An investigation of the performance of laterite in flexible pavements in a humid tropical environment

Abstract

Self-hardening of laterite especially within pavement layers is still an enigmatic subject to engineers and researchers. Being a pedogenic concretionary soil commonly used in the construction of some layers in flexible pavements in the tropics, self-hardening has great significance towards its engineering value and associated selection standards. Sesquioxide-enrichment of its fabric is thought to be responsible for this phenomenon though existing knowledge about its occurrence is still inconclusive. The general objective of the research was to study the phenomenon in Ugandan laterites in pavement layers in a humid tropical setting and determine its implications on material selection standards. The study methods were literature review, questionnaire surveys and experimentations. The experimentation method involved fieldwork to measure performance of the material in tests roads and laboratory tests on laterite samples. It was found out that self-hardening in Ugandan laterites was possible only in the immature classes such as plinthites especially when formed under distinctive tropical conditions of alternating wet and dry seasons. Such conditions exist in northern Uganda where one test road was established. However, the performance of the immature classes of the material in road pavements entailed serious performance defects. Such classes of laterite are unsuitable for pavement works. The effect of the insitu pedogenic process was evident in the particle size grading of most classes of the materials and to some extent, plasticity characteristics. The clay fraction affinity for sesquioxides through particle impregnation and nodulation converted a significant fraction of the clay fraction into pseudo-silts and pseudo-sands. This altered the grading and some plasticity characteristics of the materials but not strength. The effects of seasonal factors of climate and traffic were found to mask any self-hardening that the material may undergo. It was concluded that self-hardening in Ugandan laterites is not significant enough to warrant substantial revision of material selection specifications. The research accordingly recommended augmentation of existing selection specification through incorporation of specific gravity as a criterion and revision of expression for grading coefficient. Recognising the limitations of time and resources experienced, further areas of research have been proposed.