Seepage in Iranamadu Dam and Its Influence on the Stability

Abstract

Earth dams have played an important role in agriculture since ancient times. Failure of side slopes caused due to seepage or drawdown is one of the major considerations in the design of earth dams as the failure of a slope is catastrophic. The Iranamadu dam which was raised at different times showed a significant increase of seepage when the reservoir level increased beyond a certain elevation. In this research, the seepage problem of the Iranamadu dam was analysed using Geo-Studio (SEEP/W) software to estimate the seepage rate with the determination of the phreatic surface and to investigate the influence of the anisotropy of hydraulic conductivity of core material on seepage. For these analyses, volumetric water content function to be used in the unsaturated zone above the phreatic surface was selected from the van Genuchten model. Both steady and transient state seepage were carried out to compare the seepage rates with field measurements and the effect of anisotropy of hydraulic conductivity using the observed reservoir level history spanning over 170 days. Furthermore, the stability of the dam was assessed under seepage conditions using the SLOPE/W software. SEEP/W results show a good agreement in seepage rates with field measurements under both seepage condition when the reservoir level is above a certain elevation. Within a small range of reservoir level, some deviation of the predicted seepage rate is observed. Additionally, when the phreatic surface passes through the core section, a drop-in the seepage rate was observed due to the low permeability of the clay core. The seepage rate increased with k_y/k_x ratio and the Iranamadu dam seepage field measurements showed a good agreement when k_y/k_x ratio is 1. Therefore, the behaviour of the Iranamadu dam with respect to the seepage flow, shows a near isotropic hydraulic behaviour. The SLOPE/W results conclude that when the reservoir water level increases, the stability of the downstream slope reduced beyond the recommended value but the stability of the upstream slope was found to be satisfying the safety criteria under both steady and transient seepage conditions.