Synopsis This investigation deals with the mechanisms con- trolling the one-dimensional volume change beha- viour of saturated kaolinite and montmorillonitic clays. An attempt has been made to explain the behaviour of the clays in the light of the modified effective stress concept which takes into considera- tion the interparticle electrical attractive and repul- sive forces. Eight organic pore fluids of different dielectric properties and water have been used to vary the interparticle forces in the one-dimensional consolidation tests. In order to further confirm the nature of the mechanisms, tests have also been con- ducted in which the existing pore fluid was replaced by another of different dielectric properties, to change the force system at interparticle level. The experimental results clearly indicate that the volume change behaviour of these clays is controlled basi- cally by two mechanisms which are governed by the modified effective stress concept. In mechanism 1, the volume change is controlled by the shearing resistance at interparticle level and in mechanism 2, primarily by the long range diffuse double layer repulsive forces. Although these effects operate simultaneously, the results indicate that mechanism 1 primarily governs the volume change behaviour of non-expanding lattice type clays like kaolinite, and mechanism 2, that of the expanding lattice type clays like montmorillonite. On étudie les mécanismes qui régissent le comportement au tours du changement de volume unidimensionel d'argiles à kaolinite et à montmorillonite saturées. On essaye d'expliquer le comportement de ces argiles en fonction du concept de contrainte effective modifiée, qui tient compte des forces électriques d'attraction et de répulsion. On s'est servi de huit fluides interstitiels organiques ayant des propriétés diélectriques différentes, ainsi que de I'eau, pour faire varier ces forces entre particules dans des essais de consolidation unidimensionelle. Pour confirmer la nature de ces mécanismes, on a également fait des essais au tours desquels le fluide interstitiel existant a été remplacé par un autre fluide ayant des propriétés diélectriques différentes, pour changer le système de forces au niveau des particules. Les résultats expérimentaux indiquent clairement que le comportement au tours du changement de volume de ces argiles dépend essentiellement de deux mécanismes qui sont régis par le concept de contrainte effective modifiee. Dans le premier mécanisme, le changement de volume est régi par la résistance au cisaillement au niveau des particules et dans le second mécanisme, par des forces de répulsion à plus grande distance dans la double couche diffuse. Bien que ces effets aient lieu en même temps, les résultats indiquent que le premier mécanisme régit principalement le changement de volume du type d'argiles à réseau non dilatant, comme la kaolinite, et le second mécanisme celui du type d'argiles à réseau dilatant comme la montmorillonite.

The compressibility of pure clays under external load depends not only on the negative charges and crystallite morphology of clay minerals but also on the ion concentration, cation valency, dielectric constant and temperature of the pore fluid. The electrical double layer theory has been used by several investigators to derive the void ratio-pressure relationship of clays. This Paper presents a detailed study with regard to the Gouy—Chapman theory and the compressibility of clays. The relationship between the distance between parallel clay platelets and external pressure is shown to be almost independent of clay type. The surface area represents the clay type in showing the variations in void ratio-pressure (e–p) relationships of different clays. When the concentration of cations in the pore fluid is low, the log e–log p relationship is linear, and the compression index is a direct function of void ratio and independent of clay type. Examination of experimental results of other workers shows the validity of the Gouy—Chapman theory for predicting the e–p relationship qualitatively and, under certain circumstances, quantitatively. La compressibilité des argiles pures sous charge extérieure dépend non seulement des propriétés mécaniques des minéraux d'argile mais aussi des caractéristiques physicochimiques du fluide interstitiel. La théorie électrique des couches doubles a été employée par plusieurs rechercheurs pour représenter le rapport indice des vides/pression des argiles. Cet article offre une étude détaillée de la théorie Gouy-Chapman appliquée à la compressibilité des argiles. Le rapport entre la distance séparant les feuillets parallèles de l'argile et la pression extérieure est presque indépendant du type de sol. La zone superficielle représente le type d'argile, car elle indique les variations du rapport indice des vides/pressions des différentes argiles. Lorsque la concentration des cations dans le fluide interstitiel est basse le rapport log e/log p est linéaire et l'indice de compression est une fonction directe de l'indice des vides. Le dépouillement des résultats expérimentaux démontre que la théorie Gouy-Chapman est valable pour prédire le rapport e/p qualitativement et aussi quantitativement dans certaines circonstances.

The swelling pressures of several compacted bentonites (MX80, Febex, and Montigel) proposed for use as barrier materials in storing high-level radioactive waste in many countries were determined from the Gouy–Chapman diffuse double layer theory. The swelling pressures thus determined were compared with the reported experimental swelling pressures. The study revealed that, in general, at low compaction dry densities of the bentonites, the experimental swelling pressures are less than their theoretical counterparts, with the reverse trend at high compaction dry densities. Based on the reported experimental results for the three bentonites, relationships between the nondimensional midplane potential function, u, and the nondimensional distance function, Kd, were established. New equations for the swelling pressure were proposed on the basis of the diffuse double layer theory and the reported experimental data to compute swelling pressures of compacted bentonites. The suitability of the new equations was also verified with additionally reported experimental swelling pressures from three other bentonites (Kunigel V1, Kunigel, and bentonite S-2) that have been also proposed for use as barrier materials. Very good agreement was found in all the cases between the experimental swelling pressures and the swelling pressures obtained using the proposed equations. The use of the proposed equations is based on the weighted average valency of the cations present in bentonites, since the valency of the cations present has a significant influence on the swelling pressure.Key words: clays, compressibility, swelling pressure, expansive soils, chemical properties, radioactive waste disposal.

The use of bentonite alone or amended with natural soils for construction of liners for water-retention and waste-containment facilities is very common. The importance of bentonite content in reducing the hydraulic conductivity of liners is well recognised. The study illustrates the role of the size of the coarser fraction in controlling the hydraulic conductivity of the clay liner. It has been shown that at low bentonite contents the hydraulic conductivity of the liner varies depending on the size of the coarser fraction apart from clay content. At a given clay content, the hydraulic conductivity increases with an increase in the size of the coarser fraction. But when the clay content is more than that which can be accommodated within the voids of the coarser fractions, the hydraulic conductivity is controlled primarily by clay content alone. Four different methods of predicting hydraulic conductivity of the liners are presented. Using two constants, related to the liquid limit, the hydraulic conductivity can be predicted at any void ratio.Key words: clays, hydraulic conductivity, liquid limit, liners, void ratio.

Most of the national codes of practice do not give characterization and classi®cation of expansive soils, in spite of expansive soils being distributed very widely over almost all geographical locations in the world, causing distress to the structures founded on them and discomfort to the users. A simple user-friendly approach based on the free swell ratio, de®ned as the ratio of the sediment volume of soil in distilled water to that in carbon tetrachloride or kerosene, is formulated considering the compatibility of the results with oedometer free swell tests and the soil clay mineralogy. Statistical illustrations are provided which clearly indicate that while the assessment of soil expansivity based on index properties is an overestimation, there is a consistency in the classi®cations based on oedometer test results and the proposed approach. It is recommended that simple approaches such as the one proposed here to classify the expansive nature of soils are entered into standard codes of practice.