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GeoInsighter Summer/Fall 2006 Newsletter RETAINING WALLS AND GROUND WATER Return to the Newsletter Index The heavy storms that we suffered through recently tested many natural and human-built structures. Big storms cause problems either because of intensity (a lot of precipitation over a short period of time) or duration (precipitation lasting for an extended period of time). The worst storms provide a double whammy of intensity and duration. 
Intense storms can create flash flooding; rain falls faster than the ground or waterways can accommodate, resulting in excessive runoff. It is easy to visualize that rapidly moving water can be very damaging because of its ability to erode soil and its force against objects. However, while not so obvious, gentle rain over a long period of time can also be very damaging. The ability of the ground to absorb water diminishes during prolonged rain events; the water table rises and the water contained in the soil causes a new loading condition on buried structures. This article focuses on the effect of ground water on retaining walls, which includes basements walls (i.e., any wall that retains soil and could be subject to an increase in ground water level).
Because of the potential adverse impacts of ground water, retaining walls need to be constructed with provisions to accommodate accumulating water. Without adequate drainage, the horizontal load imparted by the combination of soil and water can crack foundation walls (especially unreinforced concrete block walls) and push retaining walls over or out of alignment, making them unsafe or visually unappealing. There are two primary objectives for including drainage provisions. First, as the water level rises behind a wall, it increases the horizontal force acting against the wall. This occurs whether the wall is a cast-in-place concrete basement wall or a modular block parking lot retaining wall. This concept should make sense because we can all understand that wet soil weighs more than dry soil. However, the horizontal force of water with no place to go can be surprisingly strong. Think of the force exerted by contained water at the bottom of an aboveground pool and imagine that force pressing on the side of a retaining wall. Second, water will migrate via the path of least resistance. If water is not routed to a planned drainage pathway, it will try to find another way out. This could be through a crack in the basement wall or a joint in the retaining wall. Unplanned water migration could, at the least, be a nuisance or, worst case, cause staining and water damage (including ice formation outdoors and mold indoors). Addressing water pressure behind a subsurface wall is typically very simple as long as it is included in the original design and construction. For both retaining and basement walls, the solution involves installing a preferential drainage pathway to allow water rising behind the wall to flow into the pathway and drain away from the wall to avoid the build up of water pressure. This requires the use of highly permeable, well draining media directly against the buried side of the wall. It should be recognized that, except in fast draining sand and gravel soil formations, water flow through soil is quite slow because of the tiny size of the flow pathways (i.e., the spaces between the soil grains that the water is driven through under pressure). Therefore, providing a faster draining, preferential drainage media is usually relatively easy. Drainage media may be soil or geosynthetics, or a combination of the two, and will allow water to drain both laterally and vertically downward. Filter fabrics are usually used to separate the natural soil from the drainage media. The bottom of the pathway often includes a perforated pipe, usually set at a level more or less equal to the bottom of the wall, that can then collect the drained water and transport it to an outlet. If sized correctly, the pipe should be able to carry water away to an outlet faster than it can flow through the soil to the pipe. It is important that the outlet for wall drainage systems be passive (i.e., by gravity) or active (i.e., using a pump within a sump with a generator backup) to ensure that the water always drains from the wall in all foreseeable weather conditions. It should be noted that it is almost always possible to install drainage systems after the fact that will have some beneficial effect on the water accumulation behind retaining walls. However, this is often very expensive because of disruption to finished areas and landscaping and often never achieves the efficiency of systems designed prior to and installed during construction. Michael C. Penney, P.E., L.S.P. Return to the Newsletter
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