Shear-thinning non-Newtonian fluid-based method for investigating cyclic stiffness degradation of marine clay

Marine clay may experience stiffness degradation and catastrophic failure when subjected to complex ocean dynamic loadings. This can result in instability, destruction, or capsizing of offshore structures. In this study, marine clay was regarded as a non-Newtonian fluid with shear-thinning behaviour, and the mechanism of progressive stiffness degradation during cyclic loading was discussed from the perspective of fluid dynamics. A series of cyclic direct simple shear tests were conducted on undisturbed marine clay obtained from three offshore sites. Further, the stiffness degradation and flow characteristics under different plasticity index (IP) and cyclic stress ratio (CSR) conditions were investigated and quantified using the stiffness degradation index (δ) and average flow coefficient (κ), respectively. The results revealed that the decrease in δ with the increasing number of cycles (N) in a semi-log scale can be categorised into three modes: (1) "linear" (nonfailure), (2) "fast–linear–fast" (failure), and (3) "linear–stable" (failure). Consequently, a two-parameter model was proposed to predict the δ of failure marine clay from different sea areas with varying IP and CSR values. Moreover, with the increase in N, κ of the nonfailure marine clay increased gradually in a very limited range, thus exhibiting illiquidity characteristics; by contrast, κ of the failure marine clay exhibited a "slow linear–exponential–rapid linear" growth pattern, thus indicating a change in liquidity from weak to strong. Finally, a unified model linking the stiffness degradation and flow characteristics of marine clay under different types and conditions was proposed, where κ at the cyclic failure state (the failure criterion is a double-amplitude shear strain of 15%) was denoted as κf. Evidently, all data points of κ/κf ∼ δ were distributed in a narrow range, and a virtually negative exponential relationship was observed between κ/κf and δ.