The Punatsangchhu-I Hydroelectric Powerplant project is located in the Southern Himalayas, about 80 km east of Bhutan’s capital Thimphu. The new dam reaches across the Punatsangchhu River, delimited by steep hills on both sides. For the excavation of the pit for the main dam construction, two temporary cofferdams were required upstream and downstream of the pit across the Punatsangchhu River bed, to seal off ground and river water ingress. Highly permeable non-cohesive soil layers needed to be sealed for the purpose of creating a seepage barrier underneath the upstream cofferdam. A two-phase cutter excavated cut-off wall (COW) was selected as the most appropriate and reliable methodology for the construction of the sealing element. Pretreatment works were carried out by means of gravity grouting and tube-à-manchette grouting, to temporarily seal the open void soil layers and to fix boulders and big cobbles, thus mitigating the risk of localised collapses of excavated COW trenches, and to minimize material overconsumption. The cut-off wall was constructed using one BAUER DHG-C hydraulic grab, and two BAUER BC 40 trench cutter units. The biggest challenges hereby were cutting through the numerous boulders, and the embedment of the COW into the steeply inclined, partially vertically dipping bedrock contour.