A long term project has been undertaken to specify the use of calcrete for road construction in Botswana, under different conditions of environment, road design and traffic. Calcrete is a rubbly or powdery calcareous material that occurs within the Kalahari sand, in association with a number of distinct landforms. Research has focused upon the distribution of calcretes and the possible use of soft types for highway engineering. This report describes the aims of the project, concentrating on the survey and mapping aspects. The mapping study compared different remote sensing techniques to identify calcrete landforms in a corridor between Jwaneng and Ghanzi (17,000 sq km), considered to be typical of southern Botswana. Tests strips of colour and multi-spectral small format aerial photography were specially flown, but it was found that the full range of calcrete landforms can be identified using standard black and white aerial photographs, which provide good detail and a sterioscopic view, and Landsat imagery, which gives good colour differentiation, especially when digital images are enhanced by computer. The engineering properties of calcrete were studied in the laboratory and on site. It is recommended that grading, hardness and plasticity are measured in the field as an initial assessment of quality. A full scale road experiment, constructed using a range of calcretes, including soft types, has successfully carried more than 100,000 equivalent standard axles of traffic, and has been used to establish a new specification for sealed calcrete bases in lightly trafficked roads. Recommendations are given for planning calcrete surveys, and a map showing potential sources of calcrete in the project area is included. (A)

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