What is the function of shear keys in the design of retaining walls?
In determining the external
stability of retaining walls, failure modes like bearing failure, sliding and
overturning are normally considered in design. In considering the criterion of
sliding, the sliding resistance of retaining walls is derived from the base
friction between the wall base and the foundation soils. To increase the
sliding resistance of retaining walls, other than providing a large self-weight
or a large retained soil mass, shear keys are to be installed at the wall base.
The principle of shear keys is as follows:
The main purpose of installation
of shear keys is to increase the extra passive resistance developed by the
height of shear keys. However, active pressure developed by shear keys also
increases simultaneously. The success of shear keys lies in the fact that the
increase of passive pressure exceeds the increase in active pressure, resulting
in a net improvement of sliding resistance.
On the other hand, friction
between the wall base and the foundation soils is normally about a fraction of
the angle of internal resistance (i.e. about 0.8φ ) where φ is the angle of internal friction of foundation soil. When
a shear key is installed at the base of the retaining wall, the failure surface
is changed from the wall base/soil horizontal plane to a plane within
foundation soil. Therefore, the friction angle mobilized in this case is φ instead of 0.8φ in the previous case and
the sliding resistance can be enhanced.