Retaining Walls Inspection – Types of Retaining Walls
Retaining walls are normally made from construction timbers, railroad ties, concrete, stone or concrete blocks. In some cases, concrete and concrete-block walls have stone or brick veneer facing. Gabion retaining wall-steel baskets are sometimes filled with stones, but as gabiuons age, the steel baskets tend to corrode and deteriorate, especially on the side facing the embankment. In time, however, soil sediment usually fills the voids between the stones and holds the wall in place.
Stone retaining walls are commonly referred to as dry or wet, depending on whether mortar was used between the stones. A dry retaining wall is called dry because it has been constructed without mortar. The stability of retaining walls depends on the weight and friction of one stone upon another. Frost heaving is not a problem with this type of wall. Since the stones are not bonded together, they will be raised and lowered together by the frost. Consequently, the bottom course of the wall is generally just about six inches below grade rather than below the frost line.
A wet wall is built with mortar between the stones. The mortar secures one stone to another, thus achieving a monolithic with greater stability. Because this wall is integral frost heaving will make it crack, so the bottom of the wall must be below the frost line to prevent frost heaving. However, wet retaining walls are less hazardous because no loose stones can be kicked out of place or fall of the top.
Retaining Walls Inspection – Provisions for Water Drainage
When constructing a retaining wall, provisions must be made for draining the water that normally accumulates behind the wall, otherwise hydrostatic pressure buildup may result in a structural failure of the wall. A continuous perforated drainpipe should be installed at the lower portion of the wall and the area should be backfilled with broken stones or gravel to provide drainage. The pipe should be directed so that the effluent flows to a suitable location away from the wall.
In many monolithic retaining walls, the perforated pipe is replaced by weep holes – holes in the wall that run from the front to the gravel backfill. The weep holes should be placed at a five- to ten-foot spacing and should be four inches in diameter – the same diameter as the drainpipes.
A wall inspection program includes checking for drainage, because retaining walls are often built without adequate drainage provisions. The gravel backfill and drainpipe might be omitted, or the weep holes might be too few or too small to be effective. Homeowners should make sure the weep holes are kept clear so that the water behind the wall can be properly drained.
A retaining wall made from construction timbers or railroad ties should be anchored into the hillside to provide the resistance to withstand the lateral forces exerted on it. If the wall is not tied back into the earth, it can buckle, bow, heave and eventually collapse.
Still, many retaining walls are designed without anchors. During a retaining walls inspection, you can tell whether anchors were used simply by looking at the wall. If tiebacks were used, end sections will be visible in the face of the wall. From a visual inspection, however, you cannot tell the length of the tiebacks. Consequently, the inspection will be focused on the condition of the wall rather than its construction, unless it is a new wall. If it is newly constructed, the property owners should have a guarantee.