A Case Study on the Weep Hole and Cutoff Wall Effect for Decreasing Uplift Pressure on Hydraulic Structures

Abstract

Many methods have been developed to provide an economic design of hydraulic structures to reduce uplift pressure. Example structures include diversion dams and concrete gravity dams; some of the most important pressure reduction strategies involve horizontal aprons or cutoff walls upstream and downstream of the dam. Weep holes are also used and can be placed between two cutoffs. The effect of weep holes and cutoff on uplift pressure is the main focus of this study. This study focuses on Yusefkand Mahabad diversion dam in Iran. The solutions are obtained by simulation using the finite element methods (FEM). The effect of weep holes and the impact of location and depth are investigated. The primary output that was used to assess the strategies were uplift pressure and exit hydraulic gradient. The weep holes were fabricated with a perforated PVC pipes with a filter envelope that releases excess pore water pressure under a hydraulic structure. This improves dam stability against overturning and prevents soil movement known as piping/internal erosion. On the other hand, a vertical cutoff wall under a hydraulic structure reduces pore water pressure by increasing flow path through the porous medium. Consequently, installation of a cutoff wall upstream of the dam is an effective way to reduce pressure and hydraulic gradient. Results show that an upstream cutoff with an 8 m depth decreases uplift force about 63% and decreases the exit hydraulic gradient 79% compared to the no-cutoff-wall case. Installing a weep hole downstream of a stilling basin decreases uplift force 8% and decreases the exit hydraulic gradient 74% more than without weep hole. Based on this study, design of diversion dams can be carried out with minimized concrete costs and hence a more economical design. In fact, a decline in uplift pressure allows a thinner concrete construction. Some other practical examples that demonstrate the positive effect of cutoff walls and weep holes under the hydraulic structures are provided in this study.