Influence of Permeation of Synthetic Groundwater Solutions on the Hydro-Mechanical Proerties of Barmer Bentonite

Abstract

The deep geological repository concept is based on “engineered barriers systems (EBS)” that are constructed in the repository and “natural barriers” provided by the surrounding geological environment. The EBS comprises of variety of sub-systems or components, such as the waste form, canister, buffer, backfill, seals, and plugs. Geological disposal is based on the concept of multiple barriers that work together to provide containment. The buffer is made up of densely compacted bentonite or bentonite-sand mix. Bentonite has both mechanical and physico-chemical functions, to fulfill as a barrier material in DGR. The bentonite buffer should hold the containers in place and prevent collapse of the excavation. A plastic deformability of the bentonite is desired to redistribute the stresses that can result from creep in the rock, and prevent transfer of excessive stresses to the canisters. The bentonite buffer must create an impermeable zone around the containers to ensure that the radionuclide released from the vitrified waste is limited by diffusive transport rather than advective transport in groundwater. Another important property of the highly compacted bentonite is its swelling potential. Its swelling potential should be as high as possible, to guarantee the sealing of any cracks occurring in the buffer material or in the storage gallery and thus ensure good imperviousness. Besides its mechanical function, bentonite buffer must sorb escaping radionuclides and thus retard their migration to the geo-environment. The bentonite buffer must retain its mechanical and physico-chemical functions over a span of several hundred thousand years to fulfill its role as a containment barrier in DGR. The bentonite buffer should maintain its physico-chemical and hydro-mechanical integrity on exposure to groundwater. Nuclear power agencies of several countries have identified suitable bentonites for use as buffer in DGR through laboratory experiments and large scale underground testing facilities. Japan has identified Kunigel VI bentonite, South Korea-Kyungju bentonite, China-GMZ bentonite, Belgium-FoCa clay, Sweden-MX-80 bentonite, Spain-FEBEX bentonite and Canada-Avonseal bentonite as candidate bentonite buffer for deep geological repository program. Bentonite from Barmer (Rajasthan State) was identified as suitable buffer for use in Indian deep geological repositories. The influence of moisture and dissolved salt migration on the physico-chemical and hydro-mechanical properties of Barmer bentonite has not been examined. The study is important to understand the clay’s behaviour under deep geological repository conditions, where, the bentonite buffer would come in contact with groundwater. Infiltration of groundwater with variable chemical composition could alter the physico-chemical and hydro-mechanical properties of the clay. The objectives of the thesis are as follows: Examine the influence of permeation of distilled water (DW) and synthetic ground water (SGW) solutions under constant volume condition on suction, physico-chemical and moisture content/dry density characteristics of compacted Barmer bentonite specimens as function of permeation period (maximum permeation period– 30 days). Examine the influence of variation in dry density and gravimetric water content as consequence of DW and SGW solution permeation on swell pressure and unconfined compression strength of Barmer bentonite specimens. Compare experimental swell pressures of re-constituted bentonite specimens with swell pressures predicted by diffuse double layer models. Examine the influence of total dissolved solids (TDS) concentration of permeating solution on the unsaturated permeability of compacted Barmer bentonite specimens.

Organization of thesis: After the first introductory chapter, a detailed review of literature is performed in Chapter 2 to review the physicochemical, mineralogical and hydro-mechanical properties of bentonites identified as buffer materials for deep geological repositories of various countries. Based on current understanding and need to perform similar studies with Barmer clay, the chapter develops the scope and objectives of the study. Chapter 3 presents a detailed experimental program of the study. Chapter 4 examines the influence of permeation of distilled water (DW) and synthetic groundwater (SGW) solutions (under constant volume conditions) on the total suction of compacted bentonite specimens at two locations in the clay. The influence of variation in dry density on the moisture migration-suction inter-relations of compacted bentonite specimens is also examined. The associated changes of DW and SGW solution migration under constant volume conditions on the physico-chemical properties, water content and dry density of compacted Barmer bentonite specimens are also examined. The experimental results brought out that matric suction mainly contributed (75 to 92 %) to total suction of the permeated specimens; the permeated specimens experienced reduction in matric suction with increase in gravimetric water content from increase in degree of saturation. Osmotic suction contributed to 10 to 25 % of the total suction of the permeated specimens and was observed to increase with gravimetric water content due to solubilization of salts contained in the voids of the compacted bentonite specimens. The total suction of compacted Barmer bentonite specimen was responsive to the total dissolved solids concentration of the permeating solutions as the specimen permeated with more saline solution (higher TDS value) exhibited lesser total suction. Upon permeation with DW and SGW solutions, the CEC of bentonite was unaltered, while, pH and TDS values were affected. Softening of the bentonite clay occurred from increase in water content and existence of compression zones (material used to seal 1mm gap in relative humidity probe aperture) that in turn facilitated dissipation of swelling stress leading to reduction in dry density values. Chapter 5 examines influence of reduction in dry density and increase in water content on the swell pressure and compression strength characteristics of compacted Barmer bentonite specimens upon DW and SGW solution migration as the results could provide insight into possible deviations from the design properties upon wetting of bentonite buffer by groundwater under deep geological repository conditions. The experimental swelling pressures are also compared with those predicted by Gouy-Chapman diffuse double layer theory. The dry density of 1.6 Mg/m specimens permeated with DW and SGW solutions reduced to 1.59 to 1.36 Mg/m and water contents increased to 18.9 to 27 % on permeation with distilled water and SGW solutions for 30 days. The reductions in dry density and increase in water content caused 30 to 70 % reductions in swell pressures and 31 to 74 % decrease in unconfined compression strength values. Specimens initially compacted to dry density of 1.8 Mg/m, experienced reduction in dry density ranging from 1.79 to 1.52 Mg/m and increase in water content from 18.6 to 24.2 % on permeation of DW and SGW solutions for 30 days. These reductions in dry density and increase in water caused the swell pressures to reduce from 4 to 55 % and unconfined compressive strengths to reduce by 31 to 67 %. Comparison of swell pressures gave -8 to 127 % variations between theoretical (from DDL theory) and experimental values due to errors associated with estimation of surface area and dissolved salt concentrations in pore water. Chapter 6 examines the influence of salinity of permeating solution on the unsaturated permeability of compacted Barmer bentonite specimens. The salinity of permeants was varied by permeating distilled water (DW) and synthetic ground water solutions under constant volume conditions over maximum period of 30 days. Experimental results showed that the saturated permeability coefficients (ksat) of specimens compacted to 1.6 Mg/m, responded to variations in TDS of the permeant. Comparatively, the ksat values of specimens compacted to 1.8 Mg/mwere unaffected by variation in TDS of the permeant. Permeation of DW and SGW solutions decreased the ksat values with time from cation hydration and growth of diffuse ion layers for both, 1.6 and 1.8 Mg/mseries specimens. Increase in gravimetric water content from DW and SGW permeation increased the kunsat values of 1.6 Mg/m specimens from reduction in total suction. Re-orientation of soil structure mobilized larger kunsat values for specimens permeated with SGW solutions than DW at similar total suction. Permeation of DW and SGW solutions had lesser impact on kunsat values of the 1.8 Mg/m specimens in comparison to the 1.6 Mg/m series specimens. Further at both densities, the influence of permeation was more evident at location closer to hydration surface. Chapter 7 summarizes the main findings of this study.