Sudhakar M. Rao

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.

The impact of cyclic wetting and drying on the swelling behaviour of stabilized expansive soils

Black cotton soil (BCS) deposits, stabilized with waste materials-wood-ash and organic matter (leaves, grass, etc.) exist in BCS areas of North Karnataka, India. These ash-modified soils (AMS) are apparently stabilized by hydrated lime produced by biochemical, dissolution, and hydration reactions. The influence of cyclic wetting and drying on the swelling behaviour of wood-ash-modified BCS and laboratory lime-treated BCS specimens are examined in this study. Such a study is required to assess the long-term behaviour of chemically stabilized soils in geotechnical applications. Cyclic wetting and drying caused the AMS specimens to become more porous and less saturated. Consequently, the cyclically wetted and dried (or desiccated) AMS specimens collapsed significantly at the experimental flooding pressures. The beneficial effects of lime-stabilization of the BCS specimens were also partially lost in cyclically wetting and drying them. The clay contents of the lime-treated BCS specimens increased on cyclic wetting and drying. The increased clay contents in turn, affected their Atterberg limits and swell–shrink potentials. Partial loss of inter-particle cementation, increased porosity, and reduced degree of saturation, also imparted small to moderate collapse potentials to the desiccated lime-treated BCS specimens.

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.]

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.

Laboratory Studies on Stabilization of an Expansive Soil by Lime Precipitation Technique

Lime stabilization prevails to be the most widely adopted in situ stabilization method for controlling the swell-shrink potentials of expansive soils despite construction difficulties and its ineffectiveness in certain conditions. In addition to the in situ stabilization methods presently practiced, it is theoretically possible to facilitate in situ precipitation of lime in soil by successive permeation of calcium chloride (CaCl2 ) and sodium hydroxide (NaOH) solutions into the expansive soil. In this laboratory investigation, an attempt is made to study the precipitation of lime in soil by successive mixing of CaCl2 and NaOH solutions with the expansive soil in two different sequences.Experimental results indicated that in situ precipitation of lime in soil by sequential mixing of CaCl2 and NaOH solutions with expansive soil developed strong lime-modification and soil-lime pozzolanic reactions. The lime-modification reactions together with the poorly de- veloped cementation products controlled the swelling potential, reduced the plasticity index, and increased the unconfined compressive strength of the expansive clay cured for 24 h. Comparatively, both lime-modification reactions and well-developed crystalline cementation products (formed by lime-soil pozzolanic reactions) contributed to the marked increase in the unconfined compressive strength of the ex-pansive soil that was cured for 7–21 days. Results also show that the sequential mixing of expansive soil with CaCl2 solution followed by NaOH solution is more effective than mixing expansive soil with NaOH solution followed by CaCl2 solution. DOI: 10.1061/(ASCE)MT .1943-5533.0000483.

Influence of Osmotic Suction on the Soil-Water Characteristic Curves of Compacted Expansive Clay

Unsaturated clays are subject to osmotic suction gradients in geoenvironmental engineering applications and it therefore becomes important to understand the effect of these chemical concentration gradients on soil-water characteristic curves (SWCCs). This paper brings out the influence of induced osmotic suction gradient on the wetting SWCCs of compacted clay specimens inundated with sodium chloride solutions/distilled water at vertical stress of 6.25 kPa in oedometer cells. The experimental results illustrate that variations in initial osmotic suction difference induce different magnitudes of osmotic induced consolidation and osmotic consolidation strains thereby impacting the wetting SWCCs and equilibrium water contents of identically compacted clay specimens. Osmotic suction induced by chemical concentration gradients between reservoir salt solution and soil-water can be treated as an equivalent net stress component, ( pπ ) that decreases the swelling strains of unsaturated specimens from reduction in ...

Classification of Expansive Soils by Sediment Volume Method

A soil expansivity classification is proposed based on the sediment volume ratio defined as the ratio of the sediment volume in 0.025% sodium chloride solution to that in kerosene. The results of the proposed classification agree well with one based on the oedometer test results of compacted soil specimens.

A RAPID METHOD TO IDENTIFY CLAY TYPE IN SOILS BY THE FREE-SWELL TECHNIQUE

This paper reports a rapid method to identify the presence of nonswelling clay, swelling clay, or a mixture of the two types of clays in a soil from the sediment volumes occupied by 10 g of dry soil in 100 mL of water and carbon tetrachloride, respectively, under no external constraint. Soils that occupy a sediment volume of 1.10 m3/Mg in carbon tetrachloride and volumes ≥ 1.50 m3/Mg in water contain both nonswelling and swelling clays. The results agree well with X-ray diffraction and infrared analysis. In the latter part of the work, the limitations in the existing definition of the free-swell index of soils is brought out. The present definition gives a negative free-swell value for soils containing nonswelling clay and a reduced value for soils containing a mixture of nonswelling and swelling clays. The free-swell index of soils is defined as the sediment volume occupied by 10 g of dry soil in 100 mL of water under no external constraint and expressed in units of m3/Mg. Further, the sediment volume of a soil (10 g) in a nonpolar solvent, is utilized to give information on the clay type present in the soil, namely, nonswelling, swelling, or a combination of both. A classification for identifying a soils expansivity based on the new definition of the free-swell index and the clay mineralogical composition of the soils is given.

MECHANISTIC APPROACH TO THE SHEAR STRENGTH BEHAVIOUR OF ALLOPHANIC SOILS

Abstract The presence of allophane minerals imparts special engineering features to the volcanic ash soils. This study examines the reasons for the allophanic soils exhibiting unusual shear strength properties in comparison to sedimentary clays. The theories of residual shear strength developed for natural soils and artificial soil mixtures and the unusual surface charge properties of the allophane particle are invoked to explain the high shear strength values of these residual soils. The lack of any reasonable correlation between ∅′ (effective stress-strength parameter) and plasticity index values for allophanic soils is explained on the basis of the unusual structure of the allophane particle. The reasons as to why natural soil slopes in allophanic soil areas (example, Dominica, West Indies) are stable at much steeper angles than natural slopes in sedimentary clay deposits (London clay areas) are explained in light of the hypothesis developed in this study.

Re-use of fluoride contaminated bone char sludge in concrete.

Managing sludge generated by treating groundwater contaminated with geogenic contaminants (fluoride, arsenic, and iron) is a major issue in developing nations. Their re-use in civil engineering applications is a possible pathway for reducing the impact on the geo-environment. This paper examines the re-use of one such sludge material, namely, fluoride contaminated bone char sludge, as partial replacement for fine aggregate (river-sand) in the manufacture of dense concrete specimens. Bone char sludge is being produced by defluoridation of contaminated groundwater in Nalagonda District, Andhra Pradesh, India. The impact of admixing 1.5-9% sludge contents on the compression strength and fluoride leaching potential of the sludge admixed concrete (SAC) specimens are examined. The compression strengths of the SAC specimens are examined with respect to strength criteria for manufacture of dense, load-bearing concrete blocks. The fluoride release potential of the SAC specimens is examined with respect to standards specific to disposal of treated leachate into inland surface water.