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.

Shanghai soft clay is a typical marine clay with specific structural characteristics. However, the effects of surface surcharge loading on structured soft clay and existing tunnels remain unclear. In this study, the effect of repeated surface surcharge loading on tunnel displacement was numerically investigated, considering the structural characteristics of Shanghai Layer 4 soft clay. An elastoplastic constitutive model (Shanghai model) that describes the mechanical properties and structural characteristics of natural clay was used to simulate the soil response. The results indicate that the first cycle of repeated surface surcharge loading had the greatest effect on the displacement of the tunnel and overlying clay; subsequent loading cycles further increased the displacement. With repeated surface surcharge loading, the displacement of the overlying clay exhibited a significant decreasing trend with increasing depth. The maximum excess pore pressure of the clay exhibited a decreasing trend with increasing loading time. The displacements of the tunnel and overlying clay were greater when the effects of the soil structure were considered. However, the initial degree of the structure had no significant effect on the accumulation of excess pore pressure when the surface surcharge loading did not reach the ultimate bearing capacity of the clay.

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.