Büyükkumla Dam, which was built on Büyükkumla Stream for drinking water purposes; is within the borders of the Gemlik District of Bursa Province, and the construction was started in 2012 and all productions were completed in 2022 and started to hold water. With the slurry trench built along the upstream and downstream route of the dam, it is aimed to prevent the groundwater that will come to the dam from the downstream and upstream regions. However, the water trapped between the downstream cofferdam and the body fill caused the water level to rise in the hull due to the underground and surface flow effect coming from the left shore in the downstream area. Due to the hydrostatic pressure effect caused by the rising water level, the slabs 15, 16, and 17 on the faceplate impermeable concrete slabs have also experienced 5-10 cm displacement. It was measured that an average of 8 lt/sec infiltration occurred from the downstream to the upstream due to this deformation. For the repair of the front face concrete slabs, two deep water wells were drilled in the area between the downstream slope of the dam body and the downstream cofferdam to drain the water accumulated in the body. After this process, it has been observed that the water level in the upstream can be lowered by evacuating the water trapped in the body. Afterward, the impermeability of the dam body was re-established by applying the methodology developed for the repair of the deformation in the concrete face. As a result, in this study, the rise of the groundwater level between the downstream cofferdam and the body of Büyükkumla Dam, causing deformation in the faceplate impermeable concrete slab by the effect of hydrostatic pressure from the body upstream, and the repair of this deformation and ensuring impermeability in the dam were discussed.