Rivers are the most dynamic geomorphic system that engineers have to cope with in the design and maintenance of bridges. The geomorphic features of the river can change dramatically with time. During major floods, significant changes can occur in a short period of time. While rivers are dynamic or can move locations, bridges do not move locations.
There are several ways in which channels can change and thereby jeopardize the stability and safety of bridges. The channel bed can scour (degrade) so that bed elevations become lower, undermining the foundation of the piers and abutments. Deposition of sediment on the channel bed (aggradation) can reduce conveyance capacity through the bridge opening. Flood waters are then forced around the bridge, attacking roadway approaches, channel banks, and flood plains. Another consequence of aggradation is that the river stage may be increased to where it exerts lateral thrust and lift on the deck and girders of the bridge (see Figures 13.1.1 and 13.1.2). The other primary way in which bridges can be adversely affected by a waterway is through bank erosion or avulsion, causing the channel to shift laterally. These phenomena of aggradation, degradation or scour, bank erosion, and lateral migration can be a result of natural or induced causes and can adversely affect the bridge (see Figure 13.1.3). Topic 13.2 presents detailed descriptions of waterway deficiencies.
Of all the bridges in the National Bridge Inventory (NBI), approximately 86% are built over waterways. Bridge inspectors need to understand the relationship between the bridge and waterway elements. This understanding involves being able to recognize and identify the streambed, embankments, floodplain, and streamflow so that an accurate assessment and record of the present condition of the bridge and waterway can be determined.
Figure 13.1.1 Failure Due to High Water Levels During Hurricane: Aerial View
Figure 13.1.2 Failure Due to High Water Levels During Hurricane: Close-Up View
Figure 13.1.3 Pier Foundation Failure
Safety is a major concern in the inspection of bridges over active waterways. Various properties can affect waterways and structures.
There are three major purposes for conducting waterway inspections.
Waterway inspections are needed to identify conditions that cause structural collapse of bridge structures. Deficient piling along with damage or deterioration to foundation members can only be detected during a waterway inspection. Entering the water and probing around the foundations is necessary to detect loss of foundation support.
Waterway inspections are conducted to create a record of the existing channel conditions adjacent to the bridge. Conditions such as channel opening width, depth at substructure elements, channel cross-section elevations, water flow velocity, and channel constriction and skew are noted and compared to previously recorded conditions.
Accessing the waterway to measure and record channel conditions may be restricted by several factors including channel width and depth, flow velocity, or pollution. These factors may require the bridge inspector to return to the site during a period of low flow. Alternatively the inspector may need to consider using an alternate means of waterway access, such as a boat, or an alternative inspection technique, such as underwater diving inspection.
Current waterway inspection data should be compared to previous inspection data in order to identify channel changes. This “tracking” of channel change over time is an important step in ensuring the safety of the bridge. Over time, vertical changes, due to either degradation or aggradation processes, or horizontal alignment changes, due to lateral migration of the channel, could result in foundation undermining, bridge overtopping, or even collapse of the structure. If major changes are found, a formal scour analysis of the site, involving a multi-disciplinary team of engineers, may be needed to estimate floodwater elevations, velocities, angle of attack, and potential scour depths. Potential threats to bridge members caused by channel changes can thus be dealt with before damage actually occurs. See Topic 13.2 for the inspection and evaluation of waterways.
According to the Hydraulic Design Series Number 6 (HDS-6) Highways in the River Environment, channels are typically well-defined and confine the streamflow during normal flow conditions (see Figure 13.1.4).
Elements of floodplains are presented in Topic 13.1.5.
Figure 13.1.4 Typical Waterway Cross Section Showing Well Defined Channel Depression
Knowledge of the type and profile of a waterway or river channel is essential to understand the hydraulics of the channel and its potential for change. The type of river may dictate certain tendencies or responses that may be more adverse than others. To aid in this understanding, various key river classes are briefly explained. Rivers can be broadly classified into four categories:
Meandering rivers consist of a series of bends connected by crossings. In general, pools exist in the bends. The dimensions of these pools vary with the size of the river, flow conditions, radius of the curvature of the bends, and type of bed and bank material. Such rivers are fairly predictable and experience relatively slow velocities. Figure 13.1.5 shows some differences between the various river categories. Figure 13.1.6 illustrates the major characteristics of a meandering river.
Figure 13.1.5 Plan View of Rivers
Braided rivers consist of multiple channels that are intertwined in braided form. At flood stages, the appearance of braiding is less noticeable. The bars dividing the multiple channels may become submerged, and the river will appear to be relatively straight. Braided rivers have steeper slopes and experience higher streamflow velocities which may cause larger scour or undermining problems.
Braided rivers can change rapidly, causing different velocity distributions, partial blockages of portions of the waterway beneath bridges, and larger quantities of debris that can be a hazard to bridges and cause accelerated scour. Figure 13.1.4 illustrates the plan view of typical rivers, including meandering, straight, and braided. This figure also relates form of river to channel type based on sediment load and relative stability of river type.
Straight rivers are something of an anomaly. Most straight rivers are in a transition between meandering and braided types. In straight rivers, any development that would flatten the gradient would accelerate change from a straight system to a meandering system. Conversely, if the gradient were increased, the channel may become braided. Therefore, in order to maintain the straight alignment over a normal range of hydrologic conditions, it may become necessary to utilize channel hydraulic control structures (Topic 13.1.7). The characteristics of straight rivers are identified in Figure 13.1.5.
Figure 13.1.6 Meandering River
Steep mountain streams are controlled by geologic formations, rock falls, and waterfalls. They experience very small changes in either plan form or profile when subjected to the normal range of discharges. The bed material of such river systems can consist of gravel, cobbles, boulders, or some mixture of these different sizes. Even though these rivers are relatively stable, they can experience significant velocity and flow changes during episodic flood events.
The floodplain is the overbank area outside the channel that carries flood flows in excess of channel capacity (see Figure 13.1.7). It is common to find bridges built within the floodplain. For many structures, the floodplain is quite large, as compared to the channel. Observations made during periods of high water can help the inspector identify the floodplain.
Elements of a Floodplain
Figure 13.1.7 Typical Floodplain
Channel characteristics are presented in detail in Topic 13.1.4.
The hydraulic opening is the entire area beneath the bridge which is available to pass flood flows (see Figure 13.1.8). The bottom of the superstructure, the two bridge abutments, and the streambed or ground elevation bounds the hydraulic, or waterway, opening. For multiple spans, intermediate supports such as piers or bents restrict the hydraulic or bridge waterway opening.
Figure 13.1.8 Hydraulic Waterway Opening
Hydraulic countermeasures are often utilized to provide protection for bridges against lateral migration of the channel and against high velocity flows and scour. A hydraulic countermeasure is a man-made or man-placed device designed to direct streamflow and protect against lateral migration or scour. These flow hydraulic control countermeasures may be utilized either at the bridge, upstream from the bridge, or downstream from the bridge. Countermeasures are designed by hydraulic and geotechnical engineers and are installed to redirect streamflow and flood flows within the watercourse and through the bridge waterway opening. Hydraulic countermeasures are broken into two distinct categories which are river training structures and armoring countermeasures.
River control structures are countermeasures designed to modify the flow to help prevent. A couple examples of river training structures are spurs and guide banks. A complete list of the various types of river training structures is located in HEC-23Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance, 3rd edition.
Spurs are linear structures, designed with properly sized and placed rocks, that projects into a channel and placed on the outside bends of the bank to protect the streambank by reducing flow velocity, inducing deposition of sediment or redirecting the flow (see Figure 13.1.10). Common applications occur on meandering streams where they are placed on the outside of the bends to redirect the flow and minimize lateral stream migration.
Guide banks are dikes which extend upstream from the approach embankment at either or both sides of the bridge opening to direct the flow through the opening (see Figure 13.1.11). Scour hole formation occurs at the upstream ends of the guide banks if left unprotected. Common scour prevention devices for guide banks include riprap.
Armoring countermeasures tend not to alter the flow significantly, but are design to resist hydraulic stresses of the design flood events. Some examples of armor countermeasures include riprap, gabions, slope stabilization, channel linings and footing aprons. A complete list of the various types of armoring countermeasures is located in HEC-23.
Layers or facings of properly sized and graded rock or broken concrete, placed or dumped to protect an abutment, pier or embankment from erosion (see Figure 13.1.9). Riprap has also been used to almost all kinds of armor which include wire-enclosed riprap partially grouted riprap, sacked concrete and concrete slabs. Riprap should be protected against subsurface erosion by filters formed either of properly graded sand/gravel or of synthetic fabrics developed and utilized to replace the natural sand/gravel filter system. It must be placed on an adequately flat slope to be able to resist the anticipated forces of the flowing flood waters. Proper design and placement of riprap is essential. This generally requires placement of the riprap on side-slopes no steeper than 1.5 to 1 vertical (1.5H:1V). Flatter side-slopes of such as 2H:1V to 3H:1V are preferable. Proper design and placement of riprap is essential. Inappropriate installations can aggravate or cause the conditions they were intended to correct or prevent.
Rectangular rock- or cobble- filled wire mesh baskets or compartmented rectangular containers, anchored together and generally anchored to the surface they are protecting (see Figure 13.1.12). Gabions may be placed on steeper slopes than riprap or may even be stacked vertically, depending upon the design procedure and site conditions.
Slope stabilization methods consist of the placement of geotextiles, wire mesh, riprap, paving, revetment, plantings or other materials on channel embankments, intended to protect the slope from erosion, slipping or caving or to withstand external hydraulic pressure (see Figure 13.1.13). It is anticipated the various stabilization methods will fill-in with sediment and help sustain plant growth. The roots from the plants contribute to stabilize the embankment or flood plain.
Channel lining is a concrete pavement that extends across the streambed. Channel linings also may be revetment mats or some other form of bed armoring. A typical revetment mat is formed by interlocking precast concrete blocks linked by cable (polyester or steel) placed on a geotextile fabric. The interlocking matrix allows for use over varying land contours and grades (see Figure 13.1.14). Channel linings may also consist of formed concrete. This type is less flexible and versatile than revetment mats and other bed armoring (see Figure 13.1.15).
Footing aprons are protective layers of material surrounding the footing of a substructure unit. Footing aprons usually consist of cast-in-place concrete (see Figures 13.1.16 and 13.1.17). Footing aprons protect footings from undermining. The aprons are not a structural element of the abutment or pier footings and are considered a structural countermeasure instead of a hydraulic countermeasure.
Figure 13.1.9 Crushed Stone Riprap
Figure 13.1.11 Guide banks Constructed on Kickapoo Creek Near Peoria, Illinois
Figure 13.1.12 Gabion Basket Serving as Slope Protection
Figure 13.1.13 Slope Stabilization
Figure 13.1.14 Concrete Revetment Mat
Figure 13.1.15 Formed Concrete Channel Lining
Figure 13.1.16 Concrete Footing Apron on a Masonry Abutment
Figure 13.1.17 Concrete Footing Apron to Protect a Spread Footing from Undermining
The bridge inspector needs to be able to correctly identify and assess waterway deficiencies when performing a bridge waterway inspection. Accurate bridge waterway inspections are vital for the safety of the motoring public. For this to happen, have a thorough understanding of the different types of waterway elements and deficiencies, as well as the various inspection techniques. See Topic 13.1 for detailed descriptions of various waterway elements.
Waterway deficiencies are properties of the waterway or substructure members that work to act negatively on the structural integrity of the bridge. They are mostly interrelated and when a change in one of these properties occurs, others are also often affected.
In general, bridges are designed so that the flow passes through the waterway parallel to the axes of the abutments and the piers. If the path of flow shifts in direction as a result of continued lateral movement so that it approaches the abutments and the piers at a significant skew angle, the capacity of the waterway can be reduced. More significantly, local scour will be increased and may lead to the failure of the structure. This depends upon the original design conditions and the degree of change resulting in misalignment in the flow with the critical elements supporting the structure. Carefully note any change in direction of the approach of the flow to the bridge and any change in the angle at which the flow hits or impinges on the abutments and piers. Also make observations of local change in flow directions and surveys of changes in bed and bank elevations. Evaluation of aerial photographs over time is extremely useful in assessing changes in waterway alignment. All of this information may be utilized to rate the severity of increasing misalignment in the flow on bridge safety.
Example of channel misalignment: If the approaching flow impinges on rectangular piers at an angle of 45 degrees versus flowing parallel to the axis of the piers, the depth of scour may be increased by a factor of two or more. The actual factor of increase depends upon the characteristics of the bed material, the pier type, and the duration of the flood.
For bridges spanning over wide floodplains, the approach angle of the low flow channel may not be significant. In these cases it is the alignment of the floodplain flow during the larger floods that will determine the magnitude of local scour.
Streamflow velocity is a major factor in the rate and depth of scour. During flood events, the streamflow velocity is increased, which produces accelerated scour rates and depths. At high streamflow velocities, bridge foundations have the greatest chance to become undermined (see Figure 13.2.1).
Figure 13.2.1 Flood Flow Around a Pier Showing High Streamflow Velocity
The streamflow velocity depends on many variables. One of these variables is the stream grade. A steep stream grade will produce high streamflow velocities, while a flat stream grade produces low streamflow velocities. Other variables that affect the streamflow velocity include the waterway alignment, the hydraulic opening, any natural or man-made changes to the stream, flooding, etc.
It is necessary to consider the adequacy of the hydraulic opening (the cross-sectional area under the bridge) to convey anticipated flows, including the design flood, without damage to the bridge. It is essential to maintain a bridge inspection file comparing original conditions in the waterway at the time the bridge was constructed to changes in the cross-sectional area of the channel under the bridge over time.
The primary method of assessing loss of cross-sectional area of the hydraulic opening is to determine channel bed elevation changes. This can be determined by a periodic survey of the channel bed or by taking soundings from the bridge. Typically, a number of survey or sounding points spaced across the bridge opening are established to determine changes in cross-sectional area. Note the lateral location of these surveyed points so that as subsequent inspections are conducted, the survey points can be repeated to maintain consistency. Photographs from key locations can be used to document debris and vegetation that can block the bridge opening.
Stream gages in the vicinity of the bridge may be useful in evaluating the adequacy of the waterway in relationship to changing hydraulic conditions. For example, stage-discharge curves based on discharge measurements by the United States Geological Survey (USGS) or other agencies and shifts in rating curves may indicate changes in channel bed elevation and cross section.
The size, gradation, cohesion, and configuration of the streambed material can affect scour rates. When comparing sands and cohesive soils, such as clays, the size of the streambed material has little effect on the depth of scour, but can affect the amount of time needed for this depth to be attained. Cohesive streambed materials that are fine usually have the same ultimate depth of scour as sand streambeds. The difference is that the cohesive streambeds take a longer period to reach this ultimate scour depth. For these reasons, the streambed type is important and correctly evaluated by the bridge inspector. Streambed rates of scour for different types of material are described later in this topic.
Substructure members on old bridges were not necessarily designed to withstand the effects of scour. Wide piers and piers skewed to the flow of the stream can contribute to an increase the depth of scour. Due to increased awareness of bridge waterway scour, recent substructure members have been designed to allow the stream to pass through with as little resistance as possible. Many newer piers have rounded or pointed noses, which can decrease the scour depth by up to 20%.
Footings that are undermined, but founded on piles are not as critical as spread footings that are undermined. Determine the substructure foundation type, in order to properly evaluate the substructure and the waterway. The foundation type may often be determined from design and/or construction drawings. In some older bridges, the foundation type is not known. In this case, advanced inspection techniques by a trained professional may be required to verify the foundation type.
The most common bridge waterway deficiency is scour, which may adversely impact bridge substructure units. Scour is the removal of material from the streambed or embankment as a result of the erosive action of streamflow.
The rate of scour will vary for different streambed materials, and for different streamflow rates. For a given streamflow rate, a streambed material will scour to a maximum depth in a given time. The following are examples for different types of streambeds and their corresponding scour rate:
There are three forms of scour considered in evaluating the safety of bridges:
Aggradation and degradation are long-term streambed elevation changes. Aggradation is the general and progressive buildup of the longitudinal profile of a channel bed due to the sediment deposition. (see Figure 13.2.2). Degradation is the general and progressive (or long-term) lowering of the channel bed due to erosion, over the relatively long channel length (see Figure 13.2.3).
Aggradation and degradation may be a result of the natural erosion and downcutting process that rivers experience through the years. This scour type may be accelerated by natural cutoffs in a meandering river, which steepens the channel gradient, increasing both the velocity of flow and hence scour. These changes may also be accelerated by various types of development or river modification, such as:
Since aggradation and degradation of the channel bed is along some considerable distance of channel, major facilities are sometimes used to control scour. These facilities can include a series of drop structures (small dam-like structures) or other scour protection of the riverbed. Presence of such structures may be indicative that the channel is experiencing scour.
Factors that may cause changes in the elevation of the streambed include:
Headcut migration is the degradation of the channel that is associated with abrupt changes in the bed elevation and then migrates upstream. Headcutting tends to form in more cohesive materials in a streambed. Cohesive materials are discussed on page 13.2.18.
Figure 13.2.2 Streambed Aggradation
Figure 13.2.3 Streambed Degradation
General scour can occur in a short time with the right conditions (see Figures 13.2.4 and 13.2.5). It is the lowering of the streambed across the waterway at the bridge which may or may not be uniform. This means it could be deeper in some parts than in others. General scour could be the result of contraction of the flow, which will result in the removal of the streambed material across all or most of the channel width or from other general scour conditions, such as flow around a bend where the scour will be concentrated near the outside of the bend.
Figure 13.2.5 Close-up of General Scour of a Pier
Changes in downstream elevation, such as at the confluence with another river which is undergoing scour of its own, can cause general scour in the upstream river. Weather events such as hurricanes can also cause general scour (see Figures 13.2.4 and 13.2.5).
General scour may reduce the degree of safety experienced by the substructures, because of the changed hydraulic conditions and the changed channel geometry. In this case, it is essential to refer to the bridge inspection file and study historical changes that have occurred in the bed elevation through the waterway. If possible, these changes are related to specific causes to assess the present safety of the bridge. These changes also provide insight as to future conditions that may be imposed by changed flow conditions, watershed development, or other conditions affecting the safety of the bridge.
Contraction scour results from the acceleration of flow due to a natural contraction, a bridge contraction, or both (see Figures 13.2.6 and 13.2.7). When the available area for stream flow at the bridge is reduced compared with the available area upstream from the bridge, velocity will increase at the bridge. Less area for flow results in faster moving water. The lowering of the streambed under the bridge due to this accelerated stream velocity is known as contraction scour. A bridge length may be shortened to reduce the initial cost of the superstructure. However, this shortened bridge results in a smaller hydraulic opening which can lead to contraction scour (see Figure 13.2.8).
Figure 13.2.6 Stream Contraction Schematic
Figure 13.2.7 Contraction Scour Photograph
Figure 13.2.8 Large number of Piers Combine to Reduce the Hydraulic Opening
Some common causes that can lead to contraction scour include:
The effects of contraction scour can be very severe.
Figure 13.2.9 Vegetation Constricting the Waterway
Figure 13.2.10 Sediment Deposits Within the Waterway Opening
Figure 13.2.11 Ice in Stream Resulting in Possible Contraction Scour
Figure 13.2.12 Debris Build-up in the Waterway
Other general scour conditions result from erosion due to streams which are meandering, braided, or straight, variable downstream control, flow around a bend or any other changes which may cause a decrease in the bed elevation. This could also result from a short-term change in downstream water surface elevation which can control the velocity through the bridge. This may occur at bridges located upstream or downstream from a confluence.
Local Scour
Local scour occurs around an obstruction that has been placed within a stream, such as a pier or an abutment which causes an acceleration of the flow and results in induced by the obstruction. Local scour can either be clear-water scour or live-bed scour.
Clear-water scour occurs when there is no bed material transport upstream of the bridge. It occurs in streams where the bed material is coarse, the stream grade is flat, or the streambed is covered with vegetation except in the location of substructure members.
Live-bed scour occurs when local scour at the substructure is accompanied by bed material transport in the upstream waterway.
The cause of local scour is the acceleration of streamflow resulting from vortices induced by obstructions (see Figure 13.2.13). Some common obstructions are:
Figure 13.2.13 Local Scour at a Pier
Figure 13.2.14 Local Scour at a Pier
Scour depths resulting from local scour are normally deeper than those from general scour, often by a factor of ten. However, if there are major changes in hydrologic conditions resulting from such factors as construction of large dams and water resources development, the general scour can be the larger element in the total scour.
Bridges in tidal situations are particularly vulnerable to local scour. A strong tidal current whose direction reverses periodically causes a complex local scour phenomenon around a bridge substructure. This local scour is caused by an imbalance between the input and output sediment transport rates around the pier, and it has a negative influence on the stability of the bridge.
To properly evaluate local scour and impacts of changes in hydrologic and hydraulic conditions on local scour, it is essential to develop and refer to that component of the bridge inspection file which deals with local scour. With each inspection, subject the critical supporting elements of the bridge to careful survey to determine the degree of local scour that has developed over time. By referring to this history of change in local scour, it can be determined whether or not the maximum local scour has occurred and the relationship of this maximum local scour to bridge safety.
If the survey of the magnitude of local scour indicates increased local scour with time and furthermore verifies that the local scour exceeds the anticipated maximum local scour when the bridge was designed, take remedial measures to protect the bridge. Surveys of local scour along the abutments and around the piers are most often done during periods of low flow when detailed measurements can be made, either by wading and probing, by probing from a boat, by the use of divers, or by sonic methods. The pattern of survey has to be established and remain the same during the life of the bridge, following either a fixed radial or a rectangular grid. Changes in magnitude of local scour can then be compared at specific points over time.
The greatest problem associated with determining the magnitude of local scour relates to maximum local scour occurring at flows near flood peak followed by a period of deposition of sediments in the scour hole after the flood peak has passed and during low-flow periods. Consequently, base the bridge rating upon maximum scour that occurred during floods but not based upon examination of bed levels around abutments and piers during low-flow periods. Hence, it is necessary to use a variety of techniques to differentiate between maximum scour that may have occurred during flood periods and apparent scour after periods of low flow.
Consider utilizing straight steel or aluminum probing rods to probe loose sediments deposited along abutments and around footings; if sediment is finer than average bed material sizes or if the sediment is easily penetrated by the rod, it is indicative that the present sediment has accumulated in the scour hole and local scour is more severe than indicated by present accumulations of sediments. Core samples may also be used to differentiate between backfill in the scour hole and the bottom of the scour hole. It may be possible to use geotechnical means as another alternative to differentiate between materials that have deposited in the scour hole and the bottom of the scour hole. It may also be necessary to use underwater surveys using divers, or perhaps to even divert water away from critical elements to allow removal of loose backfill material. The inspector can then determine the true level of maximum scour in relationship to the bridge’s supporting structural elements.
The problem of accurately determining maximum local scour and rate of change of local scour over time is one of the most difficult aspects of bridge inspection and is one of the most important aspects of evaluating bridge safety. Additional research is being conducted to provide better guidelines for investigating local scour in relationship to bridge safety.
Lateral stream migration or horizontal change in the waterway alignment is another type of erosion that can also threaten the stability of bridge crossings. Embankment instability typically results from lateral stream movement at a bridge opening and has often been the primary cause in a number of bridge collapses around the country. Bridge abutments and piers are often threatened by this type of erosion (see Figure 13.2.17).
Figure 13.2.17 Lateral Stream Migration Endangering an Abutment
Lateral stream migration often threatens bridge abutments, piers and approach roadways, particularly those that are along upstream banks at the bridge opening. Lateral stream migration can occur in four modes of bank failure:
Figure 13.2.18 Streambank Damage
Figure 13.2.19 Sloughing Streambank
Figure 13.2.20 Undermined Streambank
Lateral stream migration is very common and can result from a variety of causes. Channel changes contributing to lateral stream migration include:
Series of aerial photographs over time could be a way to check for lateral stream migration.
Figure 13.2.21 Stream Meander Changes
Figure 13.2.22 Channel Widening
When inspecting for lateral stream instability, some visual indicators are:
The resistance that a streambank has to erosion is closely related to several characteristics of the bank material. The bank material that is deposited in the stream can be classified as:
Figure 13.2.23 Schematic of Noncohesive Bank Material
Figure 13.2.24 Schematic of Cohesive Bank Material
Figure 13.2.25 Schematic of Cohesive Bank Material
Material defects that can be caused by waterway deficiencies include the deterioration and damage (i.e. abrasion, corrosion, scaling, cracking, spalling, and decay) to channel protection devices and substructure members.
As an integral part of the waterway inspection, give careful consideration to the identification of material defects. A loss of quality and quantity of materials required to provide bridge safety may occur in a variety of ways. Carefully record the changes in characteristics of materials in the bridge inspection file. Changes over time can be compared and any decision concerning maintenance requirements or replacement becomes more straightforward with historic information available.
Waterway deficiencies that are severe have the capability to cause damage to bridges. Effects of waterway deficiencies on bridge members include undermining, settlement, and failure.
Undermining is the scouring away of streambed and supporting foundation material from beneath the substructure (see Figure 13.2.26). Excessive scour often produces undermining of both piers and abutments. Such undermining is a serious condition, which requires immediate correction to assure the stability of the substructure unit. Undermining is especially serious for spread footings, but may also be cause for concern for pile foundations because loss of supporting soil around piling can reduce pile capacity. Substructure stability may be compromised, potentially leading to total bridge collapse.
End View of Pier
Side View of Pier
Figure 13.2.26 End and Side View of Scour and Undermining
The undermining of structural elements is basically an advanced form of scour. It is essential to determine whether or not undermining has a potential to develop, as well as whether it has already occurred. Address undermining immediately since it can pose an immediate threat to safety.
With small bridges, L-shaped rods can be used to probe at the base of footings to determine possible undermining. On the other hand, undermining may be very difficult to identify due to the redeposition of sediments during periods of low flow after undermining has occurred. However, in those channels where the bed is formed of coarse rock and the sediment supply to the bridge crossing is small, it is possible to inspect the footings because the backfill with fine sediments during periods of low flow generally does not occur.
For areas not accessible to effective probing from above water, it is essential to employ underwater inspection techniques utilizing divers. Whenever possible, take detailed measurements, showing the height, width, and penetration depth of the undermined cavities. Refer to Topic 13.3 for a more detailed description of underwater inspections.
Local scour and undermining is typically most severe at the upstream end of the substructure and, if not corrected, may result in differential settlement (see Figure 13.2.27).
Figure 13.2.27 Pier Settlement due to Undermining
When undermining and settlement go undetected for some length of time, the bridge may become unstable, and be subject to failure or collapse. Failure may occur over a period of time, or it may be a very rapid process occurring during a flood event.
It is necessary to identify and assemble the documentation and equipment required to conduct the waterway inspection. The required equipment will depend upon the characteristics of the river, the characteristics of the bridge, and the accessibility of the site.
Necessary information is required for a comprehensive, well-organized inspection of waterways.
Examine any previous hydraulic engineering scour evaluation studies on the bridge. These studies provide theoretical ultimate scour depths for the bridge substructure elements. Review original drawings and previous inspection report data taken from successive inspections to determine the foundation type and streambed material. Establish whether the waterway is stable, degrading or aggrading.
Become familiar with site conditions and channel protection installations. Verify if there is a change in the hydraulic opening by reviewing previous channel cross sections and profiles. Examine the photographs to determine any changes in the channel alignment.
Considering the complexity of the inspection and the equipment and materials needed to execute the inspection, develop a detailed plan of investigation, as well as forms for recording observations. Use a systematic method each time the bridge is surveyed to provide a means of accurately identifying changes that have occurred at the bridge site, which may affect the safety of the bridge.
Prior to beginning the inspection, the bridge inspector needs to understand the type and extent of the inspection required. Waterway inspections are typically accomplished by either surface inspection or underwater diving inspection.
Surface or “wading” inspection is conducted on shallow depth foundations. Submerged substructure, streambed and embankments are often accessible by inspectors using hip boots or chest waders and probing rods (see Figure 13.2.28). Additionally, boats are often used as a surface platform from which to gather waterway data, including channel cross-sections, pier soundings, etc.
Underwater diving inspection is required when the foundations are deep into water. Site conditions often require waterway and submerged substructure units to be evaluated using underwater divers, in order to obtain complete, accurate data. This is especially true when water depths are too great for wading inspection, and/or undermining of substructure elements is suspected.
Equipment required to inspect bridges is listed and described in Topic 2.4. Additional equipment may be required for the inspection of waterways. The type of equipment needed for a waterway inspection is dependent on the type of inspection. The following is a list that represents the most common waterway inspection equipment.
Refer to Topic 13.3 for additional information on underwater inspection equipment.
Figure 13.2.28 Probing Rod and Waders
Figure 13.2.29 Surface Supplied Air Diving Equipment
Give special considerations to the site conditions and the navigational controls that may adversely affect the safety of the bridge inspector and others.
Site conditions such as rapid stream flow velocity, pollution levels, safety concerns, and conditions requiring special attention need to be accounted for during a waterway inspection (see Figure 13.2.30).
Navigational control is necessary when inspecting large waterways. Notify the Coast Guard in advance of inspections where navigational controls are needed. Other navigational controls include boat traffic, operational status and condition of dolphins and fenders, dam releases (see Figure 13.2.31).
Figure 13.2.30 Rapid Flow Velocity
Figure 13.2.31 Navigable Waterway
The primary method used to inspect waterways is visual. Look at the site in the vicinity of the bridge. Also, look at the floodplain. This observation may have to be done during periods of high water flow.
After the inspector gets the general condition by visually inspecting the bridge site, the next step is to probe for any scour or undermining. Take care to adequately press the probing rod into the soil in the streambed. Sometimes scour holes are loosely filled with silt. This silt may be washed away quickly during the next period of high stream flow velocity, permitting additional scour.
Take measurements to obtain the cross section and profile. These measurements are used to analyze the area of the hydraulic opening and help determine need for and design of mitigation measures. The cross section under the bridge can be measured with a surveyor’s tape or rod. The stream profile can be measured with a hand level, survey tape and surveying rod (see Figures 13.2.32 and 13.2.33). Compare the streambed profile and hydraulic opening to previous inspections.
Figure 13.2.32 Streambed Cross-Section
Figure 13.2.33 Streambed Profile
An alternative to the sounding and scour sensing devices used during inspections is to permanently install fixed instrumentation directly on the bridge substructure. With fixed instrumentation, local scour is continuously monitored and recorded as it occurs, unaffected by washing back of silts and sands, and making information readily available to the bridge owner by setting off a beacon-type alarm on the bridge deck (or relayed back to an office). One such instrument consists of a steel rod inside of a conduit attached to the substructure unit. The rod acts as a probe, resting on the vulnerable soil supporting the substructure. As local scour occurs the soil is washed away and the rod drops a measured distance.
Other fixed instrumentation includes fixed sonar units, sliding magnetic collars, and buried “float-out” buoys, which float to the water surface after being uncovered by local scour, activating an electronic alarm system (see Figure 13.2.34).
Researchers are studying a new method for scour detection and monitoring. The new method is based on time domain reflectometer (TDR) technology, which uses pulse transmissions to show changes in a particular environment. The TDR bridge scour monitoring system consists of a probe, which is completely buried in the sediment at appropriate locations around and near the bridge pier and footings. As erosion occurs, part of the probe is exposed to water. Then, the probe reflects a specific pulse back to the TDR box, which is on the surface, indicating how much of the probe is exposed and producing wave forms to show scour depth. The probes are designed to be left at bridge sites to detect/monitor scour.
Figure 13.2.34 Scour Monitoring Collar
When inspecting the bridge waterway, three main areas are of concern. These areas include the channel under the bridge, the upstream channel, and the downstream channel.
Figure 13.2.35 Pile Bent Deterioration Normally Hidden Underwater
Figure 13.2.36 Out of Plumb Pier Column
During a waterway inspection, the superstructure can be a good indicator of existing waterway deficiencies.
The following items need to be reviewed:
Figure 13.2.37 Superstructure Misalignment
Figure 13.2.38 Drift Lodged in a Superstructure
Figure 13.2.39 Multi-Span Simply Supported Bridge
It is essential to identify any change that is observable, including changes in the gradation of riprap. It is also essential to carefully inspect the integrity of the wire basket where gabions have been used.
Disturbance or loss of embankment and embankment protection material is usually obvious from close scrutiny of the embankment. Unevenness of the surface protection is often an indicator of the loss of embankment material from beneath the protective works. However, loss of embankment material may not be obvious in the early stages of failure. Also look for irregularities in the embankment slope.
It is difficult to determine conditions of the protective works beneath the water surface. In shallow water, evidence of failure or partial failure of protective works can usually be observed. However, with deeper flows and sediment-laden flows, it is necessary to probe or sound for physical evidence to identify whether failure or partial failure exists.
Figure 13.2.41 Severe Streambed Degradation Evident at Low Water
Figure 13.2.42 Approach Roadway Built in the Floodplain
Figure 13.2.43 Stable Streambanks
Figure 13.2.44 Sediment Accumulation Redirecting Streamflow
Figure 13.2.45 Fence in Stream at Bridge
Figure 13.2.46 Waterway Alignment 1990 - 2006
Figure 13.2.47 Approach Spans in the Floodplain
Figure 13.2.48 Debris and Sediment in the Channel
A plan of action is prepared to monitor any known and potential deficiencies and address any critical findings for bridges that are determined to be scour critical. Instructions regarding the type and frequency of inspections in regards to monitoring the performance and the closing of the bridge during or after flood events are included in the scour plan of action. A schedule for the design and construction of scour countermeasures if it is determined they are needed for the bridge are also included.
Bridges over streams and rivers are subject to scour and are evaluated to determine their vulnerability to floods and to determine whether they are scour critical.
In a scour evaluation, structural, hydraulic and geotechnical engineers have to make decisions on:
A responsibility of the bridge inspector is to gather on-site data for an assessment of scour potential, that:
To accomplish these objectives, the inspector needs to recognize and understand the potential for scour and its relationship with the bridge and stream. When an actual or potential scour problem is identified by a bridge inspector, further evaluation the bridge is completed by an interdisciplinary team made up of structural, geotechnical, and hydraulic engineers.
Figure 13.2.50 Scour at a Pile Abutment
Identify and record waterway conditions at the bridge, upstream of the bridge, and downstream of the bridge. Indications that could establish a scour potential include waterway, substructure and superstructure.
Figure 13.2.51 Fast Flowing Stream
Figure 13.2.52 Scour Rates vs. Velocity for Common Streambed Materials
Figure 13.2.53 Typical Misaligned Waterway
Figure 13.2.54 Typical Large Floodplain
Figure 13.2.55 Lateral Stream Migration
Consider the following condition of bridge foundations and substructure units in the scour potential assessment:
Figure 13.2.56 Stream Alignment Not Parallel with Abutments
Figure 13.2.57 Rotational Movement and Failure Due to Undermining
Figure 13.2.58 Exposed Piling Due to Scour
Figure 13.2.59 Accelerated Flow Due to Constricted Waterway
Consider the following conditions associated with the superstructure in recognizing scour potential:
The scour evaluation is an engineering assessment of existing and potential problems and making a sound judgement on what steps can be taken to eliminate or minimize future damage.
In assessing the adequacy of the bridge to resist scour, the inspector and engineer need to understand and recognize the interrelationships between several items. The inspector can expedite the engineers’ evaluation by considering the following:
See Topic 4.2 for a detailed description of NBI Condition Rating Guidelines.
Substructure rating is a key item for rating the bridge foundations for vulnerability to scour damage. When a scour problem is found that has already occurred, considered it in the condition rating of the substructure. If the bridge is determined to be scour critical, further evaluate the condition rating for Item 60 to ensure that any existing problems have been properly considered. Be consistent with the rating factor given to Item 60 with the one given to Item 113 whenever a rating factor of 2 or below is determined for Item 113.
This item permits rating the physical channel condition affecting streamflow through the bridge waterway. Consider the condition of the channel, adjacent rip-rap, bank protection, guidebanks, and evidence of erosion, channel movement or scour in establishing the rating for Item 61.
This is an appraisal item, rather than a condition item, and permits assessment of the adequacy of the bridge waterway opening to pass flood flows.
This item permits a rating of current bridge conditions regarding its vulnerability to flood damage. A scour-critical bridge is one with abutment or pier foundations that are considered unstable due to:
When an actual or potential scour problem is identified, the bridge is to be further evaluated by an interdisciplinary team comprised of structural, hydraulic and geotechnical engineers.
In this process, the effects of a 100-year flood (a flood which has a one percent chance of occurring in any year) would be considered, but the effects of a "superflood" or 500-year flood would also be assessed and assigned to one of three conditions.
Figure 13.2.60 Scour Assessment - Safe
Figure 13.2.61 Scour Assessment - Evaluate
Figure 13.2.62 Scour Assessment - Fix
For scour critical bridges, the NBIS requires that a Plan of Action is developed for monitoring and correcting the scour problem. Monitor, in accordance with the plan, bridges which are scour critical. Such a plan would address the type and frequency of future inspections to be made and would include a schedule of timely design and construction actions for appropriate countermeasures to protect the bridge. The countermeasures might include the possibility of riprap, bed armoring, or flow-control structures or embankments.
Washouts of scour critical bridges, which appeared to be stable in the past, have still occurred. Recognizing potential problems and developing a Plan of Action for scour critical bridges will help reduce the likelihood of washouts.
The following excerpt is from a reproduction of the out-of-print Culvert Inspection Manual (Supplement to Manual 70), July 1986 — Chapter 5, Section 3:
The primary function of most culverts is to carry surface water or traffic from one side of a roadway embankment to the other side. The hydraulic design of culverts usually involves the determination of the most economical size and shape of culvert necessary to carry the design discharge without exceeding the headwater depth allowable. It is essential that the culvert be able to handle the design discharge. If the culvert is blocked with debris or the stream changes course near the ends of the culvert, the culvert may be inadequate to handle design flows. This may result in excessive ponding, flooding of nearby properties, and washouts of the roadway and embankment. In addition changes in upstream land use such as clearing, deforestation, and real estate development may change the peak flow rates and stream stability. It is therefore important to inspect the condition of the stream channel, SI&A item 61, and evaluate the ability of the culvert to handle peak flows, SI&A item 71.
The stream channel should be inspected to determine whether conditions exist that would cause damage to the culvert or surrounding properties. Factors to be checked include culvert location (horizontal and vertical alignment), scour, and accumulation of sediment and debris. These factors are closely related to each other. Poor culvert location can result in reduced hydraulic efficiency, increased erosion and sedimentation of the stream channel, and increased damage to the embankment and surrounding properties. A brief discussion of each of these factors is provided.
The upstream channel should be checked for scour that may undermine the culvert or erode the embankment. Scour that is undermining trees or producing sediment that could block or reduce the culvert opening should also be noted. The stream channel below the culvert should be checked for local scour caused by the culvert's discharge and for general scour that could eventually threaten the culvert.
The preceding paragraphs dealt with evaluating the condition of the stream channel and identifying conditions that could cause damage to the culvert or reduce the hydraulic efficiency of the culvert. A closely related condition that must be evaluated is the waterway adequacy or ability of the culvert to handle peak flows, changes in the watershed, and changes in the stream channel which might affect the hydraulic performance. Guidelines for rating SI&A item 71, Waterway Adequacy, are presented in the Coding Guide.
Figure 13.2.63 (Exhibit 63) Culvert Failure Due to Overtopping
Figure 13.2.64 (Exhibit 64) Culvert Almost Completely Blocked by Sediment Accumulation
Figure 13.2.65 (Exhibit 65) Drift and Debris Inside Timber Box Culvert
The need for underwater inspections is great. Approximately 83 percent of the bridges in the National Bridge Inventory (NBI) are built over waterways. While many of these bridges do not have foundation elements actually located in water, a great many do and most bridge failures occur because of underwater issues. Inspect underwater members to the extent necessary to determine with certainty that their condition has not compromised the structural safety of the bridge.
Several bridge collapses during the 1980’s, traceable to underwater deficiencies, have led to revisions in the National Bridge Inspection Standards (NBIS) (see Figure 13.3.1). As a result, bridge owners are required to develop a master list of bridges requiring underwater inspections.
Figure 13.3.1 Schoharie Creek Bridge Failure
According to the NBIS, underwater inspection is the inspection of the underwater portion of a bridge substructure and the surrounding channel, which cannot be inspected visually at low water by wading or probing, generally requiring diving or other appropriate techniques.
The expense of such inspections necessitates careful consideration of candidate bridge, since underwater inspection is a hands-on inspection requiring underwater breathing apparatus and related diving equipment.
Bridges that cross waterways often have foundation elements located in water to provide the most economical total design. Where these elements are continuously submerged (see Figure 13.3.2), use underwater inspection and management techniques to establish their condition so that failures can be avoided.
Figure 13.3.2 Liberty Bridge over Monongahela River
In many cases, a multi-disciplinary team including structural, hydraulic and geotechnical engineers evaluate a bridge located over water that is a candidate for underwater inspection. Underwater inspection is therefore only one step in the total investigation of a bridge.
Various factors influence the underwater bridge inspection selection criteria. In accordance with the Code of Federal Regulations (23 CFR Part 650) and the AASHTO Manual for Bridge Evaluation (MBE), all structures receive routine underwater inspections at intervals not to exceed 60 months, or 72 months with FHWA approval. This is the maximum interval permitted between underwater inspections for bridges which are in excellent condition underwater and which are located in passive, nonthreatening environments. More frequent routine and in-depth inspections may be desirable for many structures and necessary for critical structures. The bridge owner determines the inspection interval that is appropriate for each individual bridge. Factors to consider in establishing the inspection frequency and levels of inspection include:
Note those bridges that require underwater inspection on the bridges' individual inspection and inventory records. For each bridge requiring underwater inspection, include the following information as a minimum:
Originating in the offshore diving industry and adopted by the United States Navy, the designation of standard levels of inspection has gained widespread acceptance. Three diving inspection intensity levels have evolved as follows:
Routine underwater inspections normally include a 100 percent Level I inspection and a 10 percent Level II inspection, but it may include a Level II and Level III inspection to determine the structural condition of any submerged portion of the substructure with certainty.
Level I inspection consists of a close visual inspection at arm’s length with minimal cleaning to remove marine growth of the submerged portions of the bridge. This level of inspection is used to confirm the continuity of the members and to detect any undermining or elements that may be exposed that would normally be buried. Although the Level I inspection is referred to as a "swim-by" inspection, it needs to be detailed enough to detect obvious major damage or deterioration. A Level I inspection is normally conducted over the total (100%) exterior surface of each underwater element, involving a visual and tactile inspection with limited probing of the substructure and adjacent streambed. In areas where light is minimal, handheld lights may be needed. If the water clarity is poor enough that the inspector cannot inspect the member visually, a tactile inspection may be performed by making a sweeping motion of the hands and arms to cover the entire substructure.
The results of the Level I inspection provide a general overview of the substructure condition and verification of the as-built drawings. The Level I inspection can also indicate the need for Level II or Level III inspections and aid in determining the extent and the location of more detailed inspections.
Level II inspection is a detailed inspection that requires that portions of the structure be cleaned of marine or aquatic growth. In some cases, cleaning is time consuming, particularly in salt water, and needs to be restricted to critical areas of the structure. However, in fresh water, aquatic coatings can be removed by just wiping the structural element with a glove.
Generally, the critical areas are near the low waterline, near the mud line, and midway between the low waterline and the mud line. On pile structures, horizontal bands, approximately 6 to 12 inches in height, preferably 10 to 12 inches, need to be cleaned at designated locations:
Figure 13.3.3 Level II Cleaning of a Steel Pile
On large elements, such as piers and abutments, clean areas at least 1 square foot in size at three or more levels on each face of the element (see Figure 13.3.4). For a structure that is greater than 50 feet in length, clean an additional three levels on each exposed face. It is important to select the locations to clean to help minimize any potential damage to the structure and to target more critical locations. Measure and document deficient areas, including both the extent and severity of the damage.
It is intended to detect and identify high stress, damaged and deteriorated areas that may be hidden by surface growth. A Level II inspection is typically performed on at least 10% of all underwater elements. Govern the thoroughness of cleaning by what is necessary to determine the condition of the underlying material. Complete removal of all growth is generally not required.
Figure 13.3.4 Diver Cleaning Pier Face For Inspection
A Level III inspection is a highly detailed inspection of a critical structure or structural element, or a member where extensive repair or possible replacement is contemplated. The purpose of this type of inspection is to detect hidden or interior damage and loss in cross-sectional area. This level of inspection includes extensive cleaning, detailed measurements, and selected nondestructive and other testing techniques such as ultrasonics, sample coring or boring, physical material sampling, and in-situ hardness testing. The use of testing techniques is generally limited to key structural areas; areas that are suspect; or areas that may be representative of the entire bridge element in question.
A comprehensive review of all bridges contained in an agency’s inventory will indicate which bridges require underwater inspection. Many combinations of waterway conditions and bridge substructures exist. For any given bridge, the combination of environmental conditions and structure configuration can significantly affect the requirements of the inspection. It is generally accepted that there are five different types of inspections used for underwater inspections:
Underwater inspections are typically either routine or in-depth inspections.
A routine underwater (or periodic) inspection is a regularly scheduled, intermediate level inspection consisting of sufficient observations and measurements to determine the physical and functional condition of the bridge, to identify any change from initial or previously recorded conditions, and to ensure that the structure continues to satisfy present service requirements. A routine underwater inspection will incorporate Level I, Level II and a scour evaluation.
The summary guidelines for a routine underwater inspection include:
The dive team may also conduct a scour evaluation at the bridge site, including:
Figure 13.3.5 Channel Cross-Section (Current Inspection Versus Original Channel)
Figure 13.3.6 Pier Sounding Grid
Figure 13.3.7 Permanent Reference Point (Bolt Anchored in Nose of the Pier, Painted Orange)
Figure 13.3.8 Local Scour; Causing Undermining of a Pier
An initial (or inventory) inspection is the first inspection of a bridge as it becomes a part of the bridge inventory. An initial inspection is a fully documented investigation that will typically incorporate Level I and Level II inspections and a scour evaluation as required for a routine inspection. In addition, this type of inspection will provide all of the Structure Inventory and Appraisal (SI&A) and other relevant data to determine the baseline structural conditions. It also identifies and lists the existing problems and locations of existing problems or locations in the structure that may have potential problems. Aided by a prior detailed review of plans, it is during this inspection that any underwater members (or details) are noted for subsequent focus and special attention (see Figure 13.3.9).
An initial inspection may also be required when there has been a change in the configuration of the structure such as widening, lengthening, bridge replacement, or change in ownership.
Figure 13.3.9 Bascule Bridge on the Saint Croix River
Certain conditions and events affecting a bridge may require more frequent or unscheduled inspections to assess structural damage resulting from environmental or accident related causes.
The scope of the inspection is to be sufficient to determine the need for emergency load restrictions or closure of the bridge to traffic and to assess the level of effort necessary to repair the damage. The amount of effort expended on this type of inspection will vary significantly depending upon the extent of the damage. If major damage has occurred, evaluate section loss, make measurements for misalignment of members, and check for any loss of foundation support.
An in-depth inspection is a close-up, hands-on inspection of one or more members below the water level to detect any deficiencies not readily apparent using routine inspection procedures. When appropriate or necessary to fully ascertain the existence of or the extent of any deficiencies, Level III, nondestructive tests may need to be performed.
The in-depth inspection typically includes Level II inspection over extensive areas and Level III inspection of limited areas. Nondestructive testing is normally performed, and the inspection may include other testing methods, such as extracting samples for laboratory analysis and testing, boring, and probing.
One or more of the following conditions may dictate the need for an in-depth inspection:
The distinction between routine and in-depth inspections is not always clearly defined. For some bridges, such as steel pile supported structures in an actively corrosive environment, it may be necessary to include Level III, nondestructive testing inspection techniques as part of routine inspections.
A special (or interim) inspection is scheduled at the discretion of the individual in charge of bridge inspection activities. A special inspection is used to monitor a particular known or suspected deficiency (e.g., foundation settlement or scour).
Situations that may warrant a damage, in-depth or a special underwater inspection include:
Figure 13.3.10 Flood Conditions: Pier Settlement.
Figure 13.3.11 Buildup of Debris At Pier
Figure 13.3.12 Movement of a Substructure Unit
Conduct routine inspections of substructures in water at least once every 60 months. This is only applicable to substructures that are in excellent condition and for substructures that have current conditions that are considered acceptable for that timeframe and without any concerns that may require more frequent monitoring.
Structures having underwater members which are partially deteriorated or are in unstable channels may require shorter inspection intervals. Establish criteria for determining the level and frequency to which these underwater elements will be inspected base on such factors as:
Certain underwater structural elements may be inspected at greater than 60-month intervals, not to exceed 72 months, with written FHWA approval. This may be appropriate when past inspection findings and analysis justifies the increased inspection interval.
Some bridge owners, however, may shorten the underwater inspection interval to 24 or 48 months to coincide with a regular routine bridge inspection (24 to 48 months) or a fracture critical member inspection (24 months).
An underwater bridge inspection diver needs to complete an FHWA-approved comprehensive bridge inspection training course or other FHWA-approved underwater bridge inspection training course.
The underwater inspector needs to have knowledge and experience in bridge inspection. Conduct all underwater inspections under the direct supervision of a qualified bridge inspection team leader. A diver not fully qualified as a bridge inspection team leader is to be used under close supervision.
The ability of the underwater inspector to safely access and remain at the underwater work site is paramount to a quality inspection. The individual is to possess a combination of commercial diving training and experience as a working diver. This allows the inspector to meet the particular challenges of the underwater working conditions for that inspection.
Team leader requirements for those in charge of underwater inspection are the same as their top-side counterparts. See Topic 1.2 Responsibilities of the Bridge Inspector and Title 23 of the Code of Federal Regulations, Part 650, Subpart C.
Underwater bridge inspection, using either self-contained or surface-supplied equipment, is a form of commercial diving. In the United Sates, commercial diving operations are federally regulated by the Occupational Safety and Health Administration (OSHA). OSHA regulates all commercial diving operations performed inland and on the coast (through Title 29 of the Code of Federal Regulations, Part 1910, Subpart T, Commercial Diving Operations). Consult this reference for details on commercial diving procedures and safety.
The OSHA Commercial Diving Operations standard applies to all diving and related support operations that are conducted in connection to diving. The OSHA delineates diving personnel requirements, including general qualifications of dive team members. The standard also provides general and specific procedures for diving operations, and provides requirements and procedures for diving equipment and recordkeeping:
U.S Army Corp Army of Engineers Safety and Health Requirements is another safety standard that may be used and is similar to OSHA standards, except it provides more specific guidance as to the minimum dive team personnel required for various diving conditions. It also provides a more definitive requirement for diving qualifications and requires that divers be certified in the emergency administration of oxygen.
Visit www.osha.gov for more information.
American National Standards Institute (ANSI) Standards exist, which define minimum training standards for both recreational SCUBA and commercial divers. These standards provide clear-cut distinctions between recreational and commercial diver training. While not federal law, these standards constitute the consensus of both the recreational and commercial diving communities, following ANSI’s requirements for due process, consensus, and approval.
The American National Standard for Divers- Commercial Diver Training- Minimum Standard (ANSI/ACDE-01-1998) requires a formal course of study, which contains at least 625 hours of instruction. This training may come from an accredited commercial diving school, military school, or may be an equivalent degree of training achieved prior to the effective date of the Standard, which includes a documented combination of field experience and/or formal classroom instruction. Visit www.ansi.org/ for more information.
The Association of Diving Contractors International (ADC) is a non-profit organization representing the commercial diving industry. The ADC publishes “Consensus Standards For Commercial Diving Operations”, which have been developed to present the minimum standards for basic commercial diving operations conducted either offshore or inland. The Consensus Standards, in part, duplicate the ANSI standard for commercial diver training, but subdivide the minimum 625 hours of training into both a formal course of study (317 hours, minimum), and on the job training (308 hours, minimum). The ADC also formally issues OSHA-recognized Commercial Diver Certification Cards to individuals meeting minimum training standards. Visit www.adc-int.org/ for more information.
The Federal Highway Administration’s main concern is whether the diver has knowledge and experience in underwater bridge inspection. The individual employers are in the best position to determine the specific requirements of their dive teams.
The primary goal for an underwater inspection is for the dive team to complete the work safely and to perform a complete and accurate inspection. Planning for underwater bridge inspections is particularly important because of:
These factors are most influential for first-time (initial) underwater inspections that set a benchmark for future inspections. Therefore, it is important to distinguish between the first-time and follow up inspections.
The effectiveness of an underwater inspection depends on the agency’s ability to properly consider the following factors:
With these factors considered, an agency may opt for a lower level of inspection. Depending on conditions and the type of damage found, a higher level may then be necessary to determine the actual bridge condition. It is also possible that different levels may be required at various locations on the same bridge.
The steps in planning an underwater inspection include:
To aid with quality control (QC) and Quality Assurance (QA) and to ensure procedures are in place and followed, check lists have been developed to aid the bridge owner and the dive team. See Figure 13.3.3.
Figure 13.3.13 Bridge Owner's Underwater Inspection Plan Checklist
Figure 13.3.13 Bridge Owner's Underwater Inspection Plan Checklist (cont'd.)
Figure 13.3.13 Bridge Owner's Underwater Inspection Plan Checklist (cont'd.)
Figure 13.3.13 Bridge Owner's Underwater Inspection Plan Checklist (cont'd.)
The underwater portions of bridge structures can be classified into the following categories: bents, piers, abutments, caissons, cofferdams, protection devices and culverts. Proper identification is important since various elements may require different inspection procedures, levels of inspection, or inspection tools.
Bents can be divided into two groups:
Column bents have two or more columns supporting the superstructure and may in turn be supported by piling below the mud line. The column bents are typically constructed of concrete, but the piling may be timber, concrete, or steel.
Pile bents carry the superstructure loads through a pile cap, into the piles and directly to the underlying soil or rock. The piles (and pile cap) can be constructed of timber, steel, or concrete. Pile bents are generally distinguished from piers by the presence of some battered piles and also bracing which provides stability for the individual piles. See Figures 13.3.14 through 13.3.16 for photographs of pile bents of different material types.
See Topic 12.2. for detailed description of the two bent types.
Figure 13.3.14 Timber Pile Bent
Figure 13.3.15 Steel Pile Bent
Figure 13.3.16 Concrete Pile Bent
Important items to be noted by the inspector are collision damage, and material deficiencies. Scour of the river bottom material at the bottom of the piles can result in instability of the piles. The underwater inspector compares present scour and resultant pile length with that observed in previous inspections.
Piers consist of three basic elements, which are the pier cap, shaft, and footing. Piers carry superstructure loads from the pier cap to the footing, which may be a spread footing or may be supported on a deep foundation. Piers can be constructed of steel, timber, concrete, or masonry and are usually distinguished by two to four large columns or a single large shaft. As with pile bents, collision damage, material deterioration, and scour are important items to look for in an underwater inspection. It is also important for the inspector to note if the pier shaft or columns are vertical. There are four common types of piers the inspector is likely to encounter:
Figure 13.3.17 Column Pier with Solid Web Wall
Figure 13.3.18 Cantilever or Hammerhead Pier
Figure 13.3.19 Solid Shaft Pier
Abutments carry the superstructure loads to the underlying soil or rock and also retain the earth at the end of the structure. In most cases, the abutments are dry during low water periods and do not require a diving inspection. However, occasionally the abutments remain continually submerged and will require an underwater inspection (see Figure 13.3.19). Abutments can be constructed from concrete, masonry, or timber and may be supported by spread footings, piles, caissons, or pedestals. The most common abutment types include:
Scour is probably the most critical item to be aware of when performing an underwater abutment inspection. Extreme local scour (undermining) could result in a forward tilting or rotation of the abutment, especially on those abutments without deep foundations (see Figure 13.3.20).
Figure 13.3.20 Severe Flood-Induced Abutment Scour
Caissons, or drilled shafts, are enclosures which are used to build a substructure’s foundation and carry loads from the bridge through the unsound soil and water to sound soil or rock. When it is in place, a caisson can act as a pier's footing. Caissons are made from timber, reinforced concrete, steel plates, or a combination of the above materials.
Cofferdams and foundation seals are used to maintain a dry work area when constructing piers and abutments in water. Cofferdams are constructed from steel sheet piling. Once the foundation is complete, the sheeting may be removed or cut-off at the bottom of the channel.
Before a cofferdam is dewatered, a concrete seal needs to be placed below the water on top of the soil and to prevent any uplift and flooding of the dewatered cofferdam.
Dolphins, fenders, and shear fences are often placed around substructure units to protect them from impact damage (see Figure 13.3.21). They are designed to absorb some of the energy from a direct hit from a vessel. Since these systems are usually at least partially underwater, conduct a diving inspection in concert with the substructure unit inspection.
Dolphins are a group of timber piles, but may also be a group of steel or composite piles. Fenders usually consist of timber or steel members attached directly to a substructure unit or piles adjacent to the substructure unit. Shear fences are generally an extension of a fender system which consists of a series of timber piles supporting timber wales and sheeting.
Figure 13.3.21 Damaged Protective System
A culvert is a hydraulic structure normally constructed entirely below the ground and may be constructed of concrete, steel, timber, or stone masonry. Culverts that may not be inspected while dry will be inspected by diving. The underwater inspection of culvert structures present unique challenges to the inspection team, as culverts exist in a wide range of sizes, shapes, lengths, materials, and environments. Areas of special concern to the dive team when conducting culvert inspections include confined space, submerged drift and debris, and animal occupation.
Physically confined space issues arise when inspecting culverts containing individual pipes, barrels, or cells with small interior dimension, or non-linear layout. Additionally, many culverts are continually either completely submerged, or exhibit limited freeboard. In northern environments, winter inspections may also include ice as a contributing factor (see Figure 13.3.22). Conduct diving operations in physically confined space in compliance with Federal commercial diving regulations, as well as the individual agency’s Safe Practices Manual. The Occupational Safety and Health Administration (OSHA) also offers guidance for work requiring confined space entry.
Submerged drift and debris is a persistent threat to the underwater inspection team, combining with the physically confining nature of most culvert structures to greatly increase the threat of diver entanglement. The diver may be completely unaware of the presence of drift until fouled. Use surface-supplied air diving equipment when conducting diving operations in physically confined and/or debris-laden culverts.
Another threat to the diver involves animals living or seeking shelter inside the culvert. Snakes are often found in and around accumulations of sediment and drift, while, in the southeast United States, alligators often reside inside culvert structures. When inspecting a structure exhibiting debris accumulations, which partially or fully constrict one end of a culvert, approach with caution, as excited animals may try to leave the culvert in haste, while the inspector is entering.
Figure 13.3.22 Inspection of Culvert With Limited Freeboard and Ice Cover
The materials typically used in bridge substructures are concrete, timber, steel, and masonry. An estimated 75% of all underwater elements are concrete. The balance consists of timber, steel, and masonry, in descending order of use.
Plain, reinforced, and prestressed concrete are used in underwater elements. Since the majority of substructures are basically compression units, concrete is a nearly ideal material choice. Some concrete damage tends to be surface damage that does not jeopardize the integrity of the system. However, concrete deterioration that involves corrosion of the reinforcement may lead to a reduction in load carrying capacity (see Figure 13.3.23).
Cracking, delamination, spalling and chemical attack are typical for concrete substructures exposed to water. Reinforcement exposed to water and air is subjected to section loss. Scaling occurs above the water surface while abrasion occurs in the area near the water surface.
See Topic 6.2 for detailed descriptions of concrete deficiencies.
Figure 13.3.23 Concrete Deterioration
Masonry can be used in substructure units, but is seldom used as a material in newer bridges. Masonry substructures can experience cracking and delamination of the stones. Cracking of mortar joints at the normal waterline is a result of freeze-thaw damage.
See Topic 6.5 for detailed descriptions of masonry deficiencies.
Timber pile bents are typical for short span bridges in many parts of the country, particularly for older bridges. The primary cause of timber deterioration is decay, abrasion, collision, and biological organisms, such as fungi, insects, bacteria, and marine borers. The ingredients for a biological attack include suitable food, water, air, and a favorable temperature. The waterline of pile structures offers all of these ingredients during at least part of the year. Since water, oxygen, and temperature generally cannot be controlled in a marine environment, the primary means to prevent a biological attack is to deny the food source through treatment to poison the wood as a food source. Timber piles are particularly vulnerable if the treatment leaches out (which happens with age) or if the core is penetrated. Therefore, it is important to carefully inspect in the vicinity of connectors, holes, or other surface blemishes (see Figure 13.3.24).
Figure 13.3.24 Deteriorated Timber Piling
Piles used in older bridges quite often were not treated if the piles were to be buried below the mud line (eliminating the source of food and oxygen). However, in some cases, streambed scour may have exposed these piles. Take special care in differentiating between treated and untreated piles to ensure a thorough inspection of any exposed, untreated piles. With each inspection, note the diameter or circumference for each timber pile. As a minimum, make these measurements at the waterline and mud line. Make comparisons with the original pile size.
Another primary caution for inspecting underwater timber piles is that the damage is frequently internal. Whether from fungal decay or borers, timber piles may appear sound on the outside shell but be completely hollow inside. While some sources recommend hammer soundings to detect internal damage, this method is unreliable in the underwater environment. One way to inspect for such damage is to take core samples. Plug all bore holes. Ultrasonic techniques for timber piling are also available.
See Topic 6.1 for detailed descriptions of timber material deficiencies.
Underwater steel structures are highly sensitive to corrosion, particularly in the low to high water zone (see Figure 13.3.25). Whenever possible, measure steel to determine if section loss has occurred. Ultrasonic devices are particularly useful to determine remaining steel thicknesses.
Figure 13.3.25 Deteriorated Steel Piles at Splash Zone
Connections such as bolts, rivets and welds are examined for corrosion. If the steel members have a coating, check the condition of the coating and its ability to protect the steel. In addition to protecting a concrete deck, cathodic protection has been used to protect steel piles in harbor settings. These cathodic protection systems may become more popular in the future. Check to see if the system appears to be working and check the connections and power source.
See Topic 6.3 for detailed descriptions of steel material deficiencies.
One composite material, known as fiber reinforced polymer (FRP) is a mixture of fibers and resin. FRP is becoming more popular throughout the transportation community and can be used for substructure units. This material is more resistant to marine borers than timber members.
Composite materials have mechanical deficiencies similar to traditional materials, which are due to impact, abrasion or construction related events. Environmental deficiencies in composite materials include fires and ultra-violet ray degradation.
See Topic 6.6 for detailed descriptions of fiber reinforced polymer material deficiencies.
Bridges that are located in water are susceptible to damage from any vessel on the water. Damage that happens from a vessel collision may be visible on top of the water, but the extent of the underwater damage cannot be properly assessed without a detailed underwater inspection.
Damage below normal water level caused by prop wash may not be visible above the water. Examples of vessels that rotate their propellers at high speeds and may cause prop wash are ferry's leaving terminals or tugboats moving barges from their moorings. The movement may pick up bottom material and discharge it against the foundations, essentially sandblasting the material which, in time, can cause the erosion of steel and concrete surfaces.
When visibility permits, the diver visually observes all exposed surfaces of the substructure. Scraping over the surface with a sharp-tipped probe, such as a knife or ice pick, is particularly useful for detecting small cracks. With limited visibility, the diver "feels" for damage. Because orientation and location are often difficult to maintain, the diver will be systematic in the inspection. Establish regular patterns from well-defined reference points.
Typical inspection patterns include:
Major advantages of surface-to-diver communications are that the diver can be guided from the surface with available drawings, and that immediate recording of observations can be made topside along with the clarification of any discrepancies with plans.
Measure any damage encountered in detail. As a minimum for a Level II or III inspection include:
Because of the effort spent in conducting underwater inspections, combined with the time between inspections, it is particularly important to carefully document the findings. On-site recording of all conditions is essential:
Include the results in an inspection form or report. Drawings and text need to describe all aspects of the inspection and any damage found. Include recommendations on condition assessment, repairs, and time interval for the next inspection in the report. See Figure 13.3.26 for a sample underwater inspection form.
See Topic 4.4 for detailed descriptions of record keeping and documentation.
Figure 13.3.26 Sample Underwater Inspection Form
Figure 13.3.26 Sample Underwater Inspection Form (Continued)
Once a diver enters the water, their environment changes completely. Visibility decreases and is often reduced to near zero, due to muddy water and depth. In many cases, artificial lighting is required. There are times when tactile (by feel) inspections are all that can be accomplished, significantly compromising the condition evaluation of the element(s) being inspected.
The diver not only has reduced perceptual capabilities but is less mobile as well. Maneuverability is essential for underwater bridge inspections. With either self-contained or surface-supplied equipment, the diver may find it useful to adjust his/her underwater weight to near buoyancy and use swim fins for propulsion.
It is important for the diver to be able to adapt to the environment and be familiar with the diving equipment. They are to feel safe and comfortable while working underwater to be able to do an effective job on the inspection and to remain safe while performing the inspection.
Most waterways have low flow periods when current will not hinder an inspection. Plan diving inspections with this consideration in mind. Divers can work in current below 1.0 knots with relatively little hindrance. Currents may vary in direction or velocity when inspecting around submerged obstacles such as cofferdams (see Figure 13.3.27).
Figure 13.3.27 Diving Inside a Cofferdam
Waterway conditions may sometimes be too swift to allow safe diving operations (see Figure 13.3.28). For these conditions, other appropriate procedures must be used to evaluate the condition of underwater elements.
Figure 13.3.28 Excessive Current
The drift and debris that often collects at bridge substructures can be extensive (see Figure 13.3.29). This type of buildup typically consists of logs and limbs from trees that are usually matted or woven either against or within the substructure elements. Often this debris is located on the lower parts of the substructure and cannot be detected from the surface. The buildup can be so thick as to prevent access to major portions of the underwater substructure.
Address concerns such as removal, past history, and safety when dealing with the presence of drift and debris.
Since drift and debris are often under the water surface, it is difficult to estimate the time and cost required to remove and gain access. The removal of the drift and debris is required if a hands-on inspection of the underwater elements is to proceed. While in some cases debris can be removed by the inspection divers, heavy equipment, such as a hoist or underwater cutting devices, are often required.
Generally, such buildup occurs in repetitive patterns. If previous underwater inspections have been conducted, the presence of drift can be estimated based on past history. Also, certain rivers and regions tend to have a history of drift problems, while others do not. Knowledge of this record can help predict the likelihood of drift and debris accumulation. A separate drift removal team, working ahead of the dive inspection team, could possibly be utilized.
Debris build-up near a bridge creates unique safety concerns for the dive team. Occasionally, debris can be quite extensive and can lead to entanglements or sudden shifts which might entrap the diver. Divers normally approach debris from the downstream side to avoid entanglements (see Figure 13.3.29).
Bridges on many inland waterways are relatively clean and free of marine growth. In such cases, the inspection can be conducted with little extra effort from the diver other than perhaps light scraping.
In coastal waterways, the marine growth can completely obscure the substructure element and may reach several inches or more in thickness (see Figure 13.3.30). The cost of cleaning heavily infested substructures may be completely impractical. In such cases, spot cleaning and inspection may be the only practical alternative.
Figure 13.3.30 Cleaning a Timber Pile
This sometimes cold, dark, hostile underwater environment can result in a reduced physical working capacity. The diver is also totally dependent on external life support systems, which adds psychological stress. Things that can be done intuitively above water include a conscientiously planned effort and executed step-by-step procedures for underwater. For example, maintaining orientation and location during an underwater inspection requires continual attention. Typical distractions include living organisms, such as fish, snakes, and crustaceans and also environmental conditions, such as low temperatures, high current and heavy debris.
Since the majority of bridge inspections are in relatively shallow water and of relatively short duration, decompression problems rarely occur. However, multiple dives have a cumulative effect and the no-decompression time limit decreases rapidly at depths greater than 50 feet. Therefore, divers routinely track their time and depth as a safety precaution. OSHA requires that a decompression chamber be on-site and ready for use for any dive made outside the no-decompression limits or deeper than 100 feet of seawater.
Another concern for divers is vessel traffic near the area to be inspected. Someone will always be topside with the responsibility of watching boat traffic (see Figure 13.3.31). In addition, display flags indicating that a diver is down. The international code flag "A", or “Alpha” flag (white and blue), signifies that a diver is down and to stay clear of the area. OSHA requires this flag. However, it is also prudent to display the sport diver flag (white stripe on red), since it is more likely that recreational boaters will recognize this flag (see Figure 13.3.32).
Figure 13.3.31 Commercial Marine Traffic
Figure 13.3.32 Alpha (top) and Sport Diver (bottom) Flags
Diving inspectors are responsible for identifying the location of underwater elements including a description of the underwater elements. They also verify if the inspection frequency and procedures are adequate in accordance with the inspection record. Inspectors can make recommendations to improve the underwater inspection procedures listed in the inspection record if conditions have changed.
According to the AASHTO Manual for Bridge Evaluation, underwater inspections can include wading or diving inspections. The National Bridge Inspection Standards (NBIS), however, define an underwater inspection as the inspection of the underwater portion of a bridge substructure and the surrounding channel that cannot be visually inspected at low water by wading or probing, which will require diving or appropriate techniques. For this topic three general methods used to perform underwater inspections are presented:
Wading inspection is the basic method of underwater inspection used on structures over wadeable streams. The substructure units and the waterway are evaluated using a probing rod, sounding rod or line, waders, and possibly a boat. Regular bridge inspection teams can often perform wading inspections during periods of low water (see Figure 13.3.33).
Figure 13.3.33 Inspector Performing a Wading Inspection
SCUBA, an acronym for Self-Contained Underwater Breathing Apparatus, is used for many underwater inspections in this country (see Figure 13.3.34). In this mode, the diver operates independently from the surface personnel, carrying their own supply of compressed breathing gas (typically air) with the diver inhaling the air from the supplied tank and the exhaust being vented directly to the surrounding water. SCUBA diving is employed during underwater bridge inspections due to its ease of portability and maneuverability in the water. It is used where the dives have a short duration at different locations rather than a long sustained dive. This dive mode is best used at sites where environmental and waterway conditions are favorable, and where the duration of the dive is relatively short. Exercise extreme care when using SCUBA equipment at bridge sites where the waterway exhibits low visibility and/or high current, and where drift and debris may be present at any height in the water column. The use of SCUBA gear is limited to water depths of 130 feet and the time on the bottom will be limited by the amount of air the diver can carry and the amount of time based upon the no-decompression limits.
Figure 13.3.34 SCUBA Inspection Diver
As its name implies, surface-supplied diving uses a breathing gas supply that originates above the water surface and is commonly referred to as lightweight diving equipment. This breathing gas (again, typically compressed air) is transported underwater to the diver via a flexible umbilical hose. Surface-supplied equipment provides the diver with a nearly unlimited supply of breathing gas, and also provides a safety tether line and hard-wire communications system connecting the diver and above water personnel. Using surface-supplied equipment, work may be safely completed under adverse conditions that often accompany underwater bridge inspections, such as: fast current, cold and/or contaminated water, physically confined space, submerged drift and debris, and dives requiring heavy physical exertion or of relatively long duration (see Figure 13.3.35). Depths of surface-supplied dives can be conducted down to 190 feet or if the bottom times are less than 30 minutes, to a depth of 220 feet. This form of diving provides advantages such as an "unlimited" air supply and communications plus bottom times that can exceed the decompression time limits used for SCUBA. The disadvantages of this form of diving is that it requires more topside support than SCUBA and it is limited in mobility due to the connection to the surface.
Figure 13.3.35 Surface-Supplied Diving Inspection
In determining whether a bridge can be inspected by wading or whether it requires the use of diving equipment, water depth is not be the sole criteria. Many factors combine to influence the proper underwater inspection type:
Essential personal diving equipment includes:
Surface-supplied air diving equipment typically includes a compressor, which supplies air into a volume tank for storage. This compressed air is then filtered and regulated to the diver’s helmet or mask through an umbilical hose (see Figures 13.3.38 and 13.3.39). The umbilical is typically made up of several members, including, at a minimum, a breathing air hose, strength member (or safety line), communication line, and pneumofathometer hose. The pneumofathometer provides diver depth measurements to the surface (see Figure 13.3.40).
For self-contained diving, the breathing gas supply is contained within a pressurized tank, which is carried by the diver.
Figure 13.3.36 Vulcanized Rubber Dry Suit
Figure 13.3.37 Full Face Lightweight Diving Mask with Communication System
Figure 13.3.38 Surface-Supplied Air Equipment, Including Air Compressor, Volume Tank With Air Filters, and Umbilical Hoses
Figure 13.3.39 Surface-Supplied Diving Equipment Including Helmet or Hard Hat
Figure 13.3.40 Pneumofathometer Gauge
Equipment malfunction leading to loss of air supply needs to be a constant concern to the dive team. Even in shallow water, submerged drift and debris adjacent to a bridge can make an emergency ascent an arduous affair, for both the diver and the support team. As such, a reserve air supply will always be worn by the diver using surface supplied air (see Figure 13.3.39). Carbon monoxide poisoning can occur if the air intake of the surface supplied air compressor is located near the exhaust of other motorized equipment (see Figure 13.3.38).
Figure 13.3.41 Surface-Supplied Diver with a Reserve Air Tank
While not required in all situations, a two-way communication system linking the diver(s) and topside personnel greatly enhances the underwater inspection. There are two types of diver-to-surface communications: a conventional hardwire and a wireless system. In the hardwire system, the diver has a microphone and speaker connected to a surface transmitter-receiver through a cable. This is regularly used in surface-supplied diving. It can also be used when a SCUBA diver is using a full face mask with the mask tended to the surface with a strength or communication line. The wireless systems are available for use in SCUBA diving equipment. The advantage of a wireless system is that it allows the diver to have more mobility (see Figure 13.3.42), and can be used during self-contained diving operations.
There are several advantages provided to the underwater inspection team, through the use of direct two-way communication
Figure 13.3.42 Wireless Communication Box System
Figure 13.3.43 Surface Communication With Inspection Team Leader
While inspection of short-span bridges can often be accessed from shore, many bridges require a boat or barge for access. Boats may be in different sizes and types, but large enough so it can safely handle the diving equipment and personnel as well as a suitable size for the waterway conditions (see Figures 13.3.44 and 13.3.45). The boat needs to be equipped with an engine which will be dictated by the waterway conditions and the boat size.
Figure 13.3.44 Access Barge and Exit Ladder
Figure 13.3.45 Access From Dive Boat
A number of inspection tools are available. The dive team needs to have access to the appropriate tools and equipment (including both hand and power tools) as warranted by the type of inspection being conducted.
While most hand tools can be used underwater, the most useful include rulers, calipers, scrapers, probes (ice picks, dive knifes, and screwdrivers), flashlight, hammers (especially masonry and geologist’s hammers), axes, hand drills, wire brushes, incremental borers, hand saws, and pry bars (see Figure 13.3.46). These tools are usually tethered to the diver to prevent their loss underwater. Working with hand tools could be slow and may be impractical for larger jobs.
Figure 13.3.46 Diver with a Pry Bar and Diver with Hand Scraper
Power tools include both pneumatic and hydraulic tools. Pneumatic tools are not usually designed for underwater use, but can be adapted to perform the necessary tasks. Examples of pneumatic tools that can be used include pneumatic drills, chippers, hammers, scalers, and saws. Pneumatic tools are also limited to practical depths of 100 to 150 feet and can obscure the diver's vision by the bubbles produced by the tools.
Hydraulic tools are modified versions of tools used on dry land. Examples include grinders, chippers, drills, hammers and saws. The advantage of using hydraulic tools is that they do not create the bubbles that a pneumatic tool creates.
While pneumatic tools are sometimes used, hydraulic tools tend to be favored for heavy or extensive work often required during underwater inspections.
Light cleaning can be accomplished with scrapers and wire brushes. Heavier cleaning requires automated equipment such as grinders and chippers. One of the most effective means of cleaning is with the use of water blasters (see Figure 13.3.47). Take particular care with such equipment to ensure that structural damage does not result from overzealous blasting.
Figure 13.3.47 Cleaning with a Water Blaster
When inspecting underwater elements, nondestructive evaluations (NDE) may be required to determine the structural condition and may be used in Level III inspection.
For steel substructures, the inspector is often concerned with measuring the remaining thickness of any corroded members. Nondestructive evaluations for underwater steel members include:
For concrete substructures, there are several nondestructive tests for in-depth inspections that can be performed.. Nondestructive evaluations for underwater concrete members include:
Coring is a partially destructive evaluation method whose use is limited to critical areas. Cores can be taken in either concrete or timber (see Figure 13.3.48).
Figure 13.3.48 Coring Equipment
Concrete coring requires pneumatic or hydraulic equipment. Deep cores (3 feet or more) can be taken to provide an interior assessment of massive substructures (see Figure 13.3.49). Two-inch diameter cores are common, but coring tools are available in other sizes (see Figure 13.3.50). Cores not only provide knowledge about interior concrete consistency but also can be tested to determine compression strength. Be sure to select coring locations so the reinforcement is not damaged, unless a sampling of the reinforcement is desired. Patch the core holes upon completion.
Figure 13.3.49 Concrete Coring Taking Place
Ultrasonic devices (V-meters) such as those used for concrete evaluations can be used to test timber members for internal voids or material breakdown caused by marine borers or decay.
Timber coring is much simpler and less costly to perform than concrete coring (see Figure 13.3.51). While power tools are sometimes used, the most effective procedure is still to hand core with an increment borer. This approach preserves the core for laboratory as well as field evaluation. The core indicates evidence of borers or other infestation, and of void areas. Always plug the hole with a treated hardwood dowel to prevent infestation.
Color digital cameras come with a variety of lens and flash units. Popular cameras that can be used above water can be used underwater by placing them in a clear waterproof case, also known as a "housing". The boxes are constructed of clear plastic and can be used underwater (see Figures 13.3.52 and 13.3.53). There are also waterproof digital cameras that are designed specifically for underwater photography.
Figure 13.3.52 Various Waterproof Camera Housings
Figure 13.3.53 Diver Using a Camera in a Waterproof Housing
In some cases, visibility is limited and the camera needs to be placed close to the subject. Suspended particles often dilute the light reaching the subject and can reflect light back into the lens. When visibility is very low and the water is extremely turbid, clearwater boxes can be used (see Figure 13.3.54). A clearwater box is a clear plastic box that can be filled with clean water. The box can be placed up against the subject, which will displace the dirty water and allowing the camera to focus on the member being photographed.
Figure 13.3.54 Diver Using a Clearwater Box
Video equipment is available either as self-contained, submersible units or as submersible cameras (or surface video cameras in a waterproof housing) having cable connection to the surface to view on the monitor or to record (see Figure 13.3.55). The latter type allows a surface operator to direct shooting while the diver concentrates on aligning the camera only. The operator can view the monitor, control the lighting and focusing, and communicate with the diver to obtain an optimum image. Since a sound track is linked to the communication equipment, a running commentary can also be obtained.
Smaller video cameras are in plastic cases and can be used with or without the umbilical to the surface where they are monitored. Video cameras may also be attached to a staff or a truck mounted arm so it can be deployed from a bridge deck and can relay images to the monitors and recording devices.
Figure 13.3.55 Underwater Video Inspection
An extension of the video camera is a remotely operated vehicle (ROV), where the diver is eliminated and the camera is mounted on a surface controlled propulsion system (see Figure 13.3.56). Its effectiveness diminishes substantially in stream velocities greater than 1.5 knots and is limited by cloudy water, inability to determine the exact orientation and position of the camera, and difficulties the operator may have controlling the vehicle due to the current or the umbilical being tangled. The ROV cannot perform cleaning operations prior to photos being taken.
Figure 13.3.56 Remotely Operated Vehicle (ROV)
Underwater acoustic imaging can provide greatly improved images of the channel bottom conditions, undermining and submerged foundations (see Figure 13.3.57). This can aid in the planning of diving operations by detecting areas of possible damage and will allow the divers to concentrate in these areas. It can also enhance diver safety by identifying potential dive hazards before anyone would enter the water. Acoustic imaging can also provide images of an underwater element that a underwater camera may not be able to take due to the turbidity of the water. Imaging also can operate at distances of 200 feet, while cameras, even in fairly clear water, has an effective range of only a few feet.
This is also useful when an emergency evaluation of a bridge may be necessary after a bridge is damaged by a collision, especially if the water conditions (e.g., high current, low visibility, debris) preclude the use of divers.
Figure 13.3.57 Acoustic Imaging of a Pier
Divers may be able to note scour under certain conditions. The most important assessment is how much of the bent or pier is exposed when compared to plans and typical designs.
Local scour is often detectable by divers since this type of scour is characterized by holes near bents, piers, or abutments. Divers routinely check for such scour holes. A typical approach is to take depth measurements around the substructure, both directly adjacent and at concentric intervals. Note that divers typically operate in low current situations. Sediment often refills scour holes during these periods, making detection of even local scour difficult. However, since this refilled sediment is usually soft, a diver using a probing rod can often detect the soft areas indicating scour refilling.
The diver’s role is primarily to point out a potential scour problem. Almost invariably, an additional interdisciplinary engineering investigation will be needed. The diver’s primary role in scour investigation is to measure scour by one of these methods:
Although sounding-sensing devices can be used independently of diving, they are commonly part of an underwater inspection. See Advanced Inspection Methods in Topic 13.2.6 on the procedures to record the stream cross section and profile. An on-site diver can investigate questionable readings and more fully determine the channel bottom conditions.
A fathometer consists of a transducer that is suspended in the water, a sending/receiving device, and a recording device that will display the depth on paper or a display. It can be either in color or black and white. A transducer floats just below the waterline and bounces sound waves off the bottom. Depths are continuously recorded on a strip chart.
Advantages of fathometer include the following:
Disadvantages include the following:
Scour most commonly occurs during a flood. After a flood, the sediment settles, possibly refilling any scour hole that the flood may have caused. Geophysical tools can be used to measure scour after a scour hole has been refilled.
Ground-penetrating radar (GPR) equipment are also used in scour surveys (see Figure 13.3.58). They can be used to obtain high resolution, continuous subsurface profiles on land or in shallow water which is less than 25 feet deep. GPR transmits short electromagnetic pulses into the subsurface and will measure the travel time to and from the subsurface for the signal to return. Once the signal encounters an interface between two different materials, a portion of the energy will be sent back to the surface and the rest will be sent into deeper layers. These are not as effective when encountering material that is highly conductive (e.g., clay), in salt water, or water with heavy amounts of sediment.
Figure 13.3.58 Ground Penetrating Radar Record
Tuned transducers, or low-frequency sonar, is a seismic system that operates through the transmission and reception of acoustic waves. This system consists of a transmitter, a transducer towed alongside the boat, a receiver and a graphic recorder. The transmitter will produce a sound wave that is directed toward the bottom of the channel by the transducer. A portion of the sound wave will be reflected back to the surface and a portion will penetrate into the bottom of the channel. Other portions of the wave will bounce off the material once there is acoustical impedance between the layers (see Figure 13.3.59).
Figure 13.3.59 Tuned Transducer Record
When identifying a scour critical bridge, the diver has a limited role. Although divers may be able to identify the conditions during an underwater inspection, the greatest scour occurs at periods of high flow. Diver inspections include:
Figure 13.3.60 Pier Undermining, Exposing Timber Foundation Pile