Integrated approach for pavement deterioration assessments in a low latitude crystalline basement of south-western Nigeria

Abstract

An integrated approach involving electrical resistivity (ER) technique and geotechnical analysis was employed to assess the deterioration of pavement along Iwo–Osogbo road in south-western Nigeria underlain by quartzo-syenite, migmatite gneiss, and pegmatite crystalline rocks. The ER involved the use of two-dimensional (2D) electrical resistivity tomography and one-dimensional (1D) vertical electrical sounding (VES) to provide an overview of the ground conditions and probe the subsurface profile, respectively. Geotechnical analysis involving compaction, Atterberg limits, grain-size distribution (GSD), and California bearing ratio (CBR) were also carried out on the recovered samples from four test pits, two pits each dug at the smooth and deteriorated pavement portion. Whilst moisture-related distresses originating beneath the pavement were investigated through suction distribution determined from estimated soil–water characteristic curves (SWCC) employing physico-empirical model approach. At the deteriorated pavement segment; the 2D pseudosection indicated the material at and near-surface (depth less than 2.49 m) was of low resistivity values ranging between 69 and 153 Ωm, with upper bound values near surface. The resistivity further decreases with depth indicating increase in proportion of clay and or water content. The other half of the pseudosection is characterized by relatively high resistive material whose resistivity values ranged between 103 and 227 Ωm at and near surface (depth less than 1.27 m) as well as alternating bands of low (20 to 70 Ωm) and relatively high resistivity (103 to 337 Ωm) materials of varied thickness at greater depths. Conversely, the pseudosection obtained from the smooth pavement segment was characterised by high resistivity (>212 Ωm) at and near surface (depth ≤3.5 m) over the entire length. Iterative interpretation of 1D VES data indicated three to four layers earth model without association or relationship between a particular apparent resistivity curve type and deteriorated or stable portion of the road. The moisture content and Atterberg limit parameters namely plastic limit and liquid limit plots against depth suggest the soil environment beneath the pavement as unsaturated. The GSD curves indicate the soils are gap-graded comprising sand-sized and coarse particle and corroborated by GSD curve fitting. The CBR under soaked condition ranged from 16.5 to 41.1 % indicating that the subbase soil could not withstand much load. The CBR varied between 20.8 and 72.0 % indicating that the subgrade soils have a relatively medium strength whilst the subbase soil has a very high strength. The SWCCs generally indicated that the moisture content decreases with increasing matric suction resulting in suction gradient capable of generating flow. Thus, the pavement deterioration could be attributed to improper handling of the flow initiated by the suction gradient.