CFDEM simulation on the linkage effect between floor faults failure and roof collapse in ultra-thick coal seam

The ultra-thick coal seam has a large mining space and a wide range of overburden disturbance. The strong disturbance of loading and unloading caused by the roof collapse can easily exacerbate the failure of floor faults. Studying the mechanism of the linkage effect between water inrush from the floor fault and roof collapse in ultra-thick coal seam through numerical simulation is the basis for water hazard prevention and control. The key lies in understanding the coupling characteristics of progressive rock failure and fracture flow under loading and unloading. In this study, the evolution equations of tension and shear damage under loading and unloading were constructed, and combined with the yield criterion and plastic potential function with effective partial/spherical stress as the basic variable, the plastic-damage constitutive model of intact rock was obtained. The relationship between plastic displacement and strength deterioration during tension/shear/mixed loading and unloading was established, using the square tensile shear stress and B-K criterion as initial and complete fracture criteria, the fracture constitutive model of non-penetrating fracture was formed. The separation, compression and shear criteria of discrete rock blocks were put forward, and the extrusion, shear friction constitutive and dilation equations between discrete blocks were established based on experimental data. Based on the conservation of mass/momentum, the equation of state, and combined with the method of fluid volume and immersion boundary, the simulation theory of gas-water two-phase flow in the fractured rock mass was formed. Thus, the CFDEM numerical calculation program was developed, and the plastic damage, fracture, extrusion/friction, and fluid properties under loading and unloading were assigned to solid elements (rock blocks), cohesion elements (non-penetrating cracks), contact pairs (penetrating cracks), and Euler units (water and gas) respectively. According to the engineering geological conditions in the northern mining area of Ningwu coalfield, a numerical calculation model of linkage effect between water inrush from floor fault and roof collapse in extra-thick coal seam was established. The results indicate that ① the CFDEM coupling program and corresponding theoretical models can numerically achieve the transformation of overlying rocks and floor faults in ultra-thick coal seams from quasi-continuous to discrete rock, as well as the migration of groundwater in fractures. ② Under simulated conditions, the mining fractures envelope line of the fault floor of the ultra-thick coal seam is w-shaped, with the deepest point exceeding 55 m located on the fault and its hanging wall, the shallowest point 23 m located on the footwall of the fault, and the failure depth at the intact floor is 24−36 m, which has already connected to the Ordovician limestone aquifer. ③ Secondary damage is commonly observed on the floor of ultra-thick coal seam. The failure depth of the intact floor at the advanced working face is 24.0−29.3 m, but it generally increases to 31.5−36.0 m in the goaf. The total cracks opening of the fault and its hanging wall at the advanced working face is 0.34−0.86 m, but it rapidly increases to 3.6 m in the goaf, forming a dominant channel for water inrush. ④ The linkage effect of water inrush from floor fault and roof collapse is rooted in the collapse instability of key strata in overlying strata, the sinking of masonry beams and the secondary fracture, which leads to the secondary damage of floor and aggravates the risk of water inrush.