A retaining wall compensates for a difference in vertical elevation (also called grade) between two points where a slope is impractical or undesirable. The retaining wall must resist two major forces: the weight of the soil mass being retained and hydrostatic pressure from subsurface water.
The weight of the soil mass being retained varies based on the type of soil and its angle of repose. The angle of repose is the steepest slope the soil can naturally maintain without sliding downward. This angle varies depending on a soil’s sand, silt, and clay content. The angle of repose is also affected by the amount of moisture contained in soil. A retaining wall must be strong enough to resist the force of all soil filled beyond the angle of repose.
This cross-section illustrates a basic retaining wall. |
The wall is retaining all soil above the angle of repose.
|Gravity forces the soil to its natural angle of repose without the retaining wall.|
Hydrostatic pressure is exerted on a retaining wall as subsurface water builds behind it. As rain water percolates into the ground, water pressure builds behind a retaining wall as the retained soil becomes more and more saturated. The wall will eventually fail without some means of relieving this pressure.
|This cross-section illustrates how hydrostatic pressure builds behind a retaining wall.|
If too much hydrostatic pressure is exerted on the wall, it will fail.
|A drainage system installed behind the wall relieves hydrostatic pressure.|
The basic means of providing resistance against these forces are one or a combination of the following: mass of the wall, structural reinforcement, depth and design of the footing, deadmen, wall batter, and drainage systems. My next post will detail how different retaining wall materials use these methods and the advantages and disadvantages of each type of wall system.