Numerical simulation on evolution law of overburden fractures and surface cracks in crossing ditch mining of shallow coal seam

Crossing ditch mining of shallow coal seam is common in northern Shaanxi mining area, which seriously threatens the safe production and the sound ecological development. The mining-induced overburden fractures and surface cracks are developed as a result of the extension and expansion of microcracks in rock mass. In order to more systematically study the evolution law of overburden microcracks in crossing ditch mining of shallow coal seam, a numerical model of particle flow in coal mining was established with the Particle Flow Code (PFC) numerical simulation platform according to the field ditch profile and borehole data under the background of 125203 working face of Anshan Coal Mine. Meanwhile, the development characteristics, quantity change laws and force chain evolution characteristics of overburden microcracks were simulated and analyzed. Besides, the development laws of microcracks and the development mechanism of surface cracks were revealed. The results show that: the development of overburden microcracks is characterized by the dynamic evolution process of “generation-expansion and extension-aggregation in groups-penetrating to form cracks” during the crossing ditch mining of shallow coal seam. According to the basic development characteristics and distribution law of microcracks, the whole process of development could be divided into the 3 stages of the discontinuous skipping stage, the continuous penetrating stage and the horizontal expansion stage. Besides, the number of overburden microcracks increases with the increasing of advancing distance of coal working face, with exponential growth characteristics shown in the discontinuous skipping and continuous penetrating stages of microcracks, in which 547 and 2 867 microcracks are developed cumulatively, and the overburden microcracks gradually develop to the surface. Linear growth characteristics are shown in the horizontal expansion stage, in which 11 705 microcracks are developed cumulatively, and the number of microcracks deceases with the increasing height of rock stratum. During the evolution process of the overburden force chain, the failure of strong viscous force chain leads to the development of microcracks and local stress concentration. The strong viscous force chain arch gradually breaks down from bottom to top and penetrates to the surface, resulting in the extension and development of microcracks to the surface to form ground fissures. In addition, strong force chain areas are formed on both sides and the tip of microcracks that do not penetrate through the rock layers and on both sides of surface cracks, and weak (no) force chain areas are formed at the microcracks. In general, the research results could provide theoretical guidance for the crossing ditch mining of sallow coal seam and the prevention and control of damage to overburden and surface in northern Shaanxi mining area.