| With the rapid development of Chinese economy, great development also appears in construction of water conservancy and hydropower, road and rail and so. In the process of underground cavern excavation, stress distribution, surrounding rock deformation and stability would be changed by great specificity. Tunnel excavation in deep and high stress conditions, the mechanical properties and stability of the surrounding rock in particular must be pay close attention to, once there are problems, it is likely to affect the safety of the underground structure even catastrophic consequences. However, it is very difficult to accurately make out the mechanical behavior of surrounding rock and it is not easy to find out the differences of the rock deformation and stress secondary distribution between general conditions and high stress conditions. What’s the difference in the surrounding rock stability analysis methods and indicators with traditional methods, the strong exploration and further research needs to be done.In this paper, taking Jinping Ⅱ Hydropower Station diversion tunnels construction as the engineering background, there is a comprehensive study of the surrounding rock mass dynamic variation rules of the stress and displacement. And using of multiple index comprehensive evaluation to analysis the stability of surrounding rock under high stress rock burst in deep high risk tunnel. The main contents and conclusions are as follows:(1) From the geological conditions, the parameters of surrounding rock, rock classification system, a comprehensive analysis of the diversion tunnels of Jinping Ⅱ hydropower engineering geological conditions of the project area was done, the diversion tunnel section conditions on excavation and construction scheme were analyzed with the geological conditions, revealed Jinping Ⅱ Hydropower Station Diversion Tunnels with large section, large buried depth, high ground stress and high risk of rock burst characteristics. Excavation diameter of up to13m tunnel in high ground stress conditions, the surrounding rock stress redistribution of the excavation process may cause stress concentration and strong rock burst, the construction process will encounter great challenge, we must study the major problem.(2) Based on statistical analysis of the rock burst form in period of rock burst, this paper found that rock burst types are mainly spalling and burst. Rock burst occurred mainly in the surrounding rock conditions Ⅱ with large noise, mainly in arch side wall and the vault of the tunnel. Rock burst has hysteresis, it may occur in the tunnel of a certain distance from the working face which has been supported.(3) Selected the typical depth of the tunnel section, by means of numerical simulation and field measurement method, the analysis of the dynamic mechanical the surrounding rock’s behavior of tunnel excavation in high stress conditions found that around the hole the formation of stress relaxation instead of stress concentration, but because the stress transfer it will form stress concentration zone in the surrounding rock in certain depth from the wall, the safety factor of stability of surrounding rock is lower, but due to the release of stress after excavation unloading, stress will be transferred to the deep surrounding rock, it will form a large stress concentration zone in the left tunnel shoulder and right tunnel arch wall, from the point of view of energy it also formed a large elastic strain energy concentration in the stress concentration region, the region has the rockburst conditions of stress and energy. So the mechanical characteristics of surrounding rock in tunnel excavation process in high stress conditions are different from in the general conditions.(4) Analysis results of effect of increasing length and prestress of bolt on the surrounding rock stability show that, increasing the length of bolt within a certain length can improve the control effect of surrounding rock deformation and improve the stress state of surrounding rock, but with the increasing of the bolt length, bolt role gradually reduced. Using the long bolt can not give full play to bolt action, neither economic nor reasonable. Adding bolt prestressing can effectively control of the surrounding rock deformation and improve the stress state of surrounding rock, but when more than a certain value the control effect will decrease gradually.(5) This paper studied the influence of surrounding rock displacement and stress variation, variation of the bolt axial force,variation of the plastic zone volume and the energy release, the results shows that with the increase of supporting lag distance, the surrounding rock stress and displacement increase. The bearing capacity of surrounding rock increase, support resistance decreases, the surrounding rock failure volume and the energy release increase, the stability of surrounding rock reduce. Supporting lag distance of3m-6m, stress and displacement of surrounding rock release is the most dramatic, and the surrounding rock plastic zone damage volume and energy release rate changed most obviously, variation of axial force of bolt is the largest. We should pay more attention to the variation of surrounding rock when supporting lag distance of3m-6m.In the high stress of tunnel excavation, should be timely support to follow to give full play to the role of the reinforcement of the supporting structure, to improve the mechanical properties of surrounding rock and ensure the stability of surrounding rock and underground structure safety. |
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