International Journal of Geosciences, 2013, 4, 1256-1266
Published Online November 2013 (http://www.scirp.org/journal/ijg)
http://dx.doi.org/10.4236/ijg.2013.49120
Open Access IJG
Application of Geophysical Methods to Building
Foundation Studies
Folahan Peter Ibitoye1,2, Felix Vincent Ipinmoroti2, Mudasiru Salami2, Kunle Joseph Akinluwade2,3*,
Adeyinka Taofeek Taiwo2, Adelana Rasaki Adetunji2,4
1Department of Applied Geophysics, Federal University of Technology, Akure, Nigeria
2Prototype Engineering Development Institute, National Agency for Science and
Engineering Infrastructure (NASENI), Ilesa, Nigeria
3Department of Materials Science and Engineering, African University of Science and Technology (AUST), Abuja, Nigeria
4Department of Materials Science and Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria
Email: *jakinluwade@yahoo.com, folaibitoye@yahoo.com
Received August 16, 2013; revised September 19, 2013; accepted October 21, 2013
Copyright © 2013 Folahan Peter Ibitoye et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
A geophysical survey involving the electrical resistivity method utilizing the Vertical Electrical Sounding (VES) and Electri-
cal Imaging Techniques was conducted around the premises of an area within south-western Nigeria with the aim of
studying structural defects which may be responsible for future problems and characterizing the soil conditions of the
site. A total of 15 VES stations were occupied using Schlumberger Configuration with AB/2 varying from 1 to 65 m. In
the electrical imaging, dipole-dipole array was adopted and the two traverses were occupied in the S-N and E-W direc-
tions close to where wall cracks and subsurface problems were manifested. Five main geoelectric sequences were deline-
ated within the study area; these include the topsoil (clay and sandy clay), lateritic clay, weathered bedrock (clay, sandy
clay and clayey sand), fractured bedrock and fresh basement. A major discontinuity (fracture zone) was discovered along
the S-N direction, while a weak zone was also discovered along E-W direction. The result of this research has shown that
the causes of the cracks and distress on the walls within the site may have been influenced by the differential settlement re-
sulting from the incompetent subsoil materials and the fractured bedrock on which the foundation of the building was laid.
Keywords: Foundation; Bedrock; Basement; Cracks; Subsurface Instability
1. Introduction
With the growing demand for site development and un-
pleasant experience of building failure, there is increas-
ing number of necessary site investigations to reveal pos-
sible subsurface problems. Therefore, geophysical inves-
tigations are important in evaluating the physical proper-
ties of the subsurface in terms of its soil type, soil com-
petence, soil corrosivity, depth to bedrock and lithologic
sequence.
Site engineers, for reasons of cost and other considera-
tions such as assumptions in structural design, sometimes
fail to incorporate pre-construction investigations in their
job schedule. A geophysical investigation is therefore ne-
cessary for the site to reveal possible future subsurface
problems and proffer possible solutions before the erec-
tion of buildings.
2. Study Area
The Study Area, shown in Figure 1, is geographically
enclosed within latitude 7˚36'95" N to 7˚37'55" N and
longitude 4˚42'00" E to 4˚42'90" E, south-western, Ni
geria. The climate is humid tropical type with a mean
annual temperature of about 280˚C and a mean annual
rainfall of about 1600 mm [1]. Periods of high tempera-
tures are recorded annually; the first period occurs in
March-April and the second period in November-De-
cember, the coolest period is observed in the middle of
the raining season [2]. Within the Southern Ilesa area,
Nigeria; the schists are the most predominant rock type.
They are medium-grained with abundant biotite and
strongly foliated. Lenses of granular quartz are present
[3]. The quartzite and quartz mica schist are probably
younger than the gneisses and schists. Several belts of
schists occur around the study area and range from mas-
ive, granular rocks to glassy schistose varieties. The s
*Corresponding author.
F. P. IBITOYE ET AL. 1257
Figure 1. Sketch map of part of Ilesa showing the study area.
quarzite and quartz schists are resistant to weathering and
therefore occur as distinct ridges with very steep slopes,
but are highly fractured and jointed. Figure 2 is a geo-
logical map within the area [4,5]. The study area lies
within the region of undifferentiated gneiss and magma-
tite as shown on the geological map.
Two traverses were established within the premises of
the study area, which runs S-N and E-W respectively
(Figure 3). Eight Vertical Electrical Soundings (VES)
were occupied along the traverse that runs S-N, while
seven Vertical Electrical Soundings (VES) were occu-
pied along the traverse that runs E-W, the traverse
lengths are 75 m and 100 m respectively. The locations
of the VES were constrained by the manifestation of
failure at the investigated site. A total of fifteen (15) Ver-
tical Electrical Soundings (VES) with electrode separa-
tions (AB/2) ranging from 1 to 65 m were conducted
within the study area using DDR-2 resistivity meter. The
location of each of the sounding station was recorded
with the aid of ETRA F-10 (GPS) unit. The apparent
resistivity measurement at each station was plotted on
bi-logarithmic graph sheets. The curves were inspected
visually to determine the number and nature of the lay-
ering. Partial curve matching was carried out for the
quantitative interpretation of the curves. The results of
the curve matching (layer resistivities and thicknesses)
were fed into the computer as a starting model in an it-
erative forward modelling technique 1-D inversion pro-
gram [6]. From the interpretation results (layer resistivi-
ties and thicknesses), two geoelectric sections along E-W
and S-N directions and a histogram were produced. For
the combined horizontal profiling as sounding technique,
the same traverses where VES were carried out, were
also used. The Dipole-Dipole array was used for the data
acquisition. The inter-electrode spacing (a) of 5 m was
adopted while inter-dipole separation factor (n) was var-
ied from 1 - 5. The apparent resistivity values were
calculated using πn(n + 1)(n + 2)a as the geometric factor.
2-D inversion modeling of the Dipole-Dipole data was
carried out using DIPROTM Software developed by the
Korea Institute of Geoscience and Mineral Resources [7].
3. Discussion of Results
The results of this research work are presented as field
curves, histogram, geoelectric sections, pseudosections
and 2-D inversion models.
3.1. Field Curves
The interpretation of the sounding curves shows that se-
ven curve types exist, viz: HA, KH, AA, HKH, QH, HK
and KQ. The number of layers varies between 4 and 5.
KH curve type is the predominant curve type (Figure 4),
constituting 46.66% of the total, HA and AA consti-
tute 13.33%, HKH, QH, HK and KQ constitute 6.67%.
ome of the typical curve types in the area are shown in S
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F. P. IBITOYE ET AL.
1258
Figure 2. Geological map of southern part of Ilesa [4,5].
4˚42’00’’ 4˚42’10’’ 4˚42’20’’ 4˚42’30’’ 4˚42’40 ’’ 4˚42’50’’ 4˚42’60’’4˚42’70’’ 4˚42’80’’ 4˚42’90 ’’
7˚36’95’’
7˚37’00’’
7˚37’05’’
7˚37’10’’
7˚37’15’’
7˚37’20’’
7˚37’25’’
7˚37’30’’
7˚37’35’’
7˚37’40’’
7˚37’45’’
7˚37’50’’
7˚37’55’
3
4
5
4
5
6
Generator
Solar Panel Unit
Administrative/
Foundry Building
PROPOSED
CAR PARK
ROAD PATHS
TRAVERSE
FENCE
LEGEND
Building
FENC
E
ILES
A
-IFE ROAD
LOCAL
BRIDG
E
SEEPAGE
5 VES LOCATIONS
WEAK ZONE
STREAM
6
7 21
3
7
8
2
1
EXISTING BOREHOLE POINT
VEGETATION
020
40
INDUSTRIA L AREA
Figure 3. Geophysical data acquisition map of the site.
Figures 5(a)-(c). The implicatio
e predominant curve is that the underlying bedrock in
The geoelectric section along E-W direction (Figure 6)
/geologic subsurface layers com-
prising the topsoil, lateritic clay, weathered bedrock, par-
tially weathered bedrock, fractured bedrock and fresh
/sandy clay/clayey
n of KH curve type as identified six geoelectric
th
the study area is characterised by confined fractures.
3.2. Geoelectric Characteristics basement. The compositions are clay
sand topsoil (resistivity varies from 51 to 488 ohm-m and
thickness ranges from 0.42 - 2.69 m), lateritic clay
Open Access IJG
F. P. IBITOYE ET AL. 1259
Figure 4. Histogram of the VES curve types of the study area.
(a)
Open Access IJG
F. P. IBITOYE ET AL.
1260
(b)
(c)
Figure 5. (a) Typical HA sounding curve; (b) Typical KH sounding curve ; (c ) Typical HKH sounding curve.
Open Access IJG
F. P. IBITOYE ET AL.
Open Access IJG
1261
V
ES 1
V
ES 2
V
ES 3
201
273
114
105
218
378
E
64
220 1808
0510 15
10
(m)
V
.E = 0.5
V
ES 4
V
ES 5
V
ES 6
V
ES 7
274
10
20
30
966 127
334
1221
1385 1246
1018
1372
937
1128
106
240
109
488
116 146
TOPSOIL
WEATHERED BEDROCK
FRACTURED BEDROCK
LEGEND
FRESH BASEMENT
LAT ERITIC CLAY
20
869
W
51
977
626
PARTIALLY WEATHERED BEDROCK
40
0
DEPTH (M)
Figure 6. Geoelectric section along E-W direction.
(resistivity varies from 966 to 977 ohm-m and thickness
ranges from 4.10 - 5.24 m) localized at VES 2 and VES 7,
clay/sandy clay/clayey sand weathered bedrock (resistiv-
ity varies from 64 to 334 ohm-m and thickness ranges
from 6.77 - 29.08 m), partially weathered bedrock (resis-
tivity of 626 ohm-m and thickness of 30.62 m) beneath
VES 6, fractured bedrock (resistivity of 869 ohm-m) be-
neath VES 4 which is a confined fracture and fresh base-
ment (resistivity varies from 937 ohm-m to 1385 ohm-m).
The depth to rock head ranges from 7.19 m to 36.17 m.
The overburden is generally thick but thinnest at VES 4
(7.19 m) and thickest at VES 6 (36.17 m) at the western
flank, both along East-West direction. The basement re-
lief is undulating; basement depression is noticed at VES
3 (E-W) and VES 6 (E-W).
The geoelectric section along S-N direction (Figure 7)
identified five geoelectric/geologic subsurface layers
comprising the topsoil, lateritic clay, weathered bedrock,
partially weathered bedrock and fresh basement. The
compositions are sandy clay/clayey sand topsoil (resis-
tivity varies from 120 to 246 ohm-m and thickness
ranges from 0.75 - 3.52 m); lateritic clay (resistivity var-
ies from 664 ohm-m to 3205 ohm-m and thickness
ranges from 2.93 - 16.16 m); weathered bedrock (resis-
tivity varies from 91 ohm-m to 389 ohm-m), partially
weathered bedrock (resistivity of 423 ohm-m) localized
at VES 5 showing that it extends beyond the depth of
study (60 m) and fresh basement (resistivity of 3937
ohm-m).
The r
generally indicate five main geoelectric layers; namely
the topsoil (resistivity varies from 51 to 488 ohm-m and
thickness ranges from 0.42 - 3.52 m), lateritic clay (664
to 3205 ohm-m and thickness ranges from 2.93 - 16.16
m), weathered bedrock (resistivity varies from 64 ohm-m
to 393 ohm-m), fractured bedrock (resistivity of 869
ohm-m) and fresh basement (resistivity varies from 937
ohm-m to 3937 ohm-m). The topsoil generally varies in
composition from clay to sandy clay, but predominantly
composed of sandy clay. The fracture zones are generally
confined and extend beyond the depth of study.
3.3. Combined Horizontal Profiling (HP) and
Vertical Electrical Sounding (VES)
The Dipole-dipole psudosection and the 2D resistivity
structure along E-W direction are shown in Figure 8.
The 2D resistivity structure revealed four geoelectric
layers marked by A, B, C and E separated by geologic
boundaries; namely topsoil marked by A (generally green
colour except at few points with yellow and red colour);
weathered bedrock marked by B (generally green with
few yellow colour); lateritic clay marked by C (yellow
and red colour) and fresh basement marked E (red and
purple colour). The topsoil is generally thin and subsume
into the weathered bedrock in many places due to its
characterized by lateritic clay between stations 15 and 16
and sandy clay between stations 13 and 15, but gely
composed of clay at the other stations. The weathered
turated
neral
esults of the field curves and geoelectric sections bedrock (B) is characterized by two potentially sa
F. P. IBITOYE ET AL.
1262
V
ES 1
V
ES 2
V
ES 3
215
S
05
20
(m)
V.E = 0.25
V
ES 4
V
ES 5
V
ES 6
V
ES 7
245 120
233 223
TOPSOIL
LATERIT IC CLAY
FRESH BASEMENT
LEGEND
WEATHERED BEDROCK
40
N
V
ES 8
243 246
124
190
372
350
211
238
389
91
423
285
261
231
393
190
373
10
0 5m 10m 15m 20m 25m30m 35m
0
20
40
60
664
960
2751 877
2662 975 971
3205
423
1515
3937
PARTIALLY WEATHERED BEDROC
K
DEPTH (M)
Figure 7. Geoelectric section along S-N direction.
-dipole along E-W direction.
clay. Between stations 4 and 6 at depth of 2.5 m, the
resence of lateritic clay is noticed in the weathered layer
Figure 8. 2-D Modeling of dip
zones with v
ole
ery low resistivity values marked D1 and D2
respectively which could be the diagnostic effect of wet p
Open Access IJG
F. P. IBITOYE ET AL. 1263
by an oval shaped unit with a higher resistivity value
than its surroundings. This could be as a result of the
deposition that was left after weathering. The weathered
bedrock (B) is characterized by sandy clay due to its low
resistivity values but with higher portion of clay between
stations 6 and 10 which falls within the weak segment of
the investigated premises. The depth to bedrock is thicker
between stations 2 and 10, but decreases towards the wes-
tern flank (between stations 10 to 15). Between these
stations, the thickness of the overburden has decreased
but with higher resistivity thus, signifying a lower por-
tion of clay when compared to the eastern flank, the
overburden is directly underlain by competent bedrock
(E) which extends beyond the depth of study (25 m). The
Dipole-dipole pseudosection and the 2D resistivity struc-
ture along S-N are presented in Figur e 9.
The 2D resistivity structure revealed four geoelec-
tric/geologic layers marked A, B, C and D which are
separated by geologic boundaries namely; topsoil marked
A (green and yellow colour); lateritic clay marked by B
(red and purple colour); Weathered bedrock marked C
(green colour) and fractured basement (blue colour). The
topsoil is generally thin and composed of clay and sandy
clay, but subsumes into the underlying lateritic clay in
many places. A major discontinuity flanked on both sides
by regions of higher resistivity was noticed between sta-
tions 6 and 11, this discontinuity which extends beyond
the depth of study (25 m) is in form of an oval shaped
unit which has concentric inner (D) and outer (C) regions,
gradual decrease in resistivity values occurs from the
outer region (C) to the inner region (D). The fracture
zone (D) (inner region of the oval shaped uni) is the
t
Figure 9. 2-D Modeling of dip-dipole along S-N direction.
ole
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F. P. IBITOYE ET AL.
1264
zone of lowest resistivity value.
3.4. Synthesis of Results
Along E-W traverse direction, the geoelectric section and
the 2D resistivity structure shows that the topsoil is gen-
erally thin except for the significant thick lateritic clay at
VES 7 which shows similar characteristics between sta-
tions 15 and 16 on the 2D resistivity structure. The top-
soil also varies in composition from clay to sandy clay
with small portion of clayey sand. The lateritic clay be-
neath VES 2 (Figure 10) at depth of 2.45 m is shown
between stations 4 and 6 at the same depth in form of an
oval shaped unit with higher resistivity value. The wea-
thered bedrock varies in composition from clay to sandy
clay to clayey sand, but predominantly composed of san-
y clay. The saturated zone D1 at depth 2.d
w
69 m correlates
ith a low resistivity zone between VES 2 and VES 4
(Figure 10). The basement depression at VES 6 corres-
ponds with the same basement depression between sta-
tions 14 and 15, the confined fracture beneath VES 4
corresponds with the basement depression between sta-
tions 9 and 10. The geoelectric section shows that start-
ing from VES 1 to VES 4, the depth to bedrock is beyond
25 m, this correlates with the 2D resistivity structure be-
tween stations 3 and 10 which shows that the bedrock is
deeper, i.e. beyond the depth of study (25 m).
Along S-N direction, both the geoelectric section and
the 2D resistivity structure reveals that the topsoil is gen-
er
fracture zone (D) is underlain by partially weathered bed-
rock.
3.5. Subsoil Evaluation of the Study Area
From the results of the 2D resistivity structure and the
geoelectric sections, the overburden is composed of clay,
sandy clay, clayey sand and lateritic clay, but predomi-
nantly composed of sandy clay which has higher clay to
sand ratio. Due to the incompetent nature of clayey soils,
the overburden will not be able to host heavy buildings
without excavating and refilling with competent materi-
als such as sand/gravel and laterite. The underlying
ally thin (Figure 11). The lateritic clay underlying the
topsoil on the geoelectric section correlates with the lat-
eritic clay (B) on the 2D resistivity structure at the same
depth. The discontinuity noticed between stations 6 and
11 which show gradual decrease in resistivity values
from depth 2.7 m correlates with the gradual decrease in
resistivity values between VES 2 and VES 7. The outer
region (C) of the oval shaped unit on the 2D resistivity
structure correlates with the weathered bedrock on the
geoelectric section (Figure 11) while inner region (D)
correlates with the partially weathered bedrock on the
geoelectric section (which extends beyond the depth of
study (25 m) for the 2D resistivity structure) (Figure 11).
The resistivity increases with depth from the zone of low-
est resistivity (D), the partially weathered bedrock has a
higher resistivity and also at a higher depth, therefore the
VES 1 VES 2VES 3
201
273 114
105
218 37
8
E
64
220 180
8
62
6
0
51015
10
(m)
V.E = 0.5
VES 4 VES 5VES 6VES 7
274
10
20
30
96
6
127
33
4
122
1
1385 1246
1018 1372
93
7
112
8
10
6
977
24
0
10
9
48
8
116 146
TOPSOIL
WEATHERED LAYER
FRESH BASEMENT
LEGEND
FRACTURED BASEMENT
LATERITIC CLAY
20
869
W
10m
0 20m 30m 40m55m 60m
A
B
C
D1
D2
E
TOPS OIL
WEATHE RED LAYER
LATERITIC CLAY
SATURAT E D ZONES
FRESH BASEMENT
LEGEND
GEOLOGIC BOUNDARY
40
0
DEPTH (M)
Figure 10.irection. Geoelectric section and 2-D resistivity structure along E-W d
Open Access IJG
F. P. IBITOYE ET AL. 1265
V
ES 1
V
ES 2
VES 3
215
S
05
20
(m)
.E = 0.25
VES 6
V
ES 7
V
ES 4
VES 5
245
TOPSOIL
LATERITIC CLAY
FRACTURED BASEMENT
LEGEND
WEATHERED BASEMENT
40
N
V
ES 8
24
3246
124
190
372
350
211
238
38
9
120 233 223
9
1
423
28
5
261
231
393
190
373
10
0
5m
10m15m 20m
20
40
60
25m 30m 35m
0
A
B
TOPSOIL
LATERITIC CLAY
LEGEN
D
C
D
WEATHERED BASEMENT
GEOLOGIC BOU
664 960 2751 877
NDARY
2662 975 971
3205
42
1515
39373
DEPTH (M)
FRACTURE ZONE
istivity structure along S-N direction.
trical Imaging techniques. Five main geoelectric se-
quences were delineated within the study area. These
include the topsoil, lateritic clay, weathered bedrock,
fractured bedrock and fresh basement. The topsoil and
Figure 11. Geoelectric section and 2-D
basement is fractured in most places i.e., these fracture
zones are confined within the basement. Con
res
fined frac-
tu
uilding and shows as cracks on the erected walls. Along
the South-North, the topsoil is predominantly composed
of sandy clay which has high clay to sand ratio, which
manifests as cracks on the surface of the ground. Along
the East-West direction, it was noticed that the western
flank has higher resistivity values than the eastern flank,
this can result in uneven stress distribution i.e., one side
has a stronger support than the other. A major weak zone
was noticed between stations 6 and 11 on 2D resistivity
structure. It was observed that a nearby stream that flows
beneath a local bridge gradually seeps through the sandy
clay compartments (which has higher permeability than
clay) on the western flank (on top of the competent bed-
rock marked E) towards the eastern flank (lowest part of
the competent bedrock marked E) to the weak zone ob-
served between stations 6 and 11. This weak zone is
composed mainly of clay which is porous but not per-
meable resulting in the saturated zone (D1) caused by
trapped water in the clay compartments.
4. Conclusion
A geophysical investigation involving the electrical re-
sistivity method was carried out at a study location in
south-western Nigeria. The electrical resistivity method
utilized the Vertical E
weathered layer are composed of clay, sandy clay and
clayey sand formation. A major discontinuity (confined
fracture zone) was identified by the electrical imaging on
bedrock along the S-N direction. This would have been
the reason for differential settlement, since it was con-
firmed that the foundation of the structure was placed on
this weak bedrock. Moreover, the topsoil along the S-N
direction is predominantly composed of sandy clay
which has high clay to sand ratio, this manifest as cracks
on the surface of the ground. A major weak zone which
has saturated zone was also discovered along the E-W
direction. The result of this research has shown that the
causes of the cracks and distress on the walls within the
site may have been influenced by the differential settle-
ment resulting from the incompetent subsoil materials
and the fractured bedrock on which the foundation of the
building was built. In conclusion, the importance of pre-
geophysical investigation before the erection of buil-
dings cannot be overemphasized, since this will help in
designing of such proposed buildings that will be able to
withstand subsurface instability with time.
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F. P. IBITOYE ET AL.
Open Access IJG
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