Study Of Geomechanics Mechanism And Characteristics Of Surrounding Rock Mass Deformation Failure For Underground Powerhouse Of Jinping I Hydropower Station

The 21st century is a century which vigorously develops the underground construction in china. Southwest of china is rich in hydraulic power source,while alpine valleys are distributed in it. So the powerhouse of hydropower station uses mostly large underground cavities scheme. The surrounding rock stability of underground powerhouse is a major issue in engineering geological investigation,design and construction. It has particular attention since the stability problem involves whether the underground engineering could be built, and whether it influences investment and time limit.Jinping I Hydropower Station is a key cascade and control underground hydropower project,which is located in the main stream of Yalong River that is at the junction of Muli and Yanyuan in Liangshan county,Sichuan province. The diversion power generation systems layout in the right bank of the dam site, which has huge underground factory cavities . Because of its complexity and particularity beyond the human existing experience and knowledge, the design and construction of underground cavities faces many challenges.The deformation failure of surrounding rock mass which is based on the statistic analysis of field research and interior data is studied by engineering geology analysis and three dimensional numerical simulation.Firstly, based on the geological logging data and classification of discontinuities, the distribution regularity of discontinuities has been analysed. Statistical results show that discontinuities in first group are most development,second for the second group,again for the fourth group,finally for the third group. Secondly,according to circumstances of the project, three dimensional geological conceptive models in ANSYS finite element program was built. By using FLAC3D finite difference computation procedure and rapid stress boundary method,adjusting the stress boundary condition in the calculation, the optimal fitting between the calculated stress value and measured stress value is studied to obtain the initial stress field of the survey region. It superposed by self-weight stress field and tectonic stress field. Three dimensional numerical simulation results show the survey region in higher initial stress state,the calculated stress value and measured stress value has a optimal fitting in magnitude and direction.Thirdly,excavation and support of the underground caverns has been simulated according to the appropriate adjustment between the actual excavation and supporting conditions, then secondary stress filed of surrounding rock mass after excavation was obtained. The secondary stress and displacement distribution characteristics in main cross planes are analyzed.Finally, according to three dimensional numerical simulation, analysis of deformation monitoring data and the understanding of deformation failure characteristics, the formation mechanism of deformation failure in underground caverns are analyzed from rock mass structures characteristics and stress field distribution characteristics. The results of the study show combination of the discontinuities in first group and second group, first group and fourth group are probably rock mass structures factors of unloading relaxation deformation at upstream side and bend-bloat deformation at downstream side(especially hance). Higher initial stress state and gradually increased secondary stress in excavation process are probably geostress factors of comparatively large deformation. In excavation process,stress concentrated parts transferred from upstream side to downstream side and gradually increased major principal stress at downstream side process are probably factors of discrepant deformation between upstream side and downstream side.