Gemmina Di Emidio

DIFFUSION OF CALCIUM CHLORIDE IN A MODIFIED BENTONITE: IMPACT ON OSMOTIC EFFICIENCY AND HYDRAULIC CONDUCTIVITY

Chemically modified bentonites are being developed with the aim of preserving low hydraulic conductivity in the presence of potentially aggressive permeants in pollutant-containment applications. ‘Multiswellable’ bentonite (MSB) has been obtained by treating standard sodium bentonite with propylene carbonate. Research on the engineering properties of MSB has focused mainly on permeability and chemical compatibility. Solute diffusion and membrane behavior in MSB have not yet been investigated. A combined chemico-osmotic/diffusion test was performed on a MSB specimen using a 5 mM CaCl2 solution. Permeability with distilled water and with the 5 mM CaCl2 solution was measured prior to and after the chemico-osmotic/diffusion tests. The material exhibited time-dependent membrane behavior with a peak osmotic efficiency value (ω) of 0.172 that gradually shifted to zero upon breakthrough of calcium ions. Effective diffusion coefficients of calcium and chloride ions were in the range commonly described for untreated bentonite at similar porosities. After the chemico-osmotic/diffusion stage and permeation with 5 mM CaCl2, the hydraulic conductivity of MSB increased from 1.1 × 10−11 m/s to 7.0 × 10−11 m/s. The MSB was apparently converted into a calcium-exchanged bentonite at the end of the test. Prehydration and subsequent permeation might have contributed to elution of the organic additive from the clay. Further investigation is recommended to clarify the effect of prehydration on the hydraulic performance of MSB in the presence of potentially aggressive permeants.

Advances in geosynthetic clay liners: polymer enhanced clays

Bentonite clay is widely used in Geosynthetic Clay Liners (GCLs), because of its elevated sealing capacity in presence of water. However, exposure to high concentrated inorganic solutions can change the clay fabric increasing its hydraulic conductivity. To improve the hydraulic performance of GCLs, a chemical-resistant clay (HYPER clay) was developed through treatment of a natural bentonite with an anionic polymer, Sodium Carboxymethyl Cellulose (Na-CMC). The swelling ability, water adsorption capacity and hydraulic conductivity of the HYPER clay were evaluated. Test results showed the beneficial effect of the anionic-polymer treatment on the swelling, water adsorption and hydraulic performance of the clay.

Hydraulic conductivity of a dense prehydrated GCL subjected to partial desiccation

A dense prehydrated GCL (DPH) was subjected to partial but increasingly severe drying cycles alternated with permeation with a calcium solution. The permeability remained low for desiccation water contents higher than the manufacturing (prehydration) water content. Increasingly long permeation was however necessary to recover low permeability. The permeability increased irreversibly for desiccation water content lower than that of prehydration. Test results indicate the permeability of DPH GCLs subjected to dry/wet cycles coupled with cation exchange strongly depends on the extent of desiccation.

Hydraulic Conductivity of Sand-Bentonite Backfills Containing HYPER Clay

The hydraulic conductivity, k, of model sand-bentonite backfills containing HYPER clay (Na bentonite treated with carboxymethyl cellulose (CMC)) was investigated in this study. Flexible-wall tests were performed on backfill specimens composed of clean, fine sand and 2.7-5.6 % HYPER clay containing either 2 % CMC (HC2) or 8 % CMC (HC8). The geometric mean k to water (kw) for HC8 specimens decreased by nearly two orders of magnitude (from ~3x10 -9 m/s to ~3x10 - 11 m/s) with increasing HC8 content from 2.7 to 5.6 %. The geometric mean kw of 3x10 -11 m/s for the 5.6 % HC8 backfill was nearly an order of magnitude lower than kw reported previously for similar backfill specimens containing 5.7 % Na bentonite (Naturalgel ® (NG)) or 5.6 % multiswellable bentonite (MSB) due to the greater water absorption and swell capacity of HC8 relative to the NG and MSB. The 5.6 % HC2 specimens exhibited slightly lower kw relative to specimens containing 5.7 % NG or 5.6 % MSB. Also, whereas the NG and MSB specimens exhibited increases in k when the permeant liquid was changed to a 10-mM CaCl2 solution, no increases were observed for 5.6 % HC2. Although further testing is needed, the results illustrate the potential for HYPER clay to enhance the hydraulic performance of soil-bentonite vertical barriers.

Unsaturated permeability and retention curve determination from in-flight weight measurements in a bench-scale centrifuge

Centrifuges have been used in many ways to determine permeabilities, as they can significantly shorten testing times in low-permeable soils. Typically equilibrium profiles are used, or inflow-outflow measurements, or direct measurements from inside the sample, like from tensiometers or radioactive decay. Recently weight measurements of the sample outside the centrifuge were used effectively in an adapted setup. We present the results of the possibilities offered by doing transient weight measurements of a soil-sample during rotation in the centrifuge. We present the set-up of such an experiment and how the unsaturated permeability and water retention curve can be recovered from it. This eliminates the overhead of doing measurements inside the sample.

Monitoring the impact of sulfate attack on a cement-clay mix

Cement-clay mixtures are regularly employed as cut-off walls to isolate polluted soils or in ground improvement technologies. In this research, an alternative method to monitor the mechanical and hydraulic behavior of cement-clay mixtures is proposed. Bender elements were installed in a hydraulic conductivity cell, to monitor simultaneously the small-strain shear modulus (G0) and the hydraulic conductivity of a cement-treated kaolin clay mixed with blast furnace slag cement. During permeation with deionized water, an increase of G0 with time was observed, due to cement hydration (hardening). Conversely, after permeation with a sulfate solution, a sudden decrease of G0 and a gradual increase of the hydraulic conductivity were noticed. The interparticle cementation of the mixture was probably affected by contact with sulfates, which are particularly aggressive to cement. Monitoring of G0 was shown to provide valuable additional information to study the impact of sulfate attack on the mechanical and hydraulic behavior of cement-clay mixtures.

Modified clays for barriers: a review

The aim of this Specialized Lecture is to present the recent advances and issues, as well as original research, on Modified Clays for Barriers. Topics of interest include: (1) long-term hydraulic performance of modified clays for GCLs, (2) chemico-osmotic and diffusion efficiency of modified clays, (3) modeling coupled chemical-hydraulic-mechanical behavior of modified clays, (4) wet and dry ageing of modified clays, (5) use of novel bentonites for vertical barrier applications, and (6) organoclays for various barrier applications. In addition, the possible reuse of dredged sediments after polymer treatment will also be discussed. Environmental management and handling of dredged sediments are important worldwide because enormous amounts of dredged material emerge from maintenance, construction and remedial works within water systems. Usually these materials after temporary upland disposal in lagoons are disposed in landfills. The aim of this study is to analyse the possible reuse of these sediments as a low-cost alternative material for landfill covers. The mechanisms through which polymers can improve the efficiency of dredged sediments for waste containment low permeable barriers are discussed.

Wet and Dry Effects on the Hydraulic Conductivity of a Polymer Treated GCL Prototype

Geosynthetic clay liners (GCLs) are widely used to isolate pollutants because of their low hydraulic conductivity to water. However, the performance of clay barriers may be impaired by prolonged exposure to electrolytic liquids which may lead to the compression of the diffuse double layer. The consequences are the increase of permeability and the loss of self-healing capacity. Moreover, the efficiency of the liners can further deteriorate by repeated wet and dry cycles, which may lead to desiccation of the bentonite and associated cracking. Modified bentonites have been introduced to improve the resistance of clay barriers to aggressive solutions. This study deals with a polymer-amended clay, HYPER clay. HYPER clay is treated with an anionic polymer and dehydrated and it shows enhanced performance in presence of electrolyte solutions. The effect of wet and dry cycles on the hydraulic conductivity to seawater of needle-punched GCLs prototypes of treated and untreated bentonite was investigated. The prototype samples containing HYPER clay 8% showed lower permeability compared to those containing untreated bentonite. However, the temperature suggested from the standard used in this study is extremely high and it does not represent the temperature in the field.

Lime-stabilisation of high plasticity swelling clay from Ethiopia

Abstract In the present study, highly expansive clay soils from the Highlands of Ethiopia were studied to evaluate the efficiency of lime treatment to improve their mechanical properties for road subgrades. Soils treated with quick lime at 5, 7 and 9% by dry weight of the soil were cured for seven days under controlled temperature of 40 ± 2 °C and geomechanical laboratory tests were conducted to evaluate its impact on the engineering properties of the soil. Test results show substantial improvements in the properties of the soil after lime treatment. Addition of lime significantly reduces the plasticity index and swelling potential of the soil. Similarly, despite the reduction of optimum proctor dry density due to lime treatment, the unconfined compressive strength and the California bearing ratio show considerable improvements. Based on the current study, expansive soils of the studied area can be effectively stabilised for road subgrade works with the addition of 7% quick lime by dry weight of the soil. For very problematic soil, as the one investigated here, the drastic reduction of swelling potential is of particular interest for a possible application of road subgrade.

Discussion of “Chemical Compatibility of Model Soil-Bentonite Backfill Containing Multiswellable Bentonite” by Michael A. Malusis and Matthew D. McKeehan

The paper discussed presents a comprehensive and very interesting investigation regarding the chemical compatibility of soil-bentonite backfills to CaCl2 solutions. Standard (NG) as well as modified bentonites (MSB and SW101) were used as amendments. The discussion paper commented on some aspects that dealt with the paper in light of their past experience with MSB.