Dynamic Characteristics and Evolutionary Modeling of Coastal Silty Clay Under Cyclic Loading Considering Complex Drainage Conditions

Long-term cyclic loading from wind, waves, and currents alters the mechanical properties of marine soils, threatening the stability of offshore engineering structures; therefore, investigating the dynamic response of coastal silty clay under cyclic loading is crucial for the safety of marine structures. In this study, coastal silty clay was subjected to more than 300 dynamic triaxial tests to systematically examine the effects of different drainage conditions, confining pressure levels, cycle numbers, loading frequency, bias consolidation ratio (K), cyclic stress ratio (CSR), and soil density on its dynamic response. The evolution laws of cumulative plastic strain, pore water pressure, dynamic elastic modulus (E_d), and residual strength were comprehensively analyzed under multiple loading conditions. The results revealed notable differences in soil dynamic response under drained and undrained conditions. The cumulative plastic strain followed a ‘stable growth’ mode when CSR was below a critical value, transitioning to ‘destructive growth’ when it exceeded this value. Notably, the critical CSR of soil exhibiting a post-consolidation strength of less than 60 kPa was 0.6–0.8, whereas that of soil exhibiting an initial strength of more than 60 kPa exceeded 0.8 after bias consolidation. Under undrained conditions, the residual strength of soil decreased to 24% of its initial strength, recovering to 97% of the initial value under drained conditions. Based on these experimental data, an empirical model was developed to simulate the dynamic evolution of cumulative plastic strain, pore pressure accumulation, and residual strength. The data incorporated key parameters, including the number of cycles, K and CSR. Finally, the reliability and applicability of the empirical model were validated by comparing its predictions of the dynamic evolution of coastal silty clay with experimental results and relevant literature data.