Study On Surrounding Rock Quality Evaluation And The Stability For Tunnel Construction Period In High Geostress Areas

In recent years, the tunnel engineering towards to more, longer, bigger and deeper than before with the rapid development of infrastructure construction in our country. The geological conditions in the tunnel site areas changes more when the in-situ stress increases gradually. So the damage degree of the surrounding rock is increased and the damage form is varied. The complexity and the non predictability of the deformation and damage mechanism of the surrounding rock are greatly improved. Therefore, the study on the stability of the surrounding rock of the tunnel under the condition of high ground stress becomes a problem to be solved. At present, the key issues of tunnel surrounding rock under high geostress conditions is short of systematic research, such as the surrounding rock quality evaluation, layered surrounding rock stability with steep dip angle, stability analysis of surrounding rock for block rock mass, the displacement sensitivity analysis of surrounding rock mechanical parameters and rock burst prediction.The Bulungl-Gongur hydropower’s underground cavities which lie in Kirgiz of Xinjiang was taken as the research object which based on the problems mentioned above. To study the tunnel surrounding rock quality evaluation and stability study under high geostress conditions, the researcher got a new idea which used engineering geology solid mechanics theory, nonlinear science theory and numerical computation methods. In a word, the main contents and conclusions of this study is summarized as follows:(1) Evaluated quality of surrounding rock by improved extension theory which based on asymmetric approach degree criterion instead of the rule of maximum membership degree and the combination weight of the principle of minimum identification information instead of a single weight. This matter-element model has been applied in four typical surrounding rock of underground cavern of Bulungl-Gongur hydropower station and the results of evaluation were satisfactory.(2) The flexura-slip failure mechanism model of steep dip angle layered surrounding rock has been established and the limit equilibrium equation of rock plate flexural length and the safety coefficient formula have been deduced according to elastic plate energy principle. Research the influence factors of the critical bending length with practical engineering. Considering the surrounding rock parameters uncertainty, the fuzzy random reliability theory was applied to layered surrounding rock stability evaluation was put forward, calculations indicate that considering the parameters of the fuzzy randomicity is more accurate.(3) Built mechanical model of equal thickness of rock rectangular plate to calculate critical load formula under the action of the static load and the first, second dynamic instability regions under the action of the longitudinal periodic load. With that rocky rectangular plate was analyzed that whether the static and dynamic instability has occurred.(4) Applied the block theory of structural plane trace limited to analyze the key block probability. Taken three equilateral pyramid as an example. The key block probability was redefined as absolute key block probability, relative key block probability and non-key block probability by trace probability theory of structural planes. The key block sliding safety coefficient was modified by using absolute key block probability, this measure could provide theoretical basis for saving the costs of the surrounding rock which under the condition of ensuring surrounding rock’s safety.(5) Combined the laboratory test and engineering experience based on uncertainty and spatial variability of surrounding rock mechanical parameters. Built a probability distribution model which regarded density, elastic modulus, Poisson’s ratio, cohesion, internal friction angle and joints internal friction angle as factor array. A sensitivity analysis method of gray correlation displacement was got which based on the 3DEC discrete element numerical simulation. based on the probability distribution model of mechanical parameters of surrounding rock is presented. Then this method was applied in actual engineering which can provide theoretical guidance primary and secondary selection of surrounding rock parameters.(6) Combined the aid of rough set theory and cloud model and built a rock burst nonlinear science forecasting model which based on based on rough set of the FCM algorithm calculate the evaluation index weights and the comprehensive degrees of certainty which was computed by cloud model. An example showed that the results of the forecasting model, the extension theory and the Russense discriminant were similar. At last, this forecasting model was used to evaluate the rock burst in underground caverns of Bulungl-Gongur hydropower station and it played an important role to reduce the loss of rockburst.