General equations that have emerged from the principle of natural proportionality have been applied in the last years to describe the mechanical behaviour of geomaterials such as rocks, granular and fine soils and concrete. They were applied a long time ago to reconstituted samples of Weald clay tests performed at Imperial College, London, in the early 1960's. In this paper the improved theoretical equations are applied successfully to describe the deviator stress-strain and pore pressure, for the pre and post peak regions, for undisturbed overconsolidated Mexico city clay subjected to undrained triaxial compression tests. OCR up to 10 are considered. An important feature is the use of the new general sensitivity function in the pore pressure throughout the tests. This new sensitivity function is in terms of the natural shear strain and it allows the pore pressure all along the triaxial tests to be described, that is, in the pre-peak as well as the post-peak. This new sensitivity function provided by the principle of natural proportionality permits better description of the pore pressure than the empirical equation used by the second author in the past just only for the pre-peak region.
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