Control effect of thickness ratio of soft and hard composite coal seam on unloading failure

Abstract

In the engineering practices of coal seam mining and gas drainage, the unloading damage of soft and hard composite coal seam is a common problem faced by surrounding rock support and coal seam enhancement. In order to explore the mechanical properties of the composite coal seam during the unloading process and the energy evolution law during the failure process, based on the energy dissipation equation and the coal rock damage model, we applied PFC numerical simulation and laboratory tests, and studied the deformation and failure modes of coal, mechanical properties and energy dissipation and acoustic emission laws of Xintian coal body under the layered thickness; finally, the control law of unloading failure of 4# soft and hard composite coal seam thickness ratio in Xintian Coal Mine is obtained and verified by hydraulic punching on site. The results show that: under triaxial compression, the effect of different thickness ratios on the deformation and failure modes of the composite coal body can be seen in the transference of the fracture zone, the stress-strain curve of the composite coal body taking the thickness ratio of 0.45 （soft layer thickness/composite coal seam thickness） as the dividing point causes a “stepped” smooth and violent change, which is related to the size of the confining pressure; the essence of the unloading failure of the composite coal body is the fracture of the bonding bonds between the particles. What’s more, the size and the breaking of the cohesive bonds between the coal particles have a positive effect. The results of in-situ hydraulic punching are consistent with the simulation results.

Keywords

• soft-hard composite coal seam
• pfc numerical simulation
• coal seam unloading
• acoustic emission
• thickness ratio