Construction Mechanics Behavior Of Large Scale Hydropower Station Underground Caverns

21st century is the age of underground engineering. A large number of underground caverns appear as the development of economy and the advance of science and technology, and these caverns have the tendency of deep situated, big span space and high side wall. Large-scale underground powerhouse and several other chambers and tunnels make up of huge underground caverns. During the construction of these chambers and tunnels, not only will we face complex geological conditions, complex procedures are always appeared in front of us. Different constructional procedure will make the surrounding rock mass undergo different stress path, which will affect the final stability of the surrounding rock mass differently, therefore it's a complex mechanical procedure. With the background of the underground caverns of Xiangjiaba hydropower station which the author of this thesis experienced, based on nonlinear theory, the research work was carried out from several aspects, such as in-situ geological investigation, structural design, mechanical test of rock mass, theoretical analysis, in-situ construction, numerical modeling and in-situ monitoring, et al., systematic and an in-deep study was performed of the underground caverns about the optimization before construction and mechanical behavior during construction, and the main results can be summarized as following:(1) According to the in-situ geological investigation, indoor-outdoor test of rock mass and in-situ ground stress test result, with stress regression method, this thesis back analyzed the initial ground stress field of the Xiangjiaba hydropower station underground caverns with the three dimensional discrete element program 3DEC for the first time, and acquired the main infect factors and distribution of regularities, which can be used for latter engineering computation and monitoring analysis;(2) Character of energy dissipation of underground caverns was studied during construction, and energy method was used for evaluating the optimization of underground engineering constructional procedure for the first time. The energy method, together with the deformation character, stress and plastic zone distribution character of surrounding rock mass, all of these were used for synthetically evaluating the state of surrounding rock mass of several excavation schemes, and through comparing these schemes, select the best one. Based on the selected excavation scheme, computation and comprehensive evaluation were carried out for evaluating the four drafted anchoring optimization schemes, and the best one was selected. All these results were used in practice;(3) The large area underground powerhouse has the character of big span space, deep buried with complex geological condition and constructional procedure, incorporated with in-situ monitoring results, the complex mechanical character of the surrounding rock mass with several working faces pushing forward during construction was carefully studied using the three dimensional discrete element method. Results such as the surrounding rock mass'deformation character, magnitude, dip direction and dip angle of the principal stress and the maximum shear stress during construction were achieved. Based on the research work shown above, reasonable support occasion was put forward for the first time;(4) Based on the in-situ monitoring and numerical simulation, deformation and the ground stress of the surrounding rock mass at different constructional stages were systematically researched on. Aimed at the weak interlayer appear at the surrounding rock mass of the underground caverns, depend on in-situ geological investigation, monitoring material and numerical simulation, the influence of the weak interlayer to the stability of the surrounding rock mass was achieved. Research works also find that the stress value of rock anchor through weak interlayer at different parts of the surrounding rock mass shows different value, and the regularity was found. Based on all these research work shown above, associate with the. factor of construction, a new engineering phenomenon, i.e, longitudinal cracks appeared between the concrete of rock-anchored beam with the surrounding rock mass was found during the construction of the underground powerhouse. The cause analysis was conducted, and the development trend of the crack was forecasted. Results indicate that the existence of the weak interlayer at the surrounding rock mass is the principal reason, and construction is the other factor which results in the crack. The research achievement was used in practice;(5) Based on the monitoring earthquake waveform, which induced by the Wenchuan earthquake occurred at Sichuan province, south-west of China, in May 12,2008, together with the in-situ deformation and cable force monitoring results, the deformation character, plastic zones and wave velocity at different parts of the underground caverns were put into research using the two dimensional program UDEC, and the response of the underground engineering surrounding rock during this earthquake was acquired.The research work of this thesis combined with practice tightly, which aimed not only at establishing the constructional mechanical behavior method of the large-scale underground caverns, but also with purpose at solving the engineering practical problem. This work experienced the engineering stages of preliminary design and construction, which make the research contents and achievement have strikingly engineering feature, and were adopted in dynamic design optimization and construction. Several means were used and the systematically research method with its special feature, which is a successful experience with research on the mechanical behavior of underground rock mass engineering during the stages of preliminary design and construction.