A simple model for the small-strain behaviour of soils
  The constitutive equation discussed in this note eliminates some defects of linear elasticity in the description of the small-strain behaviour of soils. It is capable of representing volume changes in pure shear and different values of bulk modulus in compression and expansion. The new equation provides a simple description of soil behaviour at small strains in that it produces linear stress-strain relations that can approximate the initial part of experimental stress-strain curves. 
Jos\'e Jorge Nader
cond-mat.soft