Three-Dimensional Numerical Modeling Research On Rock Failure And Seepage Evolution Processes

Large-scale rock engineering projects may face extreme challenges such as high in-situ stress,complex geological environments,intense engineering actions,and adverse operational conditions.The numerical modeling techniques for studying the fracturing process of rocks and the seepage behavior of fractured rock masses hold significant theoretical and practical value for the safety construction and stable operation of engineering projects.Rock engineering problems are inherently three-dimensional in nature.Due to the heterogeneity and the discontinuity of rock masses,the application scenarios of two-dimensional models are very limited,except for a few special cases that allow for plane or axisymmetric simplification.Therefore,the research and development of three-dimensional modeling tools are essential.This research proposes a three-dimensional numerical model based on the rigid-body-spring method and discrete fracture network method and develops corresponding computer programs to investigate the three-dimensional deformation,cracking,failure processes of rocks,and the seepage behavior of fractured rock masses.Specifically,the main contributions are listed as follows:（1）Based on the Rigid-Body-Spring Method（RBSM）,a three-dimensional numerical model has been developed to simulate the full process of mechanical behavior of crack initiation,propagation,penetration,and failure of rocks under complex stress conditions.This model constructs distributed spring-set contacts within particle interfaces based on the Hammer integration strategy,endowing the model with the capability to describe the non-uniform aperture and stress fields caused by particle rotation.The model can reflect the characteristics of progressive cracking during the rock failure process without increasing the scale of the solution equations.Subsequently,the model was employed to investigate the strength characteristics,damage evolution,and macro-microscopic cracking mechanics of rocks under different three-dimensional stress conditions.The study examined the brittle-ductile transition and failure mode changes caused by confining pressure,as well as the influence of the intermediate principal stressσ₂ on the anisotropic deformation and fracturing behavior of rocks.To address the microscopic mechanism of the σ₂ effects in rock strength,a contact failure criterion considering lateral stress from localized cracking was established.This enhances the quantitative modeling capability of the RBSM for rock strength under three-dimensional stress conditions.（2）Based on the Discrete Fracture Network（DFN）method,a three-dimensional fracture-porous media seepage model,EMFN,has been established to simulate the seepage behavior within complex fractured rock masses.This model treats rocks as an assembly of impermeable blocks,with seepage only occurring between block interfaces.By this approach,one set of governing equations can be adopted to simultaneously describe the seepage behavior within both matrix and fractures,while naturally considering the flow exchange between them.An analytical microscopic parameter determination method is proposed for the three-dimensional Delaunay and Voronoi meshes,which enabled the model to exhibit high-precision macroscopic permeability and homogeneous isotropic local seepage behavior.Subsequently,the model is used to investigate the permeability characteristics of rock masses under complex fracture network conditions.The results show that under low fracture density conditions,the permeability of fractured rocks is significantly influenced by the connectivity of the fracture network and is accompanied by significant uncertainty.The continuum percolation theory,which takes scale effects into account,can effectively analyze the statistical laws of rock permeability.To deal with networks with high fracture density,a new semi-analytical model for the three-dimensional fractured rock mass permeability tensor is proposed based on the Snow’s permeability tensor.This model is suitable for rapidly assessing the anisotropic permeability of engineering fractured rock masses.（3）Based on the RBSM and DFN,a three-dimensional numerical model for rock permeability evolution has been established.To quantitatively describe the non-linear decrease of rock mass permeability under the influence of confining pressure,a functional relationship between the numerical model’s interface hydraulic conductivity and confining pressure is established,in which the local interface normal strain is taken as an intermediate parameter.In addition,an equivalent hydraulic aperture model was applied,remedying the model’s inadequacies in considering block elasticity and fracture roughness under the rigid particle description.Using the calibrated permeability evolution model,the full-process permeability evolution behavior of granite under different stress paths is investigated,with a focus on the effects of confining pressure and intermediate principal stress.The results show that the permeability of granite exhibits stage-wise changes during the loading process,which are closely related to the volumetric strain and cracking mode.Moreover,permeabilities in the three principal directions are influenced separately by the confining pressure effects and intermediate principal stress effects.This anisotropic permeability evolution behavior can be linked to the initiation,propagation,and coalescence processes of micro-fractures,as well as the competitive opening and closing behavior of fractures during the deformation process.