Study on Mechanical Property of Coal-Rock Combination Under Different Unloading Confining Pressure Rate

In order to study the failure characteristics of coal-rock assemblage under the condition of unloading confining pressure. This study carries out the unloading confining pressure tests of coal-rock combination under three different pressure rates. The stress-strain curves of coal-rock combination in tests were observed, and the failure, deformation, and strength characteristics were analyzed. The result shows the combination has an obvious volume expansion phenomenon dominated by the simple shear failure of the coal part in the process of unloading the confining pressure. The rock part starts to damage with the increment of the confining pressure value and the unloading rate. The fracture angle of the sample is proportional to the initial confining pressure and inversely proportional to the unloading confining pressure rate; the fragmentation and intensity degree of coal-rock combination are significantly different under different test conditions. The fragmentation degree under the unloading confining pressure rate of 0.03 MPa/s, 0.06 MPa/s, 0.09 MPa/s is increased progressively, and the opposite trend appears on intensity degree. Axial deformation is dominant in the axial compression loading stage. The axial and circumferential deformations increase slowly at the stage of constant axial pressure, and the axial deformation is relatively larger. The circumferential deformation increases sharply at the stage of decreasing axial pressure. The internal friction angle is in direct proportion to the unloading rate, the cohesion and residual strength are in inverse proportion to the unloading rate, and the residual strength is in direct proportion to the initial confining pressure.

Automated summary

This study conducted unloading confining pressure tests on coal-rock combinations under three different pressure rates. The stress-strain curves of the combinations were observed, and their failure, deformation, and strength characteristics were analyzed. The results showed that the combinations exhibited obvious volume expansion due to simple shear failure of the coal during unloading of the confining pressure. The rock part started to damage with increasing confining pressure value and unloading rate. The fracture angle was proportional to the initial confining pressure and inversely proportional to the unloading confining pressure rate, while the fragmentation and intensity degree varied significantly under different test conditions.