Macro-meso mechanical properties of gas hydrate bearing coal under triaxial compression with flexible boundary condition

To explore the influence of confining pressure on the macro-meso mechanical characteristics of gas hydrate bearing coal (GHBC) under different boundary conditions, the biaxial discrete element tests were carried out subjected to the confining pressures of 12, 16 and 20 MPa for GHBC with saturation of 80%. Firstly, the biaxial numerical models of GHBC were established for flexible and rigid boundaries, using the linear model of the rolling resistance and the parallel bonding model. These numerical models incorporated the influences of particle shape effect, the hydrate cementation and the heat-shrinkable pipe. Then, the reliability of the numerical model was verified, by comparing with the indoor test results (stress-strain curves, bulk strain curves, internal friction angle cohesion and specimen failure modes). It is found that the flexible boundary can better reflect the deviatoric, stress-axial strain, the shear expansion and the strength characteristics of the sample. Based on the established numerical model, the roles of the confining pressure and the boundary condition on the macro-meso mechanical properties of GHBC were clarified from the perspectives of the internal displacement field, the mean mechanical coordination number, the mean porosity, the contact force chain and the hydrate bond failure. The results show that: ① with the increase of confining pressure, the numerical sample with the rigid boundary mostly exhibits the single inclined plane shear failure, while that with the flexible boundary varies from the single fork shear failure to the single inclined plane shear failure. ② With the increase of confining pressure, for the two boundaries, the mean mechanical coordination numbers increase and the mean porosity decrease, leading to a denser and higher strength of the sample. ③ With the increase of confining pressure, the normal contact force between particles continues to increase, and the sample strength increases. The normal contact force distributed near the axial direction increases, while that near the horizontal direction varies weakly. The higher the confining pressure is, the more difference between the vertical and the horizontal normal contact forces have, the more prominent the anisotropy is. The normal contact force increases by 54.50%, at the flexible boundary, and increases by 45.70%, at the peak strength point, with the confining pressure increasing from 12 MPa to 20 MPa. ④ Under different confining pressures and boundary conditions, the samples fail with two different failure modes, tensile and shear. The samples mainly crack from the shear between the hydrate and coal. The research results reveal the mechanism of the influence of confining pressure on the strength deformation and failure of GHBC on the mesoscale.