Factors Affecting The Durability Of Basic Igneous Rocks As High Quality Base Course Aggregates, An Investigation Of The Karoo Dolerite Suite Of South Africa

Abstract

Covering approximately (57%) of the country's surface area the main basin of the geology of the Karoo Supergroup in South Africa has an important influence on the materials used in the construction of transport infrastructure in South Africa. The Karoo Dolerite Suite often serves as the only competent material in this otherwise sedimentary basin but on numerous occasions rapid premature failures of pavements constructed with Karoo Dolerite base courses have been attributed to the poor durability of these materials. The research presented here attempts to determine if the cause of such rapid failures can be attributed to dolerite durability deficiencies and if so if the durability of the material can be predicted using the current specifications to which these materials are required to conform with. The methodology followed was to perform comprehensive material investigations on selected pavement sections where Karoo Dolerite had been used as a base course aggregate. Eight suitable sites, ranging in age from two months to 10 years, were selected and comparative testing performed on samples of material obtained from the source quarries and pavement layers at each. Three of these sites experienced rapid failure that was suspected to be due to base course aggregate degradation. The tests performed included those specified by South African standards and a selection of new tests derived from published literature on alternative tests and proposed basic igneous rock degradation models. Well established tests were completed with relative ease while newer tests and newly proposed tests required additional development. The materials from at least two poor performance sites was shown to have undergone various degrees of degradation after quarrying which manifested itself primarily as a loss in resistance to attrition and abrasion forces. The variability in the inherent resistance of the material to these forces was also noted to have contributed to the poor performance of at least two sites. It is therefore proven that degradation of Karoo Dolerites after quarrying can occur and contribute to the poor performance of pavements but also that the variability in a source quarry can result in poor performance without material degradation. The prediction of material durability is possible but requires numerous tests to be performed on representative samples, particularly to identify variations in material properties. Petrographic properties that result in variable material properties and a susceptibility to further alteration included high degrees of deuteric and metasomatic alteration of primary minerals, especially the fine matrix minerals. It has been shown that the accurate quantification of such alteration is not feasible using easily available analysis techniques and therefore that no specifications can be based thereon. The current material specifications have been shown to not accurately predict the durability of Karoo Dolerite, primarily due to the inability to activate the mechanism by which material physical degradation occurs, namely the expansion of clay minerals within the aggregate. The only exception was the water absorption test performed on core samples, which was able to identify poor materials. Tests that were able to predict the durability included the modified versions of previously specified tests (e.g. ethylene glycol soaked aggregate impact value and ethylene glycol soaked modified durability mill index) and newly proposed testing methods (e.g. modified ethylene glycol durability index and shear wave velocity). Preliminary specifications for these have been proposed. The initial development of an aggregate expansion test has also shown a strong ability to predict Karoo Dolerite durability and is proposed for further development. Ultimately the use of poor durability Karoo Dolerite results in two changes in the material properties. The first effect is the production of more fines during construction, which can result in an unsuitable amount of fines (as identified by a low coarse sand ratio). The second effect is an increase in plasticity index and linear shrinkage of the material<0.425mm to levels considered marginal based on the current specifications. The shear strength of a Karoo Dolerite base course layer has been shown to be sensitive to such changes in plasticity index and linear shrinkage and the reduction of the current specification limit to ensure materials are non-plastic and/or non-expansive may therefore be justified.