Providing adequate drainage is important when constructing your retaining wall project. Drainage can be installed using perforated drainage pipe running perpendicular to the wall, installed at the inside base. If you use drainage pipe, the water that has saturated the soil will eventually find its way to the pipe and be directed away from the wall. The important point here is to have an exit for that water. This can be done by connecting to the home drainage system or by providing a 90° exit, through the retaining wall at the lowest point. When installing the drainage pipe place a layer of crushed stone all around the pipe to a thickness of at least one inch, two is even better. This prevents the holes in the drainage pipe from becoming clogged with soil.
In all cases, drainage of some sort should be installed on the inside base of the retaining wall. Water laden soil can be twice as heavy as dry soil. The water moves down through the soil looking for an exit route, as it is saturates the soil it will apply a tremendous amount of pressure on the backside of the retaining wall. As there is nothing on the front side of the wall to apply pressure, the wall may begin to move forward especially near the top.