Mangdechhu Hydroelectric Project is located near Trongsa town which is in the central part of Bhutan and envisages harnessing of hydropower potential of the river Mangdechhu with a total capacity of 720 MW. Ingress of groundwater from upstream and downstream cofferdam poses one of the greatest challenges in the successful excavation of the dam pit. Ingress of groundwater during foundation excavation would impose the risk of suffusion and erosion with consequent instability of the pit slopes and would affect the construction schedule, thereby increasing the cost of the project. Foundation and cut-off scheme of the 22 m high upstream cofferdam has been systematically explored and assessment of the foundation material carried out for achieving suitable seepage control measures. In view of relative change in foundation condition vis-A-vis DPR stage studies and the presence of fine-grained sand/silt horizons, the initially suggested permeation grouting methodology for the upstream cofferdam foundation was considered inadequate, and consequently, a plastic concrete cut-off wall was adopted. The paper explains the subsoil impermeabilization below the upstream cofferdam by means of plastic concrete cut-off wall and its relative effectiveness in achieving seepage control for ease of the main dam foundation excavation, thus expediting main dam concreting works, controlling cost escalation and overall maintaining project completion schedule.