Numerical Modeling Of Coupled Thermo-Mechanical-Damage Behaviors For Brittle Rocks

Geological disposal of radioactive wastes is a multi-disciplinary issue of importance for national interest. It stimulates many challenging scientific and technical issues in the rock mechanics engineering field, for example, the problem of thermo-hydro-mechanical (THM) coupling in the complex rock masses and their occurrence environment. As an important aspect of the issue of multi-field coupling for complex rock masses, the problem of thermo-mechanical-damage coupling for brittle rocks has been paid close attention by scholars throughout the world. The dissertation is based on Task B of the international cooperating project DECOVALEX-2011"Coupled mechanical thermal loading of hard rocks", and supported by National Science Foundation of China "Evolution mechanism of the transmission characteristics of surrounding rocks in the geological disposal of radioactive wastes with consideration of multi-field coupling effects (No.51179136)", and also supported by the commission of science technology and industry for national defense project "Safety assessment of the geological disposal of high-level radioactive wastes based on THMC coupling". This dissertation aims to study the damage process and the evolution of thermal conductivity induced by the thermo-mechanical coupling of brittle rock mass by means of the theory study and experimental validating, and specifically to study the evolution of damage process and thermal conductivity of brittle rocks under thermo-mechanical coupling. The main research work and achievements are summarized as follows:(1) The macro-microscopic damage mechanism of brittle rocks subjected to coupled excavation-induced and thermal-induced stresses is studied. In this paper, a micromechanical homogenization method is first introduced and the macro effective elastic tensors of cracked brittle rocks are studied. Futhermore, regarding cracked brittle rocks as a thermodynamic system, the basic internal variables which describes the process of microscopic damage for rocks are proposed. In the framework of irreversible thermodynamics, a micro-mechanical damage model based on the multi-scale homogenization method is established, and the thermal effect is taken into account in this model. Finally, the effectiveness and rationality of the proposed model is validated by by comparing with uniaxial/triaxial test results of an intact Aspo diorite sample.(2) With consideration of multi-field coupling, the evolution mechanism of the effective thermal conductivity characteristics of brittle rocks is studied in this paper. Factors which can affect the effective thermal conductivity of rocks are analyzed, including the mineral composition, porosity, saturated fluid, degree of saturation, temperature, stress and anisotropy, etc. The Wiener and Hashin-Shtrikman bounds for the effective thermal conductivity characteristics of rocks are introduced. Regarding the Representative Volume Element(RVE) of rocks as the mixture of infinite matrix and inhomogeneous ellipsoid inclusions and starting from the basic equations of the thermal conductivity problem, the general expression of the effective thermal conductivity is derived by taking use of the microscopic homogenization method, and moreover, based on the basic solution of the problem of ellipsoid inclusions, the effective thermal conductivity tensors are derived by using different homogenization schemes. The voigt and Reuss bounds of the effective thermal conductivity tensors are derived, the isotropy characteristics of the effective thermal conductivity are discussed and analyzed and the internal relations between the effective thermal conductivity characteristics and the damage process is established. The evolution law of the effective thermal conductivity characteristics of the Sweden Aspo diorite during the loading process is studied by taking use of this model. Moreover, this model is also adopted to study the effective thermal conductivity characteristics of the Gaomiaozi bentonite in our country.(3) The numerical analysis method of thermal-mechanical-damage coupling for brittle rocks is investigated. From the perspective of irreversible thermodynamics, the evolution of damage and the effective thermal conductivity characteristics are considered under conditions of temperature and stress. Based on the momentum conservation equation and the energy conservation equation and adopting the continuum analysis model, the governing equation of coupled thermo-mechanical-damage process for brittle rocks are established. The governing equations is discretized by employing the Galerkin finite element method in spatial domain and the finite difference scheme in temporal domain, the numerical model is established and a computer code is developed.(4) Specific numerical model for the thermo-mechanical-damage coupling with the Task B of international coopertive DECOVALEX-2011project is simulated. The Aspo Pillar Stability Experiment(APSE) conducted at the Aspo Hard Rock Laboratory(HRL), Sweden, is first introduced. Then, in order to review the progressive failure process of the rock pillar subjected to coupled excavation-induced and thermal-induced stresses, the distribution of temperature field, stress field and deformation field of APSE rock pillar is model, taking into account the perturbation caused by both the engineering excavation and thermal stress.