Fortifying earth dams under duress: geosynthetic reinforcement of Nagarjuna Sagar against seepage and seismic forces

Abstract
This study presents a comprehensive numerical evaluation of the right earthen embankment of the Nagarjuna Sagar Dam (NSD), focusing on its behavior under steady-state seepage, rapid drawdown, and seismic loading conditions. Located between seismic Zones II and III, the dam was analyzed using GeoStudio software specifically SEEP/W, SLOPE/W, and QUAKE/W. Steady-state seepage analysis defined the phreatic surface and served as input for slope stability assessment, which yielded safe Factors of Safety (FOS) of 2.809 (upstream) and 1.696 (downstream) under static conditions. During rapid drawdown, the upstream FOS decreased to 2.544 due to pore pressure redistribution. Seismic loading was assessed using pseudo-static analysis with a horizontal seismic coefficient of 0.1 g, consistent with IS 1893:2002 guidelines. A 10-second duration was selected to represent the bracketed duration of significant strong motion for moderate earthquakes in stable continental regions. Displacement monitoring showed maximum crest deformation of 0.107 m. The post-earthquake FOS was calculated at 1.055, indicating marginal stability. To mitigate seismic vulnerability, horizontal geosynthetic reinforcement was introduced into the upstream shell at 4 m spacing and 20 m length. This intervention improved the post-seismic FOS to 1.714. Reinforcement material properties were validated using ASTM D 4632, D 3786, D 4833, and D 4533 standards for survivability classification. Additionally, seepage losses through the embankment and toe drain were quantified using Darcy’s Law to support long-term maintenance planning. The study concludes that geosynthetic reinforcement significantly enhances seismic stability and provides a viable strategy for retrofitting zoned earth dams in moderate seismic environments.