| In order to increase the utilization and development of my country's hydropower resources,the western region is the focus of the development of important hydropower resources,and the high rock slope stability problems that need to be overcome in the construction of large hydropower stations under the complex geological conditions of the mountain and valley regions have become increasingly prominent.Rumei Hydropower Station is located in the upper reaches of the Lancang River in Mangkang County,Tibet Autonomous Region.The scale of Rumei Hydropower Station is first-class and large-scale.The retaining structure is planned to be core-wall rockfill dam,and the riverbed elevation is 2617 m.The elevation of the dam crest is 2902 m,the elevation of the foundation surface is 2587 m,and the maximum dam is 315 m high.The normal water storage level is 2895 m,and the installed capacity of the hydropower station is 2100 MW.The height of the excavation slope of the left bank dam foundation is about 350 m,the slope is about 50°,and the excavation slope is located in the weak upper unloading zone.A detailed investigation of the dam site area in the early stage shows that the dam site area of Rumei Hydropower Station has steep terrain,hard rock,strong surface weathering and unloading,and complex in-situ stress.The left bank dam foundation slope has many long,moderately inclined external faults and long cracks.And many transverse faults,the stability problem is prominent.Therefore,the research on the stability of the natural slope and excavation slope of the left bank dam foundation has great engineering significance.With the support of the mentor's scientific research project,he participated in the field survey and indoor arrangement of the middle dam site.Based on the analysis of the engineering geological conditions of the middle dam site,the dam foundation rock mass weathering unloading characteristics were systematically studied and zoned Based on the detailed survey data on site,the rock mass structure characteristics of the middle dam site are analyzed,combined with qualitative and quantitative evaluation methods,the dam foundation rock mass structure characteristics and rock mass quality are systematically studied,based on engineering geological analogy and three-dimensional numerical analysis methods.The deformation and failure characteristics of the dam foundation excavation slope and possible instability modes,and the stability of the left bank slope rock mass is systematically studied.Mainly gain the following knowledge:(1)A review of the surface geological characteristics of the middle dam site and the downstream to the 5# ditch along the left bank low line: the boundary between granite and dacite was adjusted,and the height was adjusted to be close to the middle line elevation with an elevation of 2840 m Left and right;the occurrence analysis and character analysis of the third and fourth structural planes on the left bank of the middle dam site area,and the development characteristics of the third and fourth structural planes are summarized;the gently-dipping external faults in the middle dam site are reviewed And the development and evolution characteristics of steeply dipping transverse river faults,including the spatial extension of L72.(2)Statistical analysis was carried out on the properties of grade V structural planes on the left bank of the middle dam site,including steeply inclined structural planes and gently inclined structural planes.Among them,the steep crack can be divided into 5 groups according to the trend difference,and the slow crack can be divided into 6 groups according to the trend difference.However,there are mainly 3 groups on the left bank,which are inclined to SW(group ?),inclined to SE(group VIII),and inclined to S(group IX).The average length of slow crack traces in each group is 0.65 m,with medium continuity,and rough undulations to straight rough,Wave-like roughness and microwave-like roughness are the main ones,both of which are very tight,and both have the largest proportion of non-filling and non-cemented cracks.Statistical analysis shows that the slow cracking of the dam site is gradually formed by superficial transformation on the basis of construction and structural transformation.(3)A detailed investigation and analysis on the structural characteristics of the rock mass on the left bank of the middle dam site area.The qualitative judgment shows that the strong unloading rock mass on the left bank dam foundation slope is dominated by mosaic and fragmented structures,while the weak upper unloading is mainly mosaic.Mainly,weak lower unloading is dominated by mosaic structure and sub-block structure,and unloaded rock mass is mainly sub-block structure.The statistical results show that the granite in the left bank slope has a strong structural transformation on the whole.(4)According to the structural characteristics of the slope,it is inferred that the local stability problems existing after the excavation of the left bank slope are mainly caused by the slip-cracking of the large-scale block controlled by the moderate and long fractures and the transverse faults.Among them,the low-dip fault L72 controls the stability of slope.(5)The engineering geological segmentation and basic quality evaluation of rock masses were carried out on multiple flat tunnels in the granite area and dacite area,and the spatial variation law of the rock mass of the dam foundation was preliminarily summarized.Comprehensive judgment based on evaluation methods.Quantitative evaluation mainly adopts BQ method,RMR method,HC classification method and RMY method,etc.,comprehensively judged that the rock mass quality of the strong unloading zone belongs to grade IV1,and the weak upper unloading zone The quality of the internal rock mass is ?2A,and the rock mass of the weak lower unloading zone is ?2A.Based on the field measurement data and the above analysis results,the correlation between RQD,RBI and RSI in the dam sites in the United States and China is further analyzed.According to the comparison results,the rock mass structure index(RSI)can describe the structural characteristics of rock mass in more detail,while the RBI value has ambiguity and should not be used to quantify the structural characteristics of rock mass alone.(6)Using three-dimensional numerical analysis method to study the unloading relaxation mechanism and influence depth of the dam foundation rock mass,the main understandings are as follows: 1)The calculation results without considering the unloading rock mass weakening will also show the corresponding unloading zoning phenomenon.The displacement mutation position deviates from the bottom boundary of the unloading zone qualitatively determined by the field investigation,and the calculated range without considering unloading weakening is shallower than the qualitatively judged range;2)Find the relationship between the unloading depth change and the slope structure;3)The spatial extension characteristics of L72 were analyzed,and the influence of L72 developed on the left bank on the unloading zone was studied.It was found that the slope of L72 extended 240-260 m.The body produces more tensile failure zones,and the continued extension into the slope has less impact on the slope body.(7)Based on the strength reduction method,calculate the stability of the dam foundation under natural and heavy rain conditions on the left bank.The main conclusions are as follows: 1)As the reduction factor gradually increases,the tensile-shear failure area gradually expands to the both of upper and lower zone of the slope until it penetrates.The stability factor Fs under natural conditions is 1.49;under heavy rain conditions The stability coefficient is 1.40,which meets the design requirements;2)From the calculation results,it is inferred that the natural slope of the left bank may lose its stability as a bottom sliding surface along with a slip-cracking failure;3)The left bank after the excavation of the slope The slope stability coefficients are 1.45 and 1.36 respectively under natural and heavy rain conditions.The overall stability is good,but there are also local stability problems with unfavorable structural surfaces such as L72 control.(8)Based on the three-dimensional block stability theory,analyze the characteristics of the main control structure surface in the slope body,and establish a model in combination with the airborne conditions.In this paper,the bottom slip surface is fault L72,and L15-1 and L73 are the side fracture surfaces of the movable block.,Using f8,f12 and unloading fissures as the trailing edge of the movable block respectively,to establish three possible instability block geometric models,and then further judge whether the block is a movable block according to the force of the block According to the criterion of instability mode,first obtain the sliding force and anti-sliding force of the block and calculate the stability coefficient.It is calculated that the stability coefficient of the possible movable block under the most unfavorable conditions,that is,1/4 saturation + earthquake,is at least 1.256,which meets the safety design requirements. |
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