| dc.description.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. | en_US |
