3.7 Constitutive equations
During a general loading in the ground both shear and normal stresses are likely to
change simultaneously so there will be shearing and volumetric straining together.
For soils it turns out that shearing and volumetric effects are coupled so that shearing
stresses cause volumetric strains and normal stresses cause shear strains. This is quite
surprising and we will see later how the particulate nature of soils gives rise to shear
and volumetric coupling.
A simple constitutive equation relating shearing and volumetric stress–strain
behaviour can be written as