T Thyagaraj

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 microstructural and macrostructural swelling components. The direction of osmotic flow affects the matric SWCCs. Unsaturated specimens experiencing osmotic induced consolidation and osmotic consolidation develop lower equilibrium water content than specimens experiencing osmotic swelling during the wetting path. The findings of the study illustrate the need to incorporate the influence of osmotic suction in determination of the matric SWCCs. © 2010 ASCE.

Compacted expansive soils, characterized with very low hydraulic conductivity and good contaminant retention capacity, have been widely used as barriers in landfills. They exhibit a double porosity structure with discrete interaggregate pores (macropores) and intra-aggregate pores (micropores) when compacted at optimum and dry of optimum water contents. The distribution of these macropores and micropores varies for different expansive soils depending on their grain size distribution and compaction characteristics, and thus, an in-depth study is necessary. This paper focuses on pore size distribution analysis using X-ray computed tomography (X-ray CT) and mercury intrusion porosimetry (MIP) tests on four expansive soils collected from different parts of Tamil Nadu, India. X-ray CT test gave the 2D image slices from top to bottom for all the specimens, and the acquired CT images of each soil specimen were segmented to separate the pores from the soil solids. The most probable threshold numbers for image segmentation were obtained using a newly developed methodology. The threshold numbers obtained decreased with increase in coarser fractions present in the soils. The thresholded binary images illustrated the pattern of larger pores in different expansive soils considered for the study. The MIP results showed a lower volume of macropores and a higher volume of micropores for soils with more clay content and higher dry density. A general insight into the range of macropores and micropores size distribution of compacted expansive soils with different gradation and compaction characteristics was achieved.

Compacted clay soils are used as barriers in geoenvironmental engineering applications and are likely to be exposed to salinization and desalinization cycles during life of the facility. Changes in pore fluid composition from salinization and desalinization cycles induce osmotic suction gradients between soil-water and reservoir (example, landfill/brine pond) solution. Dissipation of osmotic suction gradients may induce osmotic swelling and consolidation strains. This paper examines the osmotic swelling and consolidation behaviour of compacted clays exposed to salinization and desalinization cycles at consolidation pressure of 200 kPa in oedometer assemblies. During salinization cycle, sodium ions of reservoir fluid replaced the divalent exchangeable cations. The osmotic swelling strain developed during first desalinization cycle was 29-fold higher than matric suction induced swelling strain of the compacted specimen. Further, the diffusion controlled osmotic swelling strain was 100-fold slower than matric suction-driven swelling process. After establishment of ion-exchange equilibrium, saturated saline specimens develop reversible osmotic swelling strains on exposure to desalinization cycles. Likewise the saturated desalinated specimen develops reversible osmotic consolidation strains on exposure to cycles of salinization. Variations in compaction dry density has a bearing on the osmotic swelling and consolidation strains, while, compaction water content had no bearing on the osmotic volumetric strains. © 2012 Springer Science+Business Media Dordrecht.

Physico-chemical effects have a significant impact on the behavior of clay barriers due to the interactions between the pore fluid and clay particles, and they pose a great challenge because the efficiency of clay barriers and cover systems may be altered. Therefore, this paper highlights the effect of physico-chemical factors on shrinkage behavior of compacted expansive clay. To achieve this, the compacted clay specimens were inundated with distilled water, NaCl, and CaCl2 salt solutions in separate oedometer cells at a vertical pressure of 6.25 kPa and allowed to swell. The swollen specimens were gradually shrunk until the specimens attained constant mass, and the changes in void ratio and water content were monitored during drying. The experimental results showed that magnitude of induced osmotic suction and type of pore fluid had a significant impact on shrinkage behavior of compacted clay specimens due to the changes in soil structure. The average pore size of compacted specimens significantly decreased with the induced osmotic suction due to the reduction in size of micropores owing to the suppression of double layers. The discussions are supported by scanning electron microscopy images. Also, the experimental shrinkage data were fitted using modified van Genuchten and Fermi mathematical models, and the results showed that these models fit well with an adjusted R2 higher than 0.9962.

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 developed 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 expansive 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. © 2012 American Society of Civil Engineers.

In-situ deep Figure 14stabilization of expansive soil deposits is commonly carried out using lime piles and lime slurry injection. Recent research demonstrated the stabilization of expansive soils using lime column technique, and more recently the lime precipitation technique has emerged as the viable choice for stabilization of expansive soils. Comparison of these techniques in stabilizing the expansive soils provides a better understanding of the stabilization mechanisms and their advantages and limitations. Therefore, this paper brings out the relative efficacy of different lime treatment techniques in stabilizing the expansive soils. To achieve this objective, the laboratory model tests were carried out in expansive soil using lime pile and lime precipitation techniques in a compacted state, and lime slurry technique in a desiccated state from a central hole of diameter, d. The lime precipitation was achieved by sequential permeation of 46.2% calcium chloride and 33.3% sodium hydroxide solutions through a central hole in the compacted expansive soil. After 30 days of curing in the test moulds, the undisturbed soil specimens were collected for evaluation of the changes in physico-chemical, index and engineering properties. The experimental results reveal that the treatment is effective in stabilizing the expansive soil up to a radial distance of 0.8d from the central hole in case of lime pile treatment, whereas the lime slurry treatment up to a radial distance of 1.5d and lime precipitation treatment up to a radial distance of 2.5d. The experimental findings were supported with scanning electron microscopic images and energy dispersive X-ray spectroscopy analysis.

The present investigation examines the efficiency of an in situ lime precipitation technique in stabilizing expansive soil through laboratory-scale model tests. Expansive soil was compacted in a cylindrical mold and sequentially permeated with CaCl2 and NaOH solutions into the expansive soil through a central hole filled with coarse sand. Successive permeation of CaCl2 and NaOH solutions into the compacted expansive soil resulted in precipitation of lime in the expansive soil mass. The precipitated lime reduced the plasticity index, controlled the swell-shrink potentials, and increased the unconfined compressive strength of the expansive soil by both strong lime modification reactions and soil-lime pozzolanic reactions. The results are corroborated with microfabric studies on lime precipitation treated specimens, which showed the formation of cementation bonds.