Behavior of earth-rock cofferdam built on deep overburden foundation considering seepage effect

An increasing number of water conservancy and hydropower projects are being constructed on deep overburden foundations. Deep overburden foundations have an important influence on the mechanical behavior of earth-rock cofferdams built on them. A three-dimensional coupled seepage-stress finite element model was established to compare the two schemes under different cutoff-wall penetration depths and rapid-drawdown conditions. This paper presents a comparative analysis of the seepage and mechanical behavior of earth-rock cofferdams built on deep overburden foundations with two seepage-control schemes, namely, a composite geomembrane core-wall scheme and a composite geomembrane inclined-wall scheme. The seepage-control performance, stress and deformation characteristics, and slope stability of the cofferdams under the two seepage-control schemes are discussed. In addition, the behavior of the cutoff wall in the deep overburden under different conditions is compared. The results show that when the cutoff wall completely penetrates the overburden, it can effectively reduce the total head and lower the phreatic line in the cofferdam. For all cutoff wall depths, the seepage flow of the core wall scheme is lower than that of the inclined wall scheme. The stress distribution patterns in the cofferdam and cutoff wall are essentially the same for the core wall and inclined wall schemes. However, the tensile stress is slightly higher in the core wall scheme. During water level drawdown, the safety factor of the cofferdam is generally higher for the inclined wall scheme than for the core wall scheme.