LASU 2017

1. FOCUSING ON FOUNDATION FAILURE THROUGH GEOPHYSICAL AND GEOTECHNICAL ASSESSMENT AT MARYLAND, LAGOS, NIGERIA Adeoti, L*1., Adegbola, R.B 2, Ademilola, J.A1., Owolabi, O.O1. and Oyeniran, T.A11Department of Geosciences, University of Lagos, Akoka, Lagos, Nigeria. 2Department of Physics, Lagos State University, Ojo, Lagos, Nigeria *Corresponding Author, Email: lukuade@yahoo.com (Tel.: +2348034739175) 13th OCTOBER, 2017 LASU 2017

2. WORKFLOW • INTRODUCTION • Background to the Study • STATEMENT OF PROBLEM • AIM AND OBJECTIVES • GEOLOGY OF THE STUDY AREA • METHODOLOGY • RESULTS AND DISCUSSION • CONCLUSION • REFERENCES 13th OCTOBER, 2017 LASU 2017

3. INTRODUCTION • Background to the Study • In recent times, the statistics of failures of engineering structures such as roads, buildings, and bridges throughout the nation has increased geometrically. Several reasons speculated to be responsible for this ugly situation by the engineering community are inadequate supervision, substandard construction materials, non-compliance to building codes among others. • Unfortunately, a major reason that has always not been given serious attention in this part of the world is the lack of adequate information on the nature of subsurface conditions prior to construction exercise and after the construction exercise. This is specifically for timed post-construction monitoring information to ensure integrity (Neil and Ahmed, 2006). 13th OCTOBER, 2017 LASU 2017

4. INTRODUCTION (Contd.) • Geophysical investigation is one of the methods used in probing the subsurface prior to any engineering construction activity. The deduced soil characteristics are used as preliminary information to determine the suitability of the site for the proposed structure (Olorunfemi and Meshida, 1981). • Geophysical methods that have been found very useful in pre and post–construction geotechnical investigations include the gravity, electrical resistivity and the seismic refraction methods (e.g., Ako, 1976; Olorunfemi and Meshida, 1987; Aina et al., 1996; Boyce and Koseoglu, 1996; Adepelumi and Olorunfemi, 2000; Olorunfemi et al., 2000a&b; Akintorinwa and Adesoji, 2009; Adeoti et al., 2009; Roth et al., 2002). • The electrical resistivity method is the most commonly employed geophysical method for this purpose because it combines speed, accuracy and cost effectiveness. Hence, it is adopted for this study. 13th OCTOBER, 2017 LASU 2017

5. INTRODUCTION (Contd.) • Cone Penetrometer Test (CPT) has been the most popular and widely used geotechnical technique for obtaining “ground truth” information about the subsurface due to its Cost effectiveness, field efficiency, simplicity, reliability and the ability to provide continuous information on the soil properties with depth (Eslaamizaad and Robertson, 1998). From the cone resistance data, the resistance of soil to penetration which is an important engineering property of the subsoil can be easily determined (Adebajo, 2005). • An integrated use of subsoil geo-engineering techniques and geophysical investigation methods are therefore necessary to have an adequate knowledge of the engineering properties of the subsoil materials in a particular area. (Akintorinwa and Adesoji, 2009). • In this study, the VES and 2D resistivity imaging techniques of the electrical resistivity method were used to delineate the subsurface layers and to reliably identify underground structures while the CPT was used to characterize the stratigraphy in terms of load bearing capacity. 13th OCTOBER, 2017 LASU 2017

6. STATEMENT OF PROBLEM Distressed Distressed Distressed Sagged Plate 1c: Left Side of the Target Plate 1d: Inside Ground Floor of the Target Plate 1b: Frontal Side of the Target Collapsed Control Distressed Distressed Controlled (Before Being Distressed) Plate 1a: Rear Side of the Target Plate 1e: Stair Hall of the Target Plate 1: Target of Investigation (Study Area) 13th OCTOBER, 2017 LASU 2017

7. AIM AND OBJECTIVES The AIM of this study is to apply Electrical Resistivity geophysical method and Cone Penetration technique to examine the subsurface formation with a view to identifying the cause(s) of cracks and distresses observed on the walls of some buildings at Mende, Maryland, Lagos State. The OBJECTIVES of the study are to: • determine the subsoil geoelectric characteristics by the VES • estimate the vertical and lateral extent of the subsurface information in the area investigated with the 2D electrical resistivity imaging • characterize the subsoil in terms of its load bearing capacity with the CPT • recommend possible measures for mitigation 13th OCTOBER, 2017 LASU 2017

8. GEOLOGY OF THE STUDY AREA Fig.1: (a) Administrative Map of Lagos State showing the study area (orange box) (b) Base Map of the study area showing the 2D RI (resistivity imaging) traverses, VES and CPT data acquisition points. 13th OCTOBER, 2017 LASU 2017

9. METHODOLOGY Data Acquisition Geophysical Investigation • Vertical Electrical Sounding (VES) using Schlumberger electrode array and 2D Electrical resistivity measurement using Wenner electrode array were carried out along four traverses within the study area using PASI resistivity meter. A total of sixteen (16) VES data were acquired at predetermined distances along the four traverses. The orientation of the traverses is shown in Figure 1. Geotechnical Investigation • Cone Penetration Tests (CPT) were carried-out at eight (8) locations within the study area to determine the mechanical strength of subsurface materials using the Dutch static penetrometer. CPT 1, 2, 3, 4, 5, 6, 7 and 8 were carried out at the exact location of VES 1, 4, 5, 8, 10, 11, 14 and 15 respectively. A 2.5 tonnes, 600 steel cone with an area of 10.2 cm2 was manually used to measured the resistance of penetration into the subsurface. 13th OCTOBER, 2017 LASU 2017

10. METHODOLOGY (Contd.) Data Processing Geophysical Investigation • The VES data were plotted on log-log graphs with apparent resistivity and half electrode separation (AB/2) values on the ordinate and abscissa respectively. The layer thicknesses and resistivity values obtained from the partial curve matching technique were further refined through a computer iteration involving the WINRESIST . • The measured apparent resistivity values for the 2D data gathering were inverted for true subsurface resistivity configuration using DIPROWIN version 4.0 inversion software. The resulting 2D models of the subsurface are presented and interpreted accordingly. Geotechnical Investigation • Measurements are read on the attached gauge meter, recorded at intervals of 250 mm and presented in graphical form. The tests were terminated when the machine had achieved its maximum capacity and could no longer penetrate or when the anchorage were lifted. 13th OCTOBER, 2017 LASU 2017

11. RESULTS AND DISCUSSION Geoelectrical Investigation • The VES interpretation and correlation of the VES interpretation results along the same traverse is presented as geoelectric section in Figures 2a-2d. The geoelectric section (Figure 2a-2d) reveals five layers namely top soil, clayey sand, sandy clay, clay and sand. • The topsoil is characterized by resistivity values ranging from 12.6 – 565.2 Ωm and thickness of 0.6 to 1.7 m. Underlying the topsoil the is clayey sand in VES 1, sandy clay in VES 2,3,5,6,7,8,9 and 12, sand in VES 4, clay in VES 10,11,13,14, 15 and 16 having thickness and resistivity values ranging from 1.4 to 3.75 m and 23.1 to 105Ωm respectively. The third geoelectric layer is indicative of clay/peat with thickness of 7.9 to 21.8m and resistivity values of 8.4- 39.4Ωm. The fourth layer with a thickness of 4-33.4m and resistivity values of 7.5-177.1Ωm is interpreted as sand in VES 1, 11, and 15, sandy clay in VES 2,5 and 7, clayey sand in VES 3,6,8,10 and 13, clay/peat in VES 4,9,12,14 and 16. The fifth horizon is sand with layer resistivity values ranging from 103.5 to 442.4Ωm. The thickness could not be determined as the current terminated within this horizon. 13th OCTOBER, 2017 LASU 2017

12. RESULTS AND DISCUSSION Fig. 2a: Geoelectric section along profile AA’ with CPT interpretation superimposed Fig. 2b: Geoelectric section along profile BB’ with CPT interpretation superimposed 13th OCTOBER, 2017 LASU 2017

13. RESULTS AND DISCUSSION Fig. 2c: Geoelectric section along profile CC’ with CPT interpretation superimposed Fig. 2d: Geoelectric section along profile DD' with CPT interpretation superimposed 13th OCTOBER, 2017 LASU 2017

14. RESULTS AND DISCUSSION 2D Resistivity structures • The inverted 2D resistivity structures are presented in Figures 3a – 3d. A total spread of 200m was surveyed and a depth of 50 m was probed with resistivity values ranging from 1 -100 Ωm. Five layers which include topsoil, clay/sandy, clay,peat/clay and sandy clay/clay were identified on the 2D resistivity images. • The topsoil has a resistivity value of 23-100 Ωm and thickness of about 4m. The second layer is interpreted as clayey sand, sandy clay and sand on profile 1(Fig. 3a), sandy clay on profile 2 (Fig. 3b), sandy clay and clay on profile 3 (fig. 3c) and clay on profile 4 (fig. 3d). It is characterized by resistivity values ranging from 15 to 100 Ωm with thickness that varies from 1.5-4m. The third layer, characterized by resistivity values ranging from 2 to 18 Ωm with thickness ranging from 8-22 m is interpreted as Peat/clay. The fourth layer is interpreted as sand, clayey sand and clay on profile 1(fig. 3a), clayey sand on profile 2 (fig. 3b), clay on profile 3 and 4 (fig. 3c - 3d), has thickness values ranging from 7-13m and resistivity values ranging from 3- 100Ωm. The fifth layer, characterized by resistivity values ranging from 32-60Ωm and thickness values ranging from 18 - 30m is interpreted as clay, clayey sand and sandy clay on profile 3 and sandy clay on profile 4 (fig. 3d). 13th OCTOBER, 2017 LASU 2017

15. RESULTS AND DISCUSSION Fig. 3a: 2D Resistivity structure along traverse 1 with VES and CPT interpretation superimposed Fig. 3b: 2D Resistivity structure along traverse 2 with VES and CPT interpretation superimposed 13th OCTOBER, 2017 LASU 2017

16. RESULTS AND DISCUSSION Fig. 3c: 2D Resistivity structure along traverse 3 with VES and CPT interpretation superimposed Fig. 3d: 2D Resistivity structure along traverse 4 with VES interpretation superimposed 13th OCTOBER, 2017 LASU 2017

17. RESULTS AND DISCUSSION Geotechnical Result • Table 1 presents a standard classification of the cone end resistance value for a typical granular soil according to the work of Garg (2007) as a guide to interpret the result of the CPT tests.Figures 4a – 4d shows the plot of penetration resistance against the depth of Cone penetrometer recorded up to a maximum depth of 20m for CPT 1- 8 respectively. • The Cone resistance plots revealed two subsurface layers of peat/clay and sand with the interface at the inflection point of the rising segment. The first layer is likely an organic peat/clay with an average of cone resistance 8- 40 kg/cm² (Table 2). Beneath the 16m depth is a third layer with a cone resistance value ranging from 40-100 kg/cm² indicating a sand layer. Table 1: Cone end resistance and relative density (Garg, 2007) 13th OCTOBER, 2017 LASU 2017

18. RESULTS AND DISCUSSION Fig. 4a: A graph of depth (m) against cone resistance (kg/cm2) for CPT 1 and CPT 2. Fig. 4b: A graph of depth (m) against cone resistance (kg/cm2) for CPT 3 and CPT 4. Fig. 4c : A graph of depth (m) against cone resistance (kg/cm2) for CPT 1 and CPT 2. Fig. 4d: A graph of depth (m) against cone resistance (kg/cm2) for CPT 1 and CPT 2. 13th OCTOBER, 2017 LASU 2017

19. CONCLUSION • Electrical Resistivity method and Cone Penetration Tests were integrated with the aim of characterizing the subsurface geology and identifying the cause(s) of cracks observed on the walls of some buildings at Mende, Maryland, Lagos State. • From the interpreted results, the subsurface geoelectric units reveal the occurrence of low resistivity, incompetent organic clay up to the depth 30m. This may be responsible for the cracks observed in some part of the building in the study area. • The results also imply that shallow foundations will not be a suitable one for any proposed structure in the study area. A deep foundation involving piling through the incompetent shallow layers to the competent sand with a pile depth of 32m is recommended for any future development. 13th OCTOBER, 2017 LASU 2017

20. REFERENCES Adeoti, L., Kehinde, I., Adegbola, R.B. and Sovi, S.T. (2009). “Foundation investigation using electrical resistivity method: A case study of Iponri, Lagos State, Nigeria”. Journal of Engineering Research (JER). 14(1):50 - 57. Adepelumi, A.A. and Olorunfemi, M.O. (2000). Engineering geological and geophysical investigation of the reclaimed Lekki Peninsula, Lagos, south west Nigeria. Bulletin of Engineering Geology and the Environmental. 125 - 132. Aina, A.M., Olorunfemi, O. and Ojo, J.S. (1996). An integration of aeromagnetic and electrical resistivity method in dam site investigation. Geophysics. 61(2):349 – 356. Akintorinwa, O.J. and Adesoji, J.I. (2009). Application of geophysical and geotechnical investigations in engineering site evaluation. International Journal of Physical Sciences. 4(8):443 - 454. Ako, B.D. (1976). An integration of geophysical and geological data in dam site investigation: The case study of Opa dam. Journal of Mining and Geology 13:1 - 6. Eslaamizaad, S. and Robertson, P.K. (1998). Cone penetration test to evaluate bearing capacity of foundations in sand. Proceedings of 49th Canadian Geotechnical Society, Canada, September 23-25. 429 - 438. Garg, S.K. (2007). Physical and engineering geology. Khanna publishers, New Delhi, India. 348. 13th OCTOBER, 2017 LASU 2017

21. REFERENCES Neil, A. and Ahmed, I. (2006). A generalized protocol for selecting appropriate geophysical techniques, University of Missouri Journal of Geology and Geophysics, 1: 65 - 66. Ogbe, F.G.A. (1972 ). Stratigraphy of strata exposed in the Ewekoro quarry, western Nigeria. Cont. on African Geology. Ibadan, Nigeria. Olorunfemi, M.O. and Meshida, E.A. (1981). Engineering geophysics and its application in engineering site investigation: A case study of Ile - iIe area, The Nigerian Engineer, 24: 57 - 66. Olorunfemi, M.O. and Meshida, E.A. (1987). Engineering geophysics and its application in engineering site investigation : A case study from Ile-Ife area. The Nigerian Engineer. 22(2):57 – 66. Olorunfemi, M.O., Ojo, J.S., Sonuga, F.A., Ajayi, O. And Oladapo, M.I. (2000a). Geoelectric and electromagnetic investigation of the failed Koza and Nassarawa earth dams around Katsina, northern Nigeria. Journal of Mining. and Geology. 36:51 – 65. Olorunfemi, M.O., Ojo, J.S., Sonuga, F.A. Ajayi, O and Oladapo, M.I. (2000b). Geophysical investigation of Karkarku earth dam embarkment, Katsina, northern Nigeria. Global Journal of Pure and Applied Science. 6(1):117 – 124. Roth, J.S., Mackey, J.R., Mackey, C. and Nyquist, J.E. (2002). A case study of the reliability of multielectrode earth resistivity testing for geotechnical investigations in karst terrains, Engineering Geology, 65: 225 - 232. 13th OCTOBER, 2017 LASU 2017

22. THANK YOU 13th OCTOBER, 2017 LASU 2017