Abstract:
The volumetric change in fine-grained soil with changes in moisture content is one of the
most important phenomena. The soil shrinkage characteristic curve describes the
magnitude of the variation in bulk volume due to changes in moisture content. In other
word, the soil shrinkage characteristic curve defines the constitutive relationship between
soil volume and moisture content under drying condition and mostly represented by void
ratio versus gravimetric water content. The shape of soil shrinkage characteristic curve,
which is mainly composed of two to four stages, is influenced by the type of soil and its
structure. These are structural shrinkage and normal shrinkage when the soil is saturated,
residual shrinkage when the soil is unsaturated and subjected to volume reduction, zero
shrinkage stage when the soil is in dry state and no further reduction in volume of soil
occurs. The soil shrinkage characteristic curve is in every case necessary while defining
the soil water characteristic curves of unsaturated volume changing soil. It is important in
determining soil deformation behavior and understanding the mechanism of soil-water
interaction. The determination of soil shrinkage characteristic curve requires the
measurement of pore volume and water volume in a known volume of soil simultaneously
over the whole range of moisture content from fully saturated state to oven dry state.
Despite its importance in unsaturated soil mechanics, there is no research on determining
the soil shrinkage characteristic curve in Ethiopia. In this study, soil characterization is
done based on expansivity of fine – grained soils found in the study area. Then, the soil
shrinkage characteristic curve of volume changing soils in slurry, compacted and
undisturbed initial conditions are determined. The effect of initial conditions on soil
shrinkage characteristic curves is studied. For soil shrinkage characteristic curve
determination, the direct volume measurement method and image-based techniques of soil
volume measurement are compared. Both techniques of measuring soil volume showed
good agreement, but the image-based technique was found to save time and effort in
establishing soil shrinkage characteristic curves. The performance of existing models in
predicting the soil shrinkage characteristic curves of soils in undisturbed, compacted and
slurry condition for the study area are investigated using MATLAB
®
2016 nonlinear
programming solver called fmincon. From the comparison of these existing models, the
Leong and Wijaya (2015) model is found to be the best fitting model to soil shrinkage
characteristic curve of soils in all three initial conditions. The fitting parameters of Leong
and Wijaya (2015) model are correlated to the basic and index soil properties of soil and
developed new models for Bahir Dar volume changing soils with the help of JMP
®
Pro 14
statistical analysis software. In this study, key soil shrinkage characteristic curve
parameters such the air entry value, shrinkage limit, and minimum void ratio have been
discussed and their values reported. The water content values corresponding to the air entry
values are found to be very close to the corresponding pla stic limit values. Additionally,
the values of shrinkage limit obtained from soil shrinkage characteristic curve and from
laboratory test are compared. The shrinkage limit values from soil shrinkage characteristic
curve in slurry condition is very close to the value of shrinkage limit which is determined
by laboratory test.
Keywords: Unsaturated Soil, Soil Shrinkage Characteristic Curve, Soil Water
Characteristic Curve, Existing Models, Initial Conditions.