GEOPHYSICAL AND GEOTECHNICAL SITE CHARACTERIZATION AT LAGOS CENTRAL AREA OF LAGOS STATE, NIGERIA Oyedele, K.F., Adegbola R.B., Abidoye O.E., Titiloye-Ajose M. Presented By ABIDOYE OLANIKE ELIZABETH

Introduction Subsurface investigation (Ayolabi et. al., 2013) Some reasons why buildings are liable to collapse. Importance of Site characterization (Oyedele et al., 2011). Geotechnical and geophysical survey (Sirles et al., 2008). poor quality of building materials, salinity, old age of buildings and the most important is the subsurface conditions of the ground on which the buildings are sited. It provides subsurface information that assists civil engineers in the design of foundation of civil engineering structures. are the means of obtaining subsurface information, especially over large study areas.

Introduction The survey area is located at Adeniji Adele area Lagos State as in Plate 1 below: Plate1: Map showing the study area

Introduction Plate 2:Tilting structures close to the study area.

Literature Review Several works have been done on Site Characterization by different scholars. Geo-Assessment of Subsurface Condition in Magodo, Brook Estate, Lagos was conducted by some scholars in (Oyedele et al., 2012). Two-dimensional (2-D) model where the resistivity changes in the vertical direction, as well as in the horizontal direction along the survey line were used and discussed by many Geophysicists and recorded in (Sarma, 2014; Oyeyemi et al., 2015).

Methodology Eight Vertical Electrical Sounding (VES) points using D.C resistivity method (Terrameter PASI system) were carried out in the study area. The data were acquired using Schlumberger array and the electrode spread varies between 1-100 m for the base of the field and 1-200 m for the maximum length on the field. Two 2D lines were run across the base and length of the field using Wenner array.

Methodology Two geoelectric section profiles were generated and interpreted by using Dipro software. The geotechnical survey method used here was the Cone Penetrometer Test (CPT). The cone penetration testing (CPT) was carried out on two points using a 2.5 tons Dutch cone penetrometer anchor. Readings were taken at a depth interval of 0.25m. The bearing capacity (qt) was determined from the cone tip resistance by using equation

Methodology proposed by Bustamante and Gianeeselli (Bustamante & Gianeeselli, 1982). The equation is as follows; qt = Kb qc 1 where, Kb is an empirical bearing capacity factor that varies from 0.15 to 0.60 depending on the soil type and pile installation procedure.

Results and Discussion GEOELECTRICAL CHARACTERIZATION The VES data obtained were plotted and interpreted by the computer using iteration software known as WINRESIST as shown in Figure 1. Figure 2 shows the 2D sections got from the interpretation of the data obtained from Wenner array method with relative low resistivity values ranging from 1.3 to 1.9 ohm-metre that could be attributed to clay or peat formation.

Results and Discussion Figure 1 (i-iv): The resistivity Curves and interpreted models for VES stations 1-4

Results and Discussion Figure 2 (a & b): The Pseudo-sections of 2D at traverse 1and 2.

Results and Discussion The geoelectric sections were interpreted graphically as shown in Figure 3. The deduced layers were the topsoil, peat/clay and silty sand with peat/clay being the dominant soil type in this area with Resistivity ranging from 0.7 – 29.4 ohm-metre. GEOTECHNICAL CHARACTERIZATION Two CPT readings were acquired and processed by plotting the cone resistance against the depth in each location point using Microsoft Excel software shown in Figure 4. The bearing capacities of the different materials in the subsurface were calculated from the CPT results

Results and Discussion using the Bustamante and Gianeselli (1982) equation (Eqn. 1), this indicated that the Kb values used for the calculation are 0.375 (Bored piles) and 0.600 (Driven piles) which fall under soil type clay-silt (Table 1). Which enable us to check for competent materials in the subsurface. The competent materials at the study area were found to range between 16.25 – 16.50 m with bearing capacity ranging from 29215.8 - 31773.6 KN/m2 for both CPT 1 and CPT 2.

Results and Discussion Figure 3: Graphical Representation of the VES geoelectric sections

Results and Discussion Figure 4 (a & b): A graph of depth (m) against Cone Resistance (Kg/cm2) for CPT 1and 2

INTEGRATION OF VES, 2D IMAGING AND CPT RESULTS The electrical imaging results revealed materials with low resistivity values ranging from (1- 19 ohm-metre) which were prevalent to be clay or peat. The VES method was used to delineate an average of three geoelectric layers. The CPT indicated materials with low shear strength within the depth range of 0 to 11 m identified to compose of peat or soft clay.

Conclusion Integration of both geophysical and geotechnical methods have shown to be adequate in the successful characterization of the subsurface geology within the study area. Results obtained from both techniques employed have shown clearly that shallow foundation was not feasible in the area.

Recommendation To this end only deep foundation in the form of pilling to the competent layer at a depth greater than 11 m would be adequate for any engineering structure in the area.

References Ayolabi, E.A and Adegbola, R.B. (2013). Application of MASW in road failure investigation. Arab J geosci. 7(10) 1-7. Bustamante, M. and Gianeeselli, L. (1982). Pile bearing capacity predictions by means of static penetrometer CPT. Proceedings of the 2nd European Symposium on Penetration Testing. Amsterdam, ESOPT-II, 493-500. Oyedele, K. F., Oladele, S. and Adedoyin, O. (2011). Application of Geophysical and Geotechnical Methods to Site Characterization for Construction Purposes at Ikoyi, Lagos, Nigeria. Journal of Earth Sciences and Geotechnical Engineering, 1(1), 87-100.

References Oyedele, K. F., Oladele, S., and Okoh, C. (2012). Geo-Assessment of Subsurface Condition in Magodo, Brook Estate, Lagos Nigeria. International Journal of Advanced Scientific and Technical Research, 2(4), 731-741. Oyeyemi, K. D., Aizebeokhai, A. P. and Oladunjoye, M. A. (2015). Integrated Geophysical and Geochemical Investigations of Saline Water Intrusion in a Coastal Alluvial Terrain, Southwestern Nigeria. International Journal of Applied Environmental Sciences, 10(4), 1275-1288.

References Sarma, V. S. (2014). Electrical Resistivity(ER), Self Potential (SP), Induced Polarisation (IP), Spectral Induced Polarisation (SIP) and Electrical Resistivity Tomography (ERT) prospection in NGRI for the past 50 years-A Brief Review. J. Ind. Geophys. Union, 18(2), 245-272. Sirles, P., Neil, A., Neil, C. and Rick, H. (2008). Geophysical Methods Commonly Employed for Geotechnical Site Characterization. Transportation Research Circular, Number E- C130, 1-35.