Time-Dependent Strength Behavior of Soil-Bentonite Slurry Wall Backfill

Abstract

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. Introduction The authors were presented with a unique opportunity to sample, test and instrument a SB slurry wall under construction in Delaware City, DE for containment of contaminated groundwater at an industrial site. The wall was approximately 1000-m long and typically 12to 19-meters deep. Field testing consisted of vane shear tests that were conducted at various time increments after construction and earth pressure cells that were installed in the trench at the time of backfill placement and monitored for some time afterwards. Samples of field-mixed backfill were taken for laboratory testing. Physical property testing Samples from the site were obtained as grab samples from field-mixed SB backfill (which includes a sandy base soil and bentonite) immediately prior to backfill placement in the trench. Based upon field requirements, the bentonite conent is estimated to be approximately 5%. The material was found to be a medium to fine silty clayey sand (SC) with an average of 23.7% fines with little variability from sample to sample as shown in Fig. 1 below. The average moisture content from 12 determinations was found to be 26.7%, typical of field-mixed and placed SB backfill. igure 1 Grain size distribution of Delaware City backfill (ASTM D422) he permeability of this material, based on extensive quality control testing of the fieldlump testing he time-dependent behavior of SB backfill was investigated in the laboratory using a 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0.010 0.100 1.000 10.000 Grain Diameter (mm) Pe rc en t P as si ng (b y w ei gh t) Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Unified Soil Classification :SC