Study On Surrounding Rock Stability Of Underground Cavern Group In Yangfanggou Hydropower Station

Yangfanggou hydropower station is located in Muli County of Sichuan Xichang City.The total storage capacity of the hydropower station is 512 million and 480 thousand cubic meters,and the installed capacity is about 1500 MW.The hydropower station has now entered the construction phase is expected to be completed in 2022.The underground powerhouse uses project of excavating on Left Bank first.The ground elevation of underground cavern group is 2240~2370m,and overlying strata thickness is 197~328m.Lithology is granodiorite that intact and hard.The underground cavern group includes underground powerhouse,main-transformed cavern and tailrace surge tank designed parallelly.The direction of longitudinal axis is N5° E.According to research,there no major fracture cross the study area,but small fault and joint develop much which constitute a complex system.The randomness of structural surface’s property and location cause the possible of deformation and failure in global and local,and excavation of adjacent cavern will aggravate this possible.In addition,under the same geological conditions,the different size caverns’ deformation and failure is different which may eventually lead to the difference in the way of support.Therefore,it is very important to study the stability of the underground caverns.In this paper,Yangfanggou Hydropower Station underground caverns group as the main research object,the stability of underground caverns group as the main research object,through the underground powerhouse and main transformer cave,tailrace surge chamber of three large chambers and the auxiliary tunnels rock structure,rock quality,structural surface development characteristics of deformation and failure statistics and data collection.Analysis of rock mass structure,rock mass structure,size and surface effect of cavern group effect of underground caverns and local stability of the overall development.Through the results of this study,the possible stability problems of underground caverns are pointed out,which provide reference for the design of support.In the course of this study,we mainly collect the first-hand geological data of every cavern after excavation and participate in the construction work.Based on the study of rock mass structure and deformation and failure characteristics,taking the stability of the caverns as the goal.(1)Through the field investigation,the geological environment background of the study area is analyzed in combination with the relevant research results of the early research.(2)The rock mass structure is classified,the quality of the surrounding rock and the geological sketch work are carried out in each tunnel section,and it is arranged and preserved as the basic data.(3)To investigate the signs of deformation and failure of each tunnel section,the main control factors are analyzed and studied and the model is judged,and the comparison is made according to the size of different caverns.(4)On the basis of study,underground caverns goble and local stability are analyzed,the site for the combination of the related experimental rock mass structural plane survey data,the control of structure plane,select the appropriate calculation parameters,boundary conditions and the establishment of a scientific model,using numerical simulation software 3DEC,the stress strain and displacement etc.data analysis.The following main research results can be obtained in this paper.(1)According to the present situation of the excavated chamber,the structural planes are dominated by small faults and joints.The strike is mainly NWW,NNE and NE,and the dip angle is mainly medium steep dip.The structure of surrounding rock is mainly lumpy and massive,and the part is mainly inlay structure.The main classification of surrounding rock is class II and class III.(2)The deformation and failure modes in the study area are mainly three types: structural control,stress control and stress-structure control.The structural control type is the main structure,followed by stress structure control,and the stress control type is the least distributed.The deformation and failure of structural control mainly show collapse and slipping;stress-structure control deformation and failure type mainly show that after excavation the joint relaxation and open;stress control main by splitting spalling and crack collapse,there is no phenomenon of rock burst.(3)Through the size effect simulation of 5 different sizes of caverns,it can be concluded that displacement and depth of plastic zone are second order with respect to cavern size,rather than the linear proportional relationship in theory.When the size of the cavern increases to about 15 m,the displacement and the plastic zone depth increase sharp with the size of the cavern.The distribution of stress is roughly the same as the displacement and the distribution of the plastic zone.(4)After the analysis of the overall stability of the underground caverns,the maximum displacement of the underground chamber under the condition of no support is 7.2cm,which is located in the downstream side wall of the underground powerhouse.There is a group hole effect in the underground caverns group,which is mainly reflected in the influence of the excavation of the powerhouse on the main change.(5)The local stability analysis of 3 typical sections of the underground powerhouse,right 0+66m section is mainly affected by J150 and f123 and joint along cavern,the maximum displacement of up to 32mm;right0+05m~ left 0+35m downstream side wall affected by f83,J145,J164,the maximum displacement of up to 38mm;upstream side shoulder concrete spray layer at right 0+10m crack affected by fault f49.The above three sections through field test and numerical simulation analysis shows that the displacement even exceeds the warning value but during the excavation process with time gradually convergence that does not appear large range of deformation and failure.The monitoring data analysis results and numerical simulation results show that the overall underground cavern group is in a basically stable state,and the support should be strengthened for the local deformation sites and monitored continuously during the subsequent excavation process.