Multi-fields Coupling Analysis On Deep Rock Mass And Unloading Study On Underground Excavation

With the development of national economy, the exploration of underground space becomes deeper and deeper. Being in the conditions of high stress, temperature and seepage pressure, the mechanical behaviors of deep rock mass shows a quite difference from that of shallow rock mass. Because of the actions of various loadings, the complexity of rock medium, the uncertainty of perceptibility and a series of new mechanical properties of deep rock mass, the permeability, failure mode, strength and deformation characteristics under excavation and unloading are difficult to be interpreted with traditional theories reasonably. Consequently, it's essential to study the mechanical properties of deep rock mass under high stress, temperature and hydraulic pressure, together with the deformation regularity under excavation unloading. The content of this paper is summarized as fellows:1. Based on the study on the changes of the rock mass structure under high temperature, the expression of rock permeability and temperature is put forward with thermal stress as a bridge. According to the formula, permeability increased slowly with the rising of temperature, while increased rapidly when temperature reached a certain value. This temperature is threshold temperature and permeability of rock mass still increased rapidly when temperature beyond the threshold. The thresholds of different rock mass are different.2. Owing to the high in-site stress, permeability of deep rock mass is greatly affected by stress, for the variation of stress will lead to the variation of the effective width of fracture which will influence the permeability. Based on the variation of effective width of fracture, the expression of rock mass permeability is brought forward in the case of medium and high normal stress and shear stress. And, the formula of crack seepage and stress under three dimension stresses are obtained by virtue of seepage regularity. Additionally, the coupling equation of the stress and the seepage tensor of the joint sets and cross joint sets is deduced through tensor analysis.3. According to the mass conservation law, linear momentum equilibrium principle and energy conservation law, the governing equations for coupled Thermo-Hydro-Mechanical (THM) behaviors in deep rock mass are derived. The governing equations include those for thermal, hydrological and mechanical fields, and are based on the conservation equations for fluid and solid masses, linear momentum and energy. The interaction of THM field and the response of the deep rock mass to the THM coupling are analyzed qualitatively. Furthermore, mechanical characteristics of deep rock mass in THM coupling are simulated and corresponding qualitative research is performed. 4. Triaxial compression test and unloading test are carried out to study the failure modes and mechanical behaviors of rock mass under different confining pressure. Unloading test is distinguished from the conventional test which is by loading. The strains of rock mass under loading and unloading both increases with the growth of principal stress difference. However, with the same principal stress difference, volumetric dilatancy caused by unloading is greater than that of loading, as well as the damage of rock mass. Strength of rock mass in unloading tests is less than it in loading tests. And shear failure always occurs in rock mass under loading condition while tensional shear failure under unloading.5. The process and extent of microstructures damage inside the samples caused by loading and unloading are different under the same stress state. Failures of surrounding rock mass in the underground cavity mainly result in the unloading during excavation and which will lead to the accumulation and developing of the damage gradually. Due to different stress paths, the mechanical behaviors under unloading are distinguished from those in the tests of loading under the same stress condition. So, it is not appropriate to perform the stability evaluation of unloading rock mass according to the traditional strength criterion and it is also unreasonable to apply the mechanical parameters obtained from the loading test to the calculation of excavation engineering. Strength of rock mass will be weakened as pore water pressure will redound to the developing and coalescence of fractures and keep the confining pressure within limits. The decreasing effect pore water pressure to rock mass strength will be lowered with the increase of confining pressure.6. Excavation is a process of stress adjustment and unloading ceaselessly. Numerical software is used to simulate the excavation unloading. Variations of displacement and stress in different stages as well as the deformation regularity of different parts of rock mass are discussed. By comparison, results of numerical simulation are well in accordance with the monitoring data obtained from the multi-points extensometers and bolt stress meters which are installed in the underground power plant of Qiaoqi hydroelectric power station. And it is feasible to appraise the stability and control the deformation of surrounding rock mass in underground engineering with numerical simulation results.