Hydraulic sustainability of soil-bentonite cutoff walls subjected to cyclic wetting and drying

This study investigated the effect of cyclic wetting and drying on the backfill used in soil-bentonite (SB) cutoff walls. For this purpose, model SB vertical cutoff wall backfills were prepared comprising of a fine grained mortar sand and 2% bentonite (by total weight) and 4% bentonite (by total weight). Results of the study indicate that the volume change is influenced by the bentonite content, that is, the increase in volume change increased with increasing bentonite content.

Compressibility and Hydraulic Conductivity of Zeolite-Amended Soil-Bentonite Backfills

The effect of zeolite amendment for enhanced sorption capacity on the consolidation behavior and hydraulic conductivity, k, of a typical soil-bentonite (SB) backfill for vertical cutoff walls was evaluated via laboratory testing. The consolidation behavior and k of test specimens containing fine sand, 5.8 % (dry wt.) sodium bentonite, and 0, 2, 5, or 10 % (dry wt.) of one of three types of zeolite (clinoptilolite, chabazite-lower bed, or chabazite-upper bed) were measured using fixed-ring oedometers, and k also was measured on separate specimens using a flexible-wall permeameter. The results indicated that addition of a zeolite had little impact on either the consolidation behavior or the k of the backfill, regardless of the amount or type of zeolite. For example, the compression index, Cc, for the unamended backfill specimen was 0.24, whereas values of Cc for the zeolite amended specimens were in the range 0.19 ≤ Cc ≤ 0.23. Similarly, the k for the unamended specimen based on flexible-wall tests was 2.4 x 10-10 m/s, whereas values of k for zeolite amended specimens were in the range 1.2 x 10-10 ≤ k ≤ 3.9 x 10-10 m/s. The results of the study suggest that enhancing the sorption capacity of typical SB backfills via zeolite amendment is not likely to have a significant effect on the consolidation behavior or k of the backfill, provided that the amount of zeolite added is small (≤ 10 %).

Hydraulic Conductivity of Model Soil-Bentonite Backfills Subjected to Wet-Dry Cycling

The potential for changes in hydraulic conductivity, k, of two model soil-bentonite (SB) backfills subjected to wet-dry cycling was investigated. The backfills were prepared with the same base soil (clean, fine sand) but different bentonite contents (2.7 and 5.6 dry wt %). Saturation (S), volume change, and k of consolidated backfill specimens (effective stress = 24 kPa) were evaluated over three to seven cycles in which the matric suction, Ym, in the drying stage ranged from 50 to 700 kPa. Both backfills exhibited susceptibility to degradation in k caused by wet-dry cycling. Mean values of k for specimens dried at Ym = 50 kPa (S = 30-60 % after drying) remained low after two cycles, but increased by 5- to 300-fold after three or more cycles. Specimens dried at Ym ≥ 150 kPa (S < 30 % after drying) were less resilient and exhibited 500- to 10 000-fold increases in k after three or more cycles. The greater increases in k for these specimens correlated with greater vertical shrinkage upon drying. The findings suggest that increases in hydraulic conductivity due to wet-dry cycling may be a concern for SB vertical barriers located within the zone of a fluctuating groundwater table.

Chemical Compatibility of Model Soil-Bentonite Backfill Containing Multiswellable Bentonite

The objective of this study was to evaluate the chemical compatibility of model soil-bentonite backfills containing multiswellable bentonite (MSB) relative to that of similar backfills containing untreated sodium (Na) bentonite or a commercially available, contaminant resistant bentonite (SW101). Flexible-wall tests were conducted on consolidated backfill specimens (effective stress =34.5 kPa) containing clean sand and 4.5–5.7% bentonite (by dry weight) using tap water and calcium chloride (CaCl2) solutions (10–1,000 mM) as the permeant liquids. Final values of hydraulic conductivity (k) and intrinsic permeability (K) to the CaCl2 solutions were determined after achieving both short-term termination criteria as defined by ASTM D5084 and long-term termination criteria for chemical equilibrium between the influent and effluent. Specimens containing MSB exhibited the smallest increases in k and K upon permeation with a given CaCl2 solution relative to specimens containing untreated Na bentonite or SW101. However, none of the specimens exhibited more than a five-fold increase in k or K, regardless of CaCl2 concentration or bentonite type. Final k values for specimens permeated with a given CaCl2 solution after permeation with tap water were similar to those for specimens of the same backfill permeated with only the CaCl2 solution, indicating that the order of permeation had no significant effect on k. Also, final k values for all specimens were within a factor of two of the k measured after achieving the ASTM D5084 termination criteria. Thus, use of only the ASTM D5084 criteria would have been sufficient to obtain reasonable estimates of long-term hydraulic conductivity for the specimens in this study.