Permeability Evaluation Of Fractured Rock Mass Based On Seepage Path And Numerical Simulation

Rock mass forms a discrete fracture network in its inside by numerous discontinuous structural planes. As the fracture network may construct the potential pathways for the flow of underground fluid and radionuclide, the analysis of fluid flow in fracture rock mass is very important and necessary to geotechnical and hydrogeological engineering. The space distribution of the fractures is extremely complex, and most of the present work on the analysis of rock mass permeability has been based on the Discrete Fracture Network (DFN) model that is constructed by using Monte-Carlo method. In this work, the information of fractures including distribution of fractures and major fractures sets were obtained by the field investigation of a radioactive waste disposal site in Gansu province, China. Then, a two-dimensional fracture network model built upon the actual data was developed for permeability analysis, while the as-mentioned three-dimensional DFN model was utilized to model the seepage path as well as the permeability analysis also.The three-dimensional coordinates of fractures were obtained by using close-range photogrammetry and then converted into local two-dimensional coordinates. The two-dimensional discrete fracture network was constructed using discrete element method. Three independent regions were chosen to do the permeability analysis. By this method, the fitted permeability ellipsoid and the corresponding permeability tensor and its principal value and direction of those three regions were attained.For the three-dimensional DFN model, the study area was divided into several sub-areas, and the three-dimensional DFN model was constructed according to Monte-Carlo method and the volume density and fracture information of each sub-area. Then, on the VS2010 platform, the relatively short seepage paths of the entire study area along three principal directions were evaluated by using DFS algorithm, while the shortest ones were obtained by using Dijkstra algorithm.To gain the permeability characteristic of the study area, the calculation program was compiled by means of theory of graph to calculate the permeability coefficient of the relatively short path, as well as the shortest seepage path, along three principal directions. Moreover, through a computation of the permeable coefficient along the equivalent direction of sub-models with different scales and directions, the REV value and the equivalent permeable coefficient tensor of this three-dimension flow model were obtained. By this way, it lay a foundation for evaluating the fracture permeability in the study area.