The geometry of the joint determines the mechanical properties of the rock mass and is one of the key factors affecting the failure mode of surrounding rock masses. In this paper, a new rough discrete fractures network (RDFN) characterization method based on the Fourier transform method was proposed. The unified characterization of the complex geometric fracture network was achieved by changing the different Fourier series values, which further improved the characterization method of the RDFN model. A discrete element numerical calculation model of the complex RDFN model was established by combining MATLAB with PFC code. Numerical simulation of the anisotropic mechanical properties was performed for the RDFN model with a complex joint network. Based on the results, the geometry of the joint network has a significant influence on the strength and failure patterns of jointed rock masses. The failure modes of the opening are highly affected by the orientation of the fracture sets. The existence of the rough fracture sets could influence the failure area of different excavation situations. The study findings provide a new characterization method for the RDFN model and a new characterization approach for stability analysis of complex jointed rock masses.

Coal pillar dams are an important component of the water storage bodies of underground reservoirs. Influenced by the overlying rock pressure and water seepage, the stability of the coal pillar dam is one of the key factors affecting the stability of underground reservoirs. In this paper, an anisotropic seepage mechanical model of a coal pillar dam under plane strain was established to study the seepage stress coupling mechanism of underground reservoir No. 4 in the Daliuta Coal Mine using the COMSOL Multiphysics code. The stress field and seepage field of the coal pillar dam body were analyzed, and the influence of the principal direction of the mechanical properties of the coal pillar on the stress field, seepage field, and damaged areas of the coal pillar and goaf were discussed. According to the results, the anisotropy of the coal pillar dam body is one of the most significant factors when the principal direction of mechanical properties is θ = 45° or θ = 135°. The coal pillar damage area reaches a maximum value accounting for nearly 50%. The shear stress of the coal pillar reaches 4.69 MPa, which attains the maximum value when the principal direction angle is 90°. With increasing depth, the damaged area of the coal pillar gradually expands in the scenario of θ = 0°. When the depth increases to 160 m, the coal pillar undergoes penetration failure. In conclusion, the principal direction is the main factor affecting the stress field, seepage field displacement field, and energy evolution of the model. The anisotropy model of the equivalent continuum can account for the influence of the coal pillar structure surface, which could provide an analytical model for the stability of rock engineering.

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