Practical implications of the relation between the clay mineral content and the plasticity index of dolerite road construction material

Abstract

Naturally occurring road building materials, suitable for the higher-quality upper strata of road pavements (base and subbase layers), are systematically being depleted, resulting in the haulage of alternative material over increasing distances or the introduction of more expensive commercially manufactured materials. For this reason the naturally occurring materials have to be better assessed and utilised. The weathered rock most widely used for road construction in South Africa, yet also notorious for its variability and propensity to marginal behaviour, is dolerite. Tests developed for assessing the suitability of road building materials have, in the case of dolerite, often misled design engineers and technicians, to the point that the material is rather avoided if there is any suspicion of its quality being marginal. If the behaviour of dolerite under various environmental conditions were to be better understood, the assessment tests might also be improved, as well as being more effectively applied. This paper deals with the findings and recommendations resulting from research in this regard, especially the relationship between the Plasticity Index (PI) and the smectite content of dolerites in areas where the Weinert N-value is below 3, and its implications. It was found that there is a distinct correlation between the PI of decomposed dolerite gravel and its smectite content, and also that using the minus 0,425 mm material for the standard PI test does not adequately expose the smectite in freshly crushed dolerite rock. A rational explanation is presented for the short-term decomposition of stockpiled weathered dolerite. The results of this investigation indicate that for optimum durability, even dolerite gravel with a barely measurable smectite content should be treated (i.e. with lime) when used for base and subbase layers, and crushed dolerite rock intended for Crushed Rock (G1) base should be treated similarly.