Natural Science, 2009, 1, 2-9 NS
http://dx.doi.org/10.4236/ns.2009.11002
Structural mapping of coastal plain sands using
engineering geophysical technique: Lagos Nigeria case study
A. A. Adepelumi1, M. O. Olorunfemi2, D. E. Falebita3, O.G. Bayowa4
1,2,3Department of Geology, Obafemi Awolowo University, Ile-Ife, Nigeria; 4Department of Earth Sciences, Ladoke Akintola Univer-
sity, Ogbomoso, Nigeria. 1Phone: [phone].
Corresponding author: 1
Received 20 May 2009; revised 1 June 2009; accepted 2 May 2009.
ABSTRACT
An engineering geological survey using the
cone penetrometer and finite element method
was carried out to characterize sand-fill thick-
nesses in a reclaimed area of Lagos, SW Nigeria.
A previously developed finite element program
was modified in order to allow for predicting the
sand-fill thicknesses, and have an unders-
tanding of the geomorphic shallow structures
existing pre-sand-fill. The program was tested
using the obtained cone penetrometer test re-
sults from the Lekki-Peninsula area. The finite
element predicted thicknesses show good cor-
relation with the penetrometer obtained thick-
nesses. Six zones with thick sand-fill thick-
nesses varying from 1.25 to 6.0m were identified
from the isopach maps, these zones correlate
with major/minor depression associated with
river/stream channels and creeks. These are the
main shallow geomorphic structural features
present in the area pre-sand fill. The structural
trends of the depressions are largely influenced
by the oceanic fracture pattern.
Keywords: Sand-Fill; Finite Element; Nigeria;
Penetrometer; Depression
1. INTRODUCTION
Engineering site investigation requires determination of
thicknesses either to competent bedrock in foundation
works or of sand-fill columns in a reclaimed site. Accu-
rate mapping of bedrock topography or reliable estima-
tion of sand-fill in a reclaimed site requires that thick-
nesses are known at several test points. The more the
number of test points, the better the bedrock topography
definition and the more accurate is the estimation of spa-
tial volume. However, the more the number of test points,
the higher is the cost of investigation and the longer is
the survey duration, most especially where the survey
area runs into tens of square kilometers [1].
Geophysical methods, cone penetrometer tests, and di-
rect borehole drilling are some of the various means of
determining thicknesses or depths to a particular bedrock
[2,3,4]. Of the three methods outlined above, geophysical
methods remain the cheapest. But geophysical data inter-
pretation requires some level of control usually in terms
of subsurface information (e.g. lithological logs) obtained
from drilling. Hence geophysical investigation is often
complemented by borehole investigation with a conse-
quently increasing cost. Survey cost and duration can be
reduced if a predictive technique, with significant level of
accuracy, can be developed that utilises few initial accu-
rately determined thicknesses to predict thicknesses at
other location where tests have not been carried out.
Finite element automated approach to the prediction
of heads have been utilised by a number of authors.
These include Fenner [5], Agbede [6] and Wang and
Anderson [7]. These methods are iterative procedures
that utilise various elemental geometries such as poly-
gons, rectangles and triangles. The Finite element pro-
gram, developed by Wang and Anderson [7] was slightly
modified and used to predict formation thicknesses. The
viability of the technique was tested using cone pene-
trometer test results from a reclaimed Lekki Peninsula
area of Lagos. In the present study, the main objective is
to determine the thicknesses of sand-fill using the cone
penetrometer tests and finite element methods. And, to
have an understanding of the geomorphic shallow struc-
tures existing pre sand-fill in the reclaimed Lekki Pen-
insula area of Lagos Nigeria.
2. THE STUDY AREA
The Lekki-Peninsula area of Lagos was reclaimed by
hydraulic sand-fill. It is located within the western
coastal zone which consists largely of coastal creeks and
lagoons developed by barrier beaches associated with
sand deposition [8,9]. The study area can be found in the
south eastern part of Lagos State, southwest, Nigeria,
lying between latitude 6o 25’44.62” and 6o 27’38.16’’ N

A. A. Adepelumi et al. / Natural Science 1 (2009) 2-9 3
and longitudes 3o 27’16.70” and 3o 28’ 55.80 E (Fig. 1).
The surface geology of the study area is made up of
the Benin Formation (Miocene to Recent), recent littoral
alluvial, lagoons and coastal plain sand deposits. The
sand range in size from coarse to medium grained clean
white loose sandy soil which graded into one another
towards the lagoons and near the mouth of the larger
rivers. The low-lying beach ridges of sand called berm
and barrier beach ridges of sand are ubiquitous in the
area and are said to be derived from one or more of the
following, sand brought in along the coast and reworked
alluvial sands originally deposited by the south flowing
rivers drawing the Dahomey basement of the western
Nigerian during the late Pleistocene, Wurm-Wisconsn
[10]. The superficial deposit in the pre-sand fill is com-
posed mainly of the clay/peat deposits. The recent litto-
ral and alluvium deposits, the continental Benin sands
and the Ilaro Formations were identified as the major
aquifers. The water bearing aquifers consist of sands,
gravels or a mixture of the two [11]. Within Lagos me-
tropolis, three major aquifer zones at depths shallower
than 200 m were delineated. The first is a water table
aquifer that is prone to pollution. The second and third
aquifers are confined aquifers made up of an alternating
sequence of sand and clay. They are harnessed through
boreholes and are the basis of mini-water works in La-
gos area. The third aquifer is the most productive and
most exploited.
Figure 1. Location map of Lekki-Peninsula, Lagos, Southwestern Nigeria.

4 A. A. Adepelumi et al. / Natural Science 1 (2009) 2-9
Cone penetrometer tests were first carried out in the
study area in order to determine the hydraulic sand-fill
thicknesses. For the computation of the unknown thick-
nesses using the finite element program, the survey area
was broken into 191 triangular meshes with 147 test points.
The input data are the, number of nodes, the number of
elements and the nodal coordinates for each node.
3. METHODOLOGY
3.1. Cone Penetrometer
Cone penetrometer test is one of the most widely used
direct methods in soil testing. The application of the
method in geotechnical practice has been reviewed by
Sanglerat [12] and de Ruiter [13]. It was designed as a
control for the indirect geophysical method [14] and to
determine the properties of the insitu soil like its se-
quence or profile. The penetrometer test was carried at
one hundred stations with stations coinciding with the
nodes of the finite element triangular meshes.
The force required to drive the probe into the ground
(that is, penetration resistance) and the depth of penetra-
tion were recorded at each station. The penetration

A. A. Adepelumi et al. / Natural Science 1 (2009) 2-9 5
Figures 2a & b. Comparison of the penetrometer curves with the finite element results.
resistance in Kg/cm2 was plotted against depth of pene-
tration. In view of the envisaged resistance contrast be-
tween the sand-fill and peat/clay or sandy clay bedrock,
this method was chosen for the study. The inflection
points of the penetrometer curves were interpreted as the
interface between the different lithologies.
3.2. Finite Element Method
The program presented here is based on the application
of variational or weighted residual principle. The prob-
lem domain is visualized as a triangular element with
four nodes at the corners. The nodes are the points within
the problem domain at which thicknesses are computed

6 A. A. Adepelumi et al. / Natural Science 1 (2009) 2-9
Fenner [5]. The residual at each point in the problem
domain is a measure of the degree to which the thickness
does not satisfy the governing equation. A trial solution
t(x,y) is built up as a continuation of the basis function
Table 1. Comparison of the penetrometer tests ad finite element results.
NODE
NUMBER
PENETROME
TER
THICKNESS
(m)
FINITE
ELEMENT
THICKNESS
(m)
NODE
NUMBER
PENETROME
TER
THICKNESS
(m)
FINITE
ELEMENT
THICKNESS
(m)
NODE
NUMBER
PENETROME
TER THICK-
NESS
(m)
FINITE
ELEMENT
THICKNESS
(m)
1 1.25 - 51 2.75 - 101 2 2.07
2 - 1.5 52 2.25 2.25 102 2.5 2.32
3 1.5 - 53 2.25 - 103 2
4 1.75 - 54 2.25 2.46 104 2 1.8
5 1.75 - 55 2.25 - 105 - 1.51
6 1.5 - 56 6.75 - 106 1.4
7 - 1.75 57 1.94 1.94 107 1.37
8 1.5 - 58 1 - 108 2.25 2.1
9 - - 59 2.5 - 109 2.5 2.06
10 - - 60 - 1.5 110 2 1.76
11 2 - 61 - 1.4 111 - 1.49
12 1.75 - 62 - 4.36 112 2 -
13 1.6 - 63 2 - 113 - 1.37
14 1.75 1.75 64 5.6 - 114 - 1.33
15 3.25 3.25 65 2 3.43 115 2.25 2.16
16 2 2 66 1.7 2.36 116 2.25 2.14
17 1.7 - 67 2.14 - 117 3 1.74
18 3.4 - 68 1.5 1.63 118 2.5 1.47
19 - 3.9 69 - 1.56 119 - 1.34
20 - 3.39 70 3 - 120 - 1.3
21 2.5 2.87 71 - 2.71 121 2 2.24
22 2 2.88 72 2 2.82 122 2.75 -
23 2.5 3.2 73 3.2 3.3 123 2.75 1.81
24 2.25 2.86 74 1.2 2.26 124 - 1.47
25 - 4.42 75 2 1.89 125 2.2 -
26 - 4.36 76 1.5 - 126 - 1.31
27 2.5 3.17 77 2 - 127 - 1.25
28 2.2 - 78 2.2 2.61 128 3.5 2.01
29 2.5 3.78 79 2 2.44 129 3.5 1.79
30 2 3.03 80 1.75 2.26 130 2.5 1.46
31 - 4.49 81 1.5 1.86 131 - 1.46
32 6 - 82 - 1.63 132 - 1.27
33 3.25 3.54 83 2.2 - 133 - 1.19
34 3.5 3.77 84 - 1.49 134 3 1.88
35 3.75 4.59 85 2 1.45 135 2.25 -
36 2.2 3.05 86 2.2 - 136 2.5 1.98
37 2 - 87 3.25 2.33 137 1.25 -
38 - 2.91 88 1.75 - 138 - 1.48
39 3 - 89 1.5 1.72 139 - 1.23
40 3.5 3.77 90 - 1.55 140 - 1.12
41 3 - 91 - 1.45 141 1.6 -
42 2 2.8 92 - 1.42 142 1.5 -
43 2 3.04 93 1.5 2.29 143 2.5 -
44 - 2 94 2.25 - 144 2.5 -
45 - 2 95 2 2.17 145 1.5 -
46 2 2.5 96 2.25 1.75 146 1.2 -
47 - 2 97 - 1.53 147 1 -
48 1.75 - 98 2.25 -
49 3.2 - 99 - 1.43
50 2 - 100 - 1.39

A. A. Adepelumi et al. / Natural Science 1 (2009) 2-9 7
expressed as a series summation.
),(),(
1
yxLNtyxt
NNODE
L
L


 (1)
L = The nodal number
t = An approximate or trial solution
For the computation of the unknown thicknesses by
the program, the triangular mesh was digitized at equal
intervals of 500m. The accuracy of the predicted thick-
nesses is strongly dependent on the accuracy of the ini-
tial guess or starting thicknesses and the size of the nodal
spacing. The thicknesses were predicted for stations
(nodes) at which penetrometer tests have been carried
out and at which no penetrometer tests were carried so as
to cover the entire survey site.
4. RESULTS AND DISCUSSION
The typical cone penetration curves obtained in the study
area are shown in Fig. 2. As can be seen from the figures,
the curves generally show relatively low resistance (0-60
Kg/cm2) within the uppermost layer of loose/ uncom-
pacted dry sand. This increases to some 60–150 Kg/cm2
in the wet compacted sand, dropping sharply to between
5 and 45 Kg/cm2 in the underlying clay peat and sandy
clay horizons. The penetrometer tests delineated three
to four lithologic units. The topsoil of dry and loose sand,
wet sand, sand clay/clay or peat bedrock. It was ob-
served in general that, the first two layers constitute the
sand-fill (Fig. 2) whose thicknesses vary from 1.25 to
6.00 m (Table 1).
The finite element predicted thicknesses are presented
in Table 1. The results were compared with the pene-
trometer obtained sand-fill thicknesses. It is observed
that the finite element derived thicknesses are in good
agreement with the penetrometer test thicknesses with a
few exceptions at stations 29, 30, 35, 65, 74 and 93,
where very high percentage deviation of between 51 to
88% were obtained; these fairly large deviation may be
due to insufficient input data to act as control around
these stations.
The good correlation between the finite element pre-
dicted thicknesses and that obtained from the penetrome-
ter test results imply that, given a limited accurate thick-
nesses as input data, the finite element program would
predict the thicknesses at unknown stations to within a
reasonable level of accuracy. The close agreement also
indicates that the finite element predicted thicknesses
could be reliably used for formation thickness estimation
in the absence of sufficient penetrometer test results.
Fig. 3 is an isopach map prepared from finite element
predicted thicknesses. The map shows sand thick-
Figure 3. Isopach map of the sand-fill using the computed finite element result.
0.8
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882000 883000 884000 885000
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Scale 1:13786
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N
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882000 883000 884000 885000

8 A. A. Adepelumi et al. / Natural Science 1 (2009) 2-9
Figure 4. Isopach map of the sand-fill using the penetrometer obtained thicknesses.
nesses varying from 1.25 to 6.0 m in the area labeled t1,
t2, t3, t4, t5 and t6. This zone corresponds to basement
depression associated with river/stream channels and
creeks in the area which are the main shallow structural
features in the surveyed are pre-sand fill. The structural
trend of the depressions is largely influenced by the
oceanic fracture pattern (NW-SE, NNE-SSW, N-S, and
E-W) earlier delineated by Emery et al. [15]. This map
compares well with the isopach map prepared from
penetrometer obtained thicknesses (Fig. 4) with thick
sand zone correlating.
5. CONCLUSIONS
The qualitative and quantitative interpretation of the
penetrometer test results provided adequate information
regarding the structural disposition of the geomorphic
shallow structures existing pre-sand-fill. From the pene-
trometer test, the first two layers constitute the sand-fill
with thicknesses varying from 1.25 to 6.00 m. Good
correlations exist between the finite element, and the
penetrometer thicknesses with few exceptions. The
isopach maps show area of basement depression corre-
sponding to ancient river/stream channels and creeks
with structural disposition trending in the NE-SW,
NW-SE, NNE-SSW, N-S and E-W. The result shows that,
finite element method is an efficient means of predicting
formation thicknesses that can help considerably in re-
ducing engineering geophysics survey costs.
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0.8 1.0
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