Rigid Retaining Wall Design

The design of rigid retaining wall needs to fulfil several criteria.

The first criterion is the overturning stability. For a retaining wall, the destabilizing moment is caused by the lateral earth pressure. On the other hand, the stabilizing moment would be the weight of wall and soil above the footing (for cantilever wall). The factor of safety is defined as the ratio of stabilizing moment to destabilizing moment. It needs to be greater than 2.0 to deem the wall safe against overturning failure.

The second criterion is the sliding stability. The destabilizing force in this case is the horizontal component of the active thrust. The stabilizing forces on the other hand, is the friction developed at the base of wall. Where shear key is provided, passive pressure developed in front of the key can be included into the calculation. The factor of safety equals to the division of stabilizing force by destabilizing force. The acceptable value for factor of safety is 1.5.

The third criterion is the bearing stability. Due to the combined effect of moment and vertical force, the distribution of soil pressure underneath the wall base may not be uniform. In this case, it needs to be calculated based on the eccentricity of base resultant from the center of base. As long as the base resultant is acting within the middle third of base, the uplifting of footing will not occur. The maximum soil pressure should be less than the allowable soil bearing pressure to conclude the wall is safe in terms of bearing stability.

Other than that, the wall design needs to be checked against overall stability and structural adequacy.

In this worked example, we are required to check the stability of a gravity wall against overturning, sliding and bearing failures.

The backfill is medium sand with friction angle of 30 degree and unit weight of 18kN/m3. The wall is considered rough, where the friction angle between soil and wall is 22 degree. The backfill is sloped at 10 degree, while the wall sloping angle is 95 degree. For this scenario, the active pressure coefficient is calculated using Coulomb's theory.

Subsequently, both stabilizing and destabilizing forces are determined, and their resultant moments about the wall toe are calculated. From here, we determined the wall's factor of safety against overturning failure.

Then, we determined the factor of safety against sliding failure using both destabilizing force, and factored vertical force as stabilizing force. The factor introduced here is also known as the friction coefficient between soil-wall interface.

Lastly, we check the wall stability against bearing failure. The location of base resultant is first identified using net moment and vertical forces. No uplifting occurs since the resultant is located at the middle third of base. Then, we can proceed with determining the maximum pressure under the wall, and finally calculate the factor of safety against bearing failure.