Collapsing sand from the Kalahari region of Botswana

Mockett, L.D., Barton, M.E., Woodbridge, M.E. and Newill, D. (1992) Collapsing sand from the Kalahari region of Botswana (Contractor Report 351) TRL 73pp.

Abstract

In Botswana, approximately 1100 km of road are being upgraded from gravel to bituminous standard. Three main projects are involved in this programme and on one, the Serowe-Orapa Road, a field compaction trial was carried out using a heavy impact roller. The road is located towards the eastern edge of the Kalahari and crosses a variety of aeolian and flood-wash sands. In non-arid environments, the compaction of sand is facilitated by water added to give an optimum moisture content. However, the natural moisture content of these sands is low and the cost of importing water prohibitive and so a heavy impact roller was used in an attempt to achieve specified dry densities at the natural moisture content. The use of the impact roller and its suitability and effectiveness were evaluated by Pinard and Ookeditse (see IRRD 809088). A range of sand samples were taken from the site of a new road to represent the in-situ condition of both undisturbed areas and where compaction had been carried out. Samples were also taken at different depths below the surface and where different levels of compaction had been applied. The objectives were to investigate the physical, chemical and mineralogical properties of the sands and to compare the collapse potential of undisturbed and compacted soils. The soil fabric or microstructure using thin section analyses and scanning electron microscopy was also examined. The investigation showed that the sands, although they were non-plastic in standard tests, had varying amounts of fines (silt and clay) present which imparted considerable plasticity in the fraction of the soil passing the 63 micron sieve. The increasing fines content of the samples correlated well with increasing iron oxide content which in turn influenced the colour of the sands. Double oedometer tests according to the method of Jennings and Knight showed samples with high potential for collapse. After compaction with the impact roller the collapse potential of the near surface sample was virtually eliminated although it remained high for the deeper sample from 0.8m. This result correlated well with the lower densities and higher porosities which remained for the deeper samples. It implied that compaction at the surface was not affecting the fabric or structure of the sand at depth and this was further confirmed by the results of the scanning electron microscope examinations. The appearance of the fabric of the sands with a variety of sensitive bonding arrangements satisfied the generally accepted criteria for collapsibility. These investigations were carried out between November 1989 and April 1990 under a contract placed by the TRL with the Civil Engineering Department of Southampton University.

This record has no associated files available for download.

Contributors

Download statistics