The Coulomb theory assumes a hydrostatic distribution of pressure such that the
resultant forces R (reaction needed to hold wedge in equilibrium) and P (summation of
normal pressure times area) act at the lower third point of the planes upon which they
act, planes ab and ac, respectively. The force R acts at an angle of friction of soil on
concrete, ordinarily 25°, while P acts at an angle of friction of soil on soil, generally
assumed to be 34°. This latter angle will vary significantly from 34° to 40° or more.
Because of the different angles of friction, the theory produces an error in the result