Long-Term Stress and Consolidation Behavior of a Soil-Bentonite Slurry Trench Cutoff Wall

Soil-bentonite slurry trench cutoff walls have been used for over 50 years to control groundwater flow and contaminant transport. During the summer of 2016, a 200-m-long and 0.9-m-wide full-scale, instrumented soil-bentonite slurry trench cutoff wall was constructed to a depth of 6.2 to 7.1 m. Primary research goals for the wall included evaluation of (1) the anisotropic stresses in the backfill, (2) stress-dependency of the backfill hydraulic conductivity, and (3) the rate of consolidation. These phenomena are time dependent and affected by the construction and post construction movement, including excavation, backfilling, backfill consolidation, and long-term backfill secondary compression (creep). This paper presents the results of six years of monitoring of earth pressure cells and pore pressure sensors. After six years, effective stresses within the wall remain far below geostatic. The maximum stresses observed are transverse to the axis of the trench and attributed to lateral deformations (i.e., inward movement of the sidewalls). The lowest stresses observed are the vertical stresses, reflecting the impact of arching or side wall friction as the soft backfill moves downward within the trench. Longitudinal horizontal stresses are intermediate between the transverse horizontal stresses and vertical stresses. For the last five years, the principal phenomenon controlling changes in stress has been secondary compression (creep).