The behaviour of clay in simple shear and triaxial tests
The main objective of this work is to study the applications of critical state soil mechanics to simple shear testing. The problems with the simple shear apparatus and the approach for interpreting simple shear test results are described. Laboratory tests were conducted using the NGI simple shear apparatus, standard shear box arid computer controlled stress path testing equipment for samples with 38 mm and 100 mm diameter. The test programme comprised simple shear and shear box tests on remouled, reconstituted and undisturbed samples of Cowden till and London clay, undrained triaxial tests on reconstituted arid undisturbed London clay and stress probing tests on undisturbed London clay. Simple shear samples were tested under either constant effective vertical stress or constant volume condition. Basic results for one-dimensional compression arid for shearing obtained from 72 simple shear tests, 27 shear box tests and 16 stress path tests are presented. Analyses of the simple shear test results indicate that the critical state friction angle for horizontal planes is dependent on whether the sample was sheared under constant or constant volume condition and on the pre-shearing value of K0 which is a function of overconsolidation ratio. A theory for the critical states of simple shear tests which takes into account the influence of K0 is proposed. The normalised test data show that the overall patterns of the simple shear and triaxial tests are as predicted by the critical state model. Tangent stiffnesses instead of secant stiffnesses were calculated. For simple shear tests, the shear stiffnesses obtained from constant shearing are the same as those obtained from constant volume shearing. For London clay, the normalised shear moduli for undrained triaxial tests have the same order of magnitude as those for constant volume simple shear tests. Results of probing tests show that undisturbed London clay is linear anisotropic elastic. The elastic parameters for the constitutive equations can be measured from special stress paths.