Installation of shears keys assists to increase sliding resistance of retaining walls without the necessity to widen the base. Effect of shears keys enhances the deepening of the soil failure plane locally at the keys. Increased sliding resistance comes from difference between the active and passive forces at the sides of the keys. In case weak soils are encountered at the base level of shear keys, failure planes along the base of retaining walls because of sliding may be shifted downwards to the base level of the keys. 

Shear keys are generally designed not to be placed at the front of retaining wall footing base due to the possible removal of soils by excavation and thus lateral resistance of soils can hardly be mobilized for proper functioning of the shear keys. For shear keys located at the back of footings, it poses a potential benefit that higher passive pressures can be mobilized because of the higher vertical pressure on top of passive soils.

Fig. Different locations of shear key in retaining wall