SOME CONSIDERATIONS IN UNDRAINED TRIAXIAL TESTING OF VERY SOFT CLAY

Abstract

With the increased amount of construct ion on soft clays in Singapore, there is a need to establish an acceptable framework for laboratory testing of such clay. In this study, the feasibility of using the triaxial test to study the mechanical behaviour of very soft clay is examined using a modified Bishop-Wesley triaxial equipment. The modifications to the standard Bishop-Wesley triaxial equipment for testing include a very sensitive low capacity load cell and specially designed end-platens for the soft clay. Disturbed specimens of the marine clay were obtained from Changi reclamation site and remoulded. The effects of membrane confinement, end restraint, aspects ratio, strain rates, the storage age on the behaviour of the very soft clay are investigated. A newly developed thin, enlarged membrane and a porous polyethylene end liner are found to be effective in reducing membrane confinement and end restraint, respectively. The comparison of UU and CU test indicated that close agreement could be achieved if the specimen for the CU test was consolidated to a pressure equal to the mean effective stress. Result of tests at different effective confining pressure at the low pressure range shows that the angle of internal friction is 24° with zero intercept similar to values determined at higher stress and the ratio of shear strength to effective overburden stress is 0.24 which agrees well with field test. The behaviour of the very soft clay was also compared to the Cam-Clay model. It indicated that the very soft clay could be approximately represented by the model. A finite element program, CRISP using modified Cam-Clay was used to predict the behaviour of the soft clay under triaxial testing. It appeared that the modified Cam-Clay model will yield at an earlier strain than the experimental results, indicating that the remoulded Singapore marine clay cannot be modelled precisely by the modified Cam-Clay.