A joint study by the Ethiopian Transport Construction Authority and the Transport and Road Research Laboratory from the United Kingdom has constructed two experimental roads in Ethiopia to examine the possibility of using weathered basalt gravels as surfacing material for unpaved roads. These experiments studied gravels from several different sources under a range of traffic levels ranging from 30 to 175 vehicles per day. Test sections were placed on roads with different geometric characteristics so that performance could be studied under a range of conditions. In addition, one section of road was constructed using weathered basalt gravel stabilised with lime. The performance of the materials was assessed in terms of resistance to deformation, rate of loss of gravel and deterioration of riding quality. These were related to the physical characteristics of the gravel, including any possible change with time, and also to the traffic loading. The measurements of the road surface were based on optical levelling which was used to monitor rutting, corrugations and gravel loss. These standard measurements were supplemented by visual inspection to identify features such as surface texture or the development of potholes. Riding quality was measured with a vehicle-mounted bump integrator. Materials testing comprised density measurements and checks of grading. The results of the experiments have led to proposals for the use of weathered basalt gravels and have shown that crushing or screening is likely to be cost-effective to reduce oversize material when used on roads with traffic levels in excess of about 50 vehicles per day. The experiment also showed that certain weathered basalts stabilised with 3 per cent of lime should provide an acceptable alternative to crushed stone base construction for surface dressed roads in Ethiopia. (A)

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