Retaining walls hold back the pressure of earth embankments. Unlike basement walls, they are not braced at the top by a floor system, and must be designed so they will not topple or fail in bending at the bottom.
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Forces in a Retaining Wall
Soil pressure on a retaining wall increases with the depth of the soil it is holding back. (Forces in a Retaining Wall, below). Walls can fail in bending, by sliding, or by toppling. When engineering a wall, all three failure modes must all be analyzed.
Drainage for Retaining Walls
Soil pressures on a retaining wall increase drastically when the soil is saturated (see Soil Types for more on soil types and pressures). At the same time, the chance of soils sliding or overturning increases when soil is wet. Backfilling with poorly draining material, or failing to provide positive drainage, greatly increases the odds of wall failure. Always backfill retaining walls with free-draining granular material (sand or gravel), and provide drains that allow water to escape from behind walls (see Drainage).
Figure: Forces in a Retaining Wall
The greater the depth of the wall, the greater the total lateral force of the soil. This exerts an overturning force that is -resisted by the weight of the soil over the footing and the weight of the wall itself. Friction at the base of the footing keeps the wall from sliding.
