Retaining Wall Drainage

By James E. Laier, Ph.D., P.E., Southern Earth Sciences and Matt Nolan, Smart Drain LLC

 

The Dura Slope trench drain system provides a solution for residential, commercial, industrial and municipal drainage needs.The system is available with a variety of grate material options, including cast iron, structural foam, galvanized steel and stainless steel. Photo courtesy of NDS, Inc.

One of the main causes of retaining wall damage is excessive water buildup. Whether it is a simple retaining wall of wood, stone, or brick or a large complex retaining wall of concrete, water damage is an ever-present danger to the walls integrity. Water pressure can crack and deform retaining walls leading to costly repairs and in some cases replacement.
From a landscaping perspective, when a belt is installed vertically in the soil, it simulates the capillary action of tree roots by drawing down surface rain water and holding it underground. When placed horizontally beneath vegetation, the capillary action of the belt can be used to deliver irrigation water to the roots. When embedded horizontally below ground, with a slight slope, the siphoning action of the belt drain can be used to keep soil and construction foundations dry.

Smart Drain installation of a new retaining wall. This illustrates the typical arrangement of drainage belts behind a newly constructed gravity retaining wall. Because the efficiency of the belts increases dramatically when the drains are installed in a vertical alignment, a 10-foot spacing between two vertical belts is customary. ????Photo courtesy of Smart Drain

Under normal operating conditions the discharge rate of a single 8-inch wide belt, 10 feet long, bedded in sand, in a horizontal position with a 1 percent slope, can be expected to drain at a constant discharge rate until the water level falls far enough below the drain to break the capillary forces associated with the siphoning action.

Generally, as long as the discharge end of the drain is below the inlet end of the drain, water will be siphoned away at a constant rate of flow until the water level is below the level of the belt and capillary attraction is naturally broken. The combined effect of the capillary/siphoning action results in a consistent drainage of water at a decent flow rate.

Above and below(before and after): The wall was created from 1.75" thick flag stone. It is approximately 60 feet long on the right side of house and around 4 feet tall. Terra Flora Landscaping dug the bank out about three quarters of the way and back-filled with good top soil. Next they glued geogrid between the rocks and ran the grid back into the bank. They placed 57 clean stones in behind the wall for drainage. The base was created crush and ran 3/4", base depth was around 12" and width was around 24". ????Photo courtesy of Matt Davis, Terra Flora Landscaping Inc.

It is important for landscape contractors to find economical tools to aid them in designing and building retaining walls. Because water adds 62.4 pcf of horizontal pressure to a retaining wall for each foot of depth, trapped water forces are often far greater than earth forces retained behind gravity wall systems. For economy of construction and to minimize the potential for wall sliding and/or overturning, most designers insert small diameter weep holes, spaced 5 to 10 feet apart, along the base of their gravity walls to relieve water buildup behind the wall systems.

Landscape designers usually place filter fabric across the inside face of the weep holes and along any joints on the back side of the wall to prevent soil migration as the water drains. After the filter fabric is in place, it???~s customary to place gravel and/or a clean draining sand along the inside base of the wall to collect water that will ultimately drain through the weep holes. Once the gravel is placed, it???~s customary to fill directly behind the wall with sand. Ultimately, the best way to avoid drainage problems is to build in a reliable drainage system that will prevent water from being trapped behind the wall.