Engineering geology of cinder gravel in Ethiopia: prospecting, testing and application to low-volume roads

Expansion of the rural road network in Ethiopia requires the availability of low-cost materials for road construction, including capping layer and subgrade improvement, sub-base, base course, gravel wearing course and bituminised surface treatment. A reluctance to use cinder gravels for these purposes in the past has stemmed from the view that their properties, in terms of grading and CBR strength, are marginal and highly variable when compared to international specifications for road works. The geographical variability in Ethiopian cinder gravel geochemistry and engineering properties is described and comparisons are made with engineering geological field descriptions and cinder cone morphology, leading to the conclusion that maars and steep-sided, well-defined cones tend to yield the better-quality materials. The performance of trial sections of road constructed using cinder gravel is assessed and combined with the results of laboratory testing to develop a guideline for the wider use of the material in roadworks, either directly or through processes of blending, alternative compaction methods and cement stabilisation to yield a product that can be considered “fit for purpose” for a range of uses in low-volume road construction.