THE INFLUENCE OF PETROLOGY AND FABRIC ON THE ENGINEERING PROPERTIES OF CONCRETIONARY LATERITE GRAVEL AGGREGATES

The probable field performance of concretionary gravel aggregates is d ifficult to predict in the tropics, mainly due to the lack of understa nding of the factors that control their engineering properties. The mi neralogy and fabric of commonly used laterite gravel aggregates in Nig eria were studied to assess their influence on the engineering propert ies. Thin-section studies revealed two characteristic features which g reatly influence the physico-mechanical properties: (a) abundant voids and fissures of different sizes and (b) varying degree of iron oxide impregnation. The variations in these characteristics are due to diffe rences in the degree of lateritization. Strong and well developed grav els and pisolites are composed of 65-95% goethite/hematite and are dom inated by small voids. Weak and immature laterite gravels contain much larger pores and greater amounts of quartz and kaolinite. The quartz grains show extensive cracking which futther reduces the mechanical st rength of these laterite gravels. The void/fracture-dominated fabric o f laterite gravel aggregates is mainly responsible for their unusually high absorption/porosity and relatively low strength. In some laterit e gravels, the strength loss on wetting is mainly due to weak cementat ion and the presence of interstitial clays.