Laboratory tests on undisturbed and remolded Shanghai clay were conducted with a novel mixed boundary true triaxial apparatus. The strength and deformation behaviors of the structured soft clay at different Lode angles were carefully investigated. The relationship between the shear stress ratio and the dilatancy ratio of the clay in three different stress spaces, that is p–q space, transformed stress (TS) space, and tij modified stress space, is discussed in detail. In particular, the influences of the stress path and the structure of nondisturbed clay on the strength, deformation, and failure criterion of Shanghai clay were quantitatively evaluated. The shear strength and dilatancy of undisturbed/remolded clay decreases with the increase of the Lode angle. It is also found that the shear strength and dilatancy of the undisturbed clay are larger than those of the remolded clay, while the influence of the structure on the dilatancy decreases with an increase in the Lode angle. In the π-plane, the maximum strength of Shanghai clay under different stress paths generally obeys the spatial mobilized plane failure criterion. The coincidence of the stress–dilatancy relationship given by the TS and tij stress spaces with the test results is much better than that given by p–q stress space. Furthermore, the linearity of the plastic potential curves of the undisturbed/remolded clay is more evident in tij stress space.

The pile leg-mat foundation is a novel composite foundation designed for jack-up offshore platforms. The seismic response of such foundations in sandy seabeds is an issue that requires significant attention. In this study, shaking table tests were conducted under various input seismic waves to investigate the dynamic response of a seabed platform system subjected to seismic loads. The effects of the peak ground acceleration (PGA), seismic wave frequency, wave type, pile leg insertion depth, and pile length on the response of the seabed platform system were investigated. The results indicated that as the PGA increased, the acceleration response of the soil and platform model increased, increasing the accumulation of excess pore water pressure and the horizontal displacement of the platform model. The acceleration and horizontal displacement responses of the seismic wave with lower frequencies were higher than those with higher frequencies. The peak acceleration amplification factors under the measured and regular seismic waves showed significant differences, with the dynamic response of regular waves to the platform model being pronounced. Under seismic waves with low PGAs, the horizontal displacement of the platform model decreased with increasing pile leg insertion depth or pile length.

The mechanical behaviors of structured soft clay in consolidation tests and undrained triaxial shear tests were investigated, revealing evident structural properties of undisturbed Ningbo clay. Based on the experimental results, the limitation of the original constitutive model in simulating the structural properties of clay were discussed, and the evolution rule for the structure in the original constitutive model was revised. The parameter A that represented the ratio of plastic deviatoric strain to plastic volumetric strain was proposed to account for the distinct differences in the plastic deviatoric and volumetric strains on structural degradation. This modification enhances the ability of the constitutive model to accurately simulate the mechanical behaviors of structured soft clay. The performance of the modified constitutive model was verified by the consolidation and triaxial shear tests. Furthermore, the structure degradation of undisturbed Ningbo clay was investigated. For undisturbed Ningbo clay, the plastic volumetric strain plays a major role in causing progressive structural degradation compared to the plastic deviatoric strain.