The stiffness of a material largely determines the strains and displacements in structures,
or in the ground, as they are loaded or unloaded. Another term often used in soil
mechanics to describe the relationship between stress and strain is ‘compressibility’,
but this is basically the reciprocal of stiffness. Often there is a marked change in the
gradient of a stress–strain curve at a yield point, as shown in Fig. 3.1. This is associated