Available online at www.worldscientificnews.com
( Received 26 August 2018; Accepted 11 September 2018; Date of Publication 12 September 2018 )
WSN 110 (2018) 23-41 EISSN 2392-2192
Engineering Geological Properties of Coastal Plain Sands in Selected Areas in Ikoyi Area, a typical
Coastal Sedimentary Terrain
Abdulquadri O. Alaberea and Olaoluwa E. Emmanuelb
Department of Applied Geology, Federal University of Technology, Akure, P.M.B 704, Akure, Ondo State, Nigeria
a,bE-mail address: [email protected] , [email protected]
ABSTRACT
Coastal plain sands in some selected areas of Ikoyi, in Lagos, Dahomey basin southwestern
Nigeria have been investigated with respect to their geotechnical properties to determine their
suitability as construction materials and to recommend suitable foundation type for the proposed
developments. Representative samples collected from each sites were subjected to standard
engineering tests and CPT method was used for calculating the bearing capacity of the soil. The cone
resistance values (Qc) ranges from 8-158 kg/cm3, 20-160 kg/cm3, 32-166 kg/cm3 for Sokoto street site,
Parkview estate site, and Delta street site respectively. Correlating this result with standard penetration
test values, it shows dense soil at 3m, 10m and 45m for Delta Street site, medium dense soil at 2m and
30m for Parkview estate site, and medium dense soil at 3m, 29m and 30m for Sokoto Street site. The
undrained triaxial test revealed that undrained cohesion ranges from 47-70 KNm-2, 44-95 KNm-2, and
15-30 KNm-2 with no angle of internal friction for Sokoto Street, Delta Street and Parkview estate site
respectively. These values show that the soils have low bearing capacity due to low undrained cohesion
and angle of internal friction. The geotechnical analysis result show that for the proposed building
(tower) on the three sites, shallow foundation cannot be supported because the foundation pressure
exceeds the bearing capacities of the soil across the three sites at shallow depth and the loads needs to
be transferred to more competent soil layer at deeper depth via a non- displacement (or replacement)
pile.
Keyword: Bearing capacity, Displacement Piles, standard penetration test, cone penetrometer
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1. INTRODUCTION
Engineering properties of soils play a significant role in stability of civil engineering
construction works particularly in road construction, foundation, embankments, bridges and
dams to mention a few. This made imperative, the testing of soils on which foundation or
superstructure would be laid. The testing would determine their geotechnical suitability as a
construction material (Tomlinson, M.J. 1980). The increasing rate of building collapse, tilting
and development of huge cracks in structures requires a thoughtful solution. This could be
provided by detailed study of the geotechnical properties of the subsoil layers, calculation of
the bearing capacity of soil, and careful examination and recommendation of foundation
parameters (Robert, M. 1996).
Geotechnical properties studied includes: grain size, moisture content, specific gravity,
undrained cohesion, angle of internal friction, plastic limit, liquid limit, plasticity index, SPT
N-value and the CPT Qc values. Interest in stability of structures and protection of lives and
properties have driven several researchers to work on soils and foundation designs in different
parts of the country. Oyedele et al (2012) used resistivity tomography to evaluate the
corrosivity of coastal plain sand in Lekki area and the effect of ocean water on the rate of
corrosivity.
Oyedele et al (2011) using a combination of geotechnical and geophysical methods in
characterizing a construction site in Ikoyi area of Lagos found that the subsurface layers up to
the depth of 16 m are mechanically unstable with low penetration resistance value which may
not serve as good foundation materials. Hence, he recommended that structures in the area be
founded on stable and mechanically competent coarse sand through a displacement pile.
Oladele et al., 2012 and Alao 1983 investigated the suitability of lateritic soils in Ilorin as
construction materials. He discovered from his investigation that that the soils has high
bearing capacity with little or no volume change and therefore can be used as construction
materials in engineering works especially in dam construction, shallow foundation,
homogeneous embankments, slope stability and as sub-grade materials in road constructions.
Alao et al. (2014) investigated the geotechnical properties of several beach sands along the
coastal areas of Lagos for their foundation suitability. He found that at a depth of two metres,
ultimate bearing capacity of Beecroft beach sand is the highest followed by Suntan beach,
Bar-beach and Banana-Island in decreasing order and this soils are suitable for shallow
foundations such as strip and square footing.
Ogundalu and Oyekan (2014) used X-Ray diffraction (XRD), X-Ray photoelectron
(XPS) and scanning electron microscope (SEM) to evaluate mineralogical and geotechnical
characteristics of black cotton soils in Maiduguri and their suitability for road pavement and
construction purposes. They found that the soil strength characteristics of the black cotton
soils fall below the minimum requirement of sub base and base course and thus inimical to
foundation and road pavements in the area. Also, several authors have worked on means of
stabilizing soil layers especially the clay zones to improve on their geotechnical properties.
Fusheng et al. (2008) investigated the use of fly ash in stabilizing expansive clays.
The test results show that the plasticity index, activity, free swell, swell potential,
swelling pressure, and axial shrinkage percent decreased with an increase in fly ash or fly ash-
lime content. He also found that soils treated with fly ash show no significant change in
unconfined confined compressive strength in the first seven days, then after curing for this
period the compressive strength increases.
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2. LOCATION AND GEOLOGY OF THE STUDY AREA
Figure 1. Map showing location of the study area.
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The field test and soil sampling were carried out in boreholes drilled in Lagos within
latitude 6°27’ 11” N and longitude 3°26’8”E. Ikoyi area of Lagos is located near the eastern
margin of the Nigerian sector of the Benin basin, within the Western Nigeria coastal zone
which consists largely of coastal creeks and lagoons developed by barrier beaches associated
with sand deposition underlain by recent deposits which vary from the littoral and lagoon
sediments to the coastal belt and alluvial deposits of the major rivers (Pugh, 1954). It forms
part of the Lagos lagoon system known to be the largest of the four Lagoon system of the Gulf
of Guinea Coast (Adepelumi and Olorunfemi, 2000). Geologically, the study area is a
sedimentary basin which falls within the coastal plain sands of the Benin formation which was
latter overlain by alluvial deposits. Stratigraphy of the eastern Benin Basin has been discussed
by various workers and several classification schemes have been proposed. These notably
include those of Jones and Hockey (1964); Omatsola and Adegoke (1981) and Billman
(1992). Deposition of Cretaceous sequence in the eastern Dahomey basin began with the
Abeokuta Group, consisting of the Ise, Afowo and Araromi Formations. Tertiary sequence
include the ewekoro, akinbo, oshosun, ilaro and coastal plain sands (Benin) Formations
(Omatsola and Adegoke, 1981). The Quaternary sequence in the eastern Dahomey basin is the
Coastal Plain Sands and recent littoral alluvium which consists of poorly sorted sands with
lenses of clays. The sands are in parts cross bedded and display continental characteristics
(Durotoye, 1975). However, lithostratigraphic information from boreholes in and around
Lekki-Ikoyi area reveals that typical section of the stratigraphy consists of unconsolidated dry
and wet sand, and organic clay deposit. The deposits are sometimes inter-bedded in places
with sandy-clay or clayey-sand and mud with occasional varying proportion of vegetable
remains and peat. The environment of deposition of these sediments was suggested to be near-
shore littoral and lagoonal (Longe et al, 1987).
3. MATERIALS AND METHODOLOGY
3. 1. Sampling Methods
Soil samples was collected at three different location including sokoto, delta and
parkview site. Disturbed samples was collected at every 0.75m interval in a borehole drilled
using a percussion drilling rig till a depth of 30 m in each location. The disturbed samples was
taken to the laboratory to determine index properties of the soil such as the grain size, specific
gravity, moisture content etc. Undisturbed samples were acquired with a 100 mm internal
diameter open tube (U4 tube) fitted in a U4 bomber and lowered down the hole by means of
winch of the percussion drill rig to the depth of the soil layers to the sampled (i.e. cohesive
soil). The undisturbed samples was taken to the laboratory for the determination of shear
strength, atterberg limit, and triaxial compression test.
3. 2. Field Test
Two field test was done across the three sites and this include the cone penetrometer test
and the standard penetration test.
3. 2. 1. Standard Penetrometer Test
Standard penetration test is an in-situ field test done in a borehole using SPT hammer
and rod. The rod was lowered to the required depth in an already drilled hole and hit with the
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hammer until it fully penetrates 450 mm into the soil layer to be tested. The number of blows
required for the hammer to penetrate the soil is recorded and it is a measure of the relative
density of sand and consistency of clays. The first 150 mm reading was neglected due to
disturbance arising from the drilling the last 150 mm of the previous layer and the last 300mm
reading was averaged to get the SPT N-value.
3. 2. 2. Cone Penetrometer Test
This test was carried out using a 2.5 ton Dutch cone penetrometer equipment. It is an in-
situ test done on the study area to depths of 9.0 m, 9.5 m, and 9.5 m respectively. It involves
forcing a 600 mm CPT cone through the soil in a small diameter hole dug using a shovel by
the application of pressure from a mechanically rolled turner which passes through the 1.5m
CPT rod to the cone. The resistance to the penetration of the cone (QC) was recorded at
intervals of 0.25 m and plotted against depth to form a resistance profile. The profile was
correlated with borehole data to provide information on variation of strata with depth and
material strength across each site.
3. 3. Laboratory Test
Laboratory test carried out include tests to determine the particle size, atterberg limit,
natural moisture content, and also undrained triaxial compression test.
4. RESULTS AND INTERPRETATION
4. 1. Grain Size Distribution
The test is performed to know the percentage of different grain sizes contained within
the soil. The distribution of different grain sizes affect the engineering properties of soil which
in turn their usability as construction material. The grain size distribution shows that samples
from sokoto site is clayey-silty SAND having 78% sand, 14% silt, and 8% clay composition.
Similarly, samples from delta site could be described as gravelly silty-clayey SAND having
71% sand, 12% cay, 11% silt, and 6% gravel compositions. Whereas, samples from Parkview
site could be described as gravelly silty-clayey SAND having 75% sand, 12% silt, 8% clay,
and 5% gravel compositions.
4. 2. Specific Gravity
Table 1. Typical values of specific gravity of soil particles (after Bowles, 1979).
Soil type Specific gravity
Sand 2.65 – 2.68
Gravel 2.65 – 2.66
Clay (inorganic) 2.52 – 2.66
Clay (organic) 2.68 -2.72
Silt 2.65 – 2.66
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The result of specific gravity of the three soil samples, sokoto sit, delta site and
Parkview site are 2.69, 2.67, and 2.65. Comparing this results with some standard results or
values, the three samples could be referred to as SAND. Table 1 present the typical values of
specific gravity of soil particles.
4. 3. Atterberg Limit
The atterberg limit consistency test carried out on the soil samples show that Sokoto site
has liquid limit of 52 to 70%, plastic limit of 8 to 15%, plasticity index of 37 to 59%, while
delta site has liquid limit of 43 to 57%, plastic limit of 10 to 23%, plasticity index of 23 to
44%, and Park view site shows liquid limit of 45 to 54%, plastic limit of 6 to 13%, and
plasticity index of 35 to 44%.The plasticity index indicate inorganic clays of medium to high
plasticity for Sokoto site, gravely clay, sandy clay and silty clay.
4. 4. Natural Moisture Content
Natural moisture content of 40 to 59% was obtained for Sokoto site, 39 to 48% for Delta
site and 55 to 78% for Parkview site.
4. 5. Undrained Triaxial Compression Test
The undrained triaxial compression test is used to determine angle of internal friction
and undrained cohesion of the soil. The test shows that the soil sample from Sokoto site has an
undrained cohesion of 47 and 70 KNm-2 and angle of internal friction of 6 and 9, delta site has
an undrained cohesion of 44 and 95 KNm-2 with angle of internal friction of 5 and 8 while
Parkview site has an undrained cohesion of 15 and 30 KNm-2 with angle of internal friction of
3. These values show that the soils have low bearing capacity due to low undrained cohesion
and angle of internal friction values.
Table 2. Sokoto quick undrained triaxial test.
Table 3. Delta site quick undrained compression triaxial test
BH No Sample No Depth
(m)
Soil
Type
Moisture
content
%
Specific
gravity Cu Ø
1 21 15.00 C 59 1.56 38 1
2 25 18.00 C 52 1.63 77 3
3 30 24.00 C 40 1.69 70 2
BH No Sample No Depth
(m)
Soil
Type
Moisture
content
%
Specific
gravity Cu Ø
1 21 15.00 C 44 1.56 44 4
2 25 18.00 C 48 1.63 55 2
3 30 21.00 C 46 1.69 72 1
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Table 4. Parkview quick undrained compression triaxial test
BH
No Sample No
Depth
(m)
Soil
Type
Moisture
content
%
Specific
gravity Cu Ø
1 21 15.00 C 55 1.56 15 0
2 25 18.00 C 78 1.63 30 0
Figure 2. Borehole log for Delta Street site
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Figure 3. Borehole log for Parkview Estate site
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Figure 4. Borehole log for Sokoto Street site
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Figure 5. Penetrometer log for Sokoto Street site
(2½ ton penetrometer test log)
Cone reading (kgf/cm2)
De
pth
(m
)
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Figure 6. Penetrometer log for Delta Street site
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 50 100 150 200 250
Dep
th (
m)
Cone reading (kgf/cm2)
(2½ ton penetrometer test log)
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-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 50 100 150 200 250
Dep
th (
m)
Cone reading (kgf/cm2)
(2½ ton penetrometer test log)
Figure 7. Penetrometer log for Parkview Street site
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Figure 8. Undrained Cohesion and angle of internal friction for Delta site
Figure 9. Undrained Cohesion and angle of internal friction for Delta site
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Figure 10. Undrained cohesion and angle of internal friction for Sokoto Street
Figure 11. Undrained Cohesion and angle of internal friction for Sokoto Street.
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Figure 12. Undrained cohesion and angle of internal friction for Parkview Site
Figure 13. Undrained cohesion and angle of internal friction for Parkview site
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4. 6. Standard Penetration Test
The result of standard penetration test for delta site shows high SPT value of N42 at 3m,
high SPT value of N44 at 10m and a high SPT value of N31 at a depth of 45m. Similarly, the
test shows moderate SPT value of N18 at 2 m and 30 m respectively for park view site.
Meanwhile, the test shows high SPT value of N30 at shallow depth of 3 m, N29 at 29 m and
N22 at 45 m for Sokoto site. Correlating this result with standard penetration test values, it
shows dense soil at 3 m, 10 m and 45 m for delta site, medium dense soil at 2 m and 30 m for
park view site, and medium dense soil at 3 m, 29 m and 30 m for Sokoto site.
Table 5. Soil properties correlated with standard penetration test values
(after peck et al, 1974).
4. 7. Cone Penetrometer Test
The CPT test was carried out and terminated at a depth of 13m, 10.0m and 10.0m below
the ground surface respectively. The cone resistance value (Qc) ranges from 8-158 kg/cm3, 20-
160 kg/cm3, 32-166 kg/cm3 for sokoto site, park view site, and delta site respectively. The
result of the cone penetrometer is used to calculate ultimate bearing capacity of the soil at
specific depths. The bearing capacity is calculated using the CPT method of calculating
allowable bearing capacity:
qa = 2.7qc . Rw2 (Meyerhof method) ……. Eq. 1
Correlation between qc and SPT (N – Values) in Cohesionless Soil.
N = qC / 4 and qa = 10N, or 5.0N (If Submerged) ……… … Eq. 2
Table 6. Ultimate bearing capacity and allowable bearing capacity for square footing at depths
SITE I.D Depth
(m)
qu
(KNm-2)
qa
(KNm-2)
Delta street site 0.00 – 2.00 165 55
SPT VALUE Relative Density
0 – 4 Very loose
5 – 10 Loose
11 – 30 Medium Dense
31 – 50 Dense
Above 50 Very Dense
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Banana street site 0.00 – 3.00 156 52
Parkview estate site 0.00 -- 2.00 90 30
5. CONCLUSIONS
Based on the two field test (CPT and SPT) carried out on the three sites and several
laboratory experiment done, the allowable bearing capacity was calculated and then
foundation recommendations was made. The proposed structures across the three sites cannot
be supported on shallow foundation; however shallow foundation can only support other
ancillary facilities on the site with foundation pressure not exceeding the allowable bearing
pressure stated above. For shallow foundation utilization, it must be restricted to use of Raft
foundation; this helps to minimize differential settlement. Should the foundation pressure
exceed the allowable bearing pressure stated above, deep foundation should be considered.
The deep foundation should be a non-displacement pile (or Replacement Piles) and the depth
should be 10m for light loads or 45m for heavy load on Delta site, 9m for light loads or 45m
for heavy load on sokoto site, and 30m for park view site (the type of load depends on the
proposed use of the building and the live load).
Acknowledgement
The author is grateful to Prof. Y.A. Asiwaju-Bello for taking his time in vetting the research. I am also deeply
indebted to Mr. Seun Olabode, of the department of Applied Geology, Futa. We are also grateful to Mr. Kayode
Olalulu of KDF Konsult Ltd for providing the data and field support for this research.
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