Natural soft soils beneath transportation infrastructures sustain typical structured properties and are characterized by high sensitivity and poor engineering performance, which pose great challenges for the efficient operation of high-speed trains. Under traffic loading, soft subsoils present shakedown response and accumulate no negligible deformation. While few constitutive models are available both for the long-term behavior description of soft soils under cyclic loading and structure degradation of soil. Herein, a conceptual constitutive model within bounding surface theory framework was established to depict the dynamic behavior of soft clay under high-cycle, low-amplitude loading. The bounding surface could expand due to the hardening effect caused by contractive plastic deformation, and it could simultaneously shrink due to the weakening effect caused by both soil destructuration and excess pore water pressure. To further validate the proposed model, relevant triaxial tests were referenced. The consistent plastic deviatoric strain and excess pore water pressure from tests and the prediction confirmed the effectiveness of the proposed model. Following a comprehensive analysis of the varying internal variables during cyclic loading and a thorough investigation into the damage effects related to plastic strains, the model was considered capable of reasonably describing the structure destruction of soft soil to long-term cyclic load.

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)' with the development of plastic strains effectively represented the structural destruction process of soft clay under long-term cyclic loading conditions.