Soil bentonite (SB) slurry trench cutoff walls have been widely used in the USA to control ground water flow and the migration of contaminants in the ground water. While substantial laboratory testing has been conducted, field studies are limited. Researchers at Bucknell University were afforded the opportunity to conduct a suite of in situ tests on a SB cutoff wall constructed during the summer of 2008. Cutoff wall properties were measured in situ employing cone penetration tests (CPT), Marchetti dilatometer tests (DMT), vane shear tests (VST), and ground water level monitoring on both sides of the wall. Tests were conducted during construction and at times of 3 months, 6 months and 9 months after construction to evaluate the change in wall properties with time. In addition, bulk samples and a Shelby tube SB backfill sample were obtained during construction for laboratory testing which included water content, grain size distribution, consolidation and rigid wall hydraulic conductivity. The field and laboratory data were analyzed to develop a consistent understanding of the in situ properties of the cutoff wall backfill. The VST and CPT showed an increase in backfill shear strength over the time-frame of the study. A slight increase of shear strength with depth was also found. However, a comparison of shear strength measured compared with that predicted using typical ratios of strength to consolidation stress indicated that the in situ stress is less than that expected. Laboratory testing revealed a decreasing hydraulic conductivity with increasing consolidation stress demonstrating the importance of a reliable estimation of the stress state in the wall.