A common equation that is used to calculate the active earth pressure coefficient kA for the case of
wall friction is the Coulomb equation, which is shown in Fig. 11.4. The Coulomb equation can also
be used if the back face of the wall is sloping or if there is a sloping backfill behind the wall. Once
the active earth pressure coefficient kA is calculated, the active earth pressure resultant force PA can
be calculated by using Eq. 11.1.