Post-long-term cyclic behaviour of Coode Island Silt (CIS) containing different sand content

It is widely accepted that the post-cyclic stiffness and shear strength of marine clays may alter as a consequence of experiencing a large number of deviatoric load cycles. Most marine clays in bay areas are already undergoing long-term one-way and low amplitude ocean and wind waves either because of the seabed topography or existing infrastructures. For the engineering of such clays, particular attention should be given to the post-cyclic behaviour of the material when investigating the effect of alteration in loading regimes exerted by human-made or natural phenomena such installation of new infrastructure, earthquake, tsunami and port upgrade construction, in which the magnitude of the applied load changes. As one of the most sensitive soft soils in Melbourne, Coode Island Silt (CIS) at the northern shoreline of Port Phillip Bay comprises a considerable and variable amount of sand. This paper explores the post-cyclic constitutive behaviour of CIS containing variable sand content. To investigate the stiffness and shear strength of CIS subsequent to experiencing a large number of low amplitude cycles, a series of post-cyclic triaxial tests are performed on CIS specimens with varying sand content ratio up to 30%, immediately after applying 30,000 semi-sinusoidal load cycles. Based on the test results, it is found that the undrained shear strength of CIS, does not alter considerably as the results of long-term cyclic loading. However, a significant increase in the secant stiffness of CIS followed by very brittle yielding is observed. In the end, it is intended that results be summarised in a form applicable by industry. Hence, the possible effects of such alterations in the constitutive behaviour of CIS on the design of monopile foundations are discussed.