ABSTRACT
Road transportation greatly affects the economic growth of a country, especially developing countries like Nigeria. However, the deplorable state of Nigerian roads and the rampant road failures seen across Nigerian roads have become very devastating, as it has caused unnecessary waste of time during traffic jams, several road accidents, consistent damage to vehicles, loss of valuable properties and even human lives. Consequently, an integrated geophysical and geotechnical investigation was conducted along selected failed roads in Delta State Southern Nigeria. The method used to conduct this research was the two-dimensional electrical resistivity tomography (ERT) using the WennerSchlumberger configuration alongside Laboratory testing of soil samples for selected physical properties of the soil. Thirty (30) profiles across 25 roads were surveyed along failed and stable portions of the investigated roads. Data sets were collected using Petro Zenith Earth Resistivity Meter, and the observed resistivity data from field measurements were first processed and thereafter, inverted with the aid of the RES2DINV software. Also, laboratory test was done to obtain some selected physical properties of the collected samples of soil, which would usually suffice in a majority of geotechnical site investigations. The results show low resistivity values typical of and suspected to be expansive clay and clayey materials at different portions for most of the profiles. In Profile 1, 2 and 3, resistivity values ranging from 21.3Ωm to 300Ωm were observed to a depth of about 6.76m from the top soil. Also, profiles 5 and 6 show values from 5Ωm to 150Ωm at depth of about 5.00m from the top soil. Furthermore, Profile 17 show resistivity values of 6Ωm to 80Ωm across a depth of about 3.00m from the top soil, while resistivity values ranging from 10Ωm to 100Ωm were observed xviii to a depth of about 6.76m from the top soil in profile 21. Geotechnical results show values of specific gravity, liquid limit and plasticity index ranging from 2.1 – 2.7, 23.2% - 54.4%, and 10.1% - 29.5% respectively, indicating that the soil samples have poor to fair soil characteristics. Integrating the results, shows that there is a high level of conformity in results obtained from both methods and reveals that the unsuitable road pavement subgrade (possibly clayey soil), probable existence of geological features, ancient buried streams as well as the inadequate geotechnical properties of the tested soil samples, that failed to meet the recommended specification set by the Federal Ministry of Works and Housing for road construction in Nigeria, could be the major cause of the road failures in the study area.
