Step change mechanism of static mechanical response of coal-rock in stope under structural transient excitation

Revealing the occurrence mechanism of coal-rock mechanics response in the stope is theoretical basis for the prevention and control of coal-rock disasters. By adopting a combination of field measurements, simulation experiments, and theoretical analysis, the occurrence mechanism of step changes in the static mechanical response of coal-rock in the stope was systematically studied through the construction of static mechanical models at two steady states before and after structural transient. The results show that the mechanical response of coal-rock in the stope is closely related to the transient characteristics of overburden spatial structure. During the first weighting, the structural transient of overlying strata exhibits symmetry, leading to a symmetrical mechanical response. During periodic weighting, the structural transient region is mainly concentrated on working face side, resulting in a mechanical response that is also concentrated in coal-rock masses on the working face side. The transient change in the load transfer path of the overlying strata, induced by structural transient change, causes different instantaneous loading and unloading conditions for coal-rock in various areas of the stope. It leads to significant differences in the mechanical responses of coal-rock in these areas. Among them, the instantaneous fracture of lower hard rock layer forms instantaneous unloading on the lower surface of upper hard rock layer, causing it to sink instantly. Meanwhile, the instantaneous unloading of internal forces along fracture surface in lower hard rock layer, coupled with the transient loading induced by transferring support load from structural transient region towards goaf, causes the instantaneous rebound in some areas near goaf side of lower hard rock layer. The instantaneous subsidence of some areas in the deep surrounding rock. Affected by this, the bearing capacity of the upper surface on the lower hard rock layer increases instantaneously. It leads the peak support pressure suddenly increases, and its position transitions towards the goaf side. Meanwhile, affected by the fracture unloading and subsidence-rebound of the lower hard rock strata, the bearing capacity on the coal body decreases instantaneously. The peak value of abutment pressure on coal body decreases sharply, and its position transitions to the depth of coal body. With the increase of transfer load in structural transient region, the rebound amount of lower hard rock decreases, while the bearing capacity of the coal body increases. With the increase of action range of transfer load, the rebound amount of lower hard rock increases. The step change of coal-rock load decreases, while the range of step change increases.