General Wedge Theory of Earth Pressure

During the twenty years since the first World War the most important development in earth pressure theory has been the transition from the original theory of Coulomb to a broader concept called "the general wedge theory." According to this theory the shape of the surface of sliding and the position of the resultant earth pressure are a function of the degree and type of freedom for lateral expansion of the supported earth. Coulomb's theory represents a special case of the general wedge theory. Its validity is limited to the pressure of earth on supports such as retaining walls whose lateral resistance is independent of their amount of yield. In every other case the general wedge theory should be used. The only condition required to make this theory valid is an almost complete mobilization of the shearing resistance of the supported earth within a continuous zone extending from the foot of the bank to its upper surface. This single requirement, however, is sufficient to exclude the general wedge theory (or any other theory involving the condition of incipient failure) from application to the pressure of soft clay or other highly elastic types of soil on the timbering of cuts or tunnels.