Evaluation of yielding in unsaturated clays using an automated triaxial apparatus with controlled suction

Research in the field of unsaturated soil mechanics for high plastic clays is very active. One area of considerable current interest is development of general constitutive models for unsaturated clay based sealing materials in a framework that can be implemented in numerical modeling tools (Alonso et al. 1990, Delage and Graham 1995, Toll 1990). In particular, more quantitative information is required to define the features of yielding, failure and strain hardening for predictive modeling applications. Soil suction must be controlled and independently measured in laboratory tests. This will allow examination of behaviour along any stress path that can be expected to occur in engineering applications which will provide the necessary material information to calibrate and validate proposed constitutive frameworks. This paper presents details of laboratory tests in a custom triaxial system with stress path automation and independently controlled and measured suction (Blatz and Graham 2000, 2003). Details describing the equipment will be given along with selected results for the yield, strength, and strain hardening behavior of a high-plastic sand-clay material at suctions from 5 MPa to 160 MPa and isotropic pressures from I MPa to 6 MPa. The results demonstrate the importance of independent measurement and control of suction along well controlled loading paths for interpreting the behaviour of unsaturated high plastic clays.