This paper presents a case study of a deep soil-cement-bentonite (SCB) slurry trench cutoff wall constructed outside of Smithland, KY in 2010. Installed to a maximum depth of 56 m, this cutoff wall is the deepest known seepage barrier installed using continuous trenching. The wall was installed around the perimeter of a deep excavation to reduce long-term dewatering costs associated with construction of a hydroelectric power plant adjacent to the Ohio River. After wall construction, a dewatering system was installed inside the area enclosed by the wall to facilitate the deep excavation. Pre-construction design and construction details are presented along with the results of a post-construction assessment of the hydraulic conductivity (k) of the wall. Steady state groundwater flow measurements from the dewatering system coupled with information on the wall thickness and water levels inside and outside of the wall were used to obtain a large scale estimate of the in-situ k of the wall. The in-situ k was compared to laboratory k values measured for specimens prepared from grab samples of the as-mixed SCB backfill. Comparisons also were made to the target (design) k and the expected field mixed backfill k established during a preconstruction bench-scale study. The comparisons revealed that the in-situ k is approximately one order of magnitude less than the design k and approximately equal to the average laboratory k measured from grab samples and the expected k from the results of the bench scale study.