Abstract
Integrated geophysical investigation involving Ground Penetrating Radar (GPR) and Electrical Resistivity methods were carried out at Medina Estate, Lagos southwestern Nigeria to map the subsurface lithology in order to delineate its peat stratigraphy that has been causing foundation failure in the area. Twenty-one traverses (varying from 35-880 m in length) of Ground Penetrating Radar (GPR) survey were conducted along the streets of Medina trending NE-SW and NW-SE directions using the Mala 250 MHZ bi-static shielded antenna. Thirty-six Vertical Electrical Soundings (VES) were carried out using Schlumberger electrode array at some selected points along the established traverses within the area. The GPR data were processed into radar section using Rad Explorer software. The VES data were interpreted quantitatively using the partial curve matching method and 1-D forward modeling with Win Resist Software. Available litho-logs from boreholes drilled within the area were compared with the geophysical results. Results of the GPR survey delineated three geologic layers which include the topsoil with high amplitude, parallel to sub parallel, horizontal reflections, with thickness varying from 1 to 2 m across the entire profiles and composed of lateritic clay; peat layer with low amplitude, parallel sinuous/wavy reflections with depth of occurrence ranging from 2.0 to 8 m and clay with low amplitude, planar, horizontal, sub-parallel reflections underlying the peat layer. Vertical Electrical Sounding results revealed the presence of three geological layers which are the topsoil, peat and clay and sandy clay with layer resistivity values ranging from 20- 225 Ωm, 5 – 90 Ωm and 36 to 366 Ωm and thickness values ranging from 0.5 – 2 m, 4.0-29.0 m and infinity respectively. Borehole information confirms the occurrence of shallow peat with depth ranging from 1.5 to 9 m and clay layer with depth ranging from 9 to 21 m beneath the area. The GPR survey results correlates with the well logs acquired in the study area. Based on the correlation of the geophysical results with the well logs, the GPR gives better information about the peat layer compared to the Electrical ResistivityMethod. The information obtained from this study shows that the soils at shallow depth are organic soils which are difficult foundation materials because they exhibit very high compressibility, as such making shallow foundation impossible except some form of soil improvement is carried out. The alternative approach is the adoption of deep foundations in form of piles.
 Keywords: Foundation Failure, Geophysical Investigation, Ground Penetrating Radar, Vertical Electrical Sounding, Peat.
Highlights
The continuous incidence of foundation failure of structure is becoming alarming in Nigeria
The results of the Ground Penetrating Radar are presented as radar sections while the electrical resistivity method results are presented as resistivity sounding curves and geo-electric sections
The topsoil was characterized by high amplitude, parallel to sub parallel, horizontal reflections with thickness varying from 1 to 2m across the entire profiles and composed of lateritic clay. This was underlain by low amplitude, parallel wavy reflections and it was interpreted as peat with depth of occurrence ranging from 2 to 8 m
Summary
The continuous incidence of foundation failure of structure is becoming alarming in Nigeria. A number of factors such as subsurface geological material, inadequate information about the soil, poor foundation design and poor building materials have been attributed to this failure (Fatoba et al, 2013). This has led to the loss of life and lots of goods and properties worth millions of naira aside from the cost of rehabilitation or complete redesign and reconstruction at much higher cost. A., Department of Geology, University of Ibadan, Ibadan, Nigeria Adejato, K. O., Department of Geology, University of Ibadan, Ibadan, Nigeria Ogunkoya, A. O., Department of Geology, University of Ibadan, Ibadan, Nigeria
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