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Application of geophysical and geotechnical methods to determine the Geotechnical characteristics of a Reclaimed land Adewoyin O. O., Joshua E. O., Akinyemi M. L., Omeje M and Joel E. S. from Covenant University, Ogun State, Nigeria

Content of Presentation  Introduction  Methodology  Results and discussion  Conclusion  References

Introduction  The rampant cases of building failure have risen to a level of concern to both the government and private individuals (Oyedele and Okoh, 2011; Amadi et al ., 2012).  Often times when a building collapses, the blame is apportioned to some factors while others are left.  Land is a vital part of humans life because it is on it we have our being.

Introduction Cont’d………  Buildings on the other hand are meant to provide shelter for man and his property.  It becomes a concern when what is meant to protect man turns out to kill him.  Studies have shown that Lagos State, Nigeria has the highest number of building collapse (Olanitori, 2011).

Introduction Cont’d………  Reclaimed lands are lands recovered from the body of water by filling with sand material in order to suppress the water level.  Studies have shown that this kind of land contain mechanically unstable geological formation that can affect the foundation of the building sited on it (Ayolabi et al ., 2012).  This is because, a land like this is composed of impermeable soil layers such as clay or peat and as a result, they are naturally flooded.

Fig 1: The state of some of the buildings in the study area

Introduction Cont’d……..  In order to ensure a proper foundation system for buildings in this type of area, adequate information about the subsurface is necessary (McDowell et al ., 2002).  This information can only be obtained by conducting a comprehensive subsurface investigation with the use of adequate geophysical and geotechnical methods (Hunt, 2005).  The present study is aimed at determining the geotechnical characteristics of the subsurface soil in the study area, using both electrical resistivity and seismic refraction methods in order to determine its competence for construction purposes.

Introduction Cont’d  The area of study is along the coastal line region of Eti-osa local government area of Lagos State.  It is a gentle slope, low lying elevation of about 2 m below mean sea level.  The area is generally water logged due to water in- flow from the lagoon.

Fig 2: Topographic map of the study area with yellow circle indicating the location

Fig 3: Base map of the study area

Research Methodology  Electrical Resistivity Methods  Fig 4: Arrangement of electrodes for the vertical electrical sounding conducted in this study

Research Methodology Cont’d……..  An ABEM Terrameter SAS 1000/4000 was used (ABEM Instrument, 2000).  12 VES were carried out in the study area  The electrode spread used varied between 2 m and 260 m

Research Methodology Cont’d  The data acquired were plotted on a log-log graph sheet after which the curves obtained were curve-matched with standard curves (Reynolds, 1998; Keary et al ., 2002).  This procedure gave information on the number of layers, apparent resistivity and the thickness of each layer.  These information were further processed with WinResist computer iterative code in order to produce a one-dimensional (1-D) profile of the sounded part of the study area.

Seismic Refraction Method  In this method a 24 Channel ABEM Terraloc MK 6 seismogram was used (ABEM Instrument, 1996).

Seismic Refraction Method Cont’d  The length of each profile ranges between 50 m and 200 m.  The data acquired were processed using seisImager/2D TM software (SeisImager, 2009).  The first arrival times were picked using the Pickwin package while the travel time data were inverted with the Plotrefa package.  The purpose of this survey was to obtain subsurface information on the number of layers and the geotechnical parameters of each layer

1-D Resistivity Results and Discussion

1- D Resistivity Results and Discussion Cont’d

1- D Resistivity Results and discussion Cont’d  The electrical resistivity methods revealed four to five geoelectric layers in the study area and their lithologies are; loose dry sand, wet sand, sandy clay, sand and clay / peat.  The result of the electrical resistivity methods revealed the third geoelectric layer to be the most competent layer with resistivity ranging between 23.3 and 951.7 ohm metre. The thickness of this layer ranges between 8 and 54 m.

2- D Seismic Results and Discussion Cont’d… .

Seismic Results and Discussion Cont’d….  Seismic refraction method revealed that the third layer is the most competent in the study area.  This layer has the highest geotechnical parameters.  Bulk modulus 0.330-16.093 GPa.  Shear modulus 0.207-10.123 GPa

Seismic Results and Discussion Cont’d….  Lame’s constant 0.191 -9.345 GPa  Poisson’s ratio 0.24+/ -0.7*10^-5  This layer that is considered to be most competent is between a depth of 7 m and 18 m into the subsurface.

Conclusion  Geophysical investigations were carried out at the study site in order to determine the geotechnical characteristics of the subsurface for construction purposes.  The electrical resistivity method revealed that the materials with good geotechnical properties lied between the third and the fourth layers which is between 8 m and 54 m.

Conclusion Cont’d……..  The seismic refraction result obtained showed that the depth to the most competent layer lies between 7 m and 18 m.  Therefore, some form of arrangement must be made to transfer the weight of the building on the surface to the most competent layer at the subsurface otherwise, the building will sink.  Based on the above findings, pile foundation is recommended for lasting engineering structure in the area of study

Acknowledgements  The authors wish to appreciate the management of Covenant University for providing financial support for this research engagement.

References  ABEM Instrument AB, (1996): Equipment Manual for TERRALOC MK6 Software Version 2.21, Bromma, Sweden.  ABEM Instrument AB, (2006): Equipment Manual for ABEM Terammeter SAS 1000/4000.  Amadi, A. N., Eze, C. J., Igwe, C. O., Okunlola, I. A. and Okoye, N. O. (2012): Architects and Geologists view on the causes of building failures in Nigeria. Modern Applied Science , 6 (6): 31-37.  Ayolabi, E. A., Enoh, I. J. E. and Folorunsho, A. F. (2012): Engineering site characterisation using 2-D and 3-D tomography. Earth Science Research , 2 (1): 133-142.  Hunt, R. E. (2005): Geotechnical engineering investigation handbook. 2 nd edition, Taylor and Francis, 1-3.  Keary, P. Brooks, M. and Hill, I. (2002): An introduction to geophysical exploration. Third edition. Blackwell Science. p. 21-193.  McDowell, P. W., Barker, R. D., Butcher, A. P., Jackson, P. D., McCann, D. M., Sdipp, B. O. (2002): Geophysics in Engineering Investigation, 6, Storey’s Gate, Westminster, London.

References Cont’d………  Olanitori, M. L. (2011): Causes of structural failures of a building: Case study of a building at Oba-Ile, Akure. Journal of Building Appraisal , 6: 277-284.  Oyedele, K. F. and Okoh, C. (2011): Subsoil investigation using integrated methods at Lagos, Nigeria. Journal of Geology and Mining Research , 3 (7): 169-179.  Reynolds, J. M. (1998): An introduction to applied and environmental geophysics. Wiley, p. 417 – 450.  SeisImager (2009): SeisImager/2D TM Manual version 3.3.  WinResist (2004): WinResist Version 1.0.

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