T Thyagaraj

Physico-chemical interactions and alternate wetting and drying process have marked influence on the swell–shrink and hydraulic behaviour of compacted expansive soils used in various engineering applications. The microstructural, shrinkage cracks pattern and hydraulic behaviour of compacted expansive soils get altered due to the interactions with leachates and cyclic wet–dry process. Therefore, the effect of concentration of leachates and cyclic wetting–drying process on the swell–shrink, shrinkage cracks, microstructural and hydraulic behaviour of compacted expansive soil is studied in this theme lecture paper. The shrinkage cracks developed in clay specimen during a drying cycle were quantified using the X-ray computed tomography images and vernier caliper height measurement. And the same method was used to quantify the cracks in desiccated specimens interacted with different concentrations of interacting fluid. Scanning electron microscope and mercury intrusion porosimeter studies were carried out to understand the microstructure of salt solution-interacted specimens. The influence of physico-chemical interactions and cyclic wet–dry process on the self-healing capacity of compacted expansive soil specimens was brought out by carrying out hydraulic conductivity tests.

Compacted clays are used as barriers in landfills mainly due to their low hydraulic conductivity and self healing properties. These clays are subjected to moisture fluctuations and simultaneously exposed to leachates. Besides swelling, the shrinkage and cracks developed during drying get altered due to their exposure to contaminants and control their hydraulic behaviour. Therefore, this paper quantifies the volumetric behaviour and cracks pattern of compacted clay exposed to salt solutions during wet-dry cycles using X-ray computed tomography (XCT) imaging experiments. The results showed that the cracks volume of clay specimen interacted with 4 M NaCl solution is significantly greater than the specimens interacted with distilled water (DW) and 0.4 M NaCl solution at the end of drying cycles. XCT and digital camera imaging experiments showed that the cracks did not develop at same locations in specimen interacted with DW, indicating the self healing property of clay. However, the development of cracks at same locations in specimen interacted with 4 M NaCl solution indicates the complete deterioration of self healing property of clay. This resulted in significantly higher hydraulic conductivity of compacted specimen interacted with 4 M NaCl solution than the specimens interacted with DW and 0.4 M NaCl solution at the end of different wetting cycles.

Compacted expansive soils are widely used as engineered barriers in waste contaminant applications like landfills, brine ponds, and nuclear waste disposal sites. These liners are designed for very low hydraulic conductivity (<1 × 10−7 cm/s). Percolation of chemical waste or leachate results in physicochemical changes in compacted expansive soils which increases the hydraulic conductivity. This paper brings out the changes in swelling behavior and hydraulic conductivity of compacted expansive soil induced with osmotic gradients using NaCl and CaCl2 solutions. Multiple identical soil specimens placed in oedometer assemblies were inundated with distilled water, 0.4 and 4 M NaCl (monovalent cations), and 0.4 and 4 M CaCl2 (divalent cations) salt solutions and allowed to swell under a surcharge pressure of 12.5 kPa. Void ratio–water content plots were also traced during swelling process. Falling head permeability tests were conducted on swollen soil specimens in rigid wall oedometer permeameters under a hydraulic gradient (i) of 20. The experimental results showed that the swell potentials reduced and hydraulic conductivity increased with the increase in induced osmotic suction.

The microstructure of compacted expansive soils differs depending on the remoulding water content and compaction dry density. In particular, along the standard Proctor compaction curve, the expansive soil compacted at water content less than the optimum water content has relatively larger macropores in comparison to the expansive soil compacted at water contents greater than the optimum water content. These changes in microstructure not only influence the hydraulic conductivity but also the infiltration rates during the swelling process of compacted expansive soils. Therefore, this paper brings out the effect of remoulding water content on the infiltration rates during swelling process and hydraulic conductivity at the end of swelling process of a compacted expansive soil. The oedometric-infiltrometer test arrangement was used to determine the hydraulic response, in terms of infiltration rates and hydraulic conductivity, upon inundation of the compacted expansive soil specimens remoulded with water contents corresponding to dry and wet side of optimum water contents at the same standard Proctor compaction dry density. As expected at the end of swelling process, the swell magnitude and hydraulic conductivity were relatively higher for the clay specimen compacted at dry of optimum water content. In addition, the infiltration test results showed that the time needed for the outflow (i.e. permeation flow) to occur and attain a steady state condition was comparatively less for the compacted clay specimen compacted at dry of optimum water content than at wet side of optimum water content. The changes in dry density and water content during the swelling process of compacted clay specimens were also traced.

The present study examines the relevance of placement conditions on the behaviour of polypropylene fiber reinforced bentonite. The laboratory testing program includes conducting standard and modified Proctor compaction tests, tracing shrinkage behaviour, performing hydraulic conductivity and unconfined compression strength tests on both unreinforced and fiber reinforced bentonite. An overall acceptable zone in terms of water content and dry unit weight is established by considering the hydraulic conductivity, strength and volumetric shrinkage criteria. The experimental results showed that the volumetric shrinkage criterion controlled the overall acceptable zone for unreinforced and fiber reinforced bentonite specimens in compacted state. However, the hydraulic conductivity values of fiber reinforced bentonite specimens subjected to wet-dry cycles show that the shrinkage criteria may not be appropriate for materials such as bentonite and bentonite-enhanced clays which possess good self sealing properties; and for such materials the hydraulic conductivity needs to be evaluated after wet-dry cycles. As the basic ingredient of geosynthetic clay liners (GCLs) is bentonite clay, the relevance of the placement conditions is discussed for the GCLs.

Swelling and shrinkage of expansive soils raise concern over their performance for various geotechnical and geoenvironmental engineering applications such as top and bottom liners of landfills. Swelling results in heaving of expansive soil, whereas the shrinkage results in the formation of desiccation cracks. To overcome these volume changes in the expansive soils, they can be modified by adding sand which can reduce the swell potential and the formation of desiccation cracks. Plasticity, compaction, swelling and permeability characteristics of expansive soils get altered upon the addition of sand. In the present study, these characteristics of an expansive natural soil amended with sand and their interdependency with each other were examined through the experimental laboratory studies. Clay-sand mixtures passing through 2 mm sieve were prepared by thoroughly mixing the sand with expansive soil in three different proportions, namely low, medium and high sand content. Swell potential and hydraulic conductivity of different specimens under a surcharge load of 12.5 kN/m2 were obtained from one-dimensional swell tests and rigid wall permeability tests, respectively. Although there was a considerable variation in the sand proportion added, the consistency limit tests showed only smaller reduction in liquid limit and plasticity index because of a lesser percentage of fine sand in the total sand added. In spite of the small changes in the consistency limits, a reduction of more than 50% in swell potential and increased hydraulic conductivity was observed, indicating the more significant influence of coarser non-swelling fraction on the swell and hydraulic behaviour of clay-sand mixtures.

Compacted clay liners are an integral part of the waste landfills, which are provided to contain the leachate within the landfills and protect the surrounding environment. Generally, locally available natural soils are used for the construction of compacted clay liners if they satisfy the design criteria. However, not all soils in their natural state satisfy all the design criteria for the liner materials. Thus, there is a definite need to modify the locally available natural soils by blending with bentonite to meet the required design criteria for the liners. In view of this, the present study evaluates the suitability of an Indian red soil enhanced with bentonite as a liner material. To achieve this, a series of experiments were carried out using locally available red soil and bentonite. First, the suitability of the red soil was evaluated as a liner material. The experimental results showed that the red soil met all the selection criteria stipulated by the Environmental Protection Agencies (EPAs) for the liners except the hydraulic conductivity criterion. Therefore, the red soil was mixed with bentonite contents of 10%, 20% and 30%, and the red soil-bentonite mixtures were evaluated for their suitability for liners in their compacted state. Further, as the liners in the arid and semi-arid regions are subjected to moisture variations due to seasonal moisture fluctuations and other factors, the red soil-bentonite mixtures were subjected to wet-dry cycles, and their suitability was evaluated after wet-dry cycles. The experimental results revealed that all the red soil-bentonite mixtures met the stipulated EPA criteria for the liners in the as-compacted state. However, the red soil-bentonite mixtures with 20% and 30% bentonite contents only satisfied the hydraulic conductivity requirement even after wet-dry cycles. The experimental findings were supplemented with the microstructural insights captured through digital camera images, scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) studies.

Clay liners are integral part of both hazardous and municipal waste landfills that prevent leachate from percolating into the soil beneath and polluting it. Hence, the compacted soils must have very low hydraulic conductivity (<10–7 cm/s) to act as effective clay liners. Locally available red soil may be used as a liner material as it satisfies the design criteria. To meet the hydraulic conductivity requirement, the locally available red soil was modified by adding 10% and 20% bentonite by dry weight. Multiple identical compacted specimens were set-up in oedometric assemblies under a surcharge pressure of 12.5 kPa and were inundated with distilled water, 0.4 M CaCl2 and 0.4 M NaCl solutions to study the swelling behaviour of red soil and red soil modified with different bentonite contents. The specimens were compacted at their respective optimum moisture content values to their maximum dry unit weights. Falling head permeability tests were conducted to measure hydraulic conductivity using the rigid wall permeameters at a hydraulic gradient of 20 and surcharge pressure of 12.5 kPa. The nature of the inundating fluid and the bentonite content is seen to greatly affect the swelling behaviour and the hydraulic conductivity of the compacted red soil.