Drainage is an incredibly important aspect to any retaining wall. The force that water can apply to any structure can cause significant and costly problems, and retaining walls are no different. Retaining walls hold back significant loads that can change. The wall needs to be constructed to be able to withstand those changing loads or dynamic loads. Water entering the system and putting pressure on the wall is a dynamic load, and when not properly handled in the retaining wall construction could cause the failure of the retaining wall. This is why drainage must be considered in the construction of a retaining wall.
Retaining Wall Drainage
There are two main aspects to a retaining wall drainage system:
- Backfill
- Pipe
Drainage for a retaining wall relies on both of these aspects help to control the water that enters the system. Any water in behind the wall needs to be moved away and out of the retaining wall system. Failure to control the flow of water in behind the wall will cause hydrostatic pressure to add to the surcharge that the wall experiences. This built up dynamic load will eventually will eventually cause the wall to begin to lean forward, especially during freeze-thaw cycles.
Drainage Systems for Retaining Walls
Drainage behind a retaining wall requires both proper backfill and drainage pipe. Backfill consists of a 3/4″ angular crushed clear stone (ASTM #57) at a minimum 12″ behind the wall and continuing vertically up that wall. It is a clear stone meaning that it is a washed aggregate that is clear of fines. This allows for the water that enters the system to permeate through the backfill down to the base of the retaining wall.
Once the water gets to the base of the retaining wall, it needs to be collected and exited out of the system. Some of the water may permeate through the subsoil, though heavier clay that your region may have will cause water build up.
Pipe needs to be installed behind a retaining wall to collect that water build up at the base. This drainage pipe should be perforated, allowing the water to enter and exit the pipe on its way out of the system. The pipe should have a slope towards the lowest area or where water will be exited at a minimum slope of 1/8″ per foot. Additionally, every 50 feet of retaining wall should have an exit point through the face of the wall.
This is accomplished with a “T” in the drainage pipe with a pipe or other attachment to extend past the face of the wall at a distance that allows water to flow away from the face of the wall. The slope of the pavement or ground at the face of the wall also needs to slope away from the wall itself so that water does not flow back towards the retaining wall causing more issues.
It is important to not that no two retaining walls are the same. Some may require engineered drawings such as those that are three feet in height or greater. Depending on the load that these retaining walls must support, there may be further engineering to provide the stability that the project requires. You can always refer to the manufacturer’s engineered drawings for their wall systems in order to build a suitable wall for your project or seek outside engineering help in order to build a retaining wall that will stand the test of time.
There are also additional steps that can be taken to control the flow of the water before it is able to permeate into the backfill material and down to the base of the wall. This can include installing a low permeable soil on top of non-woven geotextile that will lay on top of the backfilled material at the top of the wall. On top of that soil would be a top soil where grass or sod can be installed to grow. This low permeability soil will allow for less water to enter the system through that backfilled clean stone.
With adding this low permeability soil, you do not want to have water sitting behind the retaining wall causing issues with the build up of hydrostatic pressure. To prevent this, an appropriate slope must be made with that soil in behind the retaining wall to an appropriate area for the water to exit to. This can further be achieved with a swale moving away from the back of the wall.
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Retaining Wall Drainage Pipe
The proper drainage pipes for retaining walls would be a flexible corrugated and perforated pipe. This flexibility allows the pipe to bend around curves in the retaining wall while also resisting any freeze-thaw cycles. Water that sits in the pipe over these freeze-thaw cycles will expand in the pipe. Having a flexible pipe allows that water to expand with the pipe itself. However, a solid PVC pipe that has a high crush rating is rigid and the expanding of that water will cause cracks to appear and the ultimate failure of that pipe. Though areas without these freeze-thaw cycles that cause the expanding and contracting of water may see benefits in the high crush rating of a PVC Schedule 40 pipe. Ultimately the pipe needs to be perforated to allow for the collection of water that enters the system through the backfill and down to the base of the retaining wall.
Retaining walls are all different. There are several aspects that must be considered when building a retaining wall, but drainage remains fairly consistent throughout as water must always be taken into consideration. Backfilling is an important aspect of that drainage design and construction, as is the installation of a drainage pipe. However, you can always refer to the manufacturer’s engineered drawings for the specific product that you are installing or consult with the manufacturer and / or engineer to create a plan for the drainage of your retaining wall. For the majority of walls that are three feet or less in height, this guide will help you with a drainage plan.