Role of Physico-chemical Processes And Micro-structural Features in Influencing Moisture Loss and Engineering Properties Of Compacted Residual Soils Exposed To Environmental Relative Humidity

Abstract

The moisture content of residual soils above the water table, and near ground surface (≤ 6 m) is influenced by the environmental relative humidity (RH). The focus of this thesis is to examine the physico-chemical and physical mechanisms of moisture retention and moisture loss in unsaturated residual soils in response to environmental RH and the implications of such changes on the engineering behavior of soils. The physicochemical drivers of moisture retention include forces and energy responsible for adsorption and desorption of water molecules on soil particle surfaces and capillaries, while the micro-structural features include, tortuosity and pore structure in influencing moisture retention and transport in unsaturated soils. In this study, laboratory experiments were performed with representative fraction of residual soil collected from Indian Institute of Science Campus, Bengaluru and five different saturated salt solutions were used to maintain environmental RH of 97%, 76%, 64.4%, 33% and 7% in the desiccators. The thermodynamics of moisture loss from the residual soil specimens were explored by performing experiments with moist powder soil specimens that were exposed to environmental RH of 33 to 97% at various constant temperatures (16 to 35°C). Distribution constant (Kc) is employed to account for the affinity of desorption of water molecules from soil particles. Analysis of laboratory results revealed that moisture desorption is an endothermic process associated with positive entropy changes and the trend of ΔGo (free energy change) variations indicated that moisture desorption is most favored at low environmental RH. Similar to SWRC (soil water retention curves), the ability of GAB (Guggenheim- Anderson-de Boer) isotherm to characterize the equilibrium soil moisture content - RH relations of compacted residual soil specimens along the drying path were explored and was successful in predicting the equilibrium water contents inspite of the variations in initial compaction conditions. The preferential desorption of water molecules from monoand multi-layers at low relative humidity required relatively larger (1.68 to 3.42 kJ/mol) desorption energy, while, from capillary condensation layer at higher relative humidity required lesser (0.08 to 0.5 kJ/mol) desorption energy. The influence of controlled environmental RH on the moisture loss behavior of compacted residual soil specimens was explored. As the RH of water in soil pores was in excess of the environmental RH of the desiccator, moisture desorption occurred from the compacted soil specimens exhibiting a falling rate segment. The moisture loss under RH gradients caused rapid increase in total suction of the specimens during the falling rate segment. Although the void ratios of the compacted specimens were unaffected by moisture loss, the micro-structure of the soils were affected; increase in very fine (< 0.002 μm) and fine pore (0.002 to 0.01 μm) contents at the expense of medium pores (0.01 to 6.0 μm) content was exhibited by most specimens. Analysis of the moisture loss - exchange period data showed that moisture loss from the compacted soil specimens is driven by diffusion and prevalence of lower RH in the environment facilitates speedier diffusion of moisture from soil pores. The thesis also develops an approach to select appropriate tortuosity (τ) equation based on dominant mode of moisture transport (liquid water or vapor) for correct prediction of moisture flux from the unsaturated soil when using Fick’s equation. Analysis of the laboratory results revealed that during the moisture loss process, as long as θ (volumetric water content) remains greater than a critical water content (θcr) value, capillary controlled flow of water dominates the moisture loss process and the τ is dependent on θ. When θ becomes less than θcr, vapor diffusion through connected air-filled pores becomes important and τ is dependent on air-filled porosity (θa). Knowledge of the final water content (wf) achieved by the unsaturated soil specimen during moisture loss under controlled environmental RH defines θcr for the soil. Good agreement is obtained between predicted and experimental moisture flux (qv) obtained by using τ values based on the dominant mode of moisture transport. Lastly, the thesis examines the influence of moisture loss under controlled environmental RH on compressive strength and collapse behavior of compacted residual soil specimens. Comparing the influence of moisture loss across specimens, it is observed that the initial water content and initial dry density had profound influence on magnitude of strength gain and stiffness respectively. The gain in strength and stiffness upon moisture loss is temporary as the improved strength and stiffness of the compacted specimens are lost on soaking. Examination of critical state stress ratios of compacted specimens upon moisture loss revealed that the critical state stress ratio for changes in net mean stress (Ma) and changes in matric suction (Mb) are strongly influenced by moisture loss. At Sr (degree of saturation) values < 1, Ma exceeds Ms (critical stress ratio at saturation) as particle aggregation at lower Sr causes the soil to behave in a coarser manner. In comparison, the Mb values are less than Ms at Sr < 1 as lowering the Sr causes the water phase to recede into the fine pores of the aggregates and the capillary bonds do not strengthen the aggregate - aggregate contact during shear. Exposure to environmental RH influences the swell and collapse tendency of the compacted residual soil specimens. At a given vertical pressure, exposure to lower RH renders the specimen more collapsible. The increased collapse potential upon moisture loss is attributed to existence of high matric suction in the unsoaked state that stabilizes the inter granular contacts; loss of inter-granular contacts on wetting leads to collapse of soil. In addition to suction, the wetting load to swell pressure ratio also influences the nature of wetting induced volumetric strains. At ratios < unity, the compacted specimen swells and at ratios > unity, the compacted specimen collapse. Further for wetting load < swell pressure ratio, the specimen with lower ratio swells more. Likewise, for wetting load > swell pressure ratio, the specimen with larger ratio collapses more.