Creep of Anisotropic Clay: Microplane Model

Undrained constant‐volume creep of anisotropically consolidated specimens of clay is mathematically described by the microplane model, which is based on the assumption that the shear strain rates on the contact planes between mutually sliding clay platelets (the microplanes) are the resolved components of the macroscopic strain rate. Thus, the microstructure is assumed to be kinematically constrained. The rate of shear on the microplanes is assumed to be governed by activation energy (rate process theory). The matrix of the current viscosities is obtained as an integral over all spatial directions involving the shear strain rates for the microplanes. This integral, which is evaluated numerically as a summation, gives the dependence of the viscosity matrix on the applied macroscopic stress. Anisotropy of the clay is characterized by a function of the spherical angles describing the relative frequency of clay platelets of various orientations. This function can be approximately estimated from X‐ray diffraction measurements. The model involves only two material parameters for the stress dependence and one for the time decay of creep rate. Satisfactory fits of test data on remolded clay samples anisotropically consolidated in the laboratory are achieved, but applicability in the field remains experimentally unverified.