Understanding the evolution of mining-induced fractures using physical and numerical modeling

The overburden rock strata fractures and collapses as the extraction of a longwall panel. Knowledge of the extents and characteristics of the fractured zone is important to evaluating many mining-induced issues including abutment pressure relief, methane production, and ground subsidence prediction. In this study, a physical model was initially created to produce the process of longwall mining with great attention focused on the caving and fracturing process of the overburden strata. A numerical model was then created according to the geological and geotechnical conditions of the physical model. Both the physical and numerical model successfully captured periodic weighting as the longwall face advances. It is found that periodic weighting does not just involve the local immediate and main roof that composes a cantilever beam right behind the longwall face, it may also involve the rupture of a rock bridge in the overburden strata above the fractured zone. The rock bridge forms and ruptures periodically as longwall mining proceeds. The permeability of a given region in the fractured zone can be evaluated by measuring the area of the voids and fractures within the region. The permeability increases as fracturing and collapse of rock mass within the region and then decreases as the collapsed rock mass compacts. The greater the horizontal-to-vertical stress ratio, the lower the maximum permeability. The more competent the main roof, the lower the maximum permeability.