Yield states and stress–strain relationships in a natural plastic clay

Natural clays are commonly anisotropic due to their mode of deposition, and lightly overconsolidated because of a variety of subsequent geological processes. They exhibit marked changes in stiffness when they yield. Yield stresses from individual tests can be generalized into a yield surface for the clay if displacements during loading are taken into account by calculating specific volumes V = (1 + e) throughout the tests.This paper describes tests on 76 mm diameter triaxial samples of natural plastic Lake Agassiz clay from Winnipeg. The samples were carefully trimmed, reconsolidated to approximately their in-situ stresses, and loaded in stress controlled tests along various stress paths in p′,q stress space.The tests produced well-defined yield envelopes in p′,q; p′, V; and q, V plots, and in corresponding plots where the stresses were normalized with respect to the vertical preconsolidation pressure σvc′. Comparison of one-dimensional yielding states with results from K0-triaxial tests that were continued to large strains suggests that the clay is cemented. At stresses inside the yield surface, the clay exhibits substantially linear elastic behaviour which is transversely isotropic. Values of bulk modulus, shear modulus, and a cross modulus which can be used to link mean effective pressures with shear strains, and deviator stresses with volumetric strains, have been evaluated in normalized form using a least-squares solution. The directions of plastic strain increment vectors after yielding have been examined. Consideration of the results suggests that they are not normal to the yield surface, and are influenced by systematic effects. Plastic compliances vary markedly with stress path direction. Keywords: clay, yield, critical state, anisotropic elasticity, associated flow, plasticity.