One-dimensional swelling tests and hydraulic conductivity tests have been performed at vertical effective stresses up to 450 kPa on Na-bentonite powder and compacted sand/Na-bentonite mixtures (5, 10 and 20% bentonite by weight) to investigate the use of bentoniteimproved soils for waste containment. It was found that bentonite powder swells to reach a final state described by a single straight line on a plot of void ratio against the logarithm of vertical effective stress, regardless of preparation technique. Swelling of sand/bentonite mixtures expressed in terms of the clay void ratio show a deviation from bentonite behaviour above a stress which depends on the bentonite content. Hydraulic conductivity data for bentonite and sand/bentonite mixtures indicate an approximately linear relationship between logarithm of hydraulic conductivity and logarithm of void ratio. A design model based on the clay void ratio, and the sand porosity and tortuosity is presented enabling the hydraulic conductivity of a mixture to be estimated.
This paper considers the engineering behaviour of bentonite-enhanced sand (BES) mixtures in relation to their performance as environmental barriers. Data on the swelling and hydraulic conductivity are presented. At low effective stresses the bentonite within BES mixtures swells sufficiently to separate the sand particles. In such states two factors affect the void ratio reached by the bentonite after swelling, the ionic concentration of the pore solution and the bentonite fabric after compaction. Bentonite swelling is very sensitive to the pore solution concentration because increasing concentration suppresses the diffuse double layer component of swelling. Remoulding during compaction can result in a slight reduction in bentonite swelling, probably because of disruption to the cluster-based fabric of bentonite. At high effective stresses the bentonite has insufficient swelling capacity to force the sand particles apart, and the sand pore volume thus limits swelling.A model to predict the swelling and hydraulic conductivity of BES in distilled water and various salt solutions is described. This model requires the swelling behaviour and hydraulic conductivity of the bentonite in the relevant solution, and the compressibility and porosity of the sand component as input parameters. Soil tortuosity is used as a fitting parameter, and is estimated from Archie's equation. Application of this model to the swelling of compacted mixtures is shown to produce a good fit with the experimental data.
The swelling behaviour and hydraulic conductivity of Na-bentonite powder and bentonite-sand mixtures (10 and 20% of bentonite by dry weight) have been measured with distilled water and various salt solutions (0.01, 0.1 and 1 mol/l concentrations), It was found that in dilute solutions, the bentonite in mixtures subjected to small confining stresses swells sufficiently to separate the sand particles and reach a clay void ratio similar to that achieved by bentonite alone. At high stresses, or in strong solutions, the bentonite in a mixture has insufficient swelling capacity to force the sand particles apart and swelling is limited by the sand pore volume. The hydraulic conductivity of a mixture depends on the bentonite void ratio, and the porosity and tortuosity of the sand matrix. A design model is proposed to predict the engineering properties of a mixture over a range of confining stresses from the properties of its constituents and the permeant.
A kinematically admissible velocity field for the mass flow of a granular material in a plane converging hopper is presented in this Paper. The proposed solution is based on assumptions suggested by the flow pattern observed during experimental work. In particular, the rupture surfaces observed during flow are incorporated into the solution. The material filling the hopper is treated as plastic, though different flow rules are specified for difference areas of the material. Stresses are not considered because the static counterpart of the kinematic solution does not contribute to the approach adopted. The main aim of the work is to provide a realistic mathematical model for understanding the velocity fields observed in experiments, rather than a complete solution based on simplifying assumptions. A criterion is suggested which may govern the transition from mass to core flow as the head of material in the hopper is reduced. Un champ de vitesse cinématiquement admissible in-situ pour l'écoulement d'une masse de materiau granulaire à travers d'une trémie convergente, est présentée dans cette étude. La solution proposée est basée sur des hypothises inspirées par l'observation experimental d'écoulement d'enchantillons. En particulier, les surfaces de rupture observées pendant l'écoulement son incorporées dans la solution. Le matériau qui remplit le trémie est supposé plastique, bien que des règles différentes d'écoulement sont déterminées pour différentes niveaux du matériau. Les contraintes ne son pas considérées parce que la contre-partie statique de la solution cinématique ne contribue pas à l'approche adoptée. Le but principal de l'étude est plus de fournir un modèle mathématique réaliste pour comprendre les champs de vitesse observés experimentalement, plutôt qu'une solution complète basée sur la simplification des hypothises. Un critère est suggéré, qui pourrait gouverner la transition de l'écoulement de la masse au noyau alors que dans le trémie la charge du materiau est réduite.
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