Depending on the relative chemical concentrations of clay pore water and landfill leachate, three osmotic flow scenarios may be anticipated to dissipate the osmotic suction gradient: outward osmotic flow, inward osmotic flow and no (osmotic) flow. This study examines the volume change behaviour of clay subjected to all osmotic flow conditions. Results show that osmotic suction gradient decreases swelling strains when the osmotic flow is outward, whereas swelling strains increase in case of inward osmotic flow. The increase or decrease in effective stress from osmotic suction gradient is transient and reduces to zero with dissipation of matric suction and osmotic suction gradient. In the no (osmotic) flow condition, the swelling strains are governed by matric suction and pore fluid osmotic suction. The experimental results bring out the dependency of swelling strains on applied stress, stress paths and osmotic suction gradient.

T. Thyagaraj

Department Of Civil Engineering

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Environmental Geotechnics

In the present study, the cyclic triaxial tests and resonant column (RC) tests were carried out, to study the effect of frequency on reconstituted marine clay saturated with distilled water and NaCl solutions. Slurry consolidated samples were prepared at 50 kPa vertical stress. Samples were isotropically consolidated at three different effective confining pressures i.e. 80, 150 and 300 kPa. The loading frequencies adopted for the study were 0.1 Hz and 1 Hz with a number of 100 loading cycles. The results show that the effect of frequency could affect the shear modulus of clay soil reconstituted with varying pore fluid chemistry. The shear modulus increases and damping ratio decreases with increase in loading frequency for clay soil reconstituted with concentrated saline solution. Furthermore, the seismic response analysis was conducted to study the effect of frequency on marine clay soil reconstituted with distilled water and electrolyte solutions using one dimensional equivalent linear DEEPSOIL software. © 2019, © 2019 Informa UK Limited, trading as Taylor & Francis Group.

International Journal of Geotechnical Engineering

Effect of frequency on cyclic response of marine clay saturated with various pore fluids

Compacted clays in landfill applications are subjected to both physico-chemical changes and wet–dry cycles. Physico-chemical interactions occur at the microstructural level between the clay particles and chemical constituents such as brine solutions or leachates generated from landfill waste. Furthermore, the volumetric changes during wetting and drying also cause microstructural changes to compacted clays. The microstructural changes due to the physico-chemical interactions and wet–dry cycles are reflected at the macrostructural level and govern the macro-behaviour of compacted clays. Therefore, this paper brings out the combined effect of the interacting fluid and wet–dry cycles on volume change, microstructure and hydraulic conductivity of compacted clay soil. To achieve this, the compacted clay specimens were inundated with distilled water and sodium chloride (NaCl) solutions during wetting cycles. The experimental results showed that the ability of clay particles to swell at microstructural level and decrease the size of macropores in compacted clay was completely lost with 4 M sodium chloride solution and with increase in wet–dry cycles. Thus, the hydraulic conductivity of compacted clay specimen inundated with 4 M sodium chloride solution was unusually high at the end of the second and further wetting cycles even when subjected to higher effective confining pressures. Copyright © ICE Publishing, all rights reserved.

Geotechnique Letters

Effect of pore fluid and wet-dry cycles on structure and hydraulic conductivity of clay

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. © 2016 American Society of Civil Engineers.

International Journal of Geomechanics

Physico-chemical effects on shrinkage behavior of compacted expansive clay

Physico-chemical effects assume significance in geoenvironmental engineering applications where compacted soils and natural soil deposits interact with chemical contaminants. The physico-chemical interactions occur at the microstructural level and affect the macrostructure and thus the volume change behaviour. This paper brings out the physico-chemical effects on the compressibility and collapse behaviour of compacted red soil. The physico-chemical effects were induced by using different fluids for specimen preparation and inundation. Salinisation of compacted specimens prepared with distilled water induces outward osmotic flow and causes induced osmotic consolidation. In addition, the diffusion of saline solutions into the compacted specimens leads to microstructural contractions at the clay particle level and results in an increase in the irreversible macrostructural strains. Consequently, the macrostructure experiences plastic hardening and this results in greater collapse strains. Scanning electron micrographs showed an increase in the size of macrovoids (loose macrostructure) with the increase in pore fluid osmotic suction owing to the microstructural contractions. Therefore, this paper also provides an insight into the behaviour of specimens prepared with saline solutions with respect to the initial loading–collapse (LC) curves and collapse behaviour when subjected to inward osmotic flow and no osmotic flow conditions. The specimens prepared with saline solutions yielded at lower vertical stress in comparison with the specimens prepared with distilled water, and the elastic response of the compacted soil was found to be independent of both matric and pore fluid osmotic suctions. The osmotic swelling, which occurs owing to dilution of pore fluid at the microstructural level, resulted in slightly lower collapse potentials in specimens subjected to inward osmotic flow condition in comparison with the no osmotic flow condition. © 2017 Thomas Telford Ltd.

Geotechnique

Physico-chemical effects on collapse behaviour of compacted red soil

In geoenvironmental engineering applications, the compacted fills often come in contact with hazardous chemical contaminants and are subjected to physico-chemical changes. These changes alter the mechanical behaviour of compacted soils due to the physico-chemical interactions at the clay particle level, which are reflected as a variation in the yield stress at the macrostructural level. Therefore, the current experimental investigation presents the effect of pore fluid osmotic suction on the compressibility and collapse behaviour of compacted clayey sand (red soil). Compaction of salt solution remoulded soil leads to the development of larger macropores owing to the microstructural contractions. The macroporosity increases with the increase in the pore fluid osmotic suction due to greater microstructural contractions. Thus, the yield stress decreases with the increase in the pore fluid suction, and the relationship between them was found to be logarithmic. The micro- and macro-structural changes occurring from the physico-chemical changes also affect the collapse, as the phenomenon of collapse is associated with macrostructural deformations. In order to eliminate the osmotic flow during collapse, no osmotic flow condition was adopted. Experimental results illustrate that the collapse increases with the increase in the pore fluid suction on account of increase in matric suction and macroporosity.

Effect of pore fluid on compressibility and collapse of clayey sand

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.

Journal	Environmental Geotechnics
Publisher	ICE Publishing
ISSN	2051803X
Open Access	No