In design, the presence of water behind the wall can have two effects. It can cause additional force to be applied to the wall from hydrostatic pressure, or its presence can cause the soil characteristics to change.
Changing soil properties has the most effect on clayey soils. Relatively dry clay will have cohesion and reduce the force on the wall. Wetter clay will lose this cohesion and apply more load to the wall.
The way that the wall is designed will effect how the water needs to be handled. If the wall was designed to for the effects of water all the way up the wall, then nothing may need to be done. If it was designed for only a certain height of water, then this must be assured.
Depending on the volume of water that is expected and the source, a few different methods of water removal can be employed:
- Weep hole
- Perforated drain pipe
- Surface water diversion
- Dewatering wells
Weep holes are small holes through the face of the retaining wall. These allow any water that is trapped behind the wall to slowly drain out. They are best for small volumes of water. They are often installed in all walls as a minimal level of assurance that water will not be trapped behind the wall.
Perforated drain pipes are usually installed at the base of a wall in combination with free draining fill or geotechnical fabric. They can drain larger volume of water, but the connection details and where the outflow is located need to be investigated.
Surface water diversions may include ditches or paved areas that keep surface water from running down a slope and being contained behind the wall. This method helps where ground water is less of a concern when compared to runoff.
Dewatering wells are only an option that is considered where the wall must be built below the groundwater level and where the force from the water can not be designed for. The wells have pumps that lower the ground water elevation in the area. These can pump large quantities of water, but they require constant monitoring and power.