Soil-bentonite slurry trench cutoff walls are widely used for seepage control, levee repair, and pollutant containment. Their widespread use in these critical applications requires a better understanding of their as-built condition and long-term behavior. The in situ hydraulic conductivity of soil-bentonite cutoff walls is stress dependent. Changes in hydraulic conductivity of the wall over time can result from changes in stress due to consolidation and load transfer though shear to the formation along the sidewalls of the trench. This paper describes a soilbentonite cutoff wall designed, constructed, and instrumented for the principal purpose of research. The cutoff wall instrumentation is designed to monitor the in situ conditions in the backfill in three dimensions (transverse, longitudinal and vertical), vertical and lateral deformations, and pore water pressures. All data is being collected as a function of time and location within the wall.