Coupled chemical-hydraulic-mechanical behaviour of bentonites

Abstract

Bentonites are clay soils characterised by a high specific surface and a permanent negative electric charge on their solid skeleton. Their common use as hydraulic and contaminant barriers for landfill and soil remediation applications, including the final disposal of nuclear waste, needs to be supported by adequate theoretical modelling of their mechanical behaviour and transport properties, in order to assess the expected performance in the long term. To this end, a theoretical approach has been proposed in order to derive constitutive equations for their coupled chemical-hydraulic-mechanical behaviour. The phenomenological parameters that govern the transport of electrolyte solutions through bentonites-that is, the hydraulic conductivity, the reflection coefficient (which is also called the chemico-osmotic efficiency coefficient), and the osmotic effective diffusion coefficient-have been measured through laboratory tests on a bentonite with porosity of 0. 81, over a range of sodium chloride concentrations in the pore solution that varied from 5 mM to 100 mM. The relevance of the osmotic phenomena has been shown to decrease when the salt concentration increases. The obtained results have been interpreted by assuming that the microscopic deviations of the pore solution state variables from their average values are negligible. In this way, it is possible to interpret the macroscopic behaviour on the basis of the physical and chemical properties of the bentonite mineralogical components.

Membrane behavior of compacted sand–bentonite mixture

“…In addition to low k, the existence of semi-permeable membrane behavior, or the ability to restrict the migration of aqueous miscible chemical species (solutes), has been demonstrated extensively for many bentonite-based barriers, including GCLs, compacted natural clay amended with bentonite, and soil-bentonite backfills for in situ vertical cutoff walls (Kemper and Rollins 1966;Barbour and Fredlund 1989;Malusis et al 2001;Malusis and Shackelford 2002;Shackelford and Lee 2003;Yeo et al 2005;Henning et al 2006;Dominijanni and Manassero 2008;Evans et al 2008;Shackelford 2009, 2011;Shackelford 2011Shackelford , 2012Bohnhoff and Shackelford 2013;Dominijanni et al 2013;Meier et al 2014;Tang et al 2014aTang et al , 2014bTang et al , 2015Shackelford and Scalia 2016). Solute (e.g., contaminant) restriction results in reduced solute mass flux and leads to chemico-osmosis, or the movement of liquid from high to low solvent (H 2 O) chemical potential (low to high solute concentration), such that the containment function of bentonite-based barriers that exhibit semi-permeable membrane behavior is enhanced Malusis et al 2003;Manassero and Dominijanni 2003;Malusis et al 2014;Shackelford , 2014.…”

Use of Geosynthetic Clay Liners for Pollutant Control

“…The proposed model provided a satisfactory interpretation of the experimental results of Malusis and Shackelford [13,14], Malusis et al [15] and Dominijanni et al [5] in terms of osmotic efficiency, , of two bentonites (see tables 2 and 3 and figure 1) tested with solutions characterized by salt (KCl, NaCl) concentrations up to a maximum of c s = 100 mM. The experimental data are in good agreement with the linear relationship, predicted by the theoretical model, relating the restrictive tortuosity factor, τ r , to the osmotic efficiency coefficient, , as follows:…”

“…Equation (2a) gives the effective specific surface, S eff , based on the single platelet specific surface, S (= 760 m 2 g -1 ), and the basic state parameter N l, AV . Moreover, another useful state parameter can be directly derived from the latter through the following equation (see also Dominijanni et al) [5]:…”

“…where τ r represents the ratio between the osmotic effective diffusion coefficient, * , D  that is measured during an osmotic test (Malusis et al [16]; Malusis and Shackelford [14]; Dominijanni et al [5]) and the effective salt diffusion coefficient, , that is obtained by extrapolating the value of * D  at  = 0. Also, the evaluation of the osmotic swelling pressure theoretically calculated on these samples by the use of the same input parameters, referring in particular to N l, AV , S eff , , 0 , sk c was in very good agreement with the related experimental results (see Manassero et al [17]).…”

“…In the case of the higher ion concentrations, the fabric of the bentonites undergoes major changes due to flocculation phenomenon under low confining stress (high void ratio), resulting in a significant increase in the average number of platelets per tactoid and a correspondent decrease in the effective specific surface and fixed charge concentration of the solid skeleton, as summarized in table 4. -Referring to a first series of hydraulic conductivity tests on different bentonites and permeant solutions, an assessment of the effective specific surface, S eff , has been performed using the following equation (Kozeny [18]; Carman [19]; Dominijanni et al [5]): …”