The 3 de Febrero Hydroelectric Project is located on the Torola River in northeastern El Salvador. The main structure is a Roller Compacted Concrete (RCC) gravity dam approximately 335 m long and 84 m high, with a crest elevation of 214.5 metres above sea level. A Grouting and Drainage Curtain (GDC) was designed and constructed to control seepage, reduce uplift pressures, and improve foundation stability. The GDC comprises drainage holes, contact grouting holes, and pressure grouting holes executed using the Grouting Intensity Number (GIN) method — a pressure–volume termination criterion — with a constant GIN value of 1500 bar·L. Construction was carried out between December 2020 and June 2022. Post-treatment Lugeon testing demonstrated a substantial permeability reduction, with final-stage verification testing confirming that 99.5% of tested intervals were classified as impermeable (≤ 3 Lugeon units). Following completion of grouting, vibrating-wire piezometers were installed to monitor conditions during first reservoir filling and long-term operation. This paper presents the grouting program design and implementation, performance verification through Lugeon testing, and the post-construction uplift pressure assessment through piezometric monitoring (November 2022 to September 2024) and finite element modelling. Particular attention is given to the right abutment zones (Monoliths M12–M16) where the most intensive treatment was applied. The integrated evidence confirms that the implemented curtain achieves its hydraulic safety objectives.