Research On Stability Of Large Earth-rock Cofferdam Under The Seepage Effection

The water resoure and hydropower projects have developed fastly in recent yearsin China, and the safety and stability of these important structures have gradually been asignificant topic in the geotechnical field. And, the cofferdam as a key pre-constructionpart of the hydraulic project will also play an important role on the the projectconstruction and the project quality, especially will influence on the people’s lives andproperty downstream. As for the earth-rock cofferdam, the water of the reservoir isimportant for the project safety considering the seepage effect and hydro-mechanicaleffect. Therefore, the earth-rock cofferdam upstream in Jinsha River as a case study,based on theoretical analysis and numerical simulation, the seepage characteristics andstability of this cofferdam have been deeply researched. The main work and the resultsare as follows:(1) Simulate the earth-rock cofferdam construction process, and then analyze itsstress settlement to obtain weir foundation settlement, total settlement and thesettlement variation and the possibility of crack is predicted and evaluated.(2) Simulate the steady seepage field of the earth-rock cofferdam consideringdifferent reservoir level, and then analyze its seepage, sensitivity, and stability to obtainvariation of the weir permeability and the weir slope minimum safety. The resultsindicated that: different reservoir level resulted in different permeability of the weir, andin the weir area penetration factor was more sensitive to the depth and permeabilitycoefficient of the cutoff wall. The stability of weir slope downstream considering steadyseepage was less than the weir slope upstream. The results indicated that the higherwater level upstream was, the smaller the safety coefficient was.(3) Simulate the unsteady seepage of the earth-rock cofferdam when the waterlevel changed, and then analyze the unsteady seepage field and stability to get variationtrend of the weir usteady permeability and minimum safety coefficient of the weir slope.The results indicated that: the changing speed of water level affecting the seepage fieldand the slope stability was very obvious. When the changing speed of water level is lessthan0.5m/d, the internal water level of the weir changed simultaneously with thereservoir water level, and the change speed of water level was greater than3.0m/d, theinternal water level of the weir changed greatly with the reservoir water level. Theresults indicated that the faster the water level changed, the lower weir slope stability was.(4) Based on the research results of the steady seepage, simulate thehydro-mechanical coupling effect of the cofferdam under design water level earth-rockcofferdam to get the force variation of the weir and minimum safety coefficients of theweir slope. The results showed that: considering hydro-mechanical coupling effect, theweir stress increased, the vertical displacement decreased, the horizontal displacementincreased, and the stability of the weir slope reduced, especially as for the area with thesmaller permeability, the coupling effect was not very obvious and also the couplingeffect generated instantaneously mechanical disturbance for the structures.