Concrete retaining walls and other underground structures act as a barrier to the natural flow of ground water. Consequently, ground water has a tendency to build up behind a retaining wall.
This has two undesirable effects. First, it creates additional force and weight that the retaining wall has to sustain. Often, this additional force is not calculated in the design of the retaining wall and, in time, may cause the wall to fail.
Second, the built-up water can infiltrate through the cold joints of the retaining wall. This can cause the reinforcing steel in the wall to rust and, over time, damage the integrity of the wall. Additionally, if the downhill space beyond the wall is used as living space, the water infiltration will cause unsatisfactory conditions, which are very expensive to correct. The visible signs of water infiltration are spalling or scaling of the concrete.
Because of these issues, designers and builders should install a drainage system, as well as a membrane, to act as a moisture barrier behind every concrete retaining wall.
Historic retaining wall drainage systems
Historically, builders have applied hot tar reinforced by mesh behind concrete or masonry walls to seal the cement. In addition, to ease the drainage of the ground water, they placed gravel, also referred to as “drain rock,” across the up-hill side of the wall. This positioning of gravel effectively creates impermeable pockets of void along the path of the ground water. Once water reaches the gravel, it drips to the base of the wall and eventually finds its way around the retaining wall and down the hill.
While this mechanism has proven to be effective, over time, it tends to fail. Tar eventually deteriorates and cracks. Soil sediments fill the gravel void and render the drain rock useless.
Modern retaining wall drainage
Today, the advancement of technology has reached a point where numerous fail-safe systems have been developed. Accordingly, the failure of retaining wall drainage systems is no longer accepted by the construction industry.
Retaining wall drainage systems
To facilitate ground water drainage, the latest technology employs panels, commonly referred to as “drainage panels” or “drainage tiles.” This type of drainage system is far more cost effective than importing and placing several cubic yards of gravel behind a retaining wall.
These drainage panels are designed to replace gravel or “drain rock.” They are made of composite material and can be placed over the waterproofing membrane and attached to the back of the retaining wall. The panels are made with “dimples” surfaced with Geotextile fabric. The fabric is placed toward the soil and acts as a filter to keep sediment out. Water drains through the fabric and drips to the base of the retaining wall through the channels produced by the dimples.
Perforated PVC pipes positioned at the base of the retaining wall can then collect the water. The PVC pipes are wrapped with filter fabric to prevent silt clogging. The perforated pipes are connected to solid PVC pipes that conduct the water downhill to a proper discharge facility. If the water is discharges on a hillside, a dissipation field is constructed.
A dissipation field is an engineered facility designed to release the water collected by perforated pipes from the uphill side of the retaining wall and then discharge it in such a way that will not cause soil erosion.
A typical engineered dissipation field is a terraced section of hillside covered by Geotextile fabric. A 20’ – 30’ section of perforated PVC pipe is laid flat on the fabric, with holes facing up. The pipe is connected to sub drains by a solid transfer pipe. The entire terraced area and PVC pipe is then covered with 6” – 12” boulders. Usually, the size and location of the pipes and the size of boulders are designed and calculated by an engineer specializing in drainage systems, based on the amount of water that will be discharged.
With a properly engineered dissipation field, water gradually seeps across a large span and won’t cause erosion by a concentrated discharge. This gradual dissipation of water also helps with the replenishment of ground water. In an effort to replenish ground water across The United States, new underground dissipation fields are being designed for subdivisions built on flat land. These underground dissipation fields collect storm water and gradually dissipate it into the aquifer.
If you would like to learn more about retaining walls drainage systems for your next project, please contact SteelCore Builders.
About SteelCore Builders
As a division of Trush Construction Company, the SteelCore Builders crew and management have been doing structural concrete and strengthening since 1987. Due to our firm’s extensive engineering expertise and exceptional reputation, we are often retained to undertake very difficult and complex projects. Our continuous research and testing of new building materials and technologies enable us to address unique situations that other firms cannot.