Constitutive modelling of the shakedown response of sensitive clay to undrained long-term cyclic loads incorporating structural destruction

Structured soft clay is characterised by high sensitivity and compressibility and accumulates excessive deformation under long-term dynamic loads, e.g., traffic loads, which likely threatens the service performance of overlying structures. In this work, to model the long-term mechanical behaviour of structured soft clay and efficiently capture its structural degradation, a new constitutive model was developed. The structural properties of soft clay, i.e., high yield strength and cohesive strength, were considered by a proposed yield surface, with their evolutions related to the combined plastic volumetric and deviatoric strains. The cyclic response of clay to undrained conditions was described through bounding surface theory. Moreover, the influence of the loading frequency on the dynamic response of clay was incorporated into the plastic modulus, and the softening effect caused by the generated excess pore water pressure (EPWP) was described by the shrinkable yield surface. Model validation was then carried out by reproducing both the accumulated strains and EPWPs of five types of reconstituted and structured soft clay. The acceptable consistency between the simulated results and experimental data and the independent and physical meaning of the featured model parameters confirmed the efficiency of the proposed model. More importantly, the evolution of the structural internal variables $S_i$ and $p_t^{\prime }$ with the development of plastic strains effectively represented the structural destruction process of soft clay under long-term cyclic loading conditions.