Study On Model And Stability Of Surrounding Rock Of Large Underground Caverns Under High Geo-Stress Condition

Recent years, stability analysis of large underground caverns has become a urgent key technology with the building and programming of many national strategic deep project and, such as hydropower station, strategic oil reserve, CO2 underground isolation, deep nuclear waste disposal, and so on. At the same time, special nonlinear mechanical behavior of wall rock is showed in the process of excavation under high geo-stress, which challenges the theory and methods of traditional rock mechanics.For the purpose of meeting the demand of national infrastructure and promoting the development of rock mechanics subject, the paper takes high geo-stress and hard rock as the main theme based on the background engineering of LaXiWa Hydropower Station underground caverns and auxiliary tunnel of JinPing II hydropower tunnels. Through roundly study of the rock constitutive model, indicator of rock's deformation and failure evaluation, obtaining method of rock mass mechanical parameters, steps of numerical simulation for engineering application, the paper forms a relatively integrated stability analysis solution of underground caverns under high geo-stress environment. In sum, the main work and conclusion including:1. Aiming at the problem that the traditional elasto-plastic constitutive models have not been successful in predicting the depth and extent of brittle failed of hard rocks under high geo-stress condition, a new hard rock constitutive model—rock mass deterioration model (RDM) is put forward in which the mechanical parameters can be dynamic update with the plastic strain. The RDM takes the damage of rock as a process of propagating of micro-cracks and the relationship between mechanical parameters deterioration and damage degree represented by general plastic strain of EDZ is established.The model accords with the essential fact that the parameters in EDZ have changed and can describe the elasto-brittle mechanical broken behavior of hard rock. Both fitting of triaxial compression test stress-strain curves of granite and simulation of EDZ of testing tunnel in JinPing II Hydropower Station prove that the model is suitable for numerical calculation of underground hard rock caverns under high geo-stress condition.2. Since it is the vector changing that the principal stress's direction and magnitude are altered with the redistributing of secondary stress in the process of excavation, a stress loosen coefficient which is used to evaluate the relaxation scope of wall rock is proposed, and a index, named principle stress rotation dissipation work (SRDW), is proposed to predict and judge the caverns'stability and damage spatial characteristics. Including the rock mass elastic strain energy, the paper summarizes the critical period in sequential dig of tunnel. The rock burst statistical analysis in auxiliary tunnel K14– K15 of JinPing II hydropower tunnels implies these indexes are applied.3. In response to the unfaithful value of mechanical parameters in numerical calculation, an integrated analysis method is builded up which is composed of sensitivity identification of mechanical parameters, parameters global optimization algorithm by GA-ANN and verification of displacement sequence gray association. The method absorbs two type measured information including depth of EDZ and deformation of wall rock and considers comprehensively the different effect of parameters related to deformation and plastic zone, the strong global research capability and the necessary of inspection.4. Considering the fact that people's perception of objective reality is a process of gradual spiral, a new method of numerical simulating application technology for wall rock stability analysis in large caverns, named PFP (preparation–feedback– prediction) is promoted from epistemological perspective. That is composed of preparative study before the construction of engineering, feedback analysis in previous part of construction and prediction of behind part of construction. The method fully utilizes the incremental information in the stepping excavation process, absorbs the advantages of numerical simulation analysis, and realizes the conversion between practice to theory and theory to practice.5. Finally, applying the research fruits mentioned above to the stability analysis of LaXiWa Hydropower Station underground large caverns, the paper summarizes the general rules of hard wall rock mechanical behaviors under high geo-stress condition in stepping excavation from stress field, displacement field, elastic energy, stress rotation dissipation work and plastic zone. The results have provided guidance for rational design and safety construction of the project.