Line Element Model For Fluid Flow In Three Dimensional Fracture Network To Determine Permeability Tensor

Seepage of rock mass attracts more and more attention in the geotechnical engineering, such as hydropower, traffic, mining, oil and other engineering fields. It also has a significant effect on slope stability, tunnel lining stability and bearing capacity of dam foundation. The discrete fracture network is more accurately and objectively than the equivalent continuous fracture network on describing the actual seepage law in rock mass because the seepage occurred mainly in fractured network. The key and practicability of discrete fracture network numerical simulation is to accurately obtain the parameters of the fracture probability distribution, to determine the permeability tensor and to reduce the computation scale. Using the assumption of disc fracture, this paper studies the high order accurate expression form of diameter distribution according to the relationship of the joint trace length and diameter distribution. The distribution is high-orderly approach the analytic function of diameter distribution. Basing on the channel morphology of the flow in the facture, the line element model of flow network is put forward. Using the permeability tensor corrected by engineering practice as the objective function, the ratio of the equivalent line element diameter and the aperture is got by the improved genetic algorithm. So we can obtain the equivalent permeability tensor of any rock mass which is given the occurrence. In addition, the calculation scale effect on the permeability tensor solution is researched. The details are as follows.Fracture size is the only parameter during numerical simulation which cannot be obtained by field measurement. The only can be abtained is the distribution form of trace length by field measurement. Assuming fracture shape is disk, the size of fracture can be expressed as disk diameter. The high order accurate expression form of diameter distribution is obtained according to the relationship of the joint trace length and diameter distribution. The numerical simulation results show that for actual trace length subjected to exponential distribution, the simulation trace length is manifested as logarithmic normal distribution form in the low order approximation, and as exponential distribution in high order approximation, which is approaching the given distribution curve.The fracture network is simulated according to the joint statistics parameters by field measurements and related evolution. Using the Depth First Search(DFS) method, the seepage channel is got by dispelling the fractures which cannot contribute to the seepage. Based on the intersection relations between fractures, the rock fracture is generalized to a three-dimensional fracture network line element model, whose nodes are the crack disk center and the cracks intersection midpoint, thus elements are the lines between the disc center and the intersection midpoint. Using the permeability tensor corrected by engineering practice as the objective function, the ratio of the equivalent line element diameter and the aperture is fitted as 11.5069 by improved genetic algorithm. As the same time, the rock mass equivalent permeability tensor which is given the occurrence is got. Comparring with the permeability tensor by using the fracture geometry parameter tensor method, the discrete fracture network simulation method is more accurate. In order to make the simulation of fracture network has more practical value, this paper researches on the efffection of reduction computation scale and the corresponding error. For the case, when the reduced scale below 0.8, the error of permeability principal value and main direction is small.