GEOTECHNICAL EVALUATIONOF FAILURE SUSCEPTIBILITY OF SOME ROADS IN SOUTHWESTERN NIGERIA

Abstract

This study investigated the contribution of traffic, depth to water table, geotechnical indices:(maximum dry density and California bearing ratio) and road cross-sectional details (slope cambering and asphalt thickness) to road failure in evaluating failure susceptibility on some roads in South-western Nigeria. The main objective of this research is to determine the causes of the perennial failure of these roads; evaluate their failure susceptibility rate and develop a predictive model. Thirty nine segments were selected based on initial reconnaissance survey. 39 monitoring wells were sunk and positioned to study groundwater regimes. Soil samples were collected from the 39 monitoring wells at different depths for laboratory test. Cross slope and asphalt thickness of the selected sections were also measured. The failure potentials termed TDRAMS index of these roads were evaluated through a model called DRAMS. Predictive model for the determination of road failure susceptibility index was calibrated using synergy between Stata and Multiple Linear Regression. The findings from the research revealed that the percentages of volume of heavy trucks in these roads were between 16% and 18%. They contributed between 60% and 80% of total equivalent traffic load which has higher damaging effect on road pavement. It was found that more than 60% of the segments studied were under the influence of ground water due to the high water tables encountered in the selected segments. The maximum dry densities (36%) and California bearing ratio (60%) of the soil samples were less than what the Highway manual specified. More than 90% of cross-section slopes and 60% of the asphalt thickness,respectively, of the selected sections fell below the recommended standards of 2.5% carriageway slope and 50 mm pavement thickness respectively. The resultant effects of the above manifested in higher TDRAMS values, which at times were as high as 350% of idea lTDRAMS (64). The study confirmed that the causes of perennial failure of roads in south-western Nigeria are as a result of combinations of factors of excessive axle loads, high water table condition and the use of substandard sub grade as materials during construction.