Numerical Investigation of 3D Distribution of Mining-Induced Fractures in Response to Longwall Mining

Extraction of coal mine methane is widely used in complex geological conditions. It does not only effectively prevent potential gas disasters, but it also alleviates energy crisis of the world. However, the high-efficiency extraction of coal mine methane is positively correlated with accurate positioning of mining-induced fracture-rich areas (relatively intensive fracture areas), which is yet to be fully understood (in particular on a 3D scale). In this paper, a simple fracture constitutive model was incorporated within a continuum code, and an innovative approach to fracture generation was proposed to get the fracture morphology. First, the code was tested against the uniaxial compression and true 3D experiments and it was shown to be capable of simulating fracture initiation and propagation on either 2D or 3D scale. This code then was used successfully to 3D longwall mining, and the numerical results were well in keeping with the field monitoring and physical modeling. The numerical results revealed that the fractured zones exhibit a 3D elliptic paraboloid shape, and mining-induced shear fractures, which dominated over the tensile fractures, had a 3D annular shape above the overlying strata, whereas a ribbon shape was observed for tensile fractures. A fracture-rich area was formed gradually at the center (exactly closer to the cut) above the gob, implying that the collapse mainly occurred in the gob roof rather than in the gob sidewall. The strengthening effect of mining-induced fractures on the permeability in longitudinal direction was markedly stronger than in transverse directions. The obtained results also suggest that the mining direction should be parallel to the maximum horizontal stress. Overall, the proposed model provides a promising tool for solving 3D complex engineering issues.