Geotechnical characterization of the Manyana black cotton soils in Botswana

The Manyana Village is divided into two types of soils. The center of the village is covered with sand that overlays clays from weathered dolerite whilst the eastern side is covered with exposed clays. Houses constructed on the eastern side of the village are prone to severe cracking attributed to the black cotton soils. The paper hints at shrink-swell mechanism of black cotton soils as a possible explanation for the cracking observed on the houses. Three laboratory methods of identifying and characterizing expansive soils are discussed followed by use of Dynamic Cone Penetration (DCP) in field-testing as well as use of indicator tests and particle sizing in the laboratory. The experimental results have shown a high field moisture content of (18 to 27%) in the eastern part of the village which was three times larger than moisture content at the village center. The clays had a plasticity index range of 12% to 30% with a clay content of 21% to 47% whilst the sandy areas were non-plastic with a clay content of at most 4%. The clay minerals identified from the samples tested were vermiculite, illite, kaolinite and montmorillonite. The soil samples from the village had low potential expansiveness whilst most samples on black cotton soil had a medium to high potential of expansiveness. The bearing capacity estimated from DCP data at depths of 1m throughout the village were 72 kPa to 275 kPa under dry conditions. A draft plasticity index map of Manyana village was produced based on the results of the investigation, the next step is to investigate the effectiveness of different soil stabilization methods for this area and taking in to consideration cost, durability and environmental impact.