Comparative Study Between Plane Element And Ring Element Model To Determine Permeability Tensor For Three-dimensional Fracture Network

Rock mass is a geological structure widely distributed in nature.Due to the different distribution characteristics and combination forms of its structural planes,rock mass parameters and characteristics are extremely complicated.The complex rock mass characteristics cause the non-uniformity and anisotropy of the seepage characteristics in the rock mass.The seepage in the rock mass fissures affects the stability of the slope,the bearing capacity of the rock mass and the stability of the lining under load.In rock mass engineering such as civil engineering,hydropower Engineering,mining engineering and other related projects have a significant impact,so the analysis of the seepage law in fractured rock mass is a research topic of important significance.In practical engineering problems,the permeability tensor,a widely recognized and used physical quantity,is used to describe the law of fracture seepage.The fracture network model is a powerful tool for calculating the permeability tensor.With the results of field tests,the permeability tensor of rock mass seepage can be obtained macroscopically.At present,there have been more systematic studies on the shape and permeability of fractured rock masses at home and abroad.The commonly used discrete network models are divided into two types:pipe flow model and surface flow model.The pipe flow model is a simplification of the two-dimensional rock mass structural plane network,and the research in this area has been relatively mature;the surface flow model is a three-dimensional structural plane network,which can more accurately reflect the distribution characteristics of rock mass structural planes and has more practical significance.Based on the surface flow model,this paper establishes a three-dimensional fracture network,divides the fracture surface into triangular units for calculation,and obtains more accurate results of permeability analysis.Through research,the following research results are obtained.(1)Fractal dimension is a index to describe the distribution characteristics of rock mass cracks.The distribution of cracks in a rock mass is determined by joint position,joint density,joint trace length,dip angle and other factors.According to the Baecher model,a three-dimensional fracture network model is established,fractures are generated in the model,and the distribution of fractures in the simulated rock mass is obtained.Then,software such as ArcGIS is used to process the fracture images,and the box dimension method is used to calculate the fractal dimension.Then study the relationship between the number of cracks and the fractal dimension and the relationship between the number of cracks and the permeability coefficient when the fissure distribution in the ring element model is different,so as to study the second-order linear change between the fractal dimension and the permeability coefficient relationship.(2)The fractured disc in the fracture model is divided into meshes,nodes and elements are generated,and a three-dimensional structural surface element model is established.After setting the water head boundary and other parameters,the finite element method is used to calculate the seepage tensor to obtain the surface element model's seepage tensor,and the result is compared with the seepage tensor calculated by the ring element model,and the error between the two is calculated.The permeability coefficients are basically the same,and the average value of the difference in the main permeability directions is about 30°.It is proved that when the loop element model is used for seepage calculation,the calculation scale is reduced and the result is accurate.(3)The penetration ellipsoid is a graphical method to visually express the penetration tensor.In the three-dimensional model,the certainty of the penetration tensor can be judged according to whether the penetration tensor can form a penetration ellipsoid in the spatial rectangular coordinate system.In this paper,a permeation ellipsoid is generated according to the permeation tensor calculated by the surface element model,and compared with the permeation ellipsoid of the ring element model,it can be seen that the two permeation ellipsoids basically coincide.