Numerical simulation and verification of unloading damage evolution characteristics of coal and rock mass during deep mining

To study the evolution characteristics of unloading damage and deformation of underlying coal and rock dur- ing the protective layer mining period,FLAC3D numerical simulation was conducted based on the actual conditions of Baode Coal Mine in Shanxi Province,China. In-situ investigations were also adopted to verify the simulation results on stress,deformation and plastic property evolution laws. The results show that the stress of the protected layer increases firstly,then decreases and after which increases again during the mining process. The spatial distribution of stress on underlying coal and rock shows an obvious “ O” shape. Influenced by the protective layer mining,the measurement points in underlying coal seam experienced four stages including original rock stress,stress concentration,mining pres- sure relief and stress recovery. The ratio of the maximum stress concentration factor and the minimum unloading stress is a constant value,and the occurrence of the two extreme values shares the same time. The stress concentration factor in front of the working face and the unloading ratio behind the working face all change reciprocally,and the change pe- riod is related to the cycle stress from the working face. In this paper,the maximum stress concentration factor is about 1. 32,at which time the value of the z-direction stress at the measuring point is the largest. The minimum pressure re- lief ratio is about 4. 4% and the z-direction stress value at the measuring point is the smallest at this time and the pres- sure relief effect is also the best. Under the effects of changing stress,the deformation of the protected layer presents the basic rule of compres-sion-recovery-expansion-retraction cycle. The final state maintains a certain expansion de- formation,which corre-sponds to the stress zoning. According to the different deformation characteristics,the underlying coal seam can be divided into original rock state zone,compression deformation zone,pressure relief expansion zone and de-formation recovery zone. The example in this paper shows that the maximum expansion deformation of No. 11 coal seam was 0. 6% . The cracks at the monitoring points developed well and the permeability increased significantly, which was conducive to gas pressure relief and extraction. Under the influence of stress changes,the plastic area in- creased in triaxial directions first,then increased in x-axis direction but y-axis and z-axis direction kept stable. With the advance of working face,the plastic zone of coal body in protected seam increased continuously in the x-axis direc- tion. By measuring the in-situ stress and expansion deformation of No. 11 coal in the mining process of 81307 working face in Baode Coal Mine,the numerical simulation results of the evolution characteristics of unloading damage and de- formation during deep mining were verified. The variation laws of in-situ stress and expansion deformation of No. 11 coal agree well with those of numerical simulation.