Soil-bentonite (SB) slurry trench cutoff walls have been used for over 30 years as subsurface vertical barriers to control ground water flow and contaminant transport. Despite the millions of square feet of cutoff wall that have been constructed, little is known about the time-dependent behavior of the backfill and, in particular, the change in shear strength with aging. From experience, practitioners know that the backfill, which is initially placed as a semi-fluid material, gains strength with time. What has not been known is whether this time-related strength gain is due entirely to consolidation of the backfill or some other mechanism. This paper presents the results of laboratory and field testing of a SB slurry trench cutoff wall. Laboratory studies included consolidation and slump testing. Field studies included testing a constructed wall using vane shear, earth pressure cells, and settlement plates. The results of these tests demonstrate an increase in shear strength within a few days of backfill placement. This initial increase in shear strength is attributed to consolidation of the low-permeability backfill. The shear strength continues to increase with time, behavior that the authors attribute to two factors. The first is secondary consolidation or creep. The second is broadly termed “aging” and is attributed in part to the thixotropic nature of the bentonite used in the backfill mixture. Recommendations for the design shear strength for SB backfill are also included.