T Thyagaraj

This paper presents a case study of a two-storey building in Oragadam, near Chennai (India), which has undergone distress due to the presence of expansive soil. Undulations in the floors, cracks in partition walls and non-uniform heave in the pavement are some of the failure patterns noticed in and around the structure. Undisturbed soil samples were collected from the site to identify the cause of distress. From the field inspection and laboratory testing it was found that the use of unsuitable fill material (expansive soil) followed by the ingress of excess water from the garden and improper planning of the location of the rainwater-harvesting system were the causes of the initiation of distress in the building.

The effect of pore fluid chemistry on the cyclic behaviour of fine grained soil is studied by performing strain controlled undrained cyclic triaxial tests on reconstituted marine clay with 0.4 M sodium chloride solution and distilled water. Specimens using different pore fluids were prepared by slurry consolidation method at vertical stress of 50 kPa. The influence of physico-chemical factors on the cyclic behaviour of reconstituted marine clay was brought out by studying the effect of number of cycles on the cyclic shear modulus degradation of clay. The effective confining pressure was taken as 150 kPa, loading frequency as 1 Hz, number of loading cycles as 100, and the strain range is kept in between 0.3% to 1%. The experimental results show that, with 0.4 M sodium chloride solution as the pore fluid, the shear modulus increases at lower strain range. The variation of damping ratio with the cyclic shear strain was also brought out in this study.

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.

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.

The experimental results of the present study on compacted expansive clay illustrate that matric suction increases with the increase in pore fluid osmotic suction due to changes in soil structure. Scanning electron micrographs show that particle stacking (aggregations) increases with increase in pore fluid osmotic suction and cation valence due to reduction in diffuse double layer thickness, and thus results in a reduction in micro pore size with a corresponding increase in macro pore size. At a given water content, the macro pore degree of saturation reduces with the increase in osmotic suction owing to the increase in macro pore size and leads to an increase in matric suction, as the matric suction is characteristic of the macro pore and the degree of saturation of the macro pore. The matric, osmotic and total suction measurements using the filter paper method also confirm that the matric and osmotic suction components are additive. This demonstrates that the method of obtaining matric suction from the difference between total suction and pore fluid osmotic suction determined using non-contact filter papers is reliable.

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.

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.

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 phenomenon of collapse assumes significance in the geotechnical engineering applications such as earth dams and highway and railway embankments as the compacted soils in these applications are susceptible to wetting-induced failures. Therefore, the present study focuses to understand the influence of placement conditions and vertical stress on the wetting-induced collapse behaviour of compacted red soil through series of single oedometer collapse tests. The loading–collapse (LC) curves of compacted red soil for different dry unit weights were established over a large range of matric suctions in order to analyse the experimental results in the context of Barcelona basic model (BBM) elasto-plastic framework and also the experimental results were supported using the concept of LC curves defined in the BBM framework. The experimental results showed a path-dependent relationship of collapse potential with the water content at a given dry unit weight and vertical stress. The position of LC curves of the compacted specimens with respect to the wetting path illustrated a decrease in the soil collapse with the increase in dry unit weight. Thus the present experimental results demonstrate that the BBM framework aids in predicting the nature of volume change and the point of maximum collapse.