| In-situ stress is one of the fundamental geological conditions of rock engineering and underground construction, and the accuracy of characteristics of the initial stress field will affect the reliability and safety of the engineering directly, the in-situ stress is affected by geological structure, topography and geomorphology, denudation, mechanical properties of rocks, underground water,temperature, and so on. What’s more, the relationship among the various factors is complex, which intensified the complexity of the distribution of the in-situ stress. And generally the deep underground caverns in western hydropower projects are in high stress or extremely high stress circumstance. So based on based on Fund project of the Central Public Welfare Scientific Research Institute:Research on in-situ stress measurment and analysis method under extremely high stress condition, National key basic research and development program (973):Evolution and Hazard Mechanism Study for High Stress Unloading Landslip(2011CB710603), Key project of National Natural Science Foundation of China: Theoretical and practical research on engineering characteristics of deep rock mass and the research project:Simulation study on the effect of rock burst prevention and control based on the rapid release of stress by blasting, the following four progressive aspects are regarded as the main study line, including:(a) in-situ stress measurement of rockmass under extremely high stress circumstance, (b) feedback analysis of tunnel surrounding rockmass which belonging to small scale engierring aera, (c) nonlinear system analysis of in-situ stress field for the whole site aear which belonging to large scale engierring aera, (d) rockburst controling measures under high stress circumstance. Theoretical research, experimental verification and application are combined to carry out the study systematically. The main contents and conclusions are listed as follows:(1) According to the difficulties encountered in rock stress measurement of the extremely high stress region, a new method for stress measurement of partial stress relief of narrow slot is put forward based on Fund project of the Central Public Welfare Scientific Research Institute: Research on in-situ stress measurment and analysis method under extremely high stress condition. Meanwhile, it’s rationlity is proved through the finite element method. This method is applied to the South side 2# scientific research experiment tunnel of Jinping Ⅱ Hydropower Plant successfully.(2) Based on the stress evolution process of surrounding rock:before excavation-excavation-surrounding rock weaken of strong relaxation zone-surrounding rock weaken of weak relaxation zone, combined with nonlinear optimization solution algorithm, feedback analysis method of initial stress field of surrounding rock stress is established with other stress information. And this method is applied to calculate the in-situ stress distributionthe for the surrounding rock in the 4# water diversion power tunnels of Jinping Ⅱ Hydropower Plant.(3) Baihetan, Wudongde, Xiaowan, Songta and Ertan locate in the western region of deep valleys, and the results of test and analysis of in-situ stress show that, the larger the acute angle between valley trend and regional principal stress is, the more intensive its stress concentration influence at the bottom of the valley is. Hard rock is favorable for in-situ stress savings, but due to the perturbation effects of faults, the rock mass stress magnitude and orientation have significant change nearby those faults.(4) For the common stress field analysis methods are not applicable for research of in-situ stress field in complicated conditions, a nonlinear system analysis method of in-situ stress field is proposed, taking the stress field in deep valley as a complex nonlinear system. Combined with the geological conditions, the qualities of the testing rockmass, the representative of various testing methods, weighted least squares, orthogonal design and intelligent optimization inversion methods, the nonlinear mapping relationship between boundary conditions (weight factors, the magnitude and direction of tectonic stress) and in-situ stress of the numerical calculation model is established for optimization solution. This method overcomes the disadvantages of merely considering some factors or without global solution in the solving process of stress field boundary conditions previously.(5) Rerferring to the analysis results of initial stress nonlinear system analysis in the deep valley dam area of Baihetan Hydropower Plant, the accordingly multiple correlation coefficient of stress test values and calculated values is R=0.89, and the phenomena of the sheet in the excavation process is consistent with the results of stress calculation. So, it’s reasonable and feasible to use the multi-source information integration calculation method to analysis the stress field of rock mass in Baihetan Hydropower Plant.(6) According to the results of measurement and analysis, the in-situ stress magnitude of the underground powerhouse of Baihetan Hydropower Plant lies between the middle to high stress level. The direction of the maximum horizontal principal stress of the underground powerhouse on the left bank is generally NEE, the stress value of the right powerhouse area is greater than the left. Under the control of regional tectonic, stress increases gradually with depth. Meanwhile, the direction of the maximum horizontal principal stress gradually changes from NNE to NWW along the vertical direction. At the bottom of the river valley, the phenomenon of ’stress concentration’ is obvious. The in-situ stress level of the slope is very low, and the direction of the maximum horizontal principal stress is approximately parallel to the river trend. The maximum horizontal principal stress direction of the slope rock mass is transferring to the direction of the regional structural position with the increase of the horizontal depth.(7) Through explicit finite difference calculation method, simulation analysis of rock burst prevention methods through of in-situ stress fast stress release by blasting for the water diversion power tunnels in Jinping Ⅱ Hydropower Plant based on the research project:Simulation study on the effect of rock burst prevention and control based on the rapid release of stress by blasting. Simulation results show that rapid pressure relief of the bursting stress can effectively reduce the stress concentration of surrounding rock in the rock burst zone, then the stress concentration areas transfer to the internal surrounding rock of unloading relaxation zone for reaching the objective of controlling rock burst.(8) Based on Rusesenes rock burst stress criterion, different stress release programs of the effectsof rock burst prevention and treatment are compared. The tilt radiation hole excavation scheme and the vertical guide chase hole plan could respectively avoid stress concentration around the contour and in front of the excavated tunnel face effectively, they have relatively reduced the possibility of rock burst. At the construction site of Jinping Hydropower Plant diversion tunnel layout corresponding stress release burst test is carried out, and a circle of blasting unloading relaxation near the stress relief hole is formed. The test results are consistent with the simulation results. |
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