Geophysical and geotechnical assessment of Obiaruku-Agbor road failure in Western Niger-Delta, Nigeria

Abstract

A geophysical and geotechnical survey was carried out along the Obiaruku-Agbor Expressway in Delta State, Nigeria to determine the causes of its deterioration. The study used a 2D electrical resistivity approach using the ABEM SAS 1000 Terrameter and yielded subsurface resistivity values ranging from 5.12 Ohm-m to 4418 Ohm-m. The geo-electric resistivity result showed that the base, sub-base, infill and embarkment soil types range from an interchange of clay,sandyclay, clay sand, fine sand and coarse sand.The high percentage of low-resistivity soil indicated that the building materials were not appropriate. Using soil samples, gathered for various geotechnical parameter tests for surface sub-base and base materials in accordance with British standards, the geotechnical characteristics of the research sites were ascertained. The findings show that A-2 and A-7 soil types are the most common types. The soils from the stable sections have a higher specific gravity (2.59-2.89) and a smaller amount of clay (16.9 %) than the soils from the failed portions of A-7-6 which suggests substandard sub-grade materials. These soils correspond to A-2-6 on the AASHTO classification system. The bulk of soils from failed sections had significant values of linear shrinkage (> 8%), a large proportion of fine particles (> 40%), and pore water pressure occurred as a result in a loss of soil index strength. The majority of fines particles in lateritic soils, low California bearing ratio (08–63% un-soaked), intermediate and high OMC above guideline for most soil samples (07.4–20.1), maximum dry density (MDD) (<2000 kg/m³), and liquid limit (20.9–58.7) are the main causes of the observed degree of instability. Poor geotechnical characteristics in the lateritic and sub-grade soils of failed sections imply that they should not be used as sub-grade materials in other engineering works, let alone in the building of new roads. It is stressed that lateritic soils are crucial for precise sub-grade soil assessment in detail sampling for highways. When these values were compared to the standard, it became clear that the main reasons for road collapse were thin pavement, inadequate drainage, water-logged sands, low-quality infill, incompetent clayey materials beneath the built road, and neglect. It is consequently advised that a well-designed, organized drainage system be built and that the soils be stabilized or replaced with materials that adhere to standards.The results of this inquiry will be useful in the repair and rehabilitation of the road's deteriorating sections and it is recommended that a thorough record of this study be kept for future reference.