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 \u3c 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

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 %)

Soil-Bentonite Slurry Trench Cutoff Wall Proposal

This is the proposal to the National Science Foundation for a soil-bentonite slurry trench cutoff wall. The proposal includes a project summary, description of the project, and the resources implemented on the project

National Science Foundation Soil-Bentonite cutoff Wall Award Abstract

This award abstract describes the purpose of constructing a soil-bentonite cutoff wall as well as the tests that were performed to analyze the performance of the wall

Assessment of backfill hydraulic conductivity in an instrumented soil-bentonite cutoff wall

The objective of this paper is to present a comparison of measured hydraulic conductivities (k) for soil-bentonite (SB) backfill within a 60-m-long section of a 200-m-long, 7-m-deep cutoff wall constructed and instrumented for studying SB backfill properties and variability at the field scale. Backfill k was measured using flexible-wall tests (70-mm diameter) on remolded specimens prepared from surface grab samples collected during construction; flexible-wall tests on undisturbed specimens collected from the wall; larger-scale rigid-wall tests (150-mm diameter) on remolded specimens prepared from grab samples; and slug tests conducted within the wall. Applied effective stresses in the laboratory tests ranged from 4–35 kPa, encompassing the range of in-situ stresses measured in the backfill after load transfer and consolidation (8–13 kPa). The results indicate low spatial variability in k for a given test type, consistent with the observed homogeneity of the backfill. Modest variability in k was observed among the different test types, with the slug tests and rigid-wall tests generally yielding slightly higher k relative to the flexible-wall tests at field-representative stresses

Assessing the influence of chemico-osmosis on solute transport in bentonite membranes based on combined phenomenological and physical modeling

The ability of bentonite-based barriers to act as semipermeable membranes that inhibit the passage of solutes (ions) is well documented. This behavior induces chemico-osmotic liquid flux that can improve the performance of such barriers by reducing solute mass flux. This paper explores the potential significance of chemico-osmosis on solute transport through bentonite membranes using a phenomenological transport framework combined with a physical model relating the macroscale transport properties (membrane efficiency coefficient, w, and hydraulic conductivity, kh) to the microscale physicochemical and fabric properties of the bentonite. The model was used to simulate the coupled transport of monovalent salt (KCl) through a geosynthetic clay liner. The results indicate that the influence of chemico-osmosis is dependent upon the void ratio of the bentonite and the extent to which clay platelets are aggregated to form tactoids. Chemico-osmosis is predicted to have an increasingly more significant impact on solute transport with increasing source concentration (Cs0), despite decreasing w with increasing Cs0