Retaining Wall Inspections
Retaining walls are used for stabilizing and controlling erosion of steeply sloped areas of the lot. In some cases, retaining walls are used in conjunction with terracing to provide a level area for recreational purposes. In either case, they should be designed to withstand the lateral pressures being exerted on them, by the soil and the hydrostatic pressures from behind the wall.
Retaining walls may be built with concrete, construction timbers, railroad ties, stone, concrete or concrete blocks. Some concrete and concrete block walls have stone or brick veneer facings. On occasion, you may find a retaining wall of steel baskets filled with stones. This is referred to as a “gabion” retaining wall. As the gabions age, the steel baskets tend to corrode and deteriorate, especially on the side facing the embankment. Over the years, the stone inside may begin to move.
The design of the retaining wall should incorporate provisions for drainage of water that normally accumulates behind the wall. Otherwise, the hydrostatic pressures built up will cause structural failure of the wall. Drainage may be provided by installing continuous perforated drain lines at the lower portion of the wall, and backfilling the areas with stone and gravel. In concrete retaining walls, the perforated pipe is replaced by weep holes in the bottom of the walls that allow the water to exit through the front of the wall. The grade or soil at the base of the drainage system behind the wall should direct any water that accumulates to the weep holes. Unfortunately, retaining walls are often built with inadequate drainage provisions. The gravel backfill or the drain line may be omitted, or the weep holes may be too few or too small to be effective. The weep holes should be kept clear so that the water behind the wall can adequately drain.
A retaining wall built with construction timbers or railroad ties should be anchored to the hillside to provide resistance to the lateral forces. If the wall is not tied back into the earth, it can bow, buckle or heave and eventually collapse. Anchoring the wall is achieved by using tiebacks or "deadman" support. A tie back is a construction timber that is placed perpendicular to the wall. The front end is flush with the wall and is fastened to the wall itself with large spikes. The rear end is fastened to a deadman or a small section of lumber perpendicular to the tieback and parallel to the wall itself. Because the construction timbers and railroad ties have open spaces between the members, weep holes may not be necessary.
Many railroad tie and timber retaining walls are not constructed with anchors. You can tell whether anchors were used by looking at the wall. If tiebacks were used, end sections will be visible in the face of the wall. However, from a visual inspection you cannot determine the length of the tie back or whether deadmen were, in fact, installed. All retaining walls should be vertical or inclined slightly towards the embankment. They should not be leaning forward or away from the embankment. When you encounter a retaining wall that is leaning, it is an indication that the wall has not withstood the lateral forces that have been exerted. Once the wall begins to lean, crack and heave, the pressure that caused its condition should be relieved.