In a paper published by the American Society of Civil Engineers in 1992, Mark Landers estimated that 60% of all bridge failures were due to scour. Of the 86 major bridge failures that occurred in the United States between 1961 and 1976, 46 were caused by scour of bridge piers. Unlike the risk posed by earthquakes, risk to bridges from scour is a universal danger – any bridge over a waterway could be subjected to scour at any time. In this edition of TheBridgeGuy, we’ll examine what scour is, why it is so dangerous, how we design for it and how we inspect for it.
Bridge scour is the erosion of sediment around bridge piers caused by fast moving water. Because bridge piers carry the weight of the bridge, they must be adequately supported at all times for the bridge to remain standing. Eroding of material from beneath foundations can cause footing instability, leading to damage or collapse. The risk to bridge piers posed by scour is a major factor in the design of bridges over waterways and can also drive the cost of bridge foundations adjacent to moving water.
The issue of bridge scour is usually handled by hydraulic and geotechnical engineers since it involves an overlap of the two disciplines. The hydraulic engineer will typically model the waterway, including the proposed restrictions imposed by the bridge itself, generating water flow rates, water surface elevations and then designing any mitigation measures necessary. The geotechnical engineer will analyze and characterize the soils, especially with respect to their susceptibility to scour. Ultimately the results are passed on to the bridge engineer for design.
Scour itself is a broad term and can be further broken down into four different categories – local scour, contraction scour, degradation and lateral migration.
Local scour is the local erosion around a particular bridge pier. The presence of a foreign object in the water can introduce vortices locally around the object. These vortices can cause local erosion around the obstacle, sometimes quite deep.
Contraction scour is gradual erosion of the stream bed across the channel due to the restricted flow caused by the bridge presence. Restrictions to flow causes higher velocities, which increases the erosive power of the water.
Degradation is similar to contraction scour but isn’t directly linked to the bridge itself. A variety of factors can cause a stream to erode over time. Periodic flooding, a rapid increase in flow and velocity, is one natural cause. Human intervention is another cause.
Lateral migration is the movement of the deepest point of the channel (known as the thalweg) transversely across the channel over time. This can occur as a result of natural changes but can become a risk to bridge abutments if the thalweg shifts toward one of the banks.
All scour can be minimized by reducing the number of piers in the water. However, it is not always practical to keep all piers out of the water, especially for longer spans. When laying out a bridge over water, it is generally a good idea to narrow the channel as little as possible as doing so will minimize the amount of contraction scour that will occur. For wide waterways, the presence of piers in the water will have little impact on velocities and thus little impact on contraction scour. Local scour then becomes the larger concern.
The shape of the column will influence the shape of the local scour hole; however, depth is usually governed more by velocity than shape. Many bridges use circular columns, leading to circular or slightly oblong scour holes. Rectangular columns will have more elongated scour holes.
Lateral migration can be predicted by hydraulic modeling. Even where there are no piers in the water, migration can still be a risk to the bridge abutments. In general, the higher the velocities, the more susceptible the channel is to migration. Sharp bends in the channel can also see more risk of migration than for straight reaches.
One of the reasons scour is so dangerous is because it can happen both slowly overtime and abruptly in one event. Water itself is a life-giving element on Earth, but it is also one of the most destructive and corrosive elements known to man. Often, the fact that scour is occurring is not entirely visible above water. A bridge known to be relatively stable with regard to scour could be completely undermined during the next flood. It is an insidious problem for many bridge owners.
Undermining of bridge foundations and loss of support isn’t the only risk related to scour.
In 1985, the Chickasawbogue Bridge near Mobile, Alabama collapse, sending a Ford van airborne, striking one of the collapsed bridge piers before sinking in 20 feet of water. The driver escaped with minor injuries.
The National Transportation Safety Board (NTSB) listed the probable cause as undetected deterioration of the steel H-piles supporting the bridge. Shortly after the collapse, the Federal Highway Administration (FHWA) required that each state implement its own underwater inspection program. This was further codified in the National Bridge Inspection Standards (NBIS) in 1988 when underwater inspection of submerged components of bridges was required every five years.
It is unclear what mechanism of deterioration led to the collapse of the bridge; however, it is probable that erosion due to scour probably had an impact.
- Driven piles rely on friction between the pile and the surrounding soil for its strength in supporting load – loss of soil due to scour could have resulted in global pile failure.
- Soil can also protect steel from corrosion – erosion due to scour could have resulted in unacceptable section loss, leading to pile section failure.
- Soil can provide lateral restraint to piles and prevent buckling – erosion of soil round the piles could have lowered the critical buckling load, leading to an overload of the piles.
Regardless of the cause of failure, the Federal Highway Administration (FHWA) recognized that scour posed a serious threat to bridge safety. Periodic inspection of underwater components ensures that any potential undermining or deterioration is noted and can be addressed. This can be done using divers, Remoted Operated Vehicles (ROVs) or by camera.
The best way to mitigate for scour is to design for it when the bridge is built. As noted previously, the parameters for scour design are a joint effort of hydraulic and geotechnical engineers. Hydraulic Engineering Circulars (HEC) 18 and 23 give specific guidance on scour design for bridges. Ultimately, the bridge engineer will receive a scour elevation for use in design.
The scour elevation is the elevation to which the groundline below water is expected to erode over the design life of the bridge. The difference between the final groundline elevation before scour occurs and the scour elevation is known as the scour depth. Because the exact time when that erosion could occur is unknown, the bridge engineer will typically take the scour elevation into account when designing the substructure at various limit states.
For new bridges, deep foundations will typically be used when scour is expected to occur. In this case, the pile or shaft must have sufficient remaining capacity to resist all applied loads even with the scour depth applied – that is, with the top X feet of material removed. The pile or shaft must also remain stable when the scour depth is applied, which is important for short pile depths where the scour elevation gets close to the pile or shaft tip elevation.
Scour design has evolved over the years. In the past, only the bridge was considered in scour design. More recently, changes to policy have attempted to protect the roadway embankments behind abutments as well. To do this, bridge foundations were lowered to be two feet below the scour elevation as well as lowering any wingwalls. Not unexpectedly, this has resulted in an increase in the cost of bridges over waterways.
Subsequent modifications to policy now allow scour countermeasures to be used in lieu of deeper foundations. Countermeasures are designed to HEC standards and are intended to prevent scour by armoring the ground in front of bridge piers. Scour protection walls are also used to protect the abutments from scour, though these can be expensive themselves.
For existing bridges built without regard for the risks posed by scour, one of the go-to solutions is to pile up large rocks, known as rip-rap, around the bridge pier. Rip-rap is effective in preventing erosion by keeping stream bed material in place, but it also helps to calm the vortices action that can create local scour holes. However, rip-rap is coming under attack by biologists as unfriendly to fish species, likely because it promotes fish predation by providing hiding places for predators and also covers up streambed material that could be used as habitat for fish.
Scour can also be exacerbated by debris collection. Over time, debris such as logs, branches and other types of flotsam can become lodged on the upstream side of bridge piers. This debris itself can become a risk to the bridge in high flows – increased area can place more lateral load on the bridge, particularly concerning if applied to the superstructure. But debris build up during floods can restrict flow, increasing erosion during such events. During emergency situations, bridge maintenance crews will attempt to remove debris that could be hazardous to the bridge and to the traveling public.
That’s all for this edition of TheBridgeGuy. Wishing everyone a Merry Christmas and a Happy New Year!
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