| Fractures of different sizes and occurrences are usually developed in engineering rock masses after a long period of complex geological process.The complex fracture network formed by the interaction of these fractures is the main channel for fluid flow in the tight rock masses,which results in complex structural characteristics and mechanical properties of the fractured rock masses.Fractured rock masses will inevitably be encountered in large-scale engineering construction such as deep geothermal development,underground storage of CO2,geological disposal of high-level radioactive waste,etc.The three-dimensional（3D）spatial distribution law of fractures determines the flow characteristics of the underground fluid and thus affects the solute transport process,which is crucial to the safe construction and long-term efficient operation of the project.In addition,the calculation of representative elementary volume（REV）of fractured rock masses is of great significance for analyzing the mechanical and hydraulic properties of fractured rock masses under the framework of equivalent continuum mechanics.Therefore,it is necessary to conduct an in-depth study on the two basic problems:seepage characteristics and REV of fractured rock masses.This thesis focuses on the calculation and prediction of permeability and REV of complex fracture rock masses.The effects of geometric characteristics of fracture network on permeability and REV are analyzed using laboratory tests,theoretical analyses,and numerical calculations.Calculation models for predicting permeability and REV based on geometric characteristics of fracture network are proposed.Based on the Beishan high-level waste repository in Gansu Province,the prediction models are applied to project.The relevant results can provide criteria for selecting continuum model in seepage calculation,and provide an empirical equation for predicting permeability.The main research achievements are as follows:（1）In terms of the construction of complex fracture network numerical models and the validation of simulation methods,two-dimensional（2D）discrete fracture network（DFN）models are established in which the fracture lengths have fractal characteristics.By comparing the fracture length distribution in the model with the field statistical distribution data,the validity of the formula containing the fractal dimension used to generate the fracture length is proved.In addition,this thesis presents an equivalent pipe network（EPN）model for characterizing fluid flow through 3D fracture networks.The EPN model is extracted from an original 3D DFN model by connecting the center of the fracture and the central point of the intersecting line between fractures to solve the problems that too long solution time and too low efficiency are caused by meshing.The seepage characteristics of large-scale,high-density,complex 3D DFN can be efficiently analyzed by the EPN model.The validity of the proposed EPN modeling approach is verified via the comparisons of the calculated permeability with analytical solutions,simulation results in the literature,and penetration test results of the 3D printed specimen.（2）In terms of the determination of the REV of the fracture network,a method to estimate the REV and directional permeability by extracting regular polygon sub-models with different orientation angles and side lengths from an original discrete fracture network model is developed.The permeability in different directions can be calculated by the same sub-model under different boundary conditions,and the efficiency of permeability calculation is improved.The influence of the fractal dimension of fracture length distribution on the REV and equivalent directional permeability of fracture networks are systematically investigated.The influences of fracture length distributions on the permeability of fractured rock masses are calculated when the distribution of fracture length in the 3D fracture network model obeys the power law function,exponential function,and lognormal function,respectively.The REVs in different seepage directions are determined,and the influences of the geometric characteristics of the fracture network on the REV are analyzed.（3）In terms of permeability prediction of the 3D fracture network,the permeability of 3D DFNs with different geometric characteristics and multiple 2D cutting planes along the flow direction of specific 3D DFNs are calculated through a large number of numerical simulations.On this basis,the relationship between the permeability of 3D DFN and the permeability of 2D cutting planes and the geometric characteristics of 3D DFN is analyzed.However,the permeability of 2D cutting planes cannot be used to predict the permeability of 3D DFN when there are continuous flow paths in cutting planes.To solve the problem,a multi-variable regression function is proposed for predicting the dimensionless equivalent permeability of the 3D model using the geometric parameters of 3D DFN and the fractal dimension of the 2D cutting plane.（4）In terms of engineering application,a 3D DFN that can effectively characterize the spatial distribution of fractures in Beishan nuclear waste geological disposal site is generated.Through a large number of simulations,two multi-variable regression functions are proposed to predict the REV size and corresponding equivalent permeability using the geometric parameters of 2D cut planes which are parallel and perpendicular to the direction of hydraulic gradient variation.The functions provide a feasible method to preliminarily predict the REV of engineering rock masses based on geometric characteristics of surface outcrops of fractures,and provide a basis for the selection of theoretical models（equivalent continuous medium model,discrete fracture network model,dual medium model）in the analysis of engineering seepage and solute transport.Based on the equivalent pipe network method,this thesis systematically studies the evolution of seepage properties of 3D DFN with different geometric characteristics.Two empirical functions for fast predicting REV and equivalent permeability based on2D outcrop data of fractured rock masses are proposed.The functions successfully predict the seepage characteristics of engineering fractured rock masses relying on the Beishan high-level radioactive waste repository in Gansu Province.There are 110 figures,19 tables,and 240 references in this thesis. |
