The effect of fines plasticity on monotonic undrained shear strength and liquefaction resistance of sands

The paper presents results of an investigation into the effects of fines plasticity on the undrained monotonic and cyclic response of sands. Monotonic and cyclic triaxial tests on mixtures of sand with 5% and 15% fines content were performed. Non-plastic and plastic fines of varying plasticity were used. At a given fines content, confining effective stress and void ratio, the results show that the undrained shear strength and cyclic resistance decrease with increasing plasticity index of fines up to a threshold value. Above this threshold value, the undrained shear strength and cyclic resistance increase with increasing plasticity index of fines. This pattern of behaviour was also reflected in the excess pore water pressure rise during both monotonic and cyclic loading. The mechanism controlling the behaviour of sands with fines and the implications of the test results to the engineering practice are finally discussed.