Abstract
The behaviour of unsaturated soils is mainly influenced by suction. When unsaturated soils display swelling properties, it becomes fundamental to investigate the impact of soil suction on the swelling stress. A survey was done across Free State province in South Africa and samples were obtained from Bloemfontein, Winburg, and Welkom. Geotechnical studies were performed on particle size definition, free swell ratio, free swell index, Atterberg limits, X-ray diffraction, proctor compaction test, soil suction measurement, and constant volume swelling tests to determine the physical and hydro-mechanical properties of the soil samples. According to the findings, at the optimum water content, the swelling stress values are in the range of 177 kPa to 326 kPa which is more than the bearing limit (~ 40 kPa) applied for most lightweight footing. Smectite is identified as the predominant clay mineral in the study areas and has a key influence on the swelling properties. A solid relation is observed between the swelling stress and the soil suctions, with a correlation coefficient value greater than 80 %.
Highlights
Introduction and BackgroundHeaving soils, predominantly clay exhibits huge volume changes as a result of soil water variation
The values of the matric suction at the optimum water content are within the range of 672 kPa to 2022 kPa and the total suction values within the range of 1036 kPa to 2814 kPa
The swelling stress values at the optimum water content are within the range of 177 kPa to 326 kPa which is greater than the bearing limit (~ 40 kPa) applied for most lightweight footing in the study areas and justify the damages to lightweight structures
Summary
Predominantly clay exhibits huge volume changes as a result of soil water variation. Foundations built on these soils are exposed to uplift forces induced by swelling stress that can cause cracks, and break up both slabs and building foundations. The behaviour of heaving soils is mainly influenced by suction, which is a free energy state of water inside the soil (Edlefsen and Anderson, 1943). Matric suction is observed as the key parameter determining unsaturated soils behaviour. It is the difference in pressure between the pore air, designated by (ua) and the pore water, designated by (uw) as given in Eq(2)
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