Design of Retaining Walls

Question

Explain the function of shear keys in the design of retaining walls?

Answer 1

In estimating the external stability of retaining walls, failure modes such as bearing failure, overturning and sliding are usually considered in design. In considering the criteria of sliding, sliding resistance of retaining walls is derived from base friction between the foundation soils and the wall base. To surge the sliding resistance of the retaining walls, rather providing a large self-weight or large retained soil mass, shear keys are installed at the wall base.

The main principle of shear keys is shown below:
Main purpose of installation of shear keys is to rise the additional passive resistance developed by height of the shear keys. Though, active pressure developed by shear keys also rises simultaneously. The success of shear keys lies in fact that the increase of passive pressure exceeds the increase in active pressure, resulting in net improvement of sliding resistance.

Instead, friction between the wall base and foundation soils is normally about a fraction of angle of internal resistance where p is the angle of internal friction of foundation soil. Whenever a shear key is installed at the base of the retaining wall, failure surface is transformed from the wall base/soil horizontal plane to a plane within foundation soil. So, the friction angle mobilized in this case is p in place of 0.8p in the previous case and the sliding resistance can be improved.