The Research Of Dam Foundation Rock Seepage And The Impact To Anti-sliding Stability Of Yagen Ⅱ Hydropower Station

The proposed Yagen Ⅱ hydropower station lies in Yajiang county of Ganziprefecture of Sichuan province. It would be the third cascade hydropower station inthe middle of Yalong river. According to design, the maximum height of concretegravity dam is153meters with254million m3reservoir capacity and990MWinstalled capacity. Under the action of hydraulic head, the problem of dam foundationseepage and rock instability may occur for the development of granite fracture in damfoundation. Therefore, it is practical and valuable to study the problem of damfoundation seepage in engineering.On the basis of analyzing the development law of structure plane in dam area andstudying the data of adit and drilling core in dam foundation, geological generalizedmodel is modeled. Combine of water pressure test and fracture statistics, penetrationparameters in different parts of dam foundation is determined. Using VisualMODFLOW software to simulate the characteristics of three-dimensional seepagefield under nature state and water storage state separately, the author analyzed theinfluence of seepage to rock mass properties. That seepage impacts on the anti-slidestability of dam foundation under water storage state is elaborated additionally withthe software of FLAC3D.Major achievement achieved as follows:The development law of structure plane is summarized, especially to structureplane with low-angle dip. The finding shows that low-angle dip structure plane withoccurrence of N40~50°W/NE∠20~25°could be tectonic structure plane, whoseoccurrence is stable, straight and extended. According to the adit data, such low-angledip fracture exists in11parts of dam foundation, developing equidistantly and cuttingwith steep dip fracture, which forms the basic geological model of dam seepage andanti-slide stability.Seepage structure of dam foundation rock is divided into following types:granular, banding and network-like structure, the latter two seepage structure of which play major roles in dam foundation seepage. Banding structure is the main channel ofdam foundation seepage,which consists of fault and influenced band. Network-likestructure beyonds secondary channel containing all types of small fractures.Simulating the tensor of seepage coefficient and water pressure test illustratesthat the permeability of dam foundation rock decreases with depth increasing andfault in fresh rock shows weak interaction to permeability. With this method, seepagecoefficient can be selected.After reservoir finishes water storage, water line of both sides of mountainshoists and then lowers. The lowering speed of groundwater line goes slowly andstabilizes gradually in5years later. The design of gravity dam drives seepagedevelops around the dam. This phenomenon appears serious in right shore. Usinganti-seepage curtain could make seepage field to extend deeply with lower leakage.Anti-seepage curtain shows better effect when leakage of dam rock is lower.The development of dam rock does harm to the coordinating among differentstructures with sliding stability. Pipe erosion of structure from groundwater with thedecreasing of confining pressure could damage the mechanical properties of countryrock. Without anti-shear cogging, gravity dam would move2mm along fh07or fh09after water storage. On the contrary, the stability of dam foundation grows better.