# Permeability Enhancement Induced by Fracture Shear Dilation During Close-Range Coal Seam Mining
## 4 Evolution of Fractures of Close-Distance Coal Seam Group
### Simulation Models  
The 3 DEC three-dimensional discrete element method was used to simulate and analyze the development law and evolution process of the overburden fracture field in the goaf, which can provide guidance for drilling and gas drainage design [42]. Figure 2 shows the numerical geometric model structure with working face 28202 as the prototype. Figure 2: Geometric model of rock strata. Coal is a sedimentary rock type, which has experienced a long deposition process. Many bedding planes and cleats in the coal mass result in anisotropic mechanical properties. These fractures typically can be divided into two types: face cleat and butt cleat. These two sets of coal cleats are perpendicular or nearly perpendicular to each other and intersect the coal to form an interconnected network throughout a coal-bed. Fractured sedimentary rock also contains two domains, fractures, and an adjoining rock matrix. Overburden movement is associated with the problem of separation fracture, and 3 DEC V4.1 software based on the discrete element has a good advantage in simulating discontinuous media. The software represents discontinuous media by discrete blocks, the discontinuities are treated as boundary surfaces between the blocks, and large displacement and rotation of the blocks along the discontinuities are allowed. Individual blocks behave as either rigid or deformable material. Deformable blocks are subdivided into a mesh of finite difference elements, and each element responds according to a prescribed linear or nonlinear stress-strain law. The relative motion of the discontinuities is also governed by linear or nonlinear force-displacement relations for movement in both the normal and shear directions. 3 DEC has several built-in material behavior models, for both the intact blocks and the discontinuities, that permit the simulation of response representative of discontinuous geologic, or similar, materials. 3 DEC is based on a Lagrangian calculation scheme that is well suited to model the large movements and deformations of a blocky system. Early distinct element codes assumed that blocks were rigid. However, the importance of including block deformability has become recognized, particularly for stability analyses of underground openings and studies of seismic response of buried structures. The Mohr-Coulomb model was formed based on the maximum shear stress to predict shear damage, and is widely used. However, the Mohr-Coulomb (M-C) strength criterion is a linear criterion, which cannot accurately describe the nonlinear properties of the rock strength under different confining pressures. Therefore, this paper attempts to incorporate the nonlinear response into the M-C strength criterion to overcome the above limitations. The modified Mohr-Coulomb strength criterion and the statistical damage constitutive model were proposed in the simulation. The damage constitutive model was used to characterize coal block damage and deformation, and a code file \"Damage_mechanical_model.dll\" for the damage model was developed using C++ UDM (User-Defined constitutive Models) subroutine. The Mohr-Coulomb slip model is used for fracture deformation in this paper. The Coulomb slip model requires six parameters: normal and shear stiffness, friction angle, cohesion, tensile strength, and dilation angle. The permeability distribution is calculated based on exported data of shear stress for all joints. The flow diagram of the calculation cycle in 3 DEC is shown in Figure 3.  
In the 3 DEC model, two sets of cleats are defined based on the geological condition in the study area. The mechanical properties of the rock mass and the discontinuities for different lithologies are provided in Table 1. The Mohr-Coulomb failure criterion was adopted to assess fault reactivation or fault slippage. The original cleats are subjected to high shear stress and tensile stress in the fractured zone under the coupling action of the overburden stress and mining-induced stress. This results in significant fracture development in the fractured zone.