Hydro-mechanical behaviour of shot-clay bentonite

Experimental tests were conducted to characterise the hydro-mechanical behaviour of shot-clay MX-80 bentonite. Granular bentonite was mixed continuously with water and shot on the walls of a tunnel section at the Grimsel Underground Research Laboratory (URL), in Switzerland. The shot-clay was placed to create a layer of bentonite in direct contact with the host rock to avoid preferential water and/or gas flow along the tunnel wall. Samples for an experimental programme were collected during the shooting. Results from an experiment, conducted under controlled total suction to analyse the hydro-mechanical behaviour of the material, are show

Water retention behaviour and microstructural evolution of MX-80 bentonite during wetting and drying cycles

MX-80 bentonite used in engineered barrier systems would be subjected to wetting and drying cycles. To assess the response of the material under such circumstances, a comprehensive experimental characterisation of the water retention behaviour of compacted MX-80 granular bentonite was performed in this study. A new methodology is proposed to investigate this behaviour under a constant volume condition for specimens prepared at different dry densities. The material was subjected to different hydraulic paths, including cyclic variations of the water content. As a result, an irreversible modification of the retention behaviour was observed when the material approached a fully saturated state during the first main wetting, and a new hydraulic domain was consequently created. The water retention capacity of the material increased as a result of such modification. Microstructural observations were performed at different stages of the hydraulic paths to relate the permanent change in the retention behaviour to the evolution of the fabric during the wetting and drying cycles. A clear transition from a double-structured to a single-structured fabric, followed by a permanent change of the microfabric, was found following the first wetting. Available data on the hydration of smectite particles were used to relate the microstructural evolution to the change in the water retention properties. This correlation shows the evolution of the active porosity at the particle level within the microstructure, which consequently affects the macroscopic response of the bentonite in terms of its water retention behaviour

Shot-clay MX-80 bentonite: An assessment of the hydro-mechanical behaviour

This study presents the results of an experimental study conducted to characterise the hydro-mechanical behaviour of shot-clay MX-80 bentonite. In the shot-clay process, granular bentonite was mixed continuously with water and shot on the walls of a tunnel section at the Grimsel Underground Research Laboratory (URL), in Switzerland. The shot-clay was placed to create a layer of bentonite in direct contact with the host rock to avoid preferential water and/or gas flow along the tunnel wall. Samples for an experimental programme were collected during the shooting. The index properties, microstructural features, swelling potential and water retention properties of the shot-clay were analysed. An experiment was then conducted under controlled total suction to analyse the hydro-mechanical behaviour of the material along a predefined stress path involving suction and confining stress variations. Based on the results of this test, the expected behaviour of the shot-clay bentonite when subjected to the environmental conditions in the repository was determined. The test results were compared with data on the observed behaviour of compacted MX-80 granular bentonite to assess the effects of the shot-clay emplacement technique on the behaviour of the MX-80 bentonite. The results highlight the role of the emplacement dry density on the behaviour of bentonite. © 2014 Elsevier B.V

Water retention and swelling behaviour of granular bentonites for application in Geosynthetic Clay Liner (GCL) systems

Geosynthetic Clay Liner (GCL) systems are used as efficient hydraulic barriers in landfills for the disposal of hazardous municipal wastes. Along with geotextiles, bentonite materials are chosen as one of the primary components of GCLs due to their high retention, adsorption, and swelling capacities. GCLs are manufactured using bentonites at a high total suction and hydrated through the uptake of liquid from the subsoil and the confined material as soon as they are installed. Bentonites may exhibit considerable volume change upon wetting. Depending on the confinement stress, the void ratio may significantly increase with a decrease in suction, particularly at higher degrees of saturation. To improve the hydraulic performance of GCLs, the swelling of the bentonites induced by the hydration should be limited when GCLs reach low suction values. The change in the hydrated void ratio is related to the pore structure evolution of the bentonites at different hydration levels. An improved understanding of the water retention and the void ratio evolution of bentonite materials during swelling is required, with particular attention given to the applications of GCL systems. The aim of this paper is to characterise the water retention and the swelling behaviour of granular bentonites for applications in GCLs. MX-80 granular bentonite, with an optimised grain size distribution, and Volclay GC-50 granular bentonite were selected, and their water retention behaviour was determined based on a new methodology. Reconstituted GCL specimens with MX-80 and Volclay GC-50 granular bentonites were tested with the same methodology to determine the water retention behaviour. An analysis of the water retention behaviour of the granular bentonites and reconstituted GCLs indicates that an adsorption mechanism controls the water retention behaviour for a wide range of total suction values. An analysis of the swelling potential of the granular bentonites indicates a significant increase in the hydrated void ratio for lower suction values. The increase in the void ratio is attributed to the modification of the smectite particles in the hydration path that results in new pore levels emerging in the bentonite structure. This change in the void ratio is expected to influence the hydraulic performance of the GCL in terms of diffusion and hydraulic conductivity

Challenging global waste management : bioremediation to detoxify asbestos

As the 21st century uncovers ever-increasing volumes of asbestos and asbestos-contaminated waste, we need a new way to stop ‘grandfather’s problem’ from becoming that of our future generations. The production of inexpensive, mechanically strong, heat resistant building materials containing asbestos has inevitably led to its use in many public and residential buildings globally. It is therefore not surprising that since the asbestos boom in the 1970s, some 30 years later, the true extent of this hidden danger was exposed. Yet, this severely toxic material continues to be produced and used in some countries, and in others the disposal options for historic uses – generally landfill – are at best unwieldy and at worst insecure. We illustrate the global scale of the asbestos problem via three case studies which describe various removal and/or end disposal issues. These case studies from both industrialised and island nations demonstrate the potential for the generation of massive amounts of asbestos contaminated soil. In each case, the final outcome of the project was influenced by factors such as cost and land availability, both increasing issues, worldwide. The reduction in the generation of asbestos containing materials will not absolve us from the necessity of handling and disposal of contaminated land. Waste treatment which relies on physico-chemical processes is expensive and does not contribute to a circular model economy ideal. Although asbestos is a mineral substance, there are naturally occurring biological-mediated processes capable of degradation (such as bioweathering). Therefore, low energy options, such as bioremediation, for the treatment for asbestos contaminated soils are worth exploring. We outline evidence pointing to the ability of microbe and plant communities to remove from asbestos the iron that contributes to its carcinogenicity. Finally, we describe the potential for a novel concept of creating ecosystems over asbestos landfills (‘activated landfills’) that utilize nature’s chelating ability to degrade this toxic product effectively