Dense clay can be used for sealing of boreholes and isolation of waste containers. Expansion of the clay to fill the space between containers and confining rock takes place by uptake of water of the clay, which thereby provides effective isolation. The issue is to prepare clay inserts with properly selected water content. The paper describes preparation of clay seals by mixing air-dry clay powder with nanoparticles of water droplets coated with very thin shells of a hydrophobic silicious substance (“dry water”). It behaves as dry powder and is easily mixed with dry clay. On compaction to the desired density the shells break into microscopic fragments while water becomes homogeneously distributed in the mass. Laboratory tests verify that the properties of clay prepared in this way are the same, or better, than of commonly saturated clay.

On peut utiliser de l'argile dense pour sceller les forages et isoler les conteneurs de déchets. L'expansion de l'argile pourremplir l'espace entre les récipients et la roche confinant a lieu par absorption d'eau de l'argile, ce qui fournit ainsi un isolement efficace. La question est de préparer des inserts d'argile avec une teneur en eau correctement sélectionnée. L'article décrit la préparation de joints d'argile en mélangeant de la poudre d'argile séchée à l'air avec des nanoparticules de gouttelettes d'eau revêtues de coquilles très minces d'une substance siliceuse hydrophobe ("eau sèche"). Il se comporte comme une poudre sèche et se mélange facilement avec de l'argile sèche. Lors du compactage à la densité souhaitée, les coquilles se cassent en fragments microscopiques tandis que l'eau se répartit de manière homogène dans la masse. Des essais en laboratoire vérifient que les propriétés de l'argile préparée de cette façon sont les mêmes, ou mieux, que les argiles couramment saturées.

Cement-based materials for use as sealants in underground waste storages must be erosion-resistant and chemically stable. Placement of highly radioactive waste (HLW) in boreholes may require that the rock is cement-grouted and stabilized by constructing concrete plugs. Where smectitic clay seals are in contact with concrete there is mutual degradation, and low-pH cement with inorganic superplasticizers, like talc, are recommended for preparing the concrete. This paper reviews our current state-of-knowledge concerning the grout and concrete sealing very deep boreholes (DBD) for purpose of high-level radioactive waste disposal. In this concept, the lower 2 km section of 4 km deep holes bored in crystalline rock could host waste-containers while the upper parts are sealed by dense clay and concrete.1 The parts of such a hole that intersect fracture-poor rock are sealed with dense expandable clay while concrete is cast where fracture zones are intersected. The paper summarizes the available experimental results concerning the performance of grouts and concrete with talc as superplasticizer in contact with smectitic clay.

The basic idea of concepts for disposal of highly radioactive waste deep in rock is that the heavy, saline, stagnant formational waters are unlikely to rise to contaminate shallow groundwater. A recent concept involves placement of the waste in the lower 2 km part of up to 4 km deep holes bored in granitic rock, and relies on the sealing capacity of engineered barriers in the form of concrete and clay in the upper parts of the holes. The parts located in fracture-poor rock are sealed with dense expandable clay, while concrete is cast where pre-grouted fracture zones are intersected. The holes will converge by creep and eventually exert the seals to radial compression. Using a new rheological model based on the Kelvin model in combination with a stochastic mechanical model, the predicted radial hole convergence causes a vanishingly small increase in pressure on the seals in the first 10,000 years. In a long time perspective they will be compressed and become less permeable. Parallel conversion of the clay minerals to become less expandable will, however, reduce this potential.

Landfills of hazardous waste like radioactive rest products with low activity, or ashes from incinerated organic waste, need to have top clay liners for minimizing penetration and percolation of precipitated rain and meltwater. Temperature and dry weather vary interchangeably with wet periods making the clay desiccate and fissure, and subsequently wetted etc. Top liners are commonly made of smectite clay, which is the best isolating soil material, undergoing swelling and shrinkage to an extent that depends on the clay content and density. The most important question is whether such liners, in unfrozen condition and covered by erosionresisting coarse soil, maintain their coherence and tightness after centuries of hydration/dehydration cycles. The present study, made on physically confined soft Iraqi clay with about 30% smectite indicates that initially homogeneous dense clay shrinks and desiccates and becomes fissured at 30oC and room RH, but partly recovers by becoming water saturated by infiltrated water. A limited number of drying and wetting sequences seem to give approximately the same change, suggesting that, under common weather conditions and lack of external disturbance, such liners retain a considerable part of their initial water tightness. Thick liners with moderately to high density and exposed to loading by overlying coarse fill are expected to serve particularly well.

KBS-3 and very deep borehole (VDH) concepts are two major types of long-term geologicaldisposal methods for high-level radioactive waste (HLW) isolating from the biosphere. TheKBS-3V concept for isolating the HLW at the depth of 400-500 m, is the officially proposedoption in Sweden and has been the subject of considerable research in the past few decades,while the VDH concept was considered as an option in the 1950s but later became discouragedbecause of insufficient experience in drilling technology. The greatest merit of the VDHconcept is that the almost stagnant groundwater in the deep boreholes prevents the transport ofthe possible release of radionuclides into the rock or up to the ground level. Since variousdisadvantages of the KBS-3V concept were found in previous research, the superiority of VDHconcept attracted the researchers to continue studying it into the late 1980s.The geological repositories of both of KBS-3V and VDH types primarily consist of a naturalbarrier (host rock) and of an engineering barrier (also known as a buffer/backfill barrier).According to the principle of IAEA and national relative research organizations, thebuffer/backfill material should have low permeability and good expandability, as well assuitable physical and sealing properties.The thesis concerns the VDH concept and is focused on the construction and performance ofthose parts of the sealed repository that are not affected by high temperature or gamma radiation.In the lower part of a VDH repository, the clay packages containing HLW will be exposed tohigh temperature (100-150 􀄇 ) in the borehole and to highly saline groundwater. In theinstallation phase of HLW, the groundwater will be pumped out and replaced by medium-softsmectite clay mud in which the HLW packages are installed vertically. During the hydrationand maturation of the clay components, the microstructural reorganization, water transport,migration of clay particles and redistribution of the density of the components take place. Thematuration determines the transient evolution of the clay seals and influences the rheologicaland soil mechanical behavior in the installation phase. The maturation of clay system alsodetermines their ultimate sealing potential of VDH repositories.This study presents the work carried out for investigating the maturation of the buffer-backfillclay in the HLW deep borehole. Initially in the study three types of clays, the Namontmorillonite,magnesium-rich and illite-smectite mixed layer clays, were examined for estimating their performance as the barrier candidate material. This is mainly presented in theliterature review. The experimental study was conducted on montmorillonite GMZ clays andI/S mixed-layer Holmehus clay. The expandability and permeability tests were carried out forinterpretation of the recorded swelling development and assessment of the effect of the salineconditions, with the goal of deriving a relationship between swelling pressure and hydraulicconductivity for different dry densities. The maturation tests of initially fully-saturatedHolmehus clay and partly saturated GMZ clay were performed. During the tests, the shearstrength mobilised by the relative movement of densified mud and migrated dense clay -contained in a perforated central tube - were determined. According to the results of shearstrength tests, the maximum operation time or the number of clay packages to be placed in asingle operation was evaluated, whilst the suitable saturation degree of the dense clay wasdiscussed as well.A model of the maturation of initially water-saturated clay seals based on Darcy’s law wasworked out and the evolution of the clay components in a lab-scale borehole using Holmehusclay were performed and compared with the experimental recordings. Good agreementsbetween the physical behaviors of the theoretical simulations and the measurements wasachieved by which the validity of the model was verified. Using the results, the hydration andsoil migration in the entire maturation process were presented in diagram. The model was alsoused for preliminary evaluation of the maturation products in real boreholes by assuming thesame Holmehus clay as used in the tests. Two constellation of borehole and dense clay withdifferent diameters, 80 cm borehole /60 cm clay and 80cm/50cm, were assumed. The resultsrespecting dry density and hydraulic conductivity of the ultimate maturation products, and thedegree of homogeneous of the buffer and backfill clay system in the assumed boreholes, arepresented and discussed. The options of different mineral types and initial physical propertiesof the candidate buffer clays provide a reference for engineering barrier design of HLW disposalin VDH.

A presently much discussed concept for disposal of highly radioactive waste (HLW) implies disposal in the lower part of very deep boreholes, called VDH here. The waste packages will be located in very salt, stagnant groundwater in rock that is much less permeable than shallow rock. A disadvantage is that some of the techniques for installation of waste and clay seals have not yet been demonstrated and that retrieval of damaged or stuck canisters is deemed difficult. The concept requires precise adaption of canister and seal positions to the rock structure, which will be known by site investigations including pilot borings for a relatively  low cost at a very early stage. The uppermost parts of the 4 km deep deposition holes may need to be supported by casings and installation of supercontainers with HLW and clay seals must be made in clay mud for stability reasons. Intersected fracture zones must be stabilized by grouting and the deposition holes plugged here by concrete cast on site. The grout and concrete are novel and have talc as superplasticizer for reaching long-term strength and chemical stability. Compared with more shallow disposal the VDH concept is competitive with respect to cost and construction time, as well as to long-term safety, since seismic and tectonic impact are less detrimental and future glaciations will cause much less disturbance. The most important value of VDH is that the groundwater that can possibly become contaminated by failing engineered barriers will stay at more than 2000 m depth and that there is no mechanism that can bring it up to the biosphere.

The matter of long-term stability of long, natural slopes in illitic clay is of great practical importance in Scandinavia and Canada and has been frequently discussed among geotechnical specialists. A remaining question is how such natural slopes can have remained stable, yet undergoing large strain, for hundreds and thousands of years, during which critical conditions have repeatedly occurred with calculated safety factors lower than or equal to unity according to common stability calculations based on plastic theory. The reason for this may be the role of creep shear strain that causes redistribution of stress and earth pressure leading to a state of equilibrium that is very sensitive to disturbance and represents a condition of near-failure. Triggering of occasional slides can be explained by temporary high porewater pressure caused by periods of intense rain, disturbance by pile driving, or loading by road construction etc, taken place in slopes that have been stable for very long periods of time. The mechanisms by which creep can lead to stable conditions of very old clay slopes can have the form of successive relative particle movements into a state where the interparticle bonds become stronger but of brittle character, according to a model based on stochastical mechanics.

Disposal of highly radioactive waste (HLW) can be environmentally acceptable if radionuclides are kept isolated from the groundwater, which has inspired planners of repositories to work out multibarrier concepts that postulate defined functions of the host rock and engineering barriers. Assessment of the role of the host rock involves groundwater flow modelling and rock mechanical analysis, which are both highly speculative and ignore future changes in rock structure, stress conditions, and groundwater flow. This makes use of engineered barriers necessary and possible technical solutions for isolating containers with highly radioactive waste by clay and concrete seals have been described in the literature. The present paper examines the performance and interaction of such seals in very deep boreholes with respect to chemical reactions and their impact on the sealing functions.

Sun protection creams based on smectite(S) and mixed-layer (IS) clays in glycerol emulsions were compared to commercial organic creams with respect to UV penetration. For very thin cream films the organic creams were superior while for thicker films the clay creams gave lower UV penetration. The investigated I/S mixed-layer clay gave more effective protection than Fe-poor S-clays because of better particle alignment and thicker particles. The content of iron compounds was found to be of greatest importance as demonstrated by tests with different amounts of Fe2O3.

Bentonite clay has attracted considerable attention as isolating material for safe disposal of high-level radioactive nuclear waste (HLW) on account of its low hydraulic conductivity and high swelling capacity. In the presently described study, three candidate smectitic clay materials were investigated and compared with respect to hydraulic conductivity and expandability. The clay samples were prepared from desiccated and crushed raw material placed and compacted in oedometer cells for saturation with distilled water and 3.5% CaCl2 solution in separate test series to a density at fluid saturation of 1200 ~ 1900 kg/m3. The samples were tested with respect to the hydraulic conductivity and swelling capacity. The MX-80 and Homehus clays matured as expected giving a successively monotonous increase in swelling pressure during the wetting process while GMZ clay showed two pressure peaks. The pressure development was different for low and high densities and can be explained by crystal expansion via interlamellar wetting, followed by osmotic swelling including establishment of electrical double-layers. The phenomena can alternatively be explained by the microstructural changes when clay minerals absorb solutions in the hydration phase. Differences in granule size distributions and mineral composition can explain different swelling abilities and permeabilities.