Numerical Simulation Using the Finite Element Method to Investigate the Effect of Horizontal Drains and Cutoff Walls on Seepage and Uplift Pressure in Heterogeneous Earth Dams

Abstract

Design of earth dams and their problems are important considerations both during and after construction because of their potential hazards and the consequences that dam failure provides for downstream populations. This study focuses on the effectiveness of using cut off walls and horizontal drains for reducing seepage flow from a typical heterogeneous earth dam. The main objective of this study is to find a suitable horizontal drain length in earth dam downstream shells and on the other hand to find the best position for cut off walls in earth dam foundations. For this purpose, various cut off wall depths and horizontal drain lengths are positioned under the earth dam at different locations.  Seepage analysis, hydraulic gradient and uplift pressure are computed by numerical simulation, using the finite element method (FEM). Results showed that increasing horizontal drain length causes a slight increase in the seepage rate and hydraulic gradient. The optimal location of a cut off wall for reducing seepage and piping/undermining is in the middle of the dam foundation. By increasing cut off wall depth, seepage from the earth dam and its foundation is reduced. Changing the location of the cut off wall has little effect on exit hydraulic gradient and it is always less than unity. Installation of a cut off wall in the middle of the foundation results in a 20% decreasing in hydraulic gradient compared to a cut off wall positioned upstream of the dam.