Parameters Optimization Of The Cutoff Wall And Seepage-stress Coupling Analysis Of Dam Body In Earth-rock Cofferdam

As an important anti-seepage structure,plastic concrete cutoff wall has good and reliable antiseepage effectiveness and mature construction methods.It is widely used in various types of earthrock cofferdams or dams for controlling seepage.It is necessary to select an optimal scheme of cutoff wall for earth-rock cofferdams,because the structure of the cutoff wall in the earth-rock cofferdam has an important influence on the seepage flow and the stability of the cofferdam.In this paper,the optimal design of the thickness and depth of cutoff wall is studied by finite element method(FEM)taking as an example of a certain earth-rock cofferdam for construction,which is located in the Pearl River Basin.And then the system optimal design result had been analyzed considering the coupling seepage and stress.The main work and achievements of this paper can be concluded as follows:1.The seepage fields based on 33 types of cutoff wall scheme which is composed of no concrete cutoff wall and 32 types of concrete cutoff wall about its thickness and its embedded depth within the weakly weathered rock were simulated.Firstly,the control index,such as the seepage per unit width of cofferdam,the water head behind the cutoff wall,the seepage gradient at the bottom of cutoff wall and the cofferdam foot,were calculated and compared with the allowable value.Secondly,normalized analysis of construction period and cost at different penetration depths of cut-off walls was developed.Finally,the optimized anti-seepage scheme of cutoff wall was obtained.The cutoff wall with 0.8 m in thickness and 2 m in embedded depth within weakly weathered rock is regarded as the optimal anti-seepage scheme for this project after considering the project input and construction convenience.2.The steady-state and non-steady-state seepage calculation of the earth-rock cofferdam with the optimized design parameters of the cut-off wall was made,then its deformation laws were introduced.The main conclusions are as follows: the seepage prevention effect of the cutoff wall is remarkable;water flows around the bottom of the cutoff wall and the shape of its path is semicircular;the water flows around the bottom of the cutoff wall in a semi-circular path;the shape of the saturation line is "Z";the saturation line lags behind the water level of the upstream reservoir;the seepage gradient at the cofferdam foot and inside the cutoff wall decreases as the water level decreases.3.The stability of cofferdam stress field was calculated,then its deformation laws were introduced.The main conclusions are as follows: when the earth-rock cofferdams are unstable,the weakest parts are located near the cofferdam foot and the connection between the upper part of the cutoff wall and its surroundings;stress concentration exists near the cutoff wall;the safety factor of the earth-rock cofferdam is between 2.10 and 2.71,which indicates that the anti-seepage facilities designed by the project are effective and safe.