Bridge: Retaining Walls
The project need for retaining walls is typically identified when cut or fill slopes fall outside the State’s right-of-way. Whenever possible, these walls should be eliminated and replaced with slopes, which are less expensive and can be less intrusive than retaining walls. Generally, retaining walls should only be considered when the value of the right-of-way or other resource being impacted exceeds the cost of the wall.
Typically, 2:1 slopes are used in roadway construction, but different slopes should be considered when attempting to eliminate walls. Depending on the circumstances:
Using reinforced slopes steeper than 2:1 such as riprap slopes or engineered Mechanically Stabilized Earth (MSE) slopes to reduce or eliminate walls. These slope types can be considered unattractive, so consideration may need to be given to the importance of aesthetics for the location.
Flattening the slope may result in larger impacts from a right-of-way perspective, but would have less visual impact and would be easier to maintain for a property owner. Instead of purchasing right-of-way for flat slopes, slope easements could be used that would still give a property owner the use of the slope area.
Length and Height
It may not be feasible to eliminate walls completely, but even eliminating portions of walls by adjusting the slope or purchasing right-of-way can result in cost savings. Retaining walls should be kept as short as possible. When feasible, the wall should be placed at the bottom of a slope instead of along the roadway or other facility edge. Placing the wall at the bottom roadway embankment slope as opposed to at the top of the slope eliminates having to place a roadway barrier on top of the wall and eliminates the need to design the wall for an impact load, which factors greatly in the structural design of the wall. A situation where retaining walls should not be placed at the bottom of a slope is when there are storm water management facilities at the top of the slope, since it is never desirable to have water stored behind a retaining wall.
Proprietary Retaining Walls
Proprietary retaining walls such as MSE Walls or Modular Block walls can be used as a cost-effective alternative to cast in place walls and to speed up construction; however, they are not practical for all situations. When proprietary retaining walls are used on a project, the contract documents should be specific in stating which ones will be allowed. Consider the following when using proprietary walls:
Soldier Pile and Lagging Walls
Soldier pile and lagging walls consist of steel piles embedded in concrete caissons, with concrete lagging spanning between the piles. A non-structural concrete fascia can be cast on the outside face of the wall for aesthetic purposes. These walls are the most cost effective when used in areas where the wall is in a cut since they do not require a separate structure for support of excavation. Height can be a concern since these walls require tiebacks when they get tall, which can become expensive and may reduce the cost savings when comparing this wall type to others. Only concrete lagging should be used in permanent soldier pile and lagging walls. While this is more expensive than timber lagging, it will be more durable and have less maintenance problems in the future.
Avoiding Environmental Impacts
Often environmental entities request that retaining walls be constructed to avoid impacts to wetlands, forests, floodplains, etc. Before agreeing to this, the benefits of avoiding the environmental feature should be compared to the wall cost and alternative mitigation. Consideration should also be given to whether or not the wall will actually reduce the impacts to the environment. For example, a tall retaining wall may not save a wetland if the wetland will be in the shadow of the wall, causing all the vegetation to die. Similarly, the wall may cut off the flow of water through the wetland, adversely impacting the wetland. It may make more sense to eliminate the wall and provide mitigation for the impacted wetland at another location, especially if the area of the wetland is relatively small.