The determination of soil parameters for design from stress path tests
The thesis concerns investigation and measurement of the elastic deformation properties of an anisotropic soil within the context of critical state soil mechanics. The soil tested is a heavily overconsolidated Gault Clay. Laboratory triaxial testing on 38 mm samples is used to measure and soil 100 mm diameter stiffness. A microcomputer-based control system has been developed for use with hydraulically-operated triaxial cells to enable stress path testing, and this is described. The axial and radial stresses and the back pressure can be varied independently to produce any desired stress path. A method of measurement of anisotropic of loading stiffness is developed using various pairs and unloading cycles, or stress path probes. tried, with isotropic, undrained cycles proving to be of most Several alternatives are uniaxial and constant pi use. For the soil tested, measured modulus values are found to be about 25% in error if anisotropy is neglected. Factors affecting stiffness measurements are assessed, including sample disturbance, soil structure, threshold and stress history effects, design of the apparatus and test procedure. Elasticity theory for cross-anisotropic soils is reviewed, particularly as it relates to the triaxial apparatus. The way in which elasticity theory is incorporated in the critical state model is discussed. Isotropic compression a~ swelling test the compression law usually used in the results question critical state model. Theoretical difficulties have been found with this law when formulating an elastic soil model within critical state theory. A series of stress probe tests is used to investigate the variation of elastic stiffness parameters with soil state. The results are compared with patterns of soil behaviour found from strain path tests.