Three Dimensional Analysis Of Seepage Flow And Performance Assessment Of Seepage Control At The Dam Site Of Mengdigou Hydropower Station

The seepage behavior of rock mass and engineering construction material is one of the key factors affecting the stability and safety of water conservancy and hydropower project. The seepage control measures, such as impervious curtain, drainage holes and filtration are the major means to control the seepage field. It is crucial to ensure the long-term stability and the operation safety of hydraulic structures, dam foundation, slope and caverns. Particular concerns are paid on the performance assessment of Mengdigou Hydropower Station on Yalong River, based on the strain-dependent hydraulic conductivity model and the Signorini variational inequalities method. The excavation- induced permeability changes in the surrounding rocks were characterized. The distribution of seepage field is analyzed, and the effectiveness of seepage control system is evaluated. The design parameters of seepage control system is finaly optimized.The major research contents of this paper are as followed:(1) Based on dam engineering geological and hydrogeological survey data, the distribution and occurrence characteristics as well as the discharge, runoff and recharge conditions of groundwater were analyzed, combined with drill pressure packer test result, researched partitioning and the permeability parameter values dam site area of rock. The results indicated that, the groundwater is mainly affected by rainfall, and recharged to river. The permeability of rock can be divide into five partitions, i.e. strong permeability, moderate permeability, upper part of weak permeability, lower part of weak permeability and slight permeability. In general, the permeability reduces with the increase of vertical distances.(2) A 3-D finite element mesh was generated, which well modelled the geological conditions, seepage control system and main structures of project area. Using the seepage analysis method which based on Signorini variational inequalities with drainage substructure analyzed the distribution of seepage field. The results indicated that, the seepage drainage system significantly reduces the pressure of dam foundation, the massive concrete block of downstream and the powerhouse. The seepage discharge has been effectively controlled, and the seepage stability of weak structure plane meets the safety requirements engineering. Seepage control schemes generally reasonable and effective.(3) Based on the optimizing analysis of the curtain grouting hole spacing, layout and drainage hole spacing, the result showed that the extending of the seepage control system of right bank in dam foundation to Fault f5 is better for the stability of right slope. The upper part of the massive concrete block and the powerhouse remain in a dry operation state over the free surface, the spacing of the corresponding drainage holes can be enlarged.(4) The permeability variation of excavation disturbed zone around the underground caverns was revealed based on strain-dependent permeability model, and its influence on seepage field distribution and effect on seepage control system were systematically studied. The result showed that, the permeability around caverns will increased 1~3 orders of magnitude after excavation, and the influence scale up to 40~50m. The free surface will be lowered and the gradient will be decreased, but the leakage tends to be increased. Therefore, the capacity of pump should increased.Research results provide a theoretical basis and technical support for the seepage control system optimization design of Mengdigou hydropower station, and and also can provide reference for other similar projects.