Buyukkumla Dam, which is a concrete faced rockfill dam is located on Buyukkumla stream, 10,00 km northwest of Gemlik district of Bursa. It is intended to provide drinking and industrial water of 19 million m 3 per year to Gemlik, Kucukkumla, Buyukkumla, Narli, Gencali, Kursunlu and Umurbey region. The dam has -10,00 m foundation level and 68,00 m crest level, where the parapet elevation is 70,00 m. The crest width and length of Buyukkumla Dam are 10,00 m and 385,00 m, respectively. The diversion is provided by a tunnel with a diameter of 3,00 m. Spillway is designed with a threshold width of 25,00 m, hand-held and uncontrolled. The excavation of the foundation of the dam was carried out below sea level. In order to make the dam body excavations in a dry environment, it was mandatory to eliminate seawater and groundwater effects. Therefore, by applying slurry trench, seawater effect was removed from the dam reservoir and the leaks under the dam were taken under control. This method was applied first time at the Aslantas Dam in Turkey and became successful to eliminate the effect of ground water level. In the slurry trench method, the purpose is to prevent leakage or to prolong the infiltration. In Buyukkumla Dam, slurry trench along the axillary axes was applied under upstream and downstream cofferdams. In this article, cutoff walls were modeled by using Seep/w program and the results of the field measurements were compared with the analysis results.