Geotechnical Properties of a mixed Geological Terrain for Road Foundation: A Case Study of Lapai-Gulu Road, Niger State, North-Central Nigeria

Abstract

A mixed geological terrain is one that comprises of both a crystalline basement complex (hard rock) and the sedimentary rocks (soft rocks), even though the contact between the two rock types are not exposed on the surface. A study was conducted on the road from Lapai to Gulu in Niger state that cut through both terrains. Sieve analysis results indicate that the soil in the hard rock areas is made up of 32% gravel, 34% sand and 34% silt/clay, with a liquid limit of 36.0-53.4%, plastic limit of 26.0-32.0%, plasticity index of 9.0-21.4% and a medium shrinkage potential with an average density of 1.96 The soft rock areas are made up of 60% gravel, 23% sand and 22% silt/clay, with a liquid limit of 35.5-39.5%, plastic limit of 25.0%, plasticity index of 10.5-12.2%, low shrinkage potential, with average density of 2.04. The soils of the hard rock area can be classified to be fair to poor and suitable as base course materials, while the soft rock areas are good to excellent and suitable as sub-base and base course materials. Foundation designs in the hard rock area would include among others excavation and improvement on natural soil conditions