开采扰动下逆冲断层滑动面应力场演化特征

复杂地质构造与煤矿冲击地压关系密切,也是世界范围内煤矿安全开采的重要课题,断层滑移失稳诱发冲击地压的机理及前兆信息是煤炭安全高效开采的理论前提和重要保障。工作面开采扰动时,以断层滑动面上应力场演化特征为研究对象,通过相似模拟和数值模拟,研究断层滑移失稳时的前兆信息。以义马千秋矿21221工作面为工程背景,建立了F_(16)逆冲断层赋存条件下的水平加载相似材料模型,运用应力监测和声发射监测的手段,分析了开采扰动下断层滑动面切应力的动态演化特征,研究了工作面开采过程中断层面上声发射事件数的分布规律;通过建立断层赋存条件下的工作面开采的数值模型,研究了断层区域岩层裂隙发育情况,分析了断层滑移失稳时切应力分布和能量释放的动态演化特征;通过相似模拟和数值模拟中应力场、声发射和能量场的分布规律,总结了断层滑移失稳的前兆信息。研究结果表明:断层滑动面切应力和声发射分布特征表现出3个不同的阶段,即水平载荷施加阶段,工作面开采阶段和断层滑动失稳阶段。载荷施加阶段,断层滑动面不断积聚能量,声发射事件数激增;工作面开采过程中,断层频繁受开采扰动,正应力与切应力两者变化不同步。开采初期,正应力处于较大值而切应力较小。随着工作面的开采,正应力与切应力的变化情况相反,正应力逐渐降低,切应力逐步增大。工作面接近断层时,断层滑动面切应力表现为逐步降低的过程中陡然增加的特征;断层滑动前期,断层构造在失稳前仍然积聚大量应变能,声发射事件数出现较少或者缺失的现象;当岩层垮落诱发断层滑移失稳时,能量在不断释放;断层区域内声发射事件数则在近似恒定不变时突然激增。断层滑移失稳前声发射数较少或恒定不变与微震监测中的"缺震"现象较为吻合,即微震事件数在断层滑移失稳前或冲击地压发生前突然出现减少,发生后激增的现象。因此,断层滑动面应力和声发射事件数的激增变化特征可作为断层滑移失稳的前兆信息。

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 precursory 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 acoustic 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 experiences 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 sharply increase when the fault slip and the collapse of rock strata occur.The relative less or constant number of acoustic emission 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 after 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.