If possible, weep holes and related drains should be assessed following a heavy rain to make sure they are working properly. If they are not discharging water, the drains should be cleaned out and observed again in the next rain. Retaining walls more than 2 feet high should be backed with drainage material, such as gravel. There should be drains at the bottom of the drainage material that should discharge the water either at the end of the wall or through pipes. These drains and the drainage material behind the wall relieve the pressure of groundwater on the wall. Failure to drain could be remedied by excavating behind the wall, replacing the drainage material and damaged drainage piping, and backfilling. In all but the driest climates, improper drainage of water from behind a retaining wall can cause the wall to fail.
Look for movement in your retaining walls. Bowing (vertical bulges), sweeping (horizontal bulges), and cracking in retaining walls can be caused by water pressure (or hydrostatic pressure). Bulging can also be a result of inadequate strength to resist the load of the earth behind the wall. Bowing and sweeping failures may be correctable if found early enough and if the cause is poor drainage.
There are other types of failures of retaining walls. Failure by over-turning (leaning from the top) or sliding may be caused by inadequate wall strength. In addition, water behind a wall can create unstable earth, especially in clay soils, and contribute to sliding. Retaining walls also fail due to settlement and heaving. Settlement occurs whenever filled earth below the wall compacts soon after the wall is built, or when wet earth caused by poor drainage dries out and soil consolidates. In cold climates, poor drainage contributes to failure by creating heaving from frozen ground. Both overturning and sliding earth may be stabilized and sometimes corrected if the amount of movement is not extreme. Settling may be corrected on small, low walls of concrete or masonry, and heaving may be controlled by proper drainage. Significant failure of any kind usually requires rebuilding or replacing all or part of a wall. Consult a qualified professional when major repairs or corrections are needed.