Experimental study on cyclic failure mode and resistance of marine clay under initial static shear stress

In the design and stability analysis of marine engineering projects such as embankments, offshore wind turbines, submarine pipelines, offshore oil platforms, and breakwaters, the initial static shear stress state of the seabed foundation soils is an essential factor that must be considered. A series of constant-volume cyclic direct simple shear tests were conducted on soft clays from the Yangtze Estuary and the Cangnan offshore region to systematically investigate the cyclic failure modes and cyclic resistence characteristics of marine clays under combined static and dynamic stress conditions. The results indicate that, based on the relationship between the initial static shear stress ratio (ISSR) and cyclic stress ratio (CSR) as well as the failure modes of the specimens, the test results can be categorized into four types: Category I: ISSR = 0, with the failure mode being cyclic mobility failure; Category II: 0 < ISSR < CSR, with the failure mode being cyclic mobility failure; Category III: 0 < ISSR < CSR, with the failure mode being accumulated plastic strain failure; Category IV: 0 < CSR < ISSR, with the failure mode being accumulated plastic strain failure. Based on fluid mechanics theory, a quantitative evaluation method for the failure modes of marine clays under combined static and dynamic stresses was established, a negative correlation between the stress reversal coefficient χ and the apparent viscosity ratio ηf/η1, where threshold stress reversal coefficient χth and threshold apparent viscosity ratio (ηf/η1)th can serve as key indicators for distinguishing between cyclic mobility failure and accumulated plastic strain failure. As ISSR increases, the cyclic resistence of marine clays gradually decreases. A new cyclic resistence evaluation method was established that accounts for the influence of ISSR and is applicable to different strain-based failure criteria. Additionally, an empirical relationship between the sensitivity coefficient ξ and plasticity index Ip in the range of 15 < Ip < 30 was proposed based on literature data.