Dynamic Distribution of Longwall Mining-Induced Voids in Overlying Strata of a Coalbed

Mining-induced voids provide essential channels for heat and mass transfer in disturbed strata. A group of dynamic distribution models of voidage was proposed to investigate the heterogeneous distribution and dynamic change of longwall mining-induced voids in overlying strata of a coalbed. It indicated that the voidage distribution has an evident periodicity and heterogeneity due to the periodical and inconsistent fractured subsidence of each key stratum with different thicknesses, strengths, and loads. Disturbed by longwall mining, the degree of difficulty of development increases successively from the pores and from the bed-separation fissures to the fractured fissures. In the caved zone, the porosity distribution along the coalbed strike and dip always presents a U-type appearance. In the fractured zone, the distribution shape of the transverse voidage alters from a narrow inverted U-type to a wide M-type, but the longitudinal voidage distribution always presents as an M-type from the narrow to the wide with time. The voids successively open and then gradually close from deep to shallow strata, and along mining operations they advance direction and present an arch-type distribution called the void-rich arch. The peak voidage near the mining face periodically extends forward with the advance of the mining operations and is slightly higher than near the starting line. The models and results might be applied to guide coalbed methane extraction, water influx prevention, coal fire prevention and control, and so on.