Simulation study on the evolution of water-conducting fractures in the overlying rock during continuous mining of multi-thick coal seams across double faults based on FDEM
Authors: Mai Longquan , Li Hao , Bai Haibo , Kang Zhiqin , Yang Zebin 
Published: September 2023

Abstract
The study of the height regularity of the development of water-conducting fissures in the mining overburden is the basis for preventing water in coal mines and mining coal under water bodies. The key to the accuracy of its numerical results lies in the realization of the continuous mining process and the transformation of quasi-continuum, random-shaped discrete bodies and their coupled mechanical response characteristics.. The elastoplastic theoretical basis was introduced under pure tension and pure shear fracture modes, and a complete rock mechanics elastoplastic constitutive was constructed. Combined with the Benzeggagh-Kenane fracture criterion, the rock mixed fracture mechanics of quasi-continuous rock mass under complex tension and shear stresses was obtained. Through this constitutive, the critical fracture displacement δF under tension and shear load is obtained, and the contact state of the rock mass is judged by δF; combined with literature data and Sargin's research results, the conditions of separation, compression, and shear friction of the discrete rock mass are derived constitutive equation. On this basis, a finite element-discrete element (FDEM) numerical program was compiled to realize the transformation process of quasi-continuum and random-shaped discrete bodies; a VUSDFLD multi-coal seam continuous mining algorithm subroutine was compiled to realize continuous mining of multi-thick coal seams. Under the condition that multiple thick coal seams pass through double faults, the evolution of water-conducting cracks in the overlying rock as the working face advances is simulated and studied. The results show that: ①The continuous deformation-fracture-discrete block interaction mechanical constitutive relationship of the mining overburden is established, the finite element-discrete element (FDEM) numerical program formed and the VUSDFLD multi-coal seam continuous mining algorithm subroutine compiled to achieve It realizes the continuous mining of multiple coal seams and completely reproduces the highly developed process of water-conducting fracture zones under the repeated disturbance of multi-thick coal seams crossing fault mining. ② The toughness of sandstone-mudstone itself is low, the surface energy of cracks is low, and it is prone to fracture and collapse, and cracks are prone to occur. Disasters are more likely to occur during coal seam mining. At the same time, repeated disturbances caused by the mining of thick coal seams will cause the original cracks to further develop and cause water inrush accidents. The development height of the water-conducting fracture zone obtained by numerical simulation is 216~226m, and the mining and fracture ratio is 14.1. Compared with the experimental results, the error in the development height of the water-conducting fracture zone obtained by numerical simulation is 10%. 3When the working face advances to the fault level, stress concentration will occur in the overlying rock on the fault zone, and the concentration will be greater than at conventional locations. is obvious, leading to local cracking, cracking and the formation of fissures in the roof rock mass. The density of strike fractures increases significantly; double faults make this effect more obvious.