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

Membrane Efficiency and Diffusive Tortuosity of a Dense Prehydrated Geosynthetic Clay Liner

In this study, a series of membrane/diffusion tests were conducted on specimens of a dense prehydrated geosynthetic clay liner (DPH GCL) subjected to KCl solutions (source concentration, Co = 8.7-160 mM) in rigid-wall cells. The source KCl solutions and de-ionized water (DIW) were circulated across the top and bottom specimen boundaries, respectively, and membrane efficiency coefficients, w, were measured based on the differential pressures induced across the specimens due to prevention of liquid flux. Also, effective salt-diffusion coefficients, Ds*, and apparent tortuosity factors, ta, were determined for each specimen based on the steady-state diffusive Cl- flux measured at the exit (bottom) boundary. The DPH GCL specimens exhibited higher w and lower Ds* (and, likewise, lower ta) relative to conventional (granular, non-prehydrated) GCL specimens tested under similar conditions. The results were consistent with the lower hydraulic conductivities, k, measured for the DPH GCL specimens and are attributed primarily to the higher bentonite dry densities in the DPH GCL specimens (1.1 Mg/m3) relative to the conventional GCL specimens (~0.4 Mg/m3), although differences in bentonite texture (powdered versus granular), bentonite type (treated versus untreated), and testing apparatus (rigid wall versus flexible wall) may have been contributing factors. Both the DPH GCL and the conventional GCL exhibited similar trends of decreasing ta with increasing w. The ta values are considered to be a function of both a matrix tortuosity factor, tm, that accounts for the geometry of the interconnected pores, and a restrictive tortuosity factor, tr, that accounts for solute exclusion due to membrane behavior. Whereas the DPH GCL exhibited a lower tm relative to the conventional GCL, both GCLs exhibited similar trends of decreasing tr with increasing w. The relationship between tr and w for both GCLs is reasonably well represented by tr = 1 – w, an expression that has been proposed for clay membranes in previous theoretical and experimental studies

2002. “Theory for Reactive Solute Transport through Clay Membrane Barriers

Abstract The theoretical development for one-dimensional, coupled migration of solutes with different ionic mobilities through clay soils that behave as ion-restrictive membranes, referred to as clay membrane barriers (CMBs), is presented. The transport formulation is based on principles of irreversible thermodynamics and accounts explicitly for coupling effects of hyperfiltration (ultrafiltration) and chemico-osmotic counter-advection associated with clay membrane behavior in the absence of electrical current. Since, by definition, no solute can enter a ''perfect'' or ''ideal'' membrane, the concept of an implicit coupling effect, such that the effective salt-diffusion coefficient, D s * approaches zero as the chemico-osmotic efficiency coefficient, x approaches unity is introduced. The theoretical development also illustrates that, even in the absence of membrane behavior, traditional advectivedispersive transport theory based on a constant value of D s * for the solutes may not be appropriate for simulating transient transport in reactive (ion exchanging) systems. This potential limitation is illustrated through simulations for solute mass flux involving the migration of a binary salt solution (KCl) through a clay barrier with exchange sites saturated with a single exchangeable cation (e.g., Na + ) that enters the pore solution upon ion exchange with the salt cation (K + ).