E-ISSN : 2541-5794 P-ISSN :2503-216X Journal of Geoscience, Engineering, Environment, and Technology Vol 4 No 4 2019 248 Ojo, O. et al./ JGEET Vol 04 No 04/2019 RESEARCH ARTICLE Integrated Approach in Geophysical Investigation of Road Failure in Crystalline Basement Environment in South-western Part of Nigeria. Olabanji Ojo 1 *, Victor Adejumo 2 , Obaromi Olubunmi 2 1 Department of Geological Sciences, Osun State University, Osogbo, Osun State, Nigeria. 2 Zibronel Geosciences, Akure, Ondo State, Nigeria * Corresponding author : adeolu.ojo@uniosun.edu.ng Tel.:+2348035626912. LinkedIn: OlabanjiOjo. Received: Oct 1, 2016. Accepted: Nov 20, 2019. DOI: 10.25299/jgeet.2019.4.4.2590 Abstract The result of the geophysical investigation carried out to access the cause of road failure and remediation measures along Ilesha-Osogbo expressway, Osun State, South-western Nigeria is presented. The study involved integrating the dipole-dipole technique of electrical resistivity method with the ground penetrating radar (GPR) method. Two dipole-dipole traverses, one long and 20 short GPR profiles were established across the failed section of the road. The dipole-dipole data were interpreted using the Diprowin software to produce the pseudo- section while the GPR data were interpreted using the RadProwin to produce the radargram. The result revealed that the failed road exhibit incompetent layer of resistivity values ranging between 17 Ωm to 171 Ωm lying between two competent layers of resistivity values greater than 200 Ωm. A combination of the dipole-dipole technique and the GPR techniques revealed the depth extend to failure to about 4.5 meters to 5 meters deep which overlie a basement rock of undifferentiated gneiss, a rock that is easily prone to weathering. The water level was observed to occur at shallow depth of about 2 meters and infiltrates the entire weathered layer. The shallow groundwater level coupled with the water-logged clayey layer derived from the weathered materials from basement rock were found responsible for the failure of this section of the road. The study recommends the excavation of the waterlogged clayey layer to a depth of about 5 to 6 meters deep into the subsurface and replacement first with heavy boulders of granitic materials and later overlaid with a thick layer of highly resistive landfill materials such as laterite. The result of the two techniques used in this work have proved to be supportive due to the integration of the double dipole technique with the GPR technique, a relatively new technique recently being introduced into road failure mapping. Keywords: Geophysical, road, failure, Idominasi, groundwater, clay 1. Introduction Failed roads generally constitute one of the major challenges to transportation especially in most developing nations of the world. It has been described as next to power supply in Nigeria (Ifabiyi and Kekere, 2013) and has been a major setback to growth and development other countries. Not only has it causes deaths of several thousands of people worldwide, it has also resulted to loss of properties worth millions of dollars annually. A greater percentage of developing nations are located in Africa and Asia. In Nigeria, this challenge is beginning to gain government attention due to concerted effort towards to putting an end to this menace. Reports show that out of 192 countries of the world, Nigeria rank 191 (FRSC, 2011) in terms of unsafe roads. Hence, death toll due to road crashes has been put at 162 deaths per 10,000 populations. Road failures in Nigeria come in form of bulges, potholes, cracks and depression, all of which makes road non-pliable for road uses who are involved in day to day delivery of goods and services. Governments at all the three tiers of government are coming up with concerted effort towards resolving these situations in the country. Roads in Nigeria has been classified into three. These include trunk A road which are highways that link state capitals together or dual carriage ways that link one part of the country to another. They constitute about 17% of the total national road network and the management of these types of road has been the responsibility of the federal government. The second, the trunk B roads which are intra-state roads which are managed by state governments constitute about 16% of the total road network of Nigeria (Ebhohimen and Luke, 2014) while the third, trunk C roads are locally constructed road that link the local communities together and serve the purpose of means of transportation for rural dwellers to move locally made goods and services from the rural area to urban markets. They constitute about 67% of the total road network in the nation. These types of roads are under the management of the third tier of the government, the local governments. The construction and ultimate maintenance of all Nigerian roads are sole responsibilities of these three tiers of government in Nigeria. Most of these roads do fail while construction is ongoing stage and also after the completion of the project. Investigation indicated that lapses exists at the design and construction level. It is unfortunate to note that most contractors fail to take into consideration a