Good quality road-building materials in the Kalahari region of Botswana are scarce. Apart from sand, the principal available road building material is calcrete, which is regarded by engineers as of poor quality for road construction. A joint ODA/Botswana funded research project was established by TRL and the Roads Department of the Ministry of Works and Communication to derive guidelines for their use. Trials were constructed and monitored over a period of nearly 15 years.
The principal outputs from the project are revised traffic limits and specifications for calcrete in road bases. Other technical conclusions included:
• The limiting condition on performance was a reduction in strength in the verge-side wheelpath caused by moisture ingress from the unsealed shoulders in the wet season.
• A cost-effective method of attaining a drier pavement environment and higher pavement strength would be to seal shoulders.
• The standard of a 4-day soaked CBR of 80 per cent is inappropriately high for low volume roads.
• The ratio of damage in the two directions indicated an exponent of approximately between 2 and 3 and is less than predicted by the 4th-power damage law.
• Kalahari sands can be used effectively as sub-base material.
An economic analysis of road projects indicated that there was approximately a 7:1 ratio in the average costs of using crushed stone compared with natural gravel road bases such as calcrete, equivalent to savings (mid 1990’s) of about £30,000 per kilometre. Sealing shoulders (producing a drier environment and higher strength in the road base) would cost £3,000 per kilometre. Savings of approximately £3,000 per kilometre were estimated where sub-base sand material is available adjacent to construction sites.

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