Where is the best position of shear keys under retaining walls?

The installation of shears keys helps to increase the sliding resistance of retaining walls without the necessity to widen the their base. The effect of shears keys enhances the deepening of the soil failure plane locally at the keys. The increased sliding resistance comes from the difference between the passive and active forces at the sides of the keys. In case weak soils are encountered at the base level of shear keys, the failure planes along the base of retaining walls due to sliding may be shifted downwards to the base level of the keys.

Shear keys are normally designed not to be placed at the front of the retaining wall footing base because of the possible removal of soils by excavation and consequently the 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 advantage that higher passive pressures can be mobilized owing to the higher vertical pressure on top of the passive soils.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.