Influence of Lode’s angle on the pore pressure generation in soils

The equation which describes generation of a pore pressure during undrained loading is derived, on the basis of a general three-invariant plasticity model. A new pore pressure parameter c, is identified. This parameter describes coupling between the mean effective stress and Lode′s angle. The pore pressure equation is applied to the three-invariant Cam clay model. Two sets of loading histories are presented: proportional and circular stress paths. Analytical solutions for tangent and secant values of the pore pressure parameters a and c are obtained. In addition, analytical solutions for the specific volume and volumetric strain during any drained stress controlled loading are derived. Furthermore, the solution for the effective stress path during undrained loading is then obtained as a special case of a drained loading, for which the rate of volume change is equal to zero. It is shown that the effective stress path for an undrained loading depends on the total stress path. Moreover, it is found that the limiting initial overconsolidation ratio below which an isotropically plastic soil does not fail when subjected to a circular stress path, depends on the compressibility and ellipticity parameters of the soil.