Chapter 17: Strength and deformation behaviour of soils

This chapter explains the basic strength and stiffness properties of soils by frequent reference to their particulate nature. The results of simple shear box tests on sand and clay are presented and compared with the Coulomb strength criterion: τ_f = c′ + σ′n tan ϕ′. It is demonstrated that the angle of shearing resistance ϕ′ depends on a number of factors, including the effective confining pressure, the initial relative density and the stress history. The key roles played by contractancy and dilatancy in controlling undrained strength are explained. The phenomena of critical state strength and residual strength are described, and the extension of the Coulomb strength criterion to the widely used Mohr–Coulomb strength criterion is explained, together with its limitations. The latter part of the chapter deals with the deformation properties of soil and its behaviour under one-dimensional compression. The important influence of inter-particle bonding is introduced, leading onto the concept of yield. In ground engineering, extensive use is made of elasticity in the solution of boundary value problems – such as the settlement of foundations. The behaviour of an ideal isotropic porous elastic material is explored in detail and compared with real soils. The important phenomenon of nonlinear stress–strain behaviour is introduced briefly.