Deformation law and failure mechanism of coal-rock-like combined body under uniaxial loading

The deformation and failure process of coal-rock combined body shows different characteristics from the sin- gle body because of the different composition structure. In order to explore the deformation and failure laws of the over- all failure process,especially the radial deformation and expansion characteristics at the different positions of the com- bined body,the static strain testing system and RMT rock mechanics testing system were used to carry out uniaxial compression test for coal-rock-like combined body. The strain characteristics of the combined specimen before its ulti- mate state were obtained by the radial strain gauges attached at different positions along the axial direction and the axi- al strain gauges in the middle of the coal or rock section,and the expansion characteristics of the coal section of combined sample were analyzed. By peeling and fragmenting the damaged specimen,the crack propagation and failure mechanism during the overall instability were described. The results show that the radial strains at different positions in combined body is different,the strains from large to small order are near the end of the coal section,in the middle of the coal section,at the joint surface,and in the middle of the rock section,indicating that the deformation of coal and rock section are influenced by each other. The radial deformation of the coal section is obviously restricted by the rock section of combined body in the pre-peak stage of stress-strain curve,while the rock section of combined body is obvi- ously subjected to the splitting and stretching action of coal section in the post-peak stage of stress-strain curve. Under the restriction of the rock section,the expansion stage of the coal section is relatively flat compared with the single coal body,and the coal section which has a smaller volume strain at expansion point can withstand longer-lasting loads after entering the expansion stage. The bursting tendency of combined body is weaker than that of single body because of the different failure mechanisms. In the excavation of the coal-rock combined rock mass,the coal body with a distance from the joint surface should be supported intensively,and the failure of rock mass caused by the fracture propagation of coal through the jointed surface should be avoided in order to ensure the overall stability of surrounding rock mass.