Triaxial Testing Study On Gassy Sandy Soil Under Constant Shear Stress Paths

Natural gas hydrate is the most promising new strategic alternative energy in the21 st century.It is very easy to destroy its harsh conditions in the process of depressurization.Thus,the slope instability of submarine energy soil containing natural gas hydrate is induced,and then lead to the submarine landslide.On the one hand,natural gas accumulates in the pores of the energy soils because of the decomposition of natural gas hydrate,which decreases the saturation of the energy soils,but not less than 85%.And natural gas exists in the form of free bubbles in the energy soils,making the energy soil evolved into a kind of gassy soil.On the other hand,the release of the gas increases the pore pressure of the energy soils and reduces the effective stress.This process can be ideal as the constant shear drained or undrained stress path.So,in view of these two aspects,the triaxial test research of the gassy soil under constant shear stress path is carried out in order to provide some early experimental basis for safe mining of the natural gas hydrate.The main research contents and achievements are as follows:(1)Two preparation methods of samples of gassy sandy soils were discussed in the study,named “Improved Method of Gas Tube” and “Gas and Liquid Displacement”.The two methods are both achieved by upgrading on the basis of the GDS standard stress path triaxial testing system.The two methods have advantages and disadvantages.Considering the research time,research content and other factors,this study selects the “Improved Method of Gas Tube” for experimental research.(2)A lot of exploratory work has been carried out in the process of the experiment.The possible tolerances in the preparation stage of the triaxial test of gassy sandy soils are analyzed,and the corresponding solutions are put forward.The test schemes for gassy sandy soils under the constant shear stress path and the conventional stress path are designed.The whole process of the test is summed up.(3)A series of conventional triaxial compression tests were carried out to investigate the relationship between the strength and saturation of the gassy sandy soils in the dense state.By comparing with the existing research conclusions of the loose and medium-dense gassy sandy soils,it is concluded that the influence of gas onthe strength of gassy sandy soils is opposite because of the different densities.According to the undrained shear behavior of saturated sand under large deformation presented by Yoshimine,a "Weak Strain Softening-Dilatancy Mode" of undrained shear behavior of gassy sandy soils is added.(4)A series of constant shear drained or undrained triaxial tests of gassy sandy soils were carried out to investigate the rule of deformation of gassy sandy soils under different cell pressure,shear stress values,different densities,different saturations and different drained conditions.