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Engineering geological and geophysical investigation of the reclaimed Lekki Peninsula, Lagos, South West Nigeria A.A. Adepelumi 7 M.O. Olorunfemi

Abstract It was hoped to develop an area of reclaimed land on the Lekki Peninsula of Lagos State. An investigation was therefore carried out in order to establish the subsurface geological/geo-electrical sequence; to identify the sand columns and determine their thickness and prepare a map of the geomorphological features of the original, pre-fill terrain. Vertical electrical resistivity soundings (VES) and penetrometer tests were carried out in a sand filled segment of the Lekki Peninsula. The information obtained showed a good correlation between the geomorphological features as interpreted from the aerial photographs and those identified from maps derived from the VES/penetrometer results. Résumé Il est prévu d’aménager une région de la péninsule de Lekki, de l’État de Lagos. Ainsi, des études ont été entreprises afin de préciser une coupe géologique et géo-électrique des terrains superficiels: identifier les séries sableuses, déterminer leurs épaisseurs et préparer une carte des caractéristiques géomorphologiques du terrain original, avant remblaiement. Des sondages électriques verticaux et des essais au pénétromètre ont été réalisés sur un secteur sableux de la péninsule de Lekki. Les données obtenues ont montré une bonne corrélation entre les caractéristiques géomorphologiques issues

Received: 28 August 1998 7 Accepted: 18 January 1999 A.A. Adepelumi 1 (Y) 7 M.O. Olorunfemi Department of Geology, Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria Telephone/Fax: c234-230171 e-mail: aadepelu6oauife.edu.ng Present address: 1 Geophysics Department, Observatorio Nacional-CNPq, R. Gal. Jose Cristino 77, CEP 20921-400 Rio de Janeiro-RJ, Brazil e-mail: [email protected]

de photo-interprétations et celles résultant de cartes établies à partir des résultats de prospection électrique et d’essais au pénétromètre. Key words Sand 7 Fill 7 Geophysics 7 Nigeria 7 Coastal 7 Reclamation Mots clés Sable 7 Remblai 7 Geophysique 7 Nigeria 7 Cótier 7 Assainissement

Introduction The Lekki Peninsula is located in the south eastern part of Lagos State, south west Nigeria, lying between latitudes 6727b and 6727b30n N and longitudes 3727b and 3730b E (Fig. 1). It is a zone of coastal creeks and lagoons (Pugh 1954; Adeyemi 1972) which is almost encompassed by the Lagos lagoon system. In order to relieve congestion in Victoria Island and Ikoyi, a section of the Lekki Peninsula was reclaimed by sand-filling in order to provide further land for development purposes. One of the main objectives of the study reported here was to determine the volume of sand pumped into the approximately 18.5 km 2 of reclaimed land shown in Fig. 1 and to establish an accurate estimate of the thicknesses of sand at as many locations as possible. As it was hoped to develop the area, it was also necessary to elucidate the subsurface geology and geophysical parameters of the superficial layers as well as the structural disposition and geomorphological features which had existed before the sand fill had been undertaken. Geophysical methods are often used in site investigation to determine the overburden thicknesses and map subsurface conditions prior to excavation and construction. Electrical resistivity and seismic exploration methods are the most common techniques used for this purpose (Kurthenecker 1934; Moore 1952; Drake 1962; Early and Dyer 1964; Burton 1976; Nunn 1979; Kearey and Brooks

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A.A. Adepelumi 7 M.O. Olorunfemi

1984; Olorunfemi and Meshida 1987). These methods are also helpful for the delineation of buried channels and bedrock depressions. In view of this and the anticipated significant resistivity contrast between the placed sand and the peat/clay or sandy clay substratum, these methods were chosen for the present study.

Geology

Fig. 1 Map of part of Lagos showing the Lekki Peninsula

Fig. 2 Map of Lekki Peninsula area showing vertical electrical sounding (VES) and penetrometer test stations

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The Lekki Peninsula area is located within the Western Nigeria coastal zone which consists largely of coastal creeks and lagoons developed by barrier beaches associated with sand deposition (Webb and Hill 1958; Adeyemi 1972). As part of the Nigerian sector of the Benin Basin, the Quaternary geology of the study area comprises the Benin Formation (Miocene to Recent), recent littoral alluvium and lagoon/coastal plain sand deposits (Durotoye 1975; Longe et al. 1987; Jones and Hockey 1964). The alluvial deposits consist mainly of sands (Jones 1960; Halsted 1971), littoral and lagoon sediments formed between two barrier beaches (Adeyemi 1972) and coastal plain sands. In

Reclaimed Lekki Peninsula, Lagos

Fig. 3 Typical Wenner VES curves

Fig. 4 Comparison of the penetrometer curves with electrical depth sounding section


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the survey area, however, the original superficial deposits tration resistances were plotted against depth of penetrawere mainly clay/peat. tion; some typical examples are given in Fig. 4 and compared with the VES interpretation model. The inflection points of the penetrometer curves were interpreted as the interfaces between the different formations. As can be Method seen from the figure, the curves generally show relatively low resistance (0–60 kg/cm 2) within the uppermost layer of The survey area was divided into a 250!250 m square grid loose/uncompacted dry sand. This increases to some network (Fig. 2). The electrical resistivity survey involved 60–15 kg/cm 2 in the wet compacted sand, dropping vertical electrical sounding (VES) and/or cone penetrom- sharply to between 5 and 45 kg/cm 2 in the underlying clay/ eter tests which were carried out at the corners of most of peat and sandy clay horizons. Three panchromatic black and white aerial photographs the grid squares. A VES inter-electrode spacing of up to 64 m was used. The apparent resistivity measurements were also available. These had been taken in 1964 at a scale were made with the ABEM SAS 300 C Terrameter Unit. of 1 : 40,000. They were interpreted by the Topcon Mirror The Wenner array was adopted and a total of 28 sounding stereoscope and used in the identification of such geomorstations employed. The VES curves obtained by plotting phological features as channels and creeks. the apparent resistivity against electrode spacing were interpreted by the partial curve matching method (Orellana and Money 1966) and computer iteration techniques. Results and Discussion A typical example is given in Fig. 3. The penetrometer tests were carried out at 100 stations, some of which were also VES stations (Fig. 2). The pene- Three geoelectric sections were drawn along profiles A-B, C-D and E-F. As shown in Figs. 5a–c. The layer resistivities and thicknesses indicated three subsurface layers. The 0.6 to 2.4 m thick upper layer was composed of dry, loose sand with resistivity values ranging between 140 and 3600 ohmFig. 5 m. Beneath this was a 0.7–5.3 m thick layer of wet sand Geological section along profiles A-B, C-D and E-F

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Fig. 6 Isopach map of the sandfilling using the VES and penetrometer test result thickness

with resistivity values varying from 3–190 ohm m. These two layers were considered to be the sand fill materials. Underlying this placed sand was a horizon of in situ sandy clay/clay or peat which was identified for depths of between 1.0 to 6.0 m. Depressions were recognised beneath stations 64 (profile A-B), 39 (profile C-D) and 23 (profile E-F). These are likely to be related to ancient river/stream channels or creeks. They are shown on the isopach map in Fig. 6 as areas of relatively thicker sand, in excess of 2.5 m (T1–T5). Table 1 compares the thicknesses of sand derived from the VES and penetrometer tests. It can be seen that the VES indicated the sand to vary from 1.53 to 6.0 m which correlates well with the results of the penetrometer tests of 1.5 to 6.0 m of sand. For most stations the differences between the two methods were less than 15%, with the exception of Stations 11, 13, 86, 103 and 137. It is of note that Station 11 is located close to the lagoon and hence is prone to saline water intrusion. The overlying sand column may have been impregnated by saline water which could have obscured the interface between the sand and the underlying clay. The study area was also divided into triangular, rectangular and trapezoidal cells and the volume of sand within

Table 1 Comparison of the penetrometer results with the vertical electrical sounding thickness Station

Penetrometer test derived sand-fill thickness (m)

VES derived thickness of sand-fill

Percentage deviation

4 11 a 12 13 17 28 32 37 39 41 51 55 56 a 58 59 64 67 70 73 76 86 88 94 103 122 135 137

1,75 2,00 1,75 1,60 1,70 2,20 6,00 2,00 5,50 3,00 2,75 2,25 2,00 1,60 2,50 5,60 2,25 3,00 3,20 1,50 2,20 1,75 2,25 2,00 2,75 2,25 1,50

1,98 4,30 1,84 2,00 1,81 2,37 6,00 2,25 5,80 3,00 3,04 2,46 1,94 1,80 2,80 6,00 2,14 3,35 3,33 1,53 2,78 1,87 2,52 2,58 2,58 2,13 1,83

13,14 115,00 5,14 25,00 6,47 7,73 0,00 12,50 5,45 0,00 10,55 9,33 –3,00 12,50 12,00 7,14 –4,89 11,67 4,10 2,00 26,36 6,86 12,00 29,00 –6,18 –5,33 22,00

a

Penetrometer lithological log derived thickness


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Fig. 7 Surface features of Lekki Peninsula area (pre-sand-fill) obtained from aerial photograph interpretation

each cell calculated. These results were then summed to give a total volume of 15,287,631 m 3 of sand. The aerial photographs were used to produce an interpretation of the likely relief prior to the sand fill operation. As can be seen from Fig. 7, this indicated four principal geomorphological features: alluvial/colluvial deposits, both major (d4, 5, 6) and minor (d1, 2, 3, 7, 8) depressions, creeks trending NE-SW (C1 and C2) and mudflats. The central and eastern parts of the study area are characterised by these mudflats, swampflats and backswamps which suggest the area must have been susceptible to periodic flooding due to its low relief. The surface relief map of the in situ clay/peat horizon was prepared from the VES and penetrometer test results and shows three major depressions (D1, D2 and D3) and three minor depressions (D3, D4 and D5); the former trending approximately NE-SE and NNE-SSW and the latter E-W and N-S. Again, it is assumed these depressions indicate former river/stream channels and creeks. The structural trend of the depressions may have been 130


influenced by the oceanic fracture pattern postulated by Emery et al (1975). A comparison of the pre-reclamation surface features interpreted from the aerial photographs and the surface relief map derived from the VES and penetrometer results indicate a good correlation, the depressions indicated on Fig. 4 generally corresponding to those indicated on Fig. 8.

Conclusions VES and penetrometer tests revealed three subsurface geoelectric layers; the upper two of dry and wet sand respectively representing the sand fill and the underlying sandy clay/clay or peat in situ deposits. The thickness of the sand varied from 1.0 to 6.0 m and using triangular, rectangular and trapezoidal cells, the total spatial volume of fill was calculated to be 15,2867,631 m 3.


Fig. 8 Surface relief map of Lekki Peninsula pre-sand-fill

A good correlation was found between the geomorphoReferences logical features interpreted from the aerial photographs and those identified from the maps derived from the VES/ Adeyemi PA (1972) Sedimentology of Lagos lagoon. Unpublished penetrometer results. The main subsurface structural special BSc thesis, Obafemi Awolo University, Ife-Ife, Osun features were six depressions, indicated on the map State, Nigeria derived from the VES/penetrometer results at positions Burton AN (1976) The use of geophysical methods in engeniering geology, Part 1: Seismic techniques. Ground Engiwhich correlated well with zones of sand more than 2.5 m neering thick. These are considered to be related to ancient river/ Drake CL (1962) Geophysics and engineering. Geophysics stream channels or creeks.

27 : 193–197 Durotoye AB (1975) Quaternary sediments in Nigeria. In: Acknowledgements The authors are grateful to Terra GeotechKogbe CA (ed) Geology of Nigeria. Elizabeth Press, Lagos, nical Engineering Co. Ltd. for providing the Penetrometer data pp 431–451 we also thank the reviewers for their comments and the editor for Early KR, Dyer KR (1964) The use of resistivity survey in founimproving the clarity of the paper. dation site underlain by Karst dolomite. Geotechnique 14 : 341–348 Emery KOE, Uchupi JP, Brown C, Mascle J (1975) Continental margin off western Africa - Angola to Sierra Leone. American Association of Petroleum Geologist Bulletin 59 : 2209–2265 Ghosh DP (1971) Inverse filter coefficients for the computation of apparent resistivity standard curves for a horizontal stratified earth. Geophysical Prospecting 19 : 749–775 Halstead LB (1971) The shoreline of lake Kainji, a preliminary survey. Journal of Mining and Geology 6 : 1–22 Jones GPA (1960) Sedimentary study of the Ngalda gravels, Bauchi province, N E Nigeria. Record of Geological survey, Nigeria, pp 29–40


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Jones HA, Hockey ED (1964) The geology of part of southwestern Nigeria. Geological Survey Nigeria Bulletin 31 : 1–101 Keary P, Brooks M (1984) An introduction to geophysical exploration. Blackwell Scientific Publication, Oxford, pp 198–217 Kurtenacker KS (1934) Some practical application of resistivity measurements to highway problem. Transaction of American Institute of Mining and Metallurgical Engineers 110 : 193–205 Longe EO, Malamo S, Olorunniwo MA (1983) Hydrogeology of Lagos Metropolis. Journal of African Earth Sciences 6 : 163–174 Moore MO, Meshida EA (1987) Geophysical methods adapted to highway engineering problems. Geophysics 17 : 505 Nunn KR (1979) Geophysical surveys at two landfill sites in the West Midlands. Symposium on the engineering behavior of industrial and urban fill. April 1979

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Olorunfemi MO, Meshida EA (1987) Engineering geophysics and its application in engineering site investigations (Case study from Ife-Ife area). The Nigerian Engineer 22 : 57–66 Orellana E, Mooney HM (1966) Master tables and curves for vertical electrical soundings over layered structures. Interciencia, Madrid Pugh JC (1954) A classification of the Nigerian coastline. Journal of the West African Science Association 1 : 3–22 Webb JE, Hill MB (1958) Ecology of Lagos lagoon (II): the topography and physical features of Lagos harbor & Lagos lagoon. Philosophical Transaction of Royal Society, London, Series B 24 : 319–33