Soil bentonite (SB) slurry trench cutoff walls have been widely used for over 40 years to control ground water flow, seepage through dams and levees, and contaminant transport. The hydraulic conductivity of SB backfill is stress dependent and the concept that the state-of-stress in SB slurry trench cutoff walls is less than geostatic was first published over 30 years ago. A full-scale, instrumented SB slurry trench cutoff wall was constructed with embedded instrumentation that directly measures the stress and pore pressure. In addition, cone penetration (CPT) and Marchetti dilatometer (DMT) tests were performed in the wall after the wall was allowed to sit for one year. The combination of direct stress measurements and in situ tests presents a unique opportunity to compare results from CPT and DMT tests to the direct measurements of stress in the wall and to predictions from available models. Finally, the authors provide summary opinions regarding cone and dilatometer testing for predicting stress in low strength materials along with practical recommendations for the design and construction of SB walls.