3D-FEM Analysis Of Structure Of Surge Shaft With Big Inner Diameter And Stability Of Surrounding Rock

With the construction of many hydropower station with big discharge and long diversion system in recent years,there is surge shaft with bigger inner diameter, more complicated structures and heavy geological conditions. As an important part of buildings in hydropower station, the working behavior and stability of surrounding rock of surge shaft is a strikingly important problem, and will influence directly the safe operation of the project. The normal design of surge shaft adopts traditional thin-cylinder method, which is based on many hypotheses and simplifications. As a result, it has difficulties in reflecting the actual working behavior of surge shaft with bigger inner diameter. Meanwhile, surge shaft is an underground structure because its body is buried by surrounding rock, and the excavation of cavern with big inner diameter will influence stability of surrounding rock. So it is necessary to research on working behavior of surge shaft lining and stability of surrounding rock with 3-D FEM.Traditional computing method combined with 3-D FEM(by means of ANSYS software) are adopted in this thesis. The structure of surge shaft with bigger inner diameter and stability of surrounding rock after excavation are analyzed in the light of specific conditions of surge shaft of Xinma hydropower station on Anning River, Liangshan State. The author has completed the following work:(1) The distribution of inner force is achieved through analysis of surge shaft structure with traditional method. The distribution of inner force accords with standard and the drafted physical dimension is reasonable. Stress and distortion is also in the reasonable scope.(2) Through analysis of surge shaft structure with linear-elastic FEM, the working behavior of surge shaft lining numerically simulated in case of internal pressure, external pressure and several different elastic modulus of surrounding rock, and so on. We can get the result that the control work condition of surge shaft is maximum surge water level, and the distribution of stress is accord with standard because most values remain within the limits of allowable tensile stress. Strong-stress discovered in local of surge shaft, so it needs reinforcing steel. It's compact relation between stress of surge shaft structure and elastic modulus of surrounding rock,(3) Being optimum to surge shaft lining thickness, its structural stress become uniform. It proves that the surge shaft size is feasible.(4) Using elastic-plastic FEM for studying these conditions of surge shaft lining, surrounding rock stress and plastic zoom. Surrounding rock is stable. Local plastic-strain zoom has been found in the structure of the floor lining. This zoom lay in front of the gate groove bottom, because of stress concentration. So it needs reinforcement.(5) At present, thin-cylinder theory and FEM are extensively applied. The comparation with two results shows that the stress value by the former calculation is larger than the value by the later. Because thin-cylinder theory assumes that the well casing is a uniform and independent structure, the calculation is limited in thin-cylinder theory. But FEM can well simulate the figure, the load, the influence of adjacent structure and the resisting effect of surrounding rock in different place. So the FEM calculation is more practicable. It suggests that FEM is used in big surge shaft design for economy and safety.(6) Under the function of ground stress field, nonlinear FEM is used to study and analyze the stress of surge shaft lining and plastic distribution, according to the order of two different excavation and support. It can get a conclusion that there isn't big range plastic zoom while cavern cutting of surge shaft and surrounding rock is stable. The stress-strain of excavation in layers is optimum to single excavation. It suggests that we should choose rational construction order to combine construction organization. It recommends excavation in layers.