Why shear keys are provided in retailing walls?

 

While designing retaining walls, to determine their stability, we check failure criteria like sliding, overturning criteria etc. In sliding criteria, the sliding resistance is found out by finding base friction between the base of the retaining wall and the soil. To increase this sliding resistance, shear keys are provided at the base of the retaining walls.

The basic principle is that shear key provide extra passive earth pressure which increases stability in sliding. But at the same time, active earth pressure is also increasing on another side. So, the shear key will serve its intended purpose only if the increase in passive earth pressure exceeds the increase in the active earth pressure due to the shear key.