Gas-driven fracture and failure of coal and rock-like materials

Abstract

Coal and rocks are porous materials containing various pores and voids. These pores and voids can be filled with gases during their formation at depth. The gases in coal would expand and cause fractures and failure after underground excavation, mining, and tunnelling.

To investigate the formation of gas inside the coal and rocks, a self-designed experimental apparatus has been designed. Hydrogen peroxide (H2O2) is used to generate gas (O2) during the hardening of cement-based materials, which is to simulate the formation of gas inside sedimentary rocks. Compaction is also applied to be the assumed in-situ stress. After hardening, rock-like solids are formed containing compressed gas. It has been found that fractures can occur in the rock-like solids containing compressed gas after disturbance.

To study the progressive gas-driven fractures, a self-designed experimental apparatus has been employed with coal briquettes. Gas decompression rate for coal briquettes containing gas dominates mainly the occurrence and intensity of fragmentation. A low gas decompression rate only causes deformation of coal samples slightly while a high gas decompression rate leads to fracture and fragmentation. The evolution of the fracture and fragmentation of coal samples corresponded to the strain changes. According to the relationship between the decompressed gas pressure and the strain of the coal sample, the critical gas pressures can be determined for the fracture and fragmentation of coal material. The major energy of the fragmentation in the experiments is the expansion energy of the free state gas inside the coal briquettes other than the adsorbed gas pressure in coal. Water content was also found to affect the outburst strongly. The critical minimum gas pressure in the coal briquettes with high water content becomes much higher than that in the coal briquettes with low water content. Water occupies the voids instead of gas, thus the gas content is not sufficient to induce a significant fracture.

Another experimental apparatus has been designed to investigate the propagation of coal and gas mixtures during outbursts. The experimental results demonstrate shock waves are generated with rapid flying of coal-gas mixtures and recorded with high-frequency gas pressure sensors at different locations along the roadway-like pipe. Stronger shock waves occur in outbursts of coal-gas mixtures in comparison with pure gas eruption for the same gas content and gas pressure. The amplitudes of outbursts are tens of times larger than those of pure gas eruption. After outbursts, pulverised coal particles erupted and scattered along the roadway-like pipe. The segregation of coal depositions was observed and quantified. According to the particle size distributions of coal depositions, the characteristic diameters, and uniformity coefficient of coal deposition were calculated and discussed. It can be concluded that more coal particles of larger size could tend to erupt during an outburst and larger coal particles move further.

A numerical investigation into gas-driven fractures and fragmentation of coal was also performed using combined finite-discrete element method. In the simulation, the gas-driven progressive fracture and failure were observed. The fractures occur at the location with a high gas pressure gradient.