Identification Of Karst Media And Evaluation Of Seepage In Liangchahe Reservoir Junction Area, Huidong County, Sichuan Province

Liangchahe reservoir is the leading reservoir for the blueprint of mackerel river mainstream cascade power station.The Cambrian series water group??₃e?lavas distribute widely in Liangchahe reservoir hub area which is a longitudinal karst valley in the same direction with tectonic line.There are several karst spring points near the hub area of Liangchahe reservoir.This fact indicates the karst space and development history may be complex in the hub area,and brings about a serious impact on the anti-seepage work and the normal operation of the reservoir project.We apply the method of exploration to analyze the karst media of longitudinal valley and to evaluate the seepage problem of this valley,then a proper way to solve the seepage problem will be determined.As a result,we can give a great example to the research of seepage problems in this type,and a reliable reference for further study about the seepage problems of reservoir in longitudinal karst valley.The geological structure is complex in hub area,which is mainly influenced by multiple phase evolution and tectonic water control.Thus,the method such as the exploration on the ground,hydrogeology drilling and geophysical exploration are used to identify the spatial structure of karst media in hub area.Meanwhile,we can find out the underground water flowing system in hub area through underground connecting tests and the characteristic of hydro-chemistry and isotope fields.Finally,adding these two aspects together,a generalized karst leakage model will be set up.Based on the generalized karst leakage model and three-dimension numerical simulation software,the dynamic prediction for the variation of groundwater seepage field can be done while before and after the water-storage of this reservoir,and the leakage quantity can be estimated after adding the anti-seepage curtain,which helps to propose a corresponding anti-seepage procedure.Our research provides some basic geological data and support to optimize seepage control design,estimate losses in a project and make a practicable anti-seepage plan for reservoir.Up to now,our main results are listed here:?1?The hub area exposed to lava for Cambrian system on two water group??₃e?,and lava stratigraphic distribution and exposed characteristics under the control of the fold structure and terrain cutting.?₃e³ at the top of the formation,overall lithology is pure carbonate rocks mixed with impure carbonate rocks,and these two components interbed at the middle and bottom parts of formation.?₃e² is interbedded by impure mass carbonate rocks and impure carbonate rocks mainly,and pure carbonate rocks and lava interbed accidentally.In the middle segment of?₃e²,the ratio of lava is obvious higher than other three segments.?₃e¹ give priority to impurity?mass?carbonate and the lava interbed,partial clip is pure carbonate rocks,the segment at the bottom of quartz sandstone.?2?The hub area?₃e¹ the global development of karst is weak or none,and overall development for karst of Cambrian system on two way water group two and three part??₃e² and?₃e³?formation is demifine.At the location of dam axis above1838 m elevation,?₃e³ karst grow mediumly to strongly.Elevation between 1750 m and 1838 m,in?₃e² the degree of karst development is weak.Below elevation of1750 m,?₃e² soluble rock and?₃e¹ impure karst grow weakly globally,but it will grow strongly in some place.In detail,they just develop some karst fissure pipes in small scale which have good connectivity,and no large karst pipe exist.?3?On the whole,the groundwater in the dam site is mainly supplied by meteoric water,which is mainly excreted in the trevally river.There is a natural underground watershed or physical water-separating boundary between xiaocha river,guancun river in adjacent valley and the groundwater in the dam site,which shows independent water system characteristics.On the left bank of the dam site,except for evaporative discharge and a few linear discharge to the trevally river,the rest of the groundwater is dispersed in karst springs.The groundwater runoff path is unobstructed and the water circulation rate is fast.The rock on both sides is mainly weakly permeable,and the dam site is not buried deep relative to the water-proof layer,which is shallow in the riverbed and the left bank,and deep in the right bank.However,the boundary between bedrock and overburden is permeable and there is a strong permeable zone.The main leakage channel is the fissure leakage along the surface karst zone.?4?There is no seepage problem in the adjacent valley downstream of the reservoir.In natural situation,the seepage amount of the reservoir under different anti-seepage conditions is calculated and simulated by analytical solution and numerical simulation respectively.And it is pointed out that the dam foundation and the right abutment are the main seepage part of the hydropower station in the karst area under the condition of no anti-seepage.?5?Based on the calculation and feasibility comparison of seepage amount of different anti-seepage schemes,the anti-seepage curtain 40 to 60 meters deep below the foundation rock of the dam foundation is selected to effectively limit the reservoir seepage in the hub area,so as to ensure the benefit and safe operation of the reservoir.