How To Design A Retaining Wall
A retaining wall can be built to any height and can be made from many different materials but its function is always the same: to resist lateral forces from the soil at the higher grade level. Retaining walls are installed in places that require a straight vertical change in ground elevation where long sloping grades are precluded due to space restrictions.
Large retaining walls are typically made from site-cast concrete or galvanized steel and usually require the skills of a licensed engineer. Retaining walls less than 3 feet in height are commonly constructed with stone masonry or heavy timbers. In residential applications, a masonry cap is frequently installed along the top of the retaining wall and the material used for the wall itself is selected based on aesthetic as well as structural considerations.
Design Considerations and Modes of Failure
Several variables factor into the structural design of a retaining wall. The grade differential and the mechanical properties of the soil behind and below the retaining wall create the primary forces. Determining the elevation of the water table and the location of the frost line are necessary elements in the design of most retaining walls. The presence of ground water will introduce complicated loadings on the retaining wall and can affect the fluid and soil mechanics of the earth bearing the wall’s footing.
A retaining wall is likely to fail in one of several ways. The structural integrity of the wall can be compromised by cracks or ruptures. The entire wall can be toppled over, or it can slide forward in an upright position. If the base of the wall is pushed outward by flowing ground water, it is referred to as undermining.
Construction Methods for Retaining Walls
The construction of a retaining wall depends on its size and design load. Pressure-treated timbers similar to railroad ties can be stacked horizontally or installed vertically to create a wall plane. Steel rods inserted into drilled holes are used to bind and reinforce the timbers.
“Deadmen” are horizontal timbers set perpendicular to the wall projecting back into the soil to provide a stabilizing effect. The use of weep holes and drainage pipes buried in a layer of crushed stone are sometimes necessary to divert ground water from behind the base of the wall. Smaller walls less than 3 feet in height usually do not require any type of footing. Larger retaining walls often employ the cantilevered reinforced concrete method which consists of a vertical wall set off-center on a horizontal footing.
The wall can be constructed of site-cast concrete or assembled with CMU (concrete masonry unit) blocks. Both types are reinforced with steel rods running horizontally through the wall and footing, and vertically down through the wall into the footing. Properly designed cantilevered retaining walls can resist the compressive and tensile loads acting on the wall and any of the forces caused by shifting soil or ground water migration.