A. Sridharan

Swelling Pressure of Clays

Experiments by three methods to determine the swelling pressure of clays showed that the conventional consolidation test gives an upper bound value, the method of equilibrium void ratios for various consolidation pressures gives the least value, and tests by the constant volume method give intermediate values. However there is no definite relationship between the three methods. The paper also shows that time effects involved in conducting the tests and the effect of stress path are both significant. Time versus swelling, and to a greater extent, time versus pressure relations can be represented by a rectangular hyperbola, which can be used to predict the ultimate swelling and the swelling pressure, respectively. Tests with varying initial conditions of molding moisture content and density have exhibited a unique relationship between swelling pressure and the initial dry density for black cotton soil. This result is in line with the osmotic pressure theory. The molding water content has marginal effect on the swelling pressure.

LIQUID LIMIT OF MONTMORILLONITE SOILS

The present work investigates the influence of compositional factors on the liquid limit of montmorillonite soils. The results showed that the percent clay fraction and surface area values did not relate with the liquid limit of the soils; the latter, however, were essentially controlled by the diffuse double layer thickness. Examination of the relations between compositional factors that affect the double layer thickness and liquid limit showed that the latter did not have a significant bearing with cation exchange capacity, while a near perfect correlation was obtained with the amount of exchangeable sodium ions present. Such a dependence resulted because of the greater ease of dissociation of adsorbed sodium ions from the clay surface in presence of water and subsequent contribution to the double layer thickness; the other exchangeable cations, namely, calcium, magnesium, and potassium, were strongly adsorbed by the clay surface and did not contribute appreciably to the double layer thickness.

Mini compaction test apparatus for fine grained soils

The standard and modified Proctor compaction tests are devised to establish dry unit weight-water content relationships for a soil under controlled conditions, such as compactive effort, water content, etc. This paper presents a mini compaction apparatus primarily for use in fine grained soils, which requires only about 1/10th volume of soil needed for the standard and modified Proctor test. Additionally, the time and effort involved in carrying out the compaction test is much less. Also, the compacted soil sample, after trimming, can be used for strength tests.

Swelling behaviour of compacted fine-grained soils

The swelling behaviour of three compacted Cyprus soils varying significantly in their physical properties have been studied with variation in compaction energies from standard Proctor compaction to modified Proctor. For comparison, studies have also been made with kaolinite and a highly plastic montmorillonitic clay. Both percent swell and swelling pressure are significantly influenced by the compaction energy. There is an unique relationship between percent swell and the swelling pressure irrespective of the soil type and the level of compaction energy. The percent swell–time relationship could be identified as a rectangular hyperbola facilitating the prediction of ultimate percent swell with a few initial results. The swell–log time relationship has three distinct phases of initial, primary, and secondary swelling. The secondary swell could be distinctly identified, and a knowledge of the rate of secondary swelling enables the prediction of long-term swelling. The study focuses on the attention of the significant influence of the compaction energy on percent swell/swelling pressure.

Mechanism of sulfate adsorption by kaolinite

Sulfate adsorption on kaolinite was followed at 0.7 μeq/ml to 99 μeq/ml solution concentrations at 30°C and at pH 6.0, and the amount of OH~ ions released and the change in surface charge were determined. Sulfate was adsorbed at positive and neutral sites by displacing OH2 and OH− groups, respectively. Adsorption of sulfate occurred predominantly at positive sites at low (0.7 μeq/ml to 0.9 /μeq/ml) solution concentrations, whereas at higher solution concentrations, the proportion of the sulfate adsorbed at the neutral sites increased, varying from 51% at 4.9 μeq/ml to 68% at 99 μeq/ml. The form of sulfate bonding was apparently governed by the level of the positive charge on the clay surface. Higher positive charge at low and intermediate levels of sulfate saturation resulted in the adsorption of sulfate as a divalent anion by forming a 6-member ring with surface Al. With a decrease in positive charge at higher levels of adsorption, the sulfate ion formed both monodentate and bidentate complexes.РезюмеАдсорбция сульфата каолинитом исследовалась для концентрации раствора между 0,7 дэкв/ мл и 99 дэкв/мл при 30°С и рН = 6,0; были определены количество выделенных ионов ОН− и из¬менения поверхностного заряда. Сульфат адсорбировался на положительных и нейтральных местах путем вытеснения групп ОН2 и ОН−, соответственно. Адсорбция сульфата происходила в главной мере на положительных местах при низких концентрациях раствора (0,7 μэкв/мл до 0,9 μ экв/мл), тогда как при больших концентрациях, пропорция сульфата адсорбированного на нейтральных местах увеличивалась, принимая значения от 51 % для 4,9 μ экв/мл до 68% для 99 μ экв/мл. Тип связи сульфата зависел, вероятно, от уровня положительного заряда на поверхности глины. Больший положительный заряд при низких и промежуточных уровнях насыщения сульфатом приводил к адсорбции сульфата в виде двухвалентного аниона путем формирования 6-членового кольца с поверхностными атомами А1. При уменьшении положительного заряда на больших уровнях адсорбции, ион сульфата образо¬вывал монозубчатые и двухзубчатые комплексы. [Е.G.]ResümeeDie Sulfatadsorption an Kaolinit wurde von 0,7 μÄquivalent/ml bis 99 μÄquivalent/ml Lösungskonzentration bei 30°C und pH 6,0 verfolgt, wobei die Menge der freigesetzten OH2-Ionen und die Veränderung der Oberflächenlagen bestimmt wurde. Das Sulfat wurde an positiven und neutralen Plätzen adsorbiert, wobei es OH2- und OH−-Gruppen ersetzte. Die Sulfatadsorption erfolgte bei niedrigen Lösungskonzentrationen (0,7 μÄquivalent/ml bis 0,9 μÄquivalent/ml) vor allem an den positiven Plätzen, während bei höheren Lösungskonzentrationen der Anteil des Sulfats, das an neutralen Plätzen adsorbiert wird, zunahm, wobei er von 51% bei 4,9 μÄquivalent/ml bis 68% bei 99 μÄquivalent/ml variierte. Die Art der Sulfatbindung wurde offensichtlich durch die Höhe der positiven Lagen auf der Tonoberfläche reguliert. Eine höhere positive Ladung führte bei niedriger oder mittelmäßiger Sulfatsättigung zu der Adsorption von Sulfat in Form eines divalenten Anions, wobei ein 6-er Ring mit dem Oberflächen-Al gebildet wurde. Wenn die positive Ladung bei höherem Adsorptionsgrad abnahm, dann bildete das Sulfation sowohl einzähnige als auch zweizähnige Komplexe. [U.W.]RésuméOn a suivi l’adsorption sulfate sur la kaolinite à des concentrations de solution de 0,7 μeq/ml à 99 μeq/ml à 30°C et au pH 6,0, et on a déterminé la quantité d’ions OH− relâchés et le changement de charge de surface. La sulfate a été adsorbée à des sites positifs et neutres par déplacement de groupes OH2 et OH−, respectivement. L’adsorption de sulfate s’est passée de façon prédominante à des sites positifs à des concentrations de solution basses (0,7 μeq/ml à 0,9 μeq/ml), tandis qu’à de plus hautes concentrations de solution, la proportion de sulfate adsorbée aux sites neutres a augmenté, variant de 51% à 4,9 μeq/ml à 68% a 99 μeq/ml. La forme de liaison de sulfate était apparemment gouvernée par le niveau de la charge positive sur la surface de l’argile. Une plus haute charge positive à des niveaux de saturation de sulfate bas et intermédiaires a résulté en l’adsorption de sulfate en tant qu’anion divalent par la formation d’un anneau à 6 membres avec l’Ai de surface. Lors d’une diminution de charge positive à des niveaux d’adsorption plus élevés l’ion sulfate a formé à la fois des complexes monodentates et bidentates. [D.J.]

Liquid limit of soil mixtures

The liquid limit test is one of the most widely used tests in the soil enginering practice. Several prroperties, including mechanical properties (for example, compressive index), have correlations with the liquid limit. In this paper detailed investigations of the liquid limit of soil mixtures have been carried out using bentonite, kaolinite, sand (coarse grained, fine grained, rounded and angular shaped), and silts. Based on the results obtained, it has been shown that the liquid limits of soil mixtures are not governed by the linear law of mxtures. While the shape of the sand was not found to influence the liquid limit, the size of the sand particles had a definite influence. Liquid limit obtained by the cone method is lesser than the limit obtained by using the Casagrande apparatus. A good relationship exists between the results of these two methods. A procedure for obtaining the liquid limit of low plastic soil has been suggested. (Author) been suggested. (Author)

Pozzolanic fly ash as a hydraulic barrier in land fills

The liner plays an important role in controlling migration of contaminants present in the leachate in waste containment systems such as land fills and impoundments. Although questions have been raised about the performance of clay liners, they are increasingly used singly or as double liners in disposal sites. Though the clay liners possess many advantages such as low permeability and large attenuative capacity, they also possess high shrinkage potential and hence can crack under unsaturated conditions causing instability and increase in leakage rates. Further, the permeability of the clay linear can increase due to clay–pollutant interaction. This study examines the potential of pozzolanic fly ash as a hydraulic barrier in land fill. The behaviour of three different types of fly ashes, showing a range of physical properties and chemical composition from three different sources are reported in the study. Geotechnical properties, needed to evaluate the use of fly ashes as barriers, such as shrinkage, compaction, permeability, consolidation and strength characteristics are reported. The results show that fly ashes possess low shrinkage and hence do not crack. Compacted fly ashes undergo very little volume changes. They also show that pozzolanic fly ashes develop good strength properties with time. Pozzolanic fly ashes containing sufficient lime develop strength even without addition of lime. Non-pozzolanic fly ashes do not develop strength even on addition of lime. Fly ashes generally consist of silt size particles and consequently possess high permeability. However, pozzolanic fly ashes with lime exhibit low permeability on curing because of the formation of gelatinous compounds which block the pores. Thus, pozzolanic fly ashes appear to be promising for construction of liners to contain alkaline leachate.

Mechanism controlling the shrinkage limit of soils

Based on the results from an extensive experimental program involving a number of natural soils as well as pure clays with extreme clay mineral types, namely kaolinite and montmorillonite, as the principal clay minerals, a hypothesis explaining the mechanisms controlling the shrinkage limit has been proposed. The shrinkage limit of a natural soil has been shown to be primarily a result of the packing phenomenon, which in turn is governed by the grain-size distribution of the soil; the shrinkage limit of pure clays appears to be affected by the fabric also. Even though clay-sized particles play an important role in the shrinkage phenomenon, there is an optimum clay content at which the shrinkage limit of a soil can become minimum. It has been further illustrated that the shrinkage limit is not at all related to the plasticity characteristics of the soil. As the factors governing the shrinkage limit are entirely different from those controlling the swelling of soils, the shrinkage limit cannot indicate the swelling behavior of the soil.

MECHANISM CONTROLLING THE VOLUME CHANGE BEHAVIOR OF KAOLINITE

The possible physical and chemical forces controlling the volume change behavior of kaolinite 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 cm3/10 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 cm3/10 g clay in benzene 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 benzene was unlikely because the protons required to impart a positive charge to the edges were not available in the nonpolar solvent. The 3688 cm−1 band in the infrared 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. That such interparticle H-bonds controlled the sediment volume and interparticle attraction in kaolinite was indicated by the decrease in sediment volume with increase in dipole moment of the solvent molecule, i.e., 25.0 cm3/10 g clay in n-heptane (dipole moment, μ = 0), 23.5 cm3/10 g clay in toluene (μ = 0.36), 17.0 cm3/10 g clay in ethanol (μ = 1.67), and 14.5 cm3/10 g clay in water (μ = 1.84).In the oedometer tests with various pore fluids, a high void ratio (i.e., volume of voids/volume of solids) of ≈ 1.3 was obtained for kaolinite in n-heptane, and hexane (μ ≅ 0) at an external pressure of 1 kg/cm2 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 (μ = 1.84) where the stronger hydrogen bond between flat layer surfaces of adjacent particles favored a parallel orientation of clay particles.The variations in void ratio-external pressure relationship indicated that kaolinite underwent lower compressibility in a solvent with low dipole moment and vice versa. Thus, the interparticle H-bond did not play a significant role in controlling the shear resistance and volume change behavior. 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.

Technical Note Reactive silica and strength of fly ashes

The use of fly ash in geotechnical engineering depends greatly on its pozzolanic reactivity. Though many factors influence the reactivity of fly ash it is well recognized that reactive silica and lime content play a major role. A new, accurate and reliable method for the determination of reactive silica content of fly ash has been established. The reactive silica content, obtained as acid soluble silica in about 2 to 3 N hydrochloric acid, is found to correlate well with unconfined compressive strength of fly ashes. The reactive silica content of fly ash is also important in the stabilization of soils using fly ash.