Threshold shear strain for cyclic degradation and pore pressure accumulation in marine clay: Insights and models for engineering applications

Threshold shear strains of marine clays represent essential soil characteristics within intricate marine dynamic settings. Precise estimation of these values provides a quantitative design basis for the advancement of marine engineering projects and geological disaster prevention. A series of constant-volume multistage strain-controlled cyclic direct simple shear tests were performed on marine clay. Consequently, the threshold shear strains for cyclic degradation (γ_td) and excess pore water pressure accumulation (γ_tp) during cyclic shearing were investigated by considering the effects of the initial effective consolidation pressure (σ_v0′) and loading frequency (f). The results demonstrate that an increase in σ_v0′ and f exhibits significant suppression effects on both the magnitude and progression rate of stiffness degradation, as well as the accumulation of excess pore water pressure. Nevertheless, the increase of σ_v0′ diminishes the effect of f, indicating that the existence of a critical σ_v0′ makes the cyclic response of the tested marine clay relatively insensitive to f. A micromechanical framework incorporating the electrical double-layer theory was established to mechanistically interpret the frequency-dependent attenuation of pore pressure accumulation. Furthermore, both γ_td and γ_tp increase with σ_v0′, while remaining independent of f, with γ_tp being notably larger than γ_td. Consequently, evaluation models of γ_td and γ_tp for clays with various I_P and σ_v0′ are proposed based on previously published data. This approach serves as a crucial reference for engineering construction practices by evaluating the necessity of considering the effect of soil cyclic degradation on the dynamic response of structures.