Permeability Distribution And Variation Of Fractured Rocks And Optimization Design Of Seepage Control In Large-scale Hydropower Projects

Over 200 m high dams constructed or under construction are mainly located in southwestern China where high mountains and deep valleys are often encountered.Influenced by tectonics,river incision,weathering and unloading,rocks are strongly unloading,in-situ geostress are high,geological environments are complex,and permeability are anistropic and spatially variable.During construction and opration of large-scale hydropower projects,the process of excavation,filling,anchorage and impounding induces intense alteration of hydraulic conductivities in rocks,and has a significant influence on the distribution characteristics of seepage field.On the other hand,the risk of leakage increases drastically under complex geological conditions,the optimization design of seepage control systems becomes a key issue in construction of large-scale hydropower projects.Thus,for the safe construction and efficient opration of large-scale hydropower projects,it is of great theoretical significance and practical value to study the permeability distribution and variation of fractured rocks and optimization design of seepage control system.Based on the Mengdigou and Baihetan hydropower projects,this thesis investigates the the permeability distribution and variation of fractured rocks by using statistical analysis and numerical simulation method,and proposes an optimization design of seepage control system.The major achievements obtained in this thesis are summarized as follows:(1)On the basis of statistical analysis of three thounds packer tests performed at Baihetan dam site,the spatial distribution and variation of hydraulic conductivity of fractured rocks are investigated.The effects of weathering,geologic structures,and rock mass quality on distribution of the hydraulic conductivity are also illustrated.The results show that the hydraulic conductivity of rocks follows a log-normal distribution.Hydraulic conductivity decreases significantly with the increase of horizontal and vertical depth.The variation of mean value of hydraulic conductivity and its logarithmic standard deviation with horizontal and vertical depth can be described by power function and Logistic function,respectively.The hydraulic conductivity of fractured rocks decreases as the degree of weathering decreases.With the increase of core recovery percentage,the permeability decreases.(2)According to deformation monitor,acoustic wave tests and borehole TV images of surrounding rocks of underground powerhouse in left bank of Baihetan hydropower station,the the basic development characteristics of excavation-induced disturbed zone(EDZ)of underground caverns is analyzed.Based on an equivalent elasto-plastic model and permeability tensor evolution model of fractured rocks,excavation-induced disturbance effect of underground caverns is modeled by the plastic yield zone and a deviatoric stress criterion.And the evolution law of permeability of rocks around caverns due to excavation is revealed.The results show that the predicted excavation disturbance results agree well with the deformation monitor,acoustic wave tests and borehole TV images.The permeability of surrounding rocks increases about two orders of magnitude due to excavation,and the influence depth can be 40 m.(3)The layout and main design parameters of seepage control system in large-scale hydropower projects in China is summarized,and an optimization design method of seepage control system in large-scale hydropower projects is proposed.On the basis of quantitative characterization of field geological conditions and mechanism of grout curtain and drainage holes,taking advantage of the elaborate simulation of seepage control system,taking reasonable control of amount of leakage,uplift pressure and seepage stability as the objective,the method can solve the quantitative optimization problem of seepage control design in large-scale hydropower projects.Taking the Mengdigou hydropower project as an example,the optimization design of seepage control factors such as extened length of the grout curtain,the rows of grouting holes and the spacing of the drainage holes is analyzed.It provides a reference for construction of the project.(4)Based on the geological and hydrogeological conditions of the Baihetan hydropower project,recharge,runoff,discharge and occurrence condition of groundwater are studied.The permeability zones and their representative value are determined.A three-dimensional finite element model representing topographic features,stratum lithology,geological structures,permeability zones,project layout and seepage control system is established.The distribution characteristics of seepage field on the left bank during impounding and considering excavationinduced disturbance effects are studied using SVA method.The effectiveness and security of seepage control system is evaluated,and the optimized design parameters of seepage control system are proposed.It shows that impounding and excavation-induced disturbance lead to changes of hydrogeological conditions and influence the distribution characteristics and seepage stability of seepage field significantly.Optimization design of seepage control system is of great significance to the maintenance of long-term safty and stability of the projects.