Characteristics of stress evolution on the thrust fault plane during the coal mining

The relationship between complex geological structure and the occurrence of coal burst in coal mine is a major concern in coal mining. It is of theoretical significance and important engineering guidance to study the mechanism and precursory information of instability fault slip and coal burst for the safe and efficient production of coal mines. The stress evolution is focused on the fault plane by the methods of physical and numerical simulation to study the precur- sory information of instability fault slip during the coal mining. Based on the engineering background of the 21221 coal mining face at Yima mining area,Henan Province,China,a physical model with F16 thrust fault structure is constructed under the horizontal loading. The dynamical evolution characteristics of the shear stress on the fault sliding plane under mining disturbance are analyzed according to the stress sensors installed on the fault plane. The acoustic emission method is employed to monitor the acoustic emission events distributed on the fault plane. In addition,a numerical coal seam mining model with F16 thrust fault structure is also established to study the rock fracture development and strain energy release characteristics in the fault influenced zone. According the physical and numerical results,the precursory information of instability fault slip is summarized by investigating the stress distribution,acoustic emission monitoring and strain energy release on the fault plane. It is revealed that three stages in terms of shear stress evolution and acous- tic emission events can be presented in the physical and numerical simulation,including horizontal pressure application stage,mining stage and fault slip stage. It can be seen that the acoustic emission events sharply increase since the strain energy continually accumulates on the fault plane during the horizontal pressure application stage. In the coal seam mining stage,due to the fault is frequently disturbed by coal seam mining,the normal stress and shear stress on the fault plane present different variation. In the earlier mining stage,the normal stress is greater than shear stress. As the advancement of coal mining,the change of normal stress and shear stress are exactly opposite. The normal stress gradually decreases while the shear stress gradually increases. In addition,the shear stress on the fault plane experi- ences the process of gradual decrease and sharp increase when the mining face is approaching the fault structure. In the fault slip stage,strain energy is accumulated again on the fault plane before the occurrence of fault slip but the acoustic emission events disappear. However,due to the continually energy release,the acoustic emission events sharp- ly increase when the fault slip and the collapse of rock strata occur. The relative less or constant number of acoustic e- mission events before the instability fault slip is consistent with the micro-seismic events in micro-seismic monitoring in which the number of micro-seismic events decrease before the occurrence of coal burst and then suddenly increase af- ter the coal burst. Therefore,the sharp increase of shear stress and acoustic emission events during the constant process can be regarded as the precursory information of instability fault slip.