Stress-Strain Modulus of Clay in Undrained Shear

Stress-strain data from laboratory shear tests on clays are used in stress distribution and soil deformation problems. The soil is frequently assumed to be an isotropic linear-elastic material having a unique Young's modulus, E. The many factors that influence the value of E, determined from undrained triaxial tests on undisturbed and remolded clays, are examined. The level of applied shear stress, the type of shear test (UU versus CU), the quality of sampling, the degree of overconsolidation, the effects of time (thixotropy, aging, and rate of strain), and the type of stress system applied prior to and during shear (direction of major principal stress, values of intermediate and minor principal stresses) are all shown to have important effects on modulus. CU tests, rather than UU tests, are generally recommended for correlating laboratory and field values of modulus, because they are usually less influenced by sample disturbance and can be better suited for evaluating the effects of different stress systems.