Study On The Dynamic Stability Analysis And Control For The Surrounding Rock Of Large Underground Cavern Group Under Complex Geological Conditions In Southwest China

The underground cavern groups in a number of hydropower projects developed at Southwest China exhibit the remarkable characteristics of unprecedented scale and complex geological conditions,strong nonlinear performances of rock masses,and numerous uncertain factors manifested in construction process.The handling of dynamic stability and control of their surrounding rocks are often encountered with crucial difficulties beyond the technical understandings and prevalent design specifications.Taking the representative underground powerhouse caverns of large-scale hydropower projects(Wudongde,Jinping I,Guandi,etc.)as engineering background,and by means of on-site investigation,theoretical analysis,on-site monitoring,numerical simulation and dynamic feedback,a systematic study has been accomplished by the author and the main achievements are presented in this thesis as follows:(1)The geological conditions,rock mass structures,and deformation characteristics of surrounding rock are summarized.The deformation and failure modes of large-scale underground caverns with typical massive structure(Guandi,Dagangshan)and large-scale layered rock mass structure(Pengshui,Wudongde,Jinping I)are studied in detail.The classification of deformation and failure modes is further proposed,i.e.,stress control type,structural surface control type,structural surface and stress composite control type.(2)Based on the structural control principle of rock mass,the dynamic analysis and optimal control system for the overall and local stability of surrounding rock during the construction of large-scale underground cavern group are proposed.It comprises a three-dimensional overall numerical inversion method considering the dynamic relaxation of surrounding rock mass and the dynamic renewal of rock structure,and a local large-scale block dynamic feedback analysis method combining geological survey analysis,field monitoring and dynamic search analysis.(3)Being focused on the underground cavern group in Wudongde Hydropower Station,the deformation characteristics,evolution law,anchorage stress characteristics and control effect of steep rock strata during layered excavation,are explored.In addition,the dynamic stability feedback analysis and anchorage mechanism based on three-dimensional numerical method are further conducted.(4)Being focused on the surge chamber of the tail water tunnel in Guandi Hydropower Station and based on the combination of key block theory,fine construction geological description,deformation monitoring and other methods,the dynamic feedback and design control for the local stability of surrounding rock during the layered excavation of large-scale underground caverns in high geo-stress area are studied.Equipped by the fast feedback,field theoretical calculation,field monitoring,verification,and optimal design,a new method for the local dynamic stability analysis of caverns is formulated.(5)Based on the field monitoring data with regard to the whole process of unloading mechanics of the surrounding rock during the dynamic design and construction period,and considering the difference between the unloading of construction blasting and the excavation of non blasting unloading,the safety grade evaluation and deformation control standard of the surrounding rock during the construction period are established.In addition,based on the analysis of the load characteristics and change rules of the anchor cable in large cavern,the risk level of the anchor cable structure and the countermeasures are established(6)Based on the long-term observation data of surrounding rock deeply anchored by the cable support system(excavation period and 5 years after completion)in Jinping I Underground Power House,the long-term safety problems of rock mass unloading and anchor cable structure support system under high geo-stress environment are studied.The stress characteristics and time effect evolution law of anchor cable supported system are analyzed,strategy for the long-term bearing risk and safety evaluation for support structure are further put forward.(7)Towards the crane beam on the rock wall of Jinping I Underground Power House,according to the excavation process of the cavern and the safety monitoring data two years after the excavation,the influence of high geo-stress unloading and deformation of the rock mass on the crane beam is analyzed,by which the deformation mechanism and the safety of the crane beam are revealed and evaluated,the corresponding countermeasures are further explored.