Gravel road material properties in Rwanda, with focus on the Southern and Eastern provinces

Abstract

In most countries, substantial proportions of the road networks are surfaced with earth or gravel. Since earth and gravel roads are not protected by a bound surface layer there can be no doubt about the weakness of earth and gravel roads. Based on their complexity no mechanistic design systems are available yet; this research work provides basic engineering performance properties of existing gravel roads which can be used by pavement engineers as a model on upcoming maintenance or design of gravel roads in Rwanda. This dissertation is presented in three parts: the first part outlines various techniques of investigations of road materials for pavement engineers working on unpaved roads; the second part presents various geotechnical tests, procedures and visual conditions used to assess the performance of actual gravel roads in the southern and eastern provinces of Rwanda; finally the third parts shows the results characterising the gravel road materials properties and a brief interpretation of their performance regarding different standards and comments for the visual condition assessment acquired. Results revealed that the materials used as pavements on gravel roads in southern and eastern provinces are laterites classify as A-2-6, 56% of the wearing courses sampled on the nine selected roads was rated as excellent; for the road bed soils the materials was classified as silt materials in group A-6 and A-7-6 with a quality which is generally reasonable. According to the weather and traffic the predicted gravel loss in southern and eastern provinces is approximately 20 to 28mm/year. From the vision condition analysis using visual condition index (VCI), 65% of gravel roads in this zone are rated as good or fair. In general, the gravel road materials in southern and eastern provinces of Rwanda perform well but due to lack of maintenance some roads deteriorate quickly. It is recommended either a periodically drag or grade of the unsurfaced road network, in order to preserve a tolerably even running surface or to provide fresh materials to replace the previous ones which are lost under the action of traffic and weather.