The system research aimed at activation laws caused by coal mining in fault footwall based on the rock strata me- chanics theory in this paper. According to the three-belt characteristics of overlying strata above mined-out area, the fault acti- vation effect was divided into the following three periods： the initial activation, the excessive activation and the ultimate activation. Corresponding activation criterion was also deduced by the mechanics theory. Results of engineering example and numerical analysis show that the activation criterion can reflect the activation laws of fault caused by coal mining.
By considering the effect of hydraulic pressure filled in wing crack and the connected part of main crack on the stress intensity factor at wing crack tip, a new wing crack model exerted by hydraulic pressure and far field stresses was proposed. By introducing the equivalent crack length lcq of wing crack, two terms make up the stress intensity factor K1 at wing crack tip： one is the component K（1） for a single isolated straight wing crack of length 2l subjected to hydraulic pressure in wing crack and far field stresses, and the other is the component K1^（2） due to the effective shear stress induced by the presence of the equivalent main crack. The FEM model of wing crack propagation subjected to hydraulic pressure and far field stresses was also established according to different side pressure coefficients and hydraulic pressures in crack. The result shows that a good agreement is found between theoretical model of wing crack proposed and finite element method （FEM）. In theory, an unstable crack propagation is shown if there is high hydraulic pressure and lateral tension. The wing crack model proposed can provide references for studying on hydraulic fracturing in rock masses.
