizontal Ellipse

The recommended measurements and evaluations for a shape inspection of horizontal ellipse are shown in Exhibit 103. The measurements are essentially the same as those recommended for a standard crown section. Shape evaluation of an ellipse is also essentially the same as the evaluation of a standard crown section except that the curvature of the bottom should also be evaluated. Marginal shape would be indicated when the bottom is flat in the center and corners are beginning to deflect downward or outward. Critical shape conditions would be indicated by reverse curvature in the bottom arc. Guidelines for rating horizontal ellipse shape culverts are provided in Exhibit 104.

Schematic of the shape inspection of a long-span horizontal ellipse

Figure 14.3.45 (Exhibit 103) Shape Inspection Long-Span Horizontal Ellipse

Table giving the condition rating guidelines for horizontal long-span culvert barrels

Figure 14.3.46 (Exhibit 104) Condition Rating Guidelines

Table giving the standard sizes for corrugated steel culverts

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute)

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for corrugated steel culverts - continued

Figure 14.3.47 Standard Sizes for Corrugated Steel Culverts (Source: American Iron and Steel Institute), continued

Table giving the standard sizes for aluminum culverts

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association)

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued


Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued


Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued


Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued

Table giving the standard sizes for aluminum culverts - continued

Figure 14.3.48 Standard Sizes for Aluminum Culvert (Source: Aluminum Association), continued