New Insights into Failure Behaviors of Tectonic Coal Under Triaxial Conditions Using Reconstituted Coal Specimens

Coal and gas outburst is tightly related to the existence of tectonic coal. Because large blocks of tectonic coals are difficult to be collected, reconstituted coal specimens are often used to study the mechanical properties. In this study, reconstituted coal specimens were prepared under different external forces. Then, triaxial compression tests were conducted under different confining pressures. The deviatoric stress, volumetric strain, axial strain and dilatancy ratio were analyzed. From the laboratory tests, the cohesion of reconstituted coal is positively related to the applied external force. Bonds are developed between coal particles. It is found that the deviatoric stress mainly contributes to bond breakage and internal friction. Under low confining pressure testing condition, the specimen is in over-consolidation state. The failure process is similar to the failure of intact coal. The deviatoric stress reaches the peak value. Then a strain-softening effect is observed with the bond breakage. But for normal consolidation scenario, the bond-breakage stress is smaller than the maximum internal friction force. With the increase of deviatoric stress, the shear stress exceeds the threshold limit value of bond breakage first and then, it exceeds the maximum internal force. Before the new unstable fracture development, extensive bond breakage happens. When the shear stress exceeds the maximum friction force, internal slippage occurs with sharp increases of dilatancy ratio. From this study, the failure behaviors of reconstituted coal are closely related to the stress conditions. The understanding of tectonic coal failure process is enhanced.