Subsurface Geotechnical Competence Evaluation Using Geoelectric Sounding and Direct Cone Penetrometer Test at Plural Garden Estate, Ilaramokin Southwestern Nigeria

Abstract

In order to evaluate the geotechnical competence of the subsurface soil materials at Plural Garden Estate, Ilaramokin Southwestern Nigeria, geotechnical investigations involving geoelectric sounding and Direct Cone Penetrometer Test (DCPT) was carried out in the estate. A total of 27 VES points and 8 DCPT points were occupied across the study area. A, H, K, Q and KH are the five sounding curve types delineated in the area. Resistivity values of the top soil, weathered layer, fractured layer and fresh bedrock vary from 65-864, 156-1698, 28-217, 433-12167 ohm-m respectively, while their thicknesses vary from 0.7-3.7, 2.4-10.5 and 6.3-40.1 m in the upper three layers respectively. The geoelectric sounding results were presented as depth slices at depths of 0.5, 0.75, 1.0 and 2.0 m competency maps. Larger part (70 to 80 %) of the surfaces (1.0 and 2.0 m) considered in the study area are characterized as moderate to high competent. The depth slice iso-resistivity maps indicated that geotechnical competence increases with depth within the shallow depths considered (0.5, 0.75, 1.0 and 2.0 m). Geotechnical test involving DCPT were done at common depth of 1.0 m to validate the 1.0 m competency map. The DCPT agreed with the geoelectrical derived 1.0 m depth slice competence map. Some zones suspected to be very low and low competence were revealed to be competent based on DCPT suggesting that the low resistivity may be due to the presence of non-plastic clay and moisture.

Full text

Indonesian Journal of Earth Sciences, Vol. 03, No. 2 (2023), 618
1
P-ISSN: 2798-1134 | E-ISSN: 2797-3549 | DOI: 10.52562/injoes.2023.618
Indonesian Journal of Earth Sciences
https://journal.moripublishing.com/index.php/injoes
| Research Article |
Subsurface Geotechnical Competence Evaluation Using Geoelectric Sounding
and Direct Cone Penetrometer Test at Plural Garden Estate, Ilaramokin
Southwestern Nigeria
Igbagbo Adedotun Adeyemo1, Andrew Ifeoluwa Afolayan1, Bisola Stella Boluwade1,
Samuel Kayode Alabi2
1Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria
2Department of Applied Geology, Federal University of Technology, Akure, Nigeria
*Correspondence: iaadeyemo@futa.eu.ng
Received: 21 April 2023 / Accepted: 5 September 2023 / Published: 21 September 2023
Abstract: In order to evaluate the geotechnical competence of the subsurface soil materials at Plural Garden Estate,
Ilaramokin Southwestern Nigeria, geotechnical investigations involving geoelectric sounding and Direct Cone
Penetrometer Test (DCPT) was carried out in the estate. A total of 27 VES points and 8 DCPT points were occupied
across the study area. A, H, K, Q and KH are the five sounding curve types delineated in the area. Resistivity values
of the top soil, weathered layer, fractured layer and fresh bedrock vary from 65-864, 156-1698, 28-217, 433-12167
ohm-m respectively, while their thicknesses vary from 0.7-3.7, 2.4-10.5 and 6.3-40.1 m in the upper three layers
respectively. The geoelectric sounding results were presented as depth slices at depths of 0.5, 0.75, 1.0 and 2.0 m
competency maps. Larger part (70 to 80 %) of the surfaces (1.0 and 2.0 m) considered in the study area are
characterized as moderate to high competent. The depth slice iso-resistivity maps indicated that geotechnical
competence increases with depth within the shallow depths considered (0.5, 0.75, 1.0 and 2.0 m). Geotechnical test
involving DCPT were done at common depth of 1.0 m to validate the 1.0 m competency map. The DCPT agreed
with the geoelectrical derived 1.0 m depth slice competence map. Some zones suspected to be very low and low
competence were revealed to be competent based on DCPT suggesting that the low resistivity may be due to the
presence of non-plastic clay and moisture.
Keywords: Subsurface; geotechnical competence; geoelectric sounding; Direct Cone Penetrometer
INTRODUCTION
Thorough investigations of the subsurface’s lithology and structures are very essential in
evaluating geotechnical competence (Coker, 2015a; Olayanju et al., 2017). Poor geotechnical
competence arising from presence of geologic structures such as fractures, joints, faults, cavities and
sinkholes are responsible for collapse of many engineering structures (Adeyemo & Omosuyi, 2012;
Adelusi et al., 2013; Adeyemo et al., 2014; Longoni et al., 2016; Ademila et al., 2020; Airen, 2021).
Presence of expansive clay minerals (such as illite, chlorite, montmorillonite, halloysite among others) in
the subsoil materials can results in differential settlement in the subsoil materials leading to foundation
failure (Egwuonwu et al., 2011; Das & Roy, 2014; Okoro et al., 2014). Other geologic conditions capable
of precipitating foundation problems are shallow depth to bedrock, poor soil strength and shallow static
water level (Ugwu & Ezema, 2013; Adeyemo et al., 2020).
Many building collapses in Nigeria have been linked to absence of pre-foundation studies. Most
of these buildings were built on sub-soils materials with inadequate bearing capacity to support the
weight of such building. The necessity of site characterization for construction purposes is very important
so as to prevent loss of valuable lives and properties that always accompany such collapse. Some non-
geological reasons why buildings may be susceptible to collapse have been advanced, which include
poor quality of building materials, non-adherence to standard practice, salinity, poor maintenance
practices, faulty design of foundation and aging of buildings (Oyedele et al., 2011; Oseghale et al., 2015;
Oyediran & Famakinwa, 2015; Pegah & Liu, 2016).
Geophysical methods are found to be very useful and reliable in assessing the suitability of an
area for the construction of an engineering structure such as buildings, roads, bridges, dams among others

Adeyemo et al. (2023)
Indonesian Journal of Earth Sciences
2
(Olayanju et al., 2017; Adeyemo et al., 2020; Pegah & Liu, 2016). Geophysical methods are suitable for
determining the depth to the bedrock, detecting the presence of bedrock structures (voids, joints,
fractures and faults), delineating presence of expansive clay minerals and other potentially dangerous
subsurface conditions before the erection of any engineering structure (Soupious et al., 2007; Ademila,
2015). For better assessment of geologic materials, geotechnical methods are combined with geophysical
investigation to assess the competence of the subsoil materials to avoid foundation failures (Owoyemi &
Awojobi, 2016; Alawode et al., 2020). Combining the two methods helps to provide control and
ground-truth information of the subsurface (Olatinsu et al., 2018). Geophysical and geotechnical methods
complement each other in site investigation (Adejumo et al., 2016; Coker, 2015b; Adiat et al., 2017;
Oladunjoye et al., 2017; Bayode & Egbebi, 2020).
In order to ensure stability of building foundations and to prolong the life span of the buildings
in the estate, proper geotechnical assessment of the proposed site is considered imperative. The research
hypothesis of this study was hinged on the fact that geotechnical investigation such as DCP test are
conventionally recognized as tools for assessing subsoil geotechnical competence, while geophysical
investigation is gradually becoming popular in subsurface geotechnical competence evaluation.
Deploying DCP test as a follow up to geoelectric sounding survey will enhance the degree of confidence
of subsoil competence evaluation and if good correlation is established between the two methods, then
geoelectric sounding methods can be relied upon for geotechnical competence evaluation in similar
geologic terrain.
Description of the study area
Ilaramokin is a fast-growing town, near Akure Southwestern, Nigeria. There is a need for an
adequate housing development that can cater for the increasing population and standard of living in the
community. Housing development improves the quality of life of residents leading to better health, jobs
creation, security and population diversity. The desire to provide adequate and befitting accommodation
for the growing population of the town led to the creation and development of the satellite estate, Plural
Garden estate along Ilesha-Akure highway.
Plural Garden Estate, Ilaramokin is located along Ilesha-Akure highway, Southwestern Nigeria.
The estate is situated about 5 kilometers west of Akure metropolis (Figure 1). The study area is located
within 732110-732189 mE (Easting) and 812025-811776 mN (Northing) of the Universal Transverse
Mercator (UTM). Ilaramokin is bounded at the north by Ikota and Ijare towns, at the east by Ipinsa and
Akure, at the south by Ipogun and Ibule towns and at the west by Igbaraoke and Ero. All these adjoining
towns are connected by the major Akure-Ilesa highway and many other minor roads.
Figure 1. Location maps of the study area; (a) Nigeria layout map (b) Basemap of Plural Estate,
Ilaramokin

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Indonesian Journal of Earth Sciences
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Plural Garden estate is situated on moderately undulating terrain with surface elevation varying
from 345 to 351 m above mean sea level (Figure 2). Ilaramokin is underlain by rocks of the Precambrian
Basement Complex of Southwestern Nigeria. The lithological units observed in the area include variably
migmatized biotite-hornblende gneiss with intercalated amphibolite. Low lying outcrops of migmatite-
gneiss complex are situated in the town while boulders of amphibolite/charnockite rocks are located in
the central and north central areas of the town. The area falls within the humid tropical climatic zone
which is characterize by two seasons. A typical wet season extends from April to October, while the dry
season extends from November to March. Annual rainfall ranges between 100 and 1500 mm, with
average wet days of about 100. Annual temperature varies between 180 and 340 °C (Iloeje, 1980).
Figure 2. Elevation map of the study area
MATERIALS & METHODS
A combination of vertical electrical sounding (VES) and dynamic cone penetrometer tests were
utilized in this work. The choice of VES technique is based on its high resolution and non-intrusive nature.
The VES technique allows for delineation of shallow to medium subsurface layers, their resistivity and
thickness values (Keller & Frischknecht, 1966; Koefeod, 1979). Layer resistivity value have been used to
infer subsurface layer lithology and geotechnical competence (Idornigie & Olorunfemi, 2006; Adiat et
al., 2017; Oladunjoye et al., 2017; Bayode & Egbebi, 2020).
The dynamic cone penetrometer test is a direct, speedy and non-intrusive means of evaluating
the subsurface layer load bearing capacity and competence. The geotechnical investigation such as DCP
test are conventionally recognized tools for assessing subsoil geotechnical competence, while geophysical
investigation methods are gradually becoming popular in subsurface geotechnical competence
evaluation. In this study, DCP test was deployed as a follow up to geoelectric sounding survey at Plural
Garden Estate, Ilaramokin Southwestern Nigeria in order to enhance the degree of confidence of subsoil
competence evaluation in the study area.
Geophysical method (Vertical Electrical Sounding)
Vertical electrical sounding (VES) using Schlumberger configuration was adopted for the
geoelectric sounding survey. Twenty-seven (27) VES positions were occupied across the study area (Figure
3). Resistance values were read off the resistivity meter and subsequently apparent resistivity values ()
are calculated using equation for Schlumberger configuration below.
Thus,   󰇣
 󰇤 (1)
When L>>>
l
i.e. 
Such that,

Adeyemo et al. (2023)
Indonesian Journal of Earth Sciences
4

  (2)
Equation 2 can also be written as;
  
 
 (3)
The apparent resistivity () values were subsequently plotted on a bi-log paper as VES curves
and then interpreted using conventional manual curve matching technique (Keller & Frischknecht, 1966;
Koefeod, 1979). The interpreted results were iterated using window Resist, a 1-D forward modelling
software (Vander Velpen, 2004). The VES results were used to generate different iso-resistivity maps of
the study area.
Figure 3. Map of study area showing VES points
Geotechnical method (Dynamic Cone Penetrometer tests)
The Dynamic Cone Penetrometer tests (Figure 4) were conducted at common depth of 1.0 m
which was guided by Vertical electrical sounding (VES) results. The Dynamic Cone Penetrometer consists
of a hammer, guide, anvil, driving rod, and cone tip. The dynamic impact for the penetration was
performed by dropping an 8 kg hammer from a free-falling height of 575 mm. The impact energy was
transferred through the driving rod with a diameter of 16 mm, and the energy transferred at the cone tip
with a diameter of 20 mm leads to the cone penetration into the subgrade. For each dynamic impact,
the DCPI is measured, which is used for continuous subgrade profiling. The DCPI can be simply obtained
using the following equation.
DCPI [mm/blow] = Dn+1- Dn (1) (4)
Where, Dn is the penetration depth of the DCP at a blow count of n. The DCPI is the only
obtainable strength index from the DCP test, and it depends on the energy transferred at the cone tip
that might reduce the reliability of the DCP results. Once the test apparatus is assembled the DCP is placed
at the test location and the initial penetration of the rod is recorded to provide a zeroing scale. While
holding the rod vertically, the weight is raised to the top of the rod 575 mm above the anvil and
dropped.

Adeyemo et al. (2023)
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5
Figure 4. Map of study area showing DCPT points
RESULTS AND DISCUSSION
Vertical Electrical Sounding Results
The Vertical Electrical Sounding (VES) results delineated three to four geoelectric layers across the
study area namely the topsoil, weathered layer, partially weathered/partially fractured basement and
presumed fresh basement. The resistivity of the top soil, weathered layer, partially weathered/partially
fractured basement and presumed fresh basement varies from 65-864, 156-1698, 28-217 and 433-12167
ohm-m respectively, while their thicknesses vary from 0.7-3.7, 2.4-10.5 and 6.3-40.1 m in the top soil,
weathered layer, partially weathered/partially fractured basement respectively. The A, H, K, Q and KH
are the five sounding curve types delineated across the area (Table 1). The KH curve is the predominant
curve type in the area with percentage of occurrence of 45%, the A curve type has 25% occurrence, the
H curve has 20% occurrence, while the K and Q curves are the least with 5% occurrence each (Table 1).
The VES results were presented as depth slice maps at different depth surfaces (0.5, 0.75, 1.0, 1.5 and 2.0
m) classified into different geotechnical competence zones (Table 2) according to Idornigie & Olorunfemi
(2006).
The 0.5 m depth slice iso-resistivity map of Ilaramokin (Figure 5) indicated that about 75% of
the estate can only be considered as moderately competent (100-350 Ωm) and highly competent (above
750 Ωm) areas, while about 25% of the area, the southwestern and southeastern parts of the area are
considered to be incompetent area (0-100 Ωm).
The 0.75 m depth slice Iso-resistivity map (Figure 6) reveals that at this depth surface, about
65% of the estate, the center region can be considered to be moderately competent (100-350 Ωm), while
about 20% of the area, the northeastern parts are considered to be competent (350-750 Ωm) and highly
competent (above 750 Ωm) areas. Some parts of the eastern and south western areas are incompetent
zones.
The 1.0 m depth slice Iso-resistivity map (Figure 7) indicates that at this depth surface, about
75% of the area can be considered to be moderately competent (100-350 Ωm), while about 25% of the
area, the northeastern parts are considered to be competent (350-750 Ωm) and highly competent
(above750 Ωm) areas. It is interesting to note that there is an increase in the area considered to be
competent at this surface; this is probably due to the shallow depth to fresh basement rocks in this area.
The 2.0 m depth slice iso-resistivity map (Figure 8) indicates that about 80% of the estate can
only be considered as moderately competent (100-300 Ωm), while about 15% of the area, the north
eastern part of the area is considered to be competent zones (400-750 Ωm) and highly competent (above
750 Ωm) zones. A small portion of the estate at the south western part is incompetent.

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Table 1. Summary of the VES Results
VES No
Easting
Northing
Resistivity
ρ1/ρ2/………/ ρn (Ωm)
Thickness
h1/h2/……./hn (m)
Curve Type
1.
732107
812029
90/780/168/5505
0.7/6.3/31.7
KH
2.
732090
812023
131/1081/217/1480
0.6/2.4/28.8
KH
3.
732068
812028
94/393/84/4432
1.0/7.3/17
KH
4.
732048
812014
89/413/104/1454
1.1/5.7/21.5
KH
5.
732030
812009
71/580/52/5924
0.9/4.2/14.4
KH
6.
732024
812009
78/312/28
1.0/10.5/6.3
KH
7.
732020
812029
80/980/68/10045
0.7/4.1/14.2
KH
8.
732016
812046
119/908/94/2185
0.9/6.1/23.2
KH
9.
732014
812068
864/1698/264
3.7/0.8
Q
10.
732012
812109
282/513/173/324
0.7/6.6/38.0
KH
11.
731956
812084
294/471/191/925
1.0/7.4/28.7
KH
12.
731904
812069
131/43/2652
5.2/17.9
H
13.
731904
812046
66/99/1800
0.8/40.5
A
14.
731710
812025
112/63/1490
1.9/15.1
H
15.
731719
811988
104/60/3231
1.2/16.7
H
16.
731727
811942
114/53/522
2.4/10.1
H
17.
731735
811902
65/39/390
1.0/7.3
H
18.
731830
811927
92/217/105/786
0.9/3.5/7.8
KH
19.
731881
811940
162/205/361
1.0/14.2
A
20.
731933
811951
88/343/146/534
1.0/5.8
KH
21.
731981
811961
105/385/141/433
1.2/4.7/6.5
KH
22.
732025
811973
108/184/7
2.4/17.5
K
23.
731826
811970
76/266/2174
0.8/26.1
KH
24.
731949
811998
155413/180/12167
2.3/8.3/40.1
AH
25.
731936
812040
99/157/421
1.2/15.1
A
26.
731868
812022
310/156/1086
1.7/19.9
H
27.
731820
812011
96/285/82/1253
0.7/2.9/13.1
KH
Table 2. Soil Competence Rating (After Idornigie and Olorunfemi, 2006)
Resistivity (Ωm)
Possible Lithology
Competence Rating
<100
Clay
Incompetent
100-350
Sandy clay
Moderately Competent
350-750
Clayey sand
Competent
>750
Sand/Laterite/Bedrock
Highly competent
The depth slice iso-resistivity maps (Figures 5-8) indicated that within the shallow depths
considered (0.5, 0.75, 1.0 and 2.0 m) in the study area geotechnical competence increases with depth.
Figure 5. 0.5 m depth slice competency map

Adeyemo et al. (2023)
Indonesian Journal of Earth Sciences
7
Figure 6. 0.75 m depth slice competency map
Figure 7. 1.0 m depth slice competency map
Figure 8. 2.0 m depth slice competency map
Geotechnical Test Results
Dynamic Cone Penetration Tests (DCPT) can assist in determining the near subsurface soil
materials bearing capacity. Soil type can also be inferred from DCPT values (Tables 3 and 4). DCPT was
carried out at 8 different locations in the study area using the Dutch type (DIN) apparatus. The DCPT
were conducted to determine the bearing capacity of the near subsurface soil materials with depth up to

Adeyemo et al. (2023)
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8
the maximum depth of 1.0 m. The tests were taken at VES points 8, 9, 10, 14, 17, 25 and 27 to validate
the results from the electrical resistivity data at 0.5m and 0.75 m (Figures 5 and 6). From the result, the
bearing capacity values at 0.5 m (Figure 9) varies from 70-326 (kN/m2). This correlates well with the
geophysical data that 55% of the estate can only be considered as moderately competent (100-300 Ωm).
Table 3. Non-Cohesive Soils Rating with bearing capacity (Bearing Values BS: 8004)
Bearing Capacity (KN/m2)
Soil
<100
Loose Sand
100-350
Medium Dense Sand
>300
Compact sand
>200
Loose gravel or sand
200-600
Medium dense sand
>600
Dense Sand/Gravel
Table 4. Cohesive Soils Rating with bearing capacity (Bearing Values BS: 8004)
Bearing Capacity (KN/m2)
Soil
<75
Soft Clays and Silts
75-150
Firm Clay
150-300
Stiff Clays
300-600
Hard Clays
200-600
Medium dense sand
>600
Dense Sand/Gravel
Figure 9. Bearing capacity result at VES 8 (left) and VES 9 (right)
Zones indicating high competency in the geophysical result at this depth was validated by DCPT
to have high bearing capacity. From the result, the bearing capacity values at 0.75 m (Figure 9) varies
from 95-186 (kN/m2). This correlates with the geophysical data that 65% of the estate can only be
considered as moderately competent. However, the DCPT could not get to the depth of 1.0 m since the
points of refusal were always reached before 1.0 m depth, and this shows that the estate is geotechnically
PROJECT: DATE: 26/08/2021
LOCATION: TEST No. VES 8
SECTION No. DCP ZERO READING 52mm
DIRECTION TEST STARTED AT
83
140
185
220
270
315
357
401
454
512
558
608
667
755
915
965
DCPT INVESTIGATION OUTPUT
FEDERAL UNIVERSITY OF TECHNOLOGY AKURE
DEPARTMENT OF APPLIED GEOLOGY
ENGINEERING GEOLOGY LABORATORY
0.00
0.25
050 100 150 200 250 300 350 400 450 500
Depth (m)
Bearing Capacity, kN/m2
PROJECT: DATE: 26/08/2021
LOCATION: TEST No. VES 9
SECTION No. DCP ZERO READING 55mm
DIRECTION TEST STARTED AT
83
140
185
220
270
315
357
401
454
512
558
608
667
755
915
965
DCPT INVESTIGATION OUTPUT
FEDERAL UNIVERSITY OF TECHNOLOGY AKURE
DEPARTMENT OF APPLIED GEOLOGY
ENGINEERING GEOLOGY LABORATORY
0.00
0.25
0.50
0 100 200 300 400 500 600 700
Depth (m)
Bearing Capacity, kN/m2

Adeyemo et al. (2023)
Indonesian Journal of Earth Sciences
9
competent at this depth slice. The higher the resistivity values the higher the bearing capacity of the soil
(Tables 5 and 6). Idornigie & Olorunfemi (2006) revealed that high resistivity depicts competent geologic
materials, such as sand or clayey sand formation, while very low resistivity suggests clay or sandy clay
materials (often less competent to support the stability of heavy engineering structures).
Table 5. Comparison apparent resistivity with bearing capacity at 0.5m
Table 6. Comparison apparent resistivity with bearing capacity at 0.75m
CONCLUSION
In order to evaluate the geotechnical competence of the subsurface soil materials at Plural Garden
Estate, Ilaramokin Southwestern Nigeria, geotechnical investigations involving geoelectric sounding and
Direct Cone Penetrometer Test (DCPT) was carried out in the estate. A total of 27 VES points and 8
DCPT were occupied across the area. Five sounding curve types (A, H, K, Q and KH) were the delineated
in the area. The KH curve is the predominant curve type in the area with percentage of occurrence of
45%, the A curve has 25% occurrence, the H curve has 20% occurrence, while K and Q curves are the
least with 5% occurrence each. The resistivity of the top soil, weathered layer, fractured layer and fresh
bedrock varies from 65-864, 156-1698, 28-217, 433-12167 ohm-m respectively, while their thicknesses
vary from 0.7-3.7, 2.4-10.5 and 6.3-40.1 m in the three upper layers respectively. The geoelectric
sounding results were presented as depth slices at depths of 0.5, 0.75, 1.0 and 2 m competency maps.
The depth slice iso-resistivity maps indicated that geotechnical competence increases with depth within
the shallow depth slices (0.5, 0.75, 1.0 and 2.0 m) considered in the study area,
Geotechnical test involving DCPT were done at common depth of 1.0 m to validate the 1.0 m
iso-resistivity depth slice competence map. DCPT characterized the soil into bearing capacity and depth
relation; the top 1 m indicated a bearing capacity between 100-600 kPa and is classified as clayey sands
with a mixture of silt and gravel overlaying 160-250 kPa probably consisting of non-cohesive sand. The
DCPT agreed with 1.0 m iso-resistivity depth slice map, however zones suspected to be very low and
low competence were revealed to be competent based on DCPT suggesting that the low resistivity may
be due to the presence of non-plastic clay and moisture content.
This study established the existence of high degree correlation between the two methods
adopted for this work. This study has revealed that VES and DCPT are complementary and thus
geoelectric sounding can be utilize successfully in assessing geotechnical competence in the absence of the
conventional geotechnical methods.
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Indonesian Journal of Earth Sciences
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