Numerical Simulation And Inverse Modeling Of Coupled Hydromechanical Processes In Geomaterials

The hydromechanical coupling process of geomaterials plays a critical role in a variety of engineering applications,such as hydropower engineering,disaster prevention and mitigation engineering,nuclear waste disposal,geologic storage of oil/gas and geothermal resource development.Due to the complexity of material composition and structural characteristics of geomaterials,the studies of mathematical description,numerical methods,and control technology of hydromechanical coupling process are still developing and improving.In this thesis,the mathematical model and numerical simulation method of the fluid flow and deformation in geomaterials are established;the return mapping scheme of elasto-plastic deformation and hydraulic hysteresis is developed;the inverse modeling procedure for evaluating the anisotropic hydraulic conductivity and its variation induced by excavation is proposed;finally the proposed methodology is adopted in the inverse modeling of the groundwater flow at the Jinping-I Hydropower Station during the reservoir impounding.This study is of great theoretical value for the multi-field coupling analysis,and of great practical use for the control of coupling process.The major achievements obtained in this thesis are summarized as follows:(1)Based on the constitutive model with consideration of inter-particle bonding as well as the water retention curve and hydraulic conductivity model for deformable unsaturated soils,this thesis proposed a numerical formulation for modeling the coupled flow-deformation processes with hydraulic hysteresis.Using the finite element method and the generalized-a time integration scheme,a return mapping scheme is developed to integrate the water retention curve model and the elasto-plastic model simultaneously within a time step,and the deformation-dependent nature of the water retention curve is considered rigorously by modifying the coefficient matrices in the discretized governing equations.The performance and efficiency of the proposed numerical formulation is validated by two existing laboratory tests and a computational example,demonstrating better performance and convergence of the proposed formulation.The proposed procedure is then applied for modeling the coupled flow-deformation processes in a soil slope under rain infiltration.The simulated results reveal the significant effects of hydraulic hysteresis on the coupled water-air two-phase flow and elasto-plastic deformation processes.The solid deformation and the evolution of the shear band would be remarkably overestimated,and the slope failure would be early predicted when neglecting the hydraulic hysteresis.(2)In the framework of hydromechanical coupling analysis of geomaterials,this thesis proposed an inverse modeling procedure for evaluating the anisotropic hydraulic conductivity and its variation induced by excavation in fractured rocks by integrating a strain-dependent hydraulic conductivity model.The time-series measurements of both hydraulic head and discharge were used to construct the objective function,which was solved with a combined method of orthogonal design,transient groundwater flow modeling,artificial neural network and genetic algorithm-based optimization for reducing the computational cost.By considering the evolution of anisotropic hydraulic conductivity in fractured rocks,the effectiveness and reliability of the inverse modeling results were successfully improved.(3)This thesis presents a case study on the transient groundwater flow behavior at the dam site of Jinping-I Hydropower Project.Utilizing the time series measurements of both hydraulic head and discharge,the inverse modeling procedure for evaluating the anisotropic hydraulic conductivity in fractured rocks was adopted to back-calculate the permeability of the foundation rocks.Based on the investigation results of water quality analysis,digital borehole imaging and tunnel geological mapping,the geological structures that lead to the leakage event in the lowest drainage tunnel in the left bank were then identified.Besides,the excavation-induced relaxation effects and their impacts on the hydraulic conductivity variations and seepage behaviors in the surrounding rocks of the underground caverns were also assessed with site characterization data and numerical simulations.On this basis,the transient seepage flow behaviors in the dam foundation and around the underground caverns during the reservoir impounding were well modeled.It has been demonstrated that the seepage control system is effective in lowering the groundwater level and limiting the amount of seepage in the dam foundation,and the leakage event does not pose a threat to the safety of the dam.