Investigation of the effect of the different configurations of double-cutoff walls beneath hydraulic structures on uplift forces and exit hydraulic gradients

Abstract

In the design of hydraulic structures, cutoff walls are needed to reduce uplift force (U) and the exit hydraulic gradient (GR). Usually two cutoff walls are required; an upstream cutoff wall is used to reduce the resultant of uplift force and a downstream cutoff wall is used to reduce the exit hydraulic gradient. This study numerically investigates double-cutoff walls beneath hydraulic structures with variations in their location and depth. Governing equations with specified boundary conditions are solved using the finite element method (FEM). Results show that if the downstream cutoff wall is deeper than the upstream cutoff wall, the resultant of uplift force will increase (compared to the no-cutoff wall case). With a constant hydraulic structure width (B), decreasing the distance between the two cutoffs (L), results in a reduction of uplift force. For L/B = 0.7, the change in the slope of the uplift force is large. Increases in the permeable depth (D) and reductions of B will lower the uplift forces. Increases in the downstream cutoff depth (d2) and L result in a reduction in the exit hydraulic gradient (GR). When the two cutoffs are located at the end of the hydraulic structure, GR is lower than when the downstream cutoff wall is located at a more downstream location. Comparisons between the available analytical solutions and computational results for two equal cutoffs show that the FEM can predict U and GR with a maximum of 5% error.