In order to optimize the efficiency and safety of gas hydrate extraction, it is essential to develop a credible constitutive model for sands containing hydrates. A model incorporating both cementation and damage was constructed to describe the behavior of hydrate-bearing cemented sand. This model is based on the critical state theory and builds upon previous studies. The damage factor Ds is incorporated to consider soil degradation and the reduction in hydrate cementation, as described by plastic shear strain. A computer program was developed to simulate the mechanisms of cementation and damage evolution, as well as the stress-strain curves of hydrate-bearing cemented sand. The results indicate that the model replicates the mechanical behavior of soil cementation and soil deterioration caused by impairment well. By comparing the theoretical curves with the experimental data, the compliance of the model was calculated to be more than 90 percent. The new state-dependent elasto-plastic constitutive model based on cementation and damage of hydrate-bearing cemented sand could provide vital guidance for the construction of deep-buried tunnels, extraction of hydrocarbon compounds, and development of resources.

Soil -structure interfaces in shallow foundations, embankments and other geo-systems are usually unsaturated, which has great influences to the performance of geo-structures. However, researches on unsaturated soilstructure interfaces are still kept in very limited numbers, particularly for the theoretical part. This paper proposes a state -dependent model for unsaturated soil -structure interfaces based on two independent stress state variables: net stress and suction. To consider the effects of initial state, stress level, and suction, a state -dependent dilatancy and suction -induced relocation of critical state line is introduced, and a rigorous, exhaustive and schematic calibration procedure of the proposed model is presented. Thereafter, simulations of direct shear tests on sand -steel, sand-geotextile, silt -steel and soil-cement interfaces with different initial states, boundary conditions and suctions are carried out. Results show that, shear strengths of interfaces increase with initial density, normal stiffness and suction, and the strain softening and dilatancy behavior are significantly enhanced by initial density and suction, while normal stiffness makes a contrary contribution. More importantly, calculations of the proposed model are fairly consistent with measurements, indicating such features of strain softening, stateand suction -dependent dilatancy, as well as the improved peak and critical shear strengths, are well captured by the model.