Geotechnical Properties of Soil Sequence for
Foundation Design at Addo, Lagos, Nigeria *1Faseki, Oluyemi Emmanuel, 2Olatinpo, Olusegun Ayobami, 3Oladimeji, Akinsile Richard
1, 2, 3Department of Geology and Mineral Sciences, University of Ilorin, P.M.B. 1515,
Ilorin, Nigeria. [email protected], 2 [email protected], 3 [email protected]
Abstract – Geotechnical characterization was undertaken in a proposed construction site along Badore Road, Addo,
Lagos, Nigeria with the aim to unravelling the geo-stratigraphy and engineering properties of the shallow formations as
foundation material. Two boreholes were drilled upto the depth of 30.0m while Cone Penetration Tests were deployed
upto depth of 6.0m. Samples from boreholes were subjected to grain size and Atterberrg Limits tests. The results revealed
that the site is underlain essentially by soft silty sandy clay at the upper layer (0.0 – 5.0m) characterized by void ratio of
0.80 – 0.84, unit weight of 17.50 – 19.0KN/m3, friction angle of 6 – 80, natural water content of 22 – 30% and average SPT-
N of 2 which is indicative of poor foundation material. The middle layer is firm to stiff silty sandy clay (5.0 – 17.0m) with
void ratio of 0.84 – 0.86, cohesion values of 30 – 32KN/m2, unit weight of 19.0 – 20.50KN/m3, friction angle of 8 – 100 and
average SPT-N value of 8 typifying low foundation material. These layers are further underlain by medium dense silty
sand (17.0 – 30.0) with void ratio of 0.43 – 0.46, unit weight of 20.0 – 21.0KN/m3, friction angle of 32 -330 and SPT-N value
of 23 which is the most competent as foundation materials. The preponderance of soil with poor engineering properties at
the shallow foundation zones (0.0 – 3.0m) precludes the adoption of shallow foundation in the area. Pile installed to
competent layer at 20.0m within the dense silty sandy layer is recommended as foundation option for consideration for
the proposed structure.
Keywords: Standard Penetration Test, Pile Safe Working Load, Geotechnical, Cone Penetration Test, Addo, Lagos
Nigeria.
I. INTRODUCTION
Subsurface geotechnical investigation is an important aspect of design process since structures are sited on the earth
surface. This is particularly important in Lagos metropolis due to the incessant occurrence of failures of structures
and the great variability in the engineering properties of the Quartenary deposits within the area making depth to
foundation unclarified. Therefore, the designing and building of structures requires a thorough understanding of
properties of available soils and rocks that will constitute the foundation and other components of the structures.
A site investigation attempts to foresee and provide against difficulties that may arise during construction because of
ground and/or other local conditions [1]. This important aspect of building construction has however been neglected
in recent times and has therefore resulted the increase in the figures of failures of buildings throughout Nigeria [2,
3]. The design of a structure which is safe, durable and has low maintenance costs depends upon an adequate
understanding of the nature of the ground on which such building is located. Site characterization usually provides
subsurface information that assists engineers in the design of foundation of civil engineering structures.
MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25
12
Detailed understanding of the physical parameters which govern the behaviour of soil may result in substantial
savings by avoiding problematic foundation conditions and costly construction methods [4, 5]. Several workers have
reported the presence of clayed and peaty material with undesirable engineering properties in different part of Lagos
metropolis [6, 2, 3, 7, 8, 9]. This further emphasized the important of pre-construction sub soil and foundation
studies within the study area. Preliminary foundation investigation works are operations which include site
investigation and foundation engineering analysis. Exploration of the subsurface can be done using remote
geophysical methods and borings/penetration tests. The boring although expensive is more reliable and helps in
recovering samples for laboratory testing and evaluation [10]. Often used together with laboratory test is in-situ
testing which is very important in geotechnical engineering as simple laboratory tests may not be reliable while
more sophisticated laboratory testing can be time consuming [11].
In-situ testing provide direct information concerning the subsurface conditions, geo-stratigraphy and engineering
properties prior to design, bids and construction on the ground. Among in-situ testing methods is the Standard
Penetration Test (SPT) and Cone Penetration Test (CPT). SPT, with its ease of performance and extensive
correlation with parameters used in foundation design is the most popular insitu method used in evaluating the
allowable bearing stress for foundation of engineering structures. SPT data have been used in correlations for unit
weight, relative density, angle of internal friction and unconfined compressive strength [12]. [13] studied the
reliability of shallow foundation design using SPT approach. The results of reliability analysis showed that the
factor of safety approach can provide an impression of degree of conservatism that is often unrealistic and therefore
submitted that the reliability based approach using SPT-N provides rational design criteria, accounting for all key
sources of uncertainty in the foundation engineering process and should be the basis of design. [14] showed that the
SPT provides three numbers that can be used to evaluate soil properties through an analysis to illustrate how the
incremental blow counts may be used to obtain more information from the test. [15] carried out a review of the
applicability of SPT as a subsurface evaluation tool and submitted that it can provide much of the information
required during a site investigation as compared to other field techniques. CPT is often used to complement SPT due
to it fastness, repeatability, strong theoretical background and ability to generate detailed subsoil profile. Many
investigators [16, 17, 7, 18, 19, 20, 21, 22, 23] have successfully employed CPT in subsurface investigation for
engineering purposes. This study will therefore deploy SPT and CPT methods in x-raying the shallow formations
and estimating their allowable bearing capacity. The focus area is Addo, along Badore Road in Eti Osa Local
Government Area of Lagos State; the site is designated for the construction of four – storey residential buildings.
This study therefore evaluates the geo-stratigraphy and geo-engineering properties of the shallow formations
underlying the sites with a view to determining the suitability of the site for the intended construction purpose.
MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25
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A. The Study Area
The focus location is Addo along Badore Road in Eti Osa Local Government area of Lagos State, Southwestern,
Nigeria. The site is designated for the construction of four – storey residential estate buildings. The area is located
within 6027’11.002”N/3023’44.9999”E (Fig. 1). Geologically, it falls extensively within the Eastern Dahomey Basin.
The Basin is a combination of inland/coastal/offshore sedimentary basin in the Gulf of Guinea [24].
Stratigraphically, the basin is divided into Abeokuta Formation, Ilaro Formation, Coastal Plain Sands and Recent
Alluvium sediments [25]. Deposition of Cretaceous sequence in the eastern Dahomey Basin began with the
Abeokuta Group, consisting of the Ise, Afowo and Araromi Formations [26]. The Ise Formation, the oldest,
unconformably overlies the basement complex and consists of conglomerates and sandstones at base and in turn
overlain by coarse to medium grained sands with interbedded kaolinite. Overlying the Ise Formation is the Afowo
Formation, which is composed of coarse to medium grained sandstones with variable but thick interbedded shales,
siltstones and claystone. The Araromi Formation overlies the Afowo Formation and is the youngest Cretaceous
sediment in the basin [26]. It is composed of fine to medium grained sandstone overlain by shales, siltstone with
interbedded limestone, marl and lignite.
The Ewekoro Formation, an extensive limestone body, overlies the Araromi Formation. The Ewekoro Formation is
overlain by the Akinbo Formation, which is made up of shale and clayey sequence. Overlying the Akinbo Formation
is Oshosun Formation which consists of greenish – grey or beige clay and shale with interbeds of sandstones. The
Ilaro Formation overlies conformably the Oshosun Formation and consists of massive, yellowish, poorly,
consolidated, cross-bedded sandstones. The Quaternary sequence in the eastern Dahomey basin is the Coastal Plain
Sands and recent littoral Alluvium [27] and consists of poorly sorted sands, silts and clay deposits with traces of peat
in parts. The sands are in parts crossbedded and show transitional to continental characteristics. The age is from
Oligocene to Recent. It directly underlies the study area and is composed of deposits which can be divided into the
littoral and lagoonal sediments of the coastal belt and the alluvial sediments of the major rivers. They consist
predominantly of unconsolidated sands, clays and mud with a varying proportion of vegetative matter (Fig. 2).
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Fig. 1: Map of the study area modified after [2]
Fig. 2: Map of surface geology and morphology of Lagos [7]
MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25
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II. MATERIALS AND METHODS
B. Boring and Standard Penetration Testing (SPT)
Two boreholes were drilled to depth 30.0m each with 250mm, 200mm and 150mm diameter steel casings using a
Percussion motorized Shell and Auger rig employing light cable percussion boring techniques with a fully equipped
motorized Pilcon Wayfarer drilling rig. The position of borehole 1 and 2 coincide with that of CPT P1 and CPTP3
respectively. CPT P1 to P2 is approximately 30.0m apart on a straight line such that the distance between CPT P1 and
P7 is 180.0m. During drilling operations, disturbed soil samples were regularly taken at depth interval of 0.75m and
whenever a change of stratum is observed. All samples recovered from the borehole were examined, identified and
classified in the field. They were later taken to the laboratory for detailed investigations where a total of 10 and 12
samples were subjected to grain size distribution test by wet sieving and Atterberg limit tests respectively. Effort
was made to ensure that all strata encountered were tested appropriately. Standard Penetration Test (SPT) was also
carried out at 1.5m intervals in both cohesive and cohesion less soils with disturbed samples recovered from the SPT
sampling tool. In carrying out the SPT test, a 50mm diameter split spoon sampler is driven into the soil using a
63.5kg hammer with a 760mm drop, and the penetration resistance is expressed as the number of blows (SPT-N-
value) required obtaining a 300mm penetration below an initial 150mm penetration seating drive. The SPT-N-values
were corrected for borehole and dilatancy where necessary and all pertinent borehole data, penetration resistance,
and sample data were recorded on the boring log sheet (figs 3 – 4). In boreholes, SPT results are routinely used to
provide an estimate of density, consistency, unconfined compressive strength and shear strength parameters.
C. Cone Penetration Testing (CPT)
CPT is a means of ascertaining the resistance of the soil. Seven CPT tests were carried out to a depth approximately
6.0m. The tests were performed using a 2.5-Ton nominal capacity manually powered CPT machine. Penetration
resistance (qc) and the depth of penetration were recorded at each station (table 1). Most of the test reached refusal
before the anchors pulled out of the subsurface. The layer sequences were interpreted from the variation of the
values of the cone resistance with depth. On the basis of the expected resistance contrast between the various layers,
inflection points of the penetrometer curves were interpreted as the interface between the different lithologies or
density variation. The cone penetration test is economical and supplies continuous records with depth.
D. Classification Tests
A series of classification tests were carried out on the samples in strict compliance with relevant geotechnical
engineering standards including British standards [28, 29, 30]. Laboratory classification tests were conducted on a
number of soil samples to verify and improve on the field identification. These tests include natural moisture
content, unit weights, Atterberg limits (liquid and plastic) and grain size distribution.
E. Analysis
The void ratio, poisson ratio and modulus of elasticity were evaluated from the method of [31]. The angle of internal
friction was estimated from penetration tests based on [31, 32, 33, 34] methods.
MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25
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Plasticity Index (PI) = LL – PL, where LL = liquid limit, PL = plastic limit
Liquidity Index (LI) = w – PL
PI
Where w = Natural water content, PL = plastic limit, PI = plasticity index.
Using the equation of ultimate bearing capacity for a driven pile;
Qf = qf x Ab + fs x As ……………………………………………………….(1)
where
Qf = ultimate load that can be applied at the top of the pile,
qf = ultimate bearing capacity of the stratum on which the pile is supported,
fs = the average shearing resistance of soil per unit area,
Ab = area of the pile at the base,
As = cylindrical surface area of the pile.
SPT-N method was employed using [35] equation for driven pile;
qf = 40 x N x D (limited by 400N)…………………………………………..(2)
B
fs = 2Na where
N is the SPT-N at the vicinity of the base of the pile,
Na is the average SPT-N value over the embedded depth of the pile.
Since the above equation is applicable for driven piles, the value of qf obtained was further multiplied by 0.33 while
that of fs was multiplied by 0.5 to derived corresponding values for bored piles.
Allowable bearing capacity:
qa = qu/F.S ……………………………………………………………………(3)
where F.S. is factor of safety = 2.5.
III. RESULTS AND DISCUSSION
1). Soil Stratigraphy
Results of boring and Standard Penetration Test (SPT) are presented as borehole lithologic logs (Figs 2 and 3) while
the classification tests results are summarized in Tables 2 and 3. A careful interpretation of the results revealed the
occurrence of three geotechnical layers from ground surface to about 30.0m depth which consists essentially of soft
silty sandy clay (0.0 – 5.0m), firm to stiff silty sandy clay/peat (5.0 – 17.0m) and medium dense silty sand (17.0 –
30.0m) which exemplify alluvial sediments of creek environments. The first layer is soft silty sandy clay with
thickness ranging between 4.50 – 5.0m. The second layer composed of firm to stiff sandy clay with thickness
approximately 11.50 – 12.0m sandwiched with peaty materials. The third layer is medium dense light grey silty
sand. It is pertinent to mention that only the third layer is competent as foundation material.
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2). Penetration Tests
The cone penetrometer depth probed varied from 4.75 – 6.0m with resistance values range of 2-102kg/cm2 (table 1).
The topsoil to approximately 2.50m have very low resistance of 2kg/cm2 which is undesirable as foundation material
while soil at depths approximately 2.50 – 4.25m have resistance ranging from 2 – 38kg/cm2 interpreted to be very
low foundation material since it is <40kg/m2 pointing to their probable unsuitability as foundation materials for the
proposed structure. At depths 5.0 – 6.0m, the silty sandy clay has resistance range of 56 – 102kg/m2 which typify
low to medium foundation material. The Standard Penetration Test revealed that the upper layer of silty sandy clay
has SPT-N value of 1 interpreted to be very soft and therefore unsuitable as foundation material. The middle layer of
silty sandy clay with peat intercalations has an average SPT-N value range of 7 – 12 interpreted as firm to stiff clay
which is indicative of low foundation material thereby corroboting the result of cone penetration test. The presence
of the fibrous peat intercalations within the essentially silty sandy clay deposit in the middle layer is suggestive of
very low strength characteristic due to its extremely soft nature which made collection of its undisturbed sample
impossible, hence these strata are considered unsuitable as foundation material. The lower layer of silty sand has
SPT-N value range of 17 – 33 interpreted to be medium dense pinpointing it relative suitability as foundation
material. The layer is the most competent for foundation consideration.
3). Classification Tests
The soils (table 2) are generally classified as poorly graded sand since they all have their coefficient of uniformity
less than 6 except for sample Bad2 B2 which is well graded [36, 37]. Also, with natural water content (22 -68%), the
silty sandy clay at depths approximately 0.0 – 17.0m is indicative of low to high compressibility and settlement
potential under imposed load. The plasticity index (table 3) range of 16 – 57% are indicative of low to high plasticity
of the essentially clayed material [38] and is therefore classified as low to medium foundation material. Also, the
liquidity index of the clay which ranges from 0.15 – 0.70 is quite low and is suggestive of a clay with little or no
potential for liquefaction under sudden load [39].
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TABLE 1: Cone penetration test data
Cone Readings(kg/cm2)
Depth(m) P1 P2 P3 P4 P5 P6 P7
0.25 2 2 2 2 2 2 2
0.50 2 2 2 2 2 2 2
0.75 2 2 2 2 2 2 2
1.00 2 2 2 2 2 2 2
1.25 2 2 2 2 2 2 2
1.50 2 2 2 2 2 2 2
1.75 2 2 2 2 2 2 2
2.00 2 2 2 2 2 2 2
2.25 2 2 2 2 2 2 2
2.50 2 2 2 2 2 2 2
2.75 2 10 6 2 2 2 2
3.00 2 10 4 2 4 2 2
3.25 2 10 10 8 8 2 2
3.50 2 14 15 10 12 2 2
3.75 2 20 28 14 12 2 2
4.00 2 20 30 26 16 2 2
4.25 18 26 38 34 26 22 2
4.50 80 44 50 46 34 72 36
4.75 100 60 72 56 42 84 58
5.00 - 70 100 67 56 92 72
5.25 - 80 - 84 66 - 88
5.50 - 95 - 92 75 - -
5.75 - - - 96 - - -
6.00 - - - 102 - - -
MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25
19
30.00
24.00
27.00
21.00
18.00
15.00
12.00
9.00
6.00
3.00
0.00
Depth(m) Sample N0 Legend(S.P.T)Values“N”
Strata Description Thickness(m)
1
2
3
4
5
67
8
9
1011
12
13
1415
1616
17
1819
20
21
2223
24
25
26
27
28
29
3031
32
33
34
35
36
37
38
3940
41
11.50
26
End of borehole
Soft dark grey Silty Sandy Clay
31
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
-------....---------------------------------...--------------------------------....---------------------------------.....-------
.------.-----------------.......----------------------------.....-----------------Y----------.......---------------......------------------------------.....--------------------------------------.......Yy.......................yy----------------------------------------yy...------------------
1 V
9
9
17
21
12
13.50
7
Dense light grey Silty Sand
Firm to stiff dark grey Silty Sandy Clay with tracesof Peat between 5.25m and 12.75m
1
5.0
Fig. 3: Log of borehole 1
MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25
20
30.00
24.00
27.00
21.00
18.00
15.00
12.00
9.00
6.00
3.00
0.00
Depth(m) Sample N0 Legend(S.P.T)Values“N”
Strata Description Thickness(m)
1
2
3
4
5
67
8
9
1011
12
13
1415
1616
17
1819
20
21
2223
24
25
26
27
28
29
3031
32
33
34
35
36
37
38
3940
41
12.0
25
End of borehole
Soft dark grey Silty Sandy Clay
32
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
.............
-------....---------------------------------...--------------------------------....---------------------------------.....-------
.------.-----------------.......----------------------------.....-----------------Y----------.......---------------......------------------------------.....--------------------------------------.......Yy.......................yy----------------------------------------yy...----------------------------------
1 V
10
9
17
22
12
13.50
6
Dense light grey Silty Sand
Firm to stiff dark grey Silty Sandy Clay with tracesof Peat between 5.25m and 12.75m
1
4.50
Fig. 4: Log of borehole 2
MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25
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TABLE 2: Grain size distribution results parameters
Serial
No
Sample
Designation
Depth
(m)
Effective
size (d10)
Effective
size (d30)
Effective
size (d60)
Coefficient of
Uniformity (Cu)
Coefficient of
Curvature (Cc)
Remark
1 Bad1 B1 16.50 0.18 0.28 0.40 2.22 1.09 SP
2 Bad2 B1 18.00 0.15 0.30 0.50 3.33 1.20
SP
3 Bad3 B1 21.00 0.10 0.30 0.50 5.00 1.80
SP
4 Bad4 B1 24.00 0.15 0.18 0.22 1.6 1.0 SP
5 Bad5 B1 27.00 0.15 0.20 0.50 3.3 0.5 SP
6 Bad1 B2 16.50 0.20 0.40 0.70 3.5 1.1 SP
7 Bad2 B2 17.25 0.20 0.60 1.50 7.5 1.2 SW
8 Bad3 B2 22.50 0.10 0.30 0.50 5.0 1.8 SP
9 Bad4B2 24.00 0.15 0.30 0.40 2.7 1.5 SP
10 Bad5 B2 25.50 0.15 0.25 0.40 2.7 1.0 SP
TABLE 3: Atterberg Limit Test Results
S/N Sample No Depth (m) Natural Water
Content
(%)
Liquid Limit
(%)
Plastic Limit
(%)
Plasticity
Index(%)
Liquidity Index
(%)
1 Bad6 B1 0.75 28 50 18 32 0.31
2 Bad7 B1 1.50 30 48 16 32 0.50
3 Bad8 B1 5.25 68 84 30 54 0.70
4 Bad9 B1 12.00 40 49 20 29 0.69
5 Bad10 B1 13.50 39 86 31 55 0.15
6 Bad11 B1 15.00 27 51 20 31 0.23
7 Bad6 B2 1.50 31 46 16 30 0.50
8 Bad7 B2 4.50 30 51 16 35 0.40
9 Bad8 B2 5.50 63 86 29 57 0.60
10 Bad9 B2 13.50 22 45 15 30 0.23
11 Bad10 B2 15.00 30 53 22 31 0.26
12 Bad11 B2 15.75 31 49 19 30 0.40
V. GEOTECHNICAL ENGINEERING PARAMETERS
MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25
22
The summary of geotechnical engineering parameters derived from integration of boring, penetration and laboratory
tests results is presented in table 4. The void ratio of 0.82 – 0.85 for the upper and middle silty sandy clay layer is
considered high pointing to a very poor foundation material that will be prone to high initial settlement under load.
Cohesion value of 32KN/m2 is also considered low while an angle of internal friction of 90 for the middle layer of
firm clay is attributable to the presence of silt in the essentially clayed deposit. The lower layer of dense silty sand
with low void ratio of 0.45 and relatively high angle of internal friction of 330 is considered the most competent
foundation material exemplified by it average SPT-N value of 32.
TABLE 4: Summary of Engineering Properties
Str
atu
m D
epth
Ran
ge(m
)
Th
ick
nes
s(m
Geotechnical Engineering Parameters
Typ
e of
Soi
l
Ave
rage
CP
T V
alu
e (k
g/cm
2 )
Ave
rage
SP
T V
alu
e
Voi
d R
atio
Coh
esio
n (
kN
/m2 )
Un
it W
eigh
t(k
N/m
3 )
An
gle
of I
nte
rnal
Fri
ctio
n
Est
imat
ed
mod
u
Ela
stic
ity(
kN
/m2 )
Poi
sson
Rat
io
0.0 – 5.0 5.0 Soft Silty
Sandy
Clay
7 2 0.82 - 18.50 7 3,225 0.5
5.0 – 17.0 11.50 Firm Silty
Sandy
Clay
33 8 0.85 32 19.50 9 5,344 0.5
17.0 – 30.0 8.50 Dense
Silty Sand
- 23 0.45 - 20.50 33 25,972 0.3
TABLE 5: Pile Safe Working Load
Pile Type Driven Pile Bored Pile
Diameter (mm) Founding Depth (m) Ultimate Pile
Capacity
(KN)
Safe Working Load
(KN)
Ultimate Pile
Capacity
(KN)
Safe Working Load
(KN)
300 20.0 898 359 368 147
400 20.0 1408 563 561 225
500 20.0 2024 810 790 316
600 20.0 2753 1101 1056 422
MAYFEB Journal of Civil Engineering Vol 1 (2016) - Pages 12-25
23
The relatively resistive sandy deposits is comparable to the sandy deposits reported as occurring from 35.0 – 60.0m
in several parts of southwestern Lagos by [40, 41, 42] as medium coarse grained in texture, characterized by SPT-N
values of 25 to 50 indicating its competence as foundation soils. Since pile is usually installed to at least 5 times pile
diameter into the bearing stratum, pile foundation installed at 20.0m within the dense silty sandy layer is considered
appropriate for foundation of engineering structure in the area, this is in tandem with [2, 18] which established the
presence of mechanically competent layer at 16.0m in southwestern part of the study area. Such pile installed at that
depth could generate anticipated safe working loads of 359.0kN – 1101.0kN and 147.0kN – 422.0kN under driven
and bored piles respectively at assumed diameters of 300.0 – 600.0mm (table 5).
VI. CONCLUSIONS
Penetration and geotechnical laboratory tests has revealed that the study area is underlain predominantly by soft silty
sandy clay (0.0 – 5.0m) at the upper layer underlain by firm to stiff silty sandy clay layer (5.0 – 17.0m) with peat
intercalations at 5.25 and 12.75m. Beneath these layers is medium dense silty sandy layer (17.0 – 30.0m) which is
considered the most competent foundation layers. From engineering assessment, the upper and middle silty sandy
clay layers with poor geotechnical properties have the potential to settle appreciably under foundation loads and are
therefore considered inappropriate for support of the proposed structure. The preponderance of soil with poor
geotechnical properties at the shallow foundation zones precludes the adoption of shallow foundation in the area.
Hence, pile installed to at least 20.0m within the medium dense silty sandy layer is recommended for the proposed
structure.
ACKNOWLEDGMENTS
The authors are grateful to Optimal Gems Resources Limited, Ikeja for providing equipment for the fieldwork.
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