Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 7 RETAINING WALL
7.4.2.2 Coulomb’s Theory
Coulomb Theory is also based on limit equilibrium of a plane wedge of soil. However, the theory
takes into consideration the effects of wall friction, sloping wall face as well as the sloping backfill.
The pressures calculated by using these coefficients are commonly known as the Coulomb earth
pressures. Since Coulomb’s method is based on limit equilibrium of a wedge of soil, only the
magnitude and direction of the earth pressure is found. Pressure distributions and the location of the
resultant are assumed to be triangular. Coulomb’s coefficients of lateral pressures are as follows with
their related terms and pressures diagrams shown in Figure 7.7
K a =
cos 2 - θ
(7.11)
cos 2 sin- θ sin- β
θ cosθ+ δ
cos- δ cos- β
cos 2 + θ
K p =
(7.12)
cos 2
θ cosθ - δ
Figure 7.7 Schematic Of Coulomb’s Theory Plane Failure Wedge of Soil
(a) Active Condition (b) Passive Condition
7-8 March 2009