Constitutive modeling of three-dimensional non-coaxial characteristics of clay

A simple critical state-based constitutive model is proposed to represent the non-coaxial plastic deformations of saturated clay subjected to monotonic shearing with fixed principal stress directions. While extensively reported in experimental studies, this particular type of soil response has not been adequately addressed by constitutive modeling. The presented model employs a revised three-dimensional non-coaxial flow rule that relies on introducing a reference stress tensor for decomposing stress rate and consequently defining the non-coaxial flow direction even when stress rate is colinear with the current stress. Undrained hollow cylinder torsional shear tests are performed on undisturbed Shanghai clay specimens. Such experimental observations, combined with complementary test results on Wenzhou clay, validate the proposed model. The comparisons show that the proposed model can reasonably represent the non-coaxial plastic flows of clays under monotonic shearing characterized by different fixed principal stress directions and various intermediate stress levels.