A review on the interaction mechanism between coal bursts and fault structure instability from the perspective of multi physical field

The study on the interaction mechanism between coal bursts and fault structure instability from the perspective of multi physical field has great scientific significance and practical value for ensuring safe and efficient mining of coal resources. Human excavation activities can disturb fault structures formed by intense compression such as crustal movement. A large amount of elastic strain energy accumulated during the fault formation will be released due to the reactivation and instability of fault structure, which is bound to induce dynamic disasters such as coal bursts. The geological occurrence environment of fault structure when a coal burst happens is introduced from the point of view of reasons and the state of fault structure formation, and the high level stress environment. The model of the occurrence of coal burst related to fault structure is summarized. Two key scientific problems are concluded: the mechanical mechanism of fault slip under mining disturbance and the precursor characteristics of multi physical field of coal burst induced by fault instability. A physical model with the existence of fault structure and extra thick rock strata is established with the mining activities considered. A point by point integration calculated methodology of fault relative slip is deduced. The concept of fault structure potential energy characterizing the correlation between the sliding friction of fault and energy release is proposed. The dynamic evolution characteristics of fault structural displacement field, stress field and energy field under mining disturbance are analyzed. The characteristics of fault instability and the mechanical mechanism of fault induced coal burst are systematically explained. The results show that a nonlinear dynamic evolution from local static slip to global violent slip can be observed on fault structure under the mining disturbance. Meanwhile, the relative displacement between the fault hanging wall and footwall changes from slow slip to sudden and sharp slip under the mining disturbance, and the maximum displacement on the fault suddenly moves from the top of fault to the middle near the coal seam, which causes serious damage to mining safety. The evolution of the normal stress and shear stress is from synchronous development to differentiated appearance, that is, the normal stress increases and the shear stress decreases or the shear stress increases and the normal stress decreases. Under the lateral pressure and overburden subsidence pressure, the transformation from maximum sliding static friction to sliding instability friction occurs at the interaction zone between fault structure and overlying extra thick rock. The abrupt change of fault displacement field and the differentiation of stress field both occur during this transformation. The appearance of the maximum displacement on the fault plane indicates the severe slips of fault structure. Before the fault instability occurs, the fault structure remains relatively stable because the overlying extra thick rock near the fault is not easy to be fractured, and the fault structure potential energy is accumulated for a long time but not released. The monitoring data shows that the number of micro seismic events, acoustic emission events and electromagnetic radiation intensity exist the missing phenomenon. The value of fault structure potential energy suddenly increases from almost zero after the fault instability occurs, and the severe fault slips and coal bursts are usually accompanied. In conclusion, the coupling effect of overlying strata movement and fault slip is the main cause of fault slip under the condition of existence of fault structure, the high level stress environment and mining disturbance. The evolution characteristics of multiple physical fields can be used as the precursory characteristics of coal burst, such as the sudden change from a steady state of displacement field, the differentiation between normal and shear stress of stress field, and the sharp increase and hysteresis of energy field.