Linkage between swelling pressure, total suction of saturated bentonites and suction of saturating aqueous solutions

In deep geological repositories, compacted bentonites have been proposed for use as barrier materials for isolating nuclear waste. The prevailing hydro-mechanical-chemical boundary conditions in the repositories can affect the swelling capacity of compacted bentonites. The present study examines the linkage between the swelling pressure of saturated bentonites, the total suction of saturated bentonites and the suction of hydrating fluids. An equation describing the linkage between these parameters was derived based on the thermodynamics of soil moisture. To validate the derived relationship, laboratory tests involving constant volume swelling pressure tests and total suction measurements after the swelling pressure tests were carried out on a selected Ca-Mg-rich bentonite. Seven bentonite samples with compaction dry density ranging from 1.20 to 1.83 Mg/m3 were hydrated with deionized water, whereas four samples with a dry density of 1.59 Mg/m3 were hydrated with 0.0001 to 2.0 M CaCl2 solutions. The total suctions of the saturated bentonite and the hydrating fluids were determined by using a chilled-mirror hygrometer. The total suction of the saturated bentonite was found to be affected by the compaction dry density and the suction of the hydrating fluids. The experimental results from the present study and from the literature demonstrated that the total suction of saturated bentonites minus the suction of the hydrating fluids used to saturate bentonites is approximately equal to the swelling pressure of saturated bentonites

SOME STUDIES ON THE STRENGTH OF PARTLY SATURATED CLAYS

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Computation of hydraulic conductivity of montmorillonitic clays by diffuse double layer theory

Knowledge of hydraulic conductivity of clays is very important in many cases and in waste containment system in particular. Several investigations have been carried out on hydraulic conductivity in the past and it has been brought out that the hydraulic conductivity is strongly influenced by the pore fluid chemistry apart from the soil type and void ratio. Based on the theoretical considerations, the effect of dielectric constant of the pore fluid, valency and concentration of cations in the pore fluid, it is surmised that the hydraulic conductivity is significantly influenced by diffuse double layer thickness, which reduces the pore volume through which flow takes place. A study of Gouy-Chapman diffuse double layer theory show that the diffuse double layer thickness is negligible for non-polar fluid with very low dielectric constant (e.g. CCl4). Considering the thickness of diffuse double layer contributed by CCl4 as zero, the equivalent thickness of diffuse double layer as affected by dielectric constant, valency, concentration, hydrated radius of cation and specific surface has been estimated and presented for montmorillonitic clays. The effective void ratio has been evaluated and a unique relationship between effective void ratio and hydraulic conductivity has been brought out for montmorillonitic clays with different pore medium chemistry. The present study throws more light on the fundamental mechanisms controlling the hydraulic conductivity in clays

Liquid limit of mixtures of smectite, kaolinite and quartz powder with water and NaCl solution

Clay mineralogy plays an important role in determining the mechanical properties of clayey materials. Several liquid limit and water uptake tests were performed on kaolinite-smectite mixtures for different quartz powder content and water and / or NaCl content as pore fluids. To interpret the results, the diffuse double layer theory is used. The experimental results show the effect of quartz powder in reducing the liquid limit of smectite with water and 1 M NaCl as fluid. A linear relationship is obtained from the tests. The quartz powder acts only to dilute the liquid limit values. To better understand the behavior of the clays regarding the liquid limit, predicted values for the liquid limit of all the mixtures with water and NaCl as pore fluid have been developed assuming the linear law to be valid. The prediction is very good for water, but it becomes less good with lower liquid limit values/higher quartz contents. The correlation with NaCl as fluid is not that good when the quartz content is high. A correlation of water uptake and liquid limit is also presented. The data fit is good. The overall results confirm that the effect of the pore fluid type is significant when the smectite content in the mixture is high

Mechanism controlling the volume change behavior of kaolinite

The possible physical and chemical forces controlling the volume change behavior of kaolinite [1318-74-7] were ascertained from the sediment volume of kaolinite in various solvents under no external load condition and from conventional oedometer measurements of kaolinite in several pore fluids. The minimum sediment volume of $14.5 cm^3/10 g$ clay occupied by kaolinite in water where repulsive (R) forces were dominant indicated that the R contribution was insignificant for kaolinite. The maximum sediment volume of $25.0 cm^3/10 g$ clay in $C_6H_6$, where coulombic attraction forces were significant, suggested that electrostatic attraction between silicate sheets and midplane cations and van der Waals forces were not appreciable for kaolinite. The positive edge-negative face bonding of kaolinite particles in $C_6H_6$ was unlikely. The $3688 cm^{-1}$ band in the IR spectrum of a kaolinite-dimethylamine sample decreased by $10 cm^{-1}$ on H-bond formation of the solvent molecule with the exposed structural hydroxyls of the octahedral sheet. The adsorbed solvent molecules likely H-bonded with an adjacent clay particle as indicated by the decrease in sediment volume with increase in dipole moment of the solvent molecule. In the oedometer tests with various pore fluids, a high void ratio of approximate 1.3 was obtained for kaolinite in n-heptane, and hexane (\mu \simeq 0) at an external pressure of $1 kg/cm^2$ probably because the weakly bonded kaolinite particles were randomly oriented. At the corresponding applied pressure, a lower void ratio of 0.88 resulted in water (\mu = 1.84) where the stronger H-bond between flat layer surfaces of adjacent particles favored a parallel orientation of clay particles. The volume change behavior was essentially controlled by frictional forces and clay fabric. In nonpolar solvents the random arrangement of kaolinite particles and the frictional forces mobilized a high shear resistance on the application of a consolidation pressure, resulting in a lower compressibility. In a solvent with high dipole moment the parallel array of clay particles mobilized less shear resistance and produced a greater compression

Response of circular footings to vertical vibrations

In this paper, analog solutions are presented for the response of a circular footing resting on an elastic half-space with uniform and parabolic contact pressure distributions and subjected to frequency dependent and frequency independent excitations. In addition, an analog solution to a rigid circular footing subjected to frequency dependent excitation is also presented. The results have been compared with the rigorous solution of Sung and the agreement is found to be good

Characterization of Ariake and other marine clays

Characterisation of marine clays is an important aspect of understanding their behaviour. Investigation showed that there is a good linear relationship between percent clay and liquid limit and plastic limit. Sand and silt fraction has only diluted the magnitude of liquid limit and plastic limit. The ratio of plastic limit to liquid limit is almost a constant irrespective of percent sand fraction and is a function of clay mineral type or mixture of clay minerals, associated cations and ion concentration. Marine clays of Japan around Ariake bay (Hachirogata, Ariake – Ushiya, Meguri, Okishin and Yamaashi)have shown good correlation between plastic and liquid limits and this ratio ranged between 0.35 to 0.50 as against 0.07 (Limontmorillonite)to 0.88 (halloysite) of pure clay minerals. Cochin marine clay behaviour compares well with Ariake-Ushiya, although their geological history is entirely different. The revised plasticity chart with liquid limit and plastic limit as coordinates appears to be a better representation of fine grained soils

Effect of wetting and drying on shear strength

Climatic changes, such as repeated wetting and drying over geological ages, lead to the degeneration of the parent rock to form soil. Simultaneously these actions can also cause some aggregation of soil particles and the production of bonds, called desiccation bonds, which impart an intrinsic effective stress to the soil. This intrinsic effective stress can influence the shear strength behavior of soils subjected to such climatic actions. When a soil is subjected to repeated wetting and drying in the laboratory, an intrinsic effective stress is imparted to it which results in greater shear strength and in a stiffer stress-strain response. This intrinsic effective stress may be attributed to chemical bonds