PFC numerical analysis on crack initiation mechanism of fracture grouting in top of Ordovician limestone

The advanced regional grouting is an effective means to prevent and control the water hazard of limestone in coal seam floor, and the fracture grouting projects is a key link to determine the effect of the grouting in floor against water hazard. However, due to the unclear understanding of the fracture initiation mechanism of the injected limestone stratum, the grouting pressure, grout water-cement ratio and other grouting parameters in the process of fracture grouting are not effectively controlled. Thus, it is difficult to guarantee the grouting effect. In order to solve the above problems, numerical simulation calculation was conducted for the fracture grouting in top of Ordovician limestone with the Particle Flow Code (PFC) numerical simulation software, considering the grout water-cement ratio, in-situ stress, and the dip angle and width of weak surface and crack. The results show that: The fracture initiation pressure decreases with the increase of the grout water-cement ratio (1∶1, 2∶1, 3∶1), but increases with the decrease of the difference between the maximum and minimum principal stresses (9, 12 and 15 MPa) under the weak surface and fracture conditions. Besides, the crack initiation pressure decreases with the increase of the width of the weak surface or crack (3, 8, 15 mm) and the angle between the weak surface/crack and the maximum principal stress (30°, 60°, 90°). The initiated cracks propagate along the direction parallel to the maximum principal stress. The fracture initiation pressure decreases with the decrease of strength ratio of weak surface to matrix (0.30, 0.03). In case of weak surface with high strength, the fracture initiation pressure is greater than that under the fracture condition, and the cracks propagate along the matrix. When the grouting hole is located at both ends of the weak surface or in the middle of the crack, the cracks are initiated along the weak surface or at both ends of the crack. When the grouting hole is located in the middle of the weak surface, the cracks propagate along the matrix in the middle position. Generally, the research results are helpful to the effective control of fracture grouting in the advanced regional for floor reconstruction against water hazard, and guide the selection of grouting parameters, thus providing technical support for the development of deep coal resources.