Geoelectrical and geotchnical investigation of failed section of Orsu-Ihiala Road , Imo State SouthEastern Nigeria
Date
2019-12
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Federal University of Technology, Owerri
Abstract
Geophysical and Geotechnical investigation of the causes of the consistent failure of Orsu-Ihiala road has been studied. The study area is part of the Anambra Basin consisting of Eocene Ameke formation, the Oligocene Ogwashi-Asaba Formation and the recent Meander belt. 2-D electrical resistivity tomography of the subsurface was carried out using Wenner method (for a =3, 6, 9 and 12 respectively), 6VES was run along the failed and partially stable part of the road using Schlumberger method (with maximum AB/2 = 55m). Three (3) samples were collected at depth of 1.5m (subgrade) from the failed part of the road (collected from Amanachi, Orlu LGA headquarters and Amagriget) and were analyzed at Arab contractors limited. The 2-D pseudo resistivity section at the subgrade (less than 2m) shows that the area is underlain by moderate to low resistivity (10Ωm - 135Ωm) which is interpreted as clayey topsoil and clay. These zones coincide with the fairly stable and failed portion of the road. At the same depth resistivity was observed to be decreasing NW-SE, implying the clay content decrease along the NW -SE direction on the road. Analysis of VES suggested 3-4 geoelectric layers defined as clayey topsoil, clay, clayey sand and sand with resistivity of 96Ωm - 136Ωm, 54Ωm -135Ωm, 288Ωm to 980Ωm and 2780Ωm - 32800Ωm respectively.The geotechnical results reveal that the soil sample is made up of granular material with highly compressible silty clay belongs to group A-2, subgrade A-2-6 (sample S01) and group A-7, subgroup A-7-5 (sample S02 and S03) as per AASHTO classification. The result of the Atterberg Limit showed that the plastic index (PI) is relatively low (16.9%, 7.1% and 14.9% for S01, S02 and S03 respectively). The LL for S01, S02 and S03 are 38.5%, 42.5% and 46% respectively and are referred to as intermidate plasticity. The maximum dry density (MDD) and optimum water content (OMC) are 1.776g/cm3 and 14.4% for S01, 1.713g/cm3 and14.6% for S02 and 1.692g/cm3 and14.4% for S03. Low MDD with high OMC is an indication that the soil is generally loose. The California Bearing Ratio (soaked) for sample S01, S02 and S03 are 2.1%, 3.4% and 16.2% respectively. S01 and S02 fall within FMW specification while S03 is above the specification. The specific gravity of studied samples is 2.401gm/cm3 , 2.586gm/cm3 and2.413gm/cm3 forS01, S02 and S03 respectively. Which indicates moderate organic content and moderate porosity (especially sample S01). The moisture content of the samples is 18.2%, 24.2%, 25.4% for S01, S02 and S03 respectively. The material is generally rated as a fair to poor subgrade material (as per general specification Roads and Bridges, Revised 1997, FMW, Nigeria). 2-D pseudo resistivity secton, VES and the geotechnical result shows that the road was founded on a subgrade (at depth 1.5m) clayey material. And that the claycontent of the subgrade material decreases along the NW-SE profile resulting from transition from Recent sand rich meander belt to clay rich Ameke Formation. The road failed basically due to the clayey material found at the subgrade and poor drainage construction. This project therefor shows that geophysics complement Geotechnical analysis and used be carried out prior to construction of engineering structures.
Description
This thesis is for the award of Master of Science (M. Sc.) in Geophysics
Keywords
Geoelectrical, geotechnical, geophysical, resistivity, Orsu-Ihiala, South-Eastern Nigeria, Department of Geology
Citation
Ajah, N. J. (2019). Geoelectrical and geotechnical investigation of failed section of Orsu-Ihiala road , Imo State SouthEastern Nigeria {Unpublished Master's Thesis}, Federal University of Technology, Owerri