A prediction model and numerical simulation of the location of the longwall face during the highest possible failure period of gob gas ventholes

The failure of gob gas ventholes (GGVs) is the main cause of shortening the highly efficient drainage period of GGVs. Therefore, in order to improve the stability of GGVs, it is of great significance to investigate the location of a longwall face when a GGV is in the highest possible failure period. Based on the probability integral method, this paper obtained a modified equation for calculating the amount of horizontal movement of an overlying stratum during longwall mining, and in this equation, the influence of the distance between the overlying stratum and the coal seam on the subsidence coefficient of the stratum was taken into account. In addition, this paper established a prediction model of the location of the working face during the highest possible failure period of GGVs. Furthermore, taking Zhangbei coal mine as an example, this paper investigated the distribution of the maximal amount of relative horizontal movement (RHM) of the contiguous strata along a venthole and explored the changing characteristics of the maximal amount of RHM of the overlying contiguous strata as the distance of the working face passing GGV increases. In addition, this paper also researched the distribution of shear stress exerted on casing along a venthole during coal mining and the changing characteristics of shear stress as working face advances using numerical simulations. The results showed that when the longwall face passed the venthole by approximately 30-70 m, GGVs were in the highest possible failure period, which is the early stage of longwall face passing GGV. Zhangbei and Xieqiao coal mines in Huainan mining area of China carried out the engineering application of extracting depresssurized gas with GGVs. The measured failure data of ventholes 11418-1, 11418-2, 1242(1)-2 and 1242(1)-3 validated the prediction.