integration of geophysical and geotechnical investigations

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Journal of Applied Sciences 2(3), 2009

241

Integration of Geophysical and Geotechnical Investigations for a Proposed Lecture

Room Complex at the Federal University of Technology, Akure, SW, Nigeria

Akintorinwa, O. J. and Adeusi F. A.

Department of Applied Geophysics,Federal University of Technology, P.M.B. 704, Akure. Nigeria

E-mail address for correspondence: [email protected]

______________________________________________________________________________________

Abstract : A foundation studies has been undertaken at the newly proposed School of Earth and Mineral

Sciences lecture complex, Federal University of Technology, Akure. The study is aimed at evaluating thecompetence of the near surface Formation as foundation materials. Geophysical and geotechnical methods

of investigation were adopted. The Vertical Electrical Sounding (VES), using schlumberger configuration

and soil analysis techniques were adopted. A total of 24 Vertical Electrical Sounding (VES) and five soilsamples from different location within the study area were used for the study.

The geophysical results revealed three distinct geoelectric sequences which comprises of topsoil, weathered

layer and bedrock. The topsoils composed of sandy clay/ clayey sand/ laterites while the weathered layercomposed of clay/sandy clay. The geoelectric section across the study area shows the undulation nature of

the bedrock topography with depth of rock head between 2 and 21 m. There is no evident of geologicalfeature such as fracture/fault within the bedrock which can aid subsidence in the area. The geotechnical

results show that the soil has relatively high clay content. Based on the consistency limits of the soils withinthe area, the soil generally indicate low to medium plasticity, hence, the soils are expected to exhibit low to

medium swelling potential. It can however be concluded that the subsoils on or within which engineeringstructures will be founded within the study area are competent.

______________________________________________________________________________________

INTRODUCTION

The statistics of failures of structures such as road, buildings, dam and bridges throughout the nation hasincreased geometrically. The need for pre-foundation studies has therefore become very imperative so as to

prevent loss of valuable lives and properties that always accompany such failure. Foundation study usually provides subsurface information that normally assists civil engineers in the design of foundation of civil

engineering structures.

Geophysical methods such as the Electrical Resistivity (ER), Seismic Refraction, Electromagnetic (EM),

Magnetic and Ground Penetrating Radar are used singly or in combinations for engineering siteinvestigation. The applications of such geophysical investigation were determination of depth to bedrock,

structural mapping and evaluation of subsoil competence.

The management of the Federal University of Technology, Akure, recently allocated a site for a proposed

lecture room complex for the School of Earth and Mineral Sciences (SEMS). The site is located within a basement complex area with variable overburden. The need to provide information in the subsurface

sequence and structure disposition necessary for foundation design necessitated an integrated geophysicaland geotechnical investigations of the site whose results are presented in this paper.

Ozean Journal of Applied Sciences 2(3), 2009ISSN 1943-2429© 2009 Ozean Publication

mailto:[email protected]:[email protected]:[email protected]


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Description of the Environment of the Investigated Site

The study area is located within Federal University of Technology, Akure. It lies between latitudes7

o18′03′′ N - 7

o18′06′′N and Longitudes 5

o08′02′′E - 5

o08′05′′E. The topography is low lying. The site is

located within the sub-equatorial climatic belt with tropical rain-forest vegetation. The mean annualtemperature is 24

0C-27

0C, while the annual rainfall, varies between 1500mm and 3500mm (Adeleke and

Goh Cheng Leong, 1978). The Federal University of Technology, Akure is underlain by rocks of thePrecambrian Basement Complex of Southwestern Nigeria (Rahaman, 1989). The crystalline rocks are

porphyritic granite, biotite granite, charnockite, quartzite and gneiss migmatite (Fig 1). Gneiss migmatiteand biotite granite are the major outcrop that occur within the study area, while charnockite occurs as a

discrete body in other part of the area. The proposed site for the lecture rooms is underlain by biotitegranite (see Fig. 1).

METHODOLOGY

Four traverses were established across the study area (Fig. 2). Six (6) Vertical Electrical Sounding (VES)stations were occupied along each of the traverses. A total of 24 sounding were carried out using the

Schlumberger configuration. The electrode spacing (AB/2) was varied from 1-65 m. The co-ordinate ofeach of the sounding stations in Universal Traversal Mercaton (UTM) co-ordinate was recorded with the

aid of the “GARMIN 12” channel personnel navigation geographic position system (GPS) unit.

The apparent resistivity values were plotted against electrode spacing (AB/2) on a bi-logarithmic graph

sheet to generate depth sounding curves. The field curves were then inspected visually for identification ofthe curve type. Partial curve matching was carried out on the field curves. The interpretation results (layer

resistivity and thicknesses) were fed into computer for 1-D computer assisted interpretation involvingResist version 1.0 software (Varder Velper, 1988). The final interpreted results were used for the

preparation of geoelectric sections, histograms and maps.

Five disturbed soil samples were collected at different locations at a depth not exceeding 1m within the site

as shown in Figure 2. These samples were preserved in polythene bags and transported to the laboratory.The natural moisture content of the samples collected from the field was determined in the laboratory

within a period of 24 hours after collection. This was followed by air drying of the samples by spreading

them out on trays in a fairly warm room for four days. Large soil particles (clods) in the samples were broken with a wooden mallet. Care was taken not to crush the individual particles. Methods of testing soilsfor engineering parameters were conducted in accordance with B.S. 1377 for all the soil samples collected.

The tests include natural moisture content, grain size analysis, liquid limit, plastic limit, linear shrinkage,and compaction test.

RESULTS AND DISCUSSIONS

Geophysical Results

Five curve types were identified within the study area. These include A, H, QH, HA and KH type with theH as the predominant curve type (Fig. 3). The typical curve types are as shown in Figures 4. Table 1 gives

the summary of the VES interpretation. The number of layers varies between three layers and four layers.

Geoelectric Parameters

The VES interpretation results were used to prepare 2-D geoelectric sections displayed in Figure 5. The

geoelectric sections delineate maximum of four geoelectric/geologic subsurface layers comprising the topsoil, weathered layer, partly weathered/fractured basement and fresh bedrock. The top soil is composed of

sandy clay/clayey sand/laterite with resistivity values varying from 172-698 Ώm and thickness of between0.6-4.1 m. The weathered layer ranges in composition from clay and sandy clay with resistivity values that

vary between 42 and 230 Ωm.


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Iso-resistivity and Isopach Map of the Topsoil

Figures 6 and 7 show the iso-resistivity and isopach map of the topsoil respectively. As revealed by themap, the topsoil is composed of sandy clay/ clayey sand with exception of a portion at the northwestern

parts of the area which is lateritic. The predominant resistivity value of this layer ranges from 200-400 Ωm(Fig. 8a). The Isopach map of the topsoil (Fig. 7) shows that the topsoils are generally thin with the highest

thickness up to 2.9 m in the Northeastern parts of the area. The predominant thickness of the topsoil isgenerally < 2 m (Fig. 8b).

Isopach Map of the Overburden

The Isopach map of the overburden is as shown in Figure 9. From the map, the thickness of the overburdenranges from 2 to 21 m with predominant thickness in the range of 5-10 m (Fig. 10). The overburden is

appreciably thick to house the proposed structure foundation.

Geotechnical Results

Table 2 shows the summary of the geotechnical results. The natural moisture content of tested soil samples

ranges from 7.63 - 12.03%. |This shows that the natural moisture content of the soil in the area is relativelylow at its natural state. Moisture variation is generally determined by intensity of rain, depth of collection

of sample and texture of the soil (Jegede, 2000). From the grading curves (Figure 11) the soils can beclassified as well graded soil. The tested soils have percentage finer (percentage passing 0.075 mm) ranges

from 32-45%. Generally the tested soils have percentage passing 0.075 mm of more than 35% with averageof 38% which is not far from the maximum of 35% recommended by Federal Ministry of Works and

Housing (FMWH) (1972) for a foundation material, hence; the soils can be generally rated as fair to goodsub-grade foundation material. The soils that are largely made up of fine particle are likely to have poor

geotechnical properties as foundation materials than soils that are largely made up of coarse particle.

As shown in Table 2, the Liquid Limit of the soil samples ranges from 26-33%. The Plastic Limit ranges

from 14-21%, and the Plasticity Index of soils ranges from 5-12%. The tested soil samples are of mediumconsistency limits indicating low percentage of clay content in the soil. Generally, soils having high values

of liquid and plastic limits are considered poor as foundation materials. The plastic index of all the soilsample are lower than 20% maximum Federal Ministry of Works and Housing (FMWH) (1972), hence it

shows a good engineering property since the higher the plastic index of a soil, the less the competency ofthe soil as a foundation material.

The linear shrinkage value of the tested soils ranges from 9-11% (Table 2). Brink et al (1992) suggestedthat soils with linear shrinkage below 8% would be inactive, inexpensive and are good as foundation

material. The linear shrinkage of all tested soils are greater than 8%, hence the soils are likely to besubjective to swelling and shrinkage during alternate dry and wet seasons of the humid tropical climatic

condition of the south western Nigeria. This must be taken into cognizance in the design of the foundation.The maximum dry density (MDD) and optimum moisture content OMC) of the soils ranges between 1553-

1868kg/m3

and 14-23% respectively. These values show that, the soils respond gradually to compaction.The importance of compaction is to improve the desirable load bearing properties of soil as a foundation

material.


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Subsurface Engineering Evaluation of the Study Area

Determined depth to the basement rockhead varies from 2- 21 m (Fig. 9) for most parts of the area,overburden thicknesses are generally less than 10 m. Areas with high thicknesses are found at the northern

parts of the studied area. There are no indications of any major linear structure such as fracture/faults (Fig.5). However, VES 18 show unconfined fractures. The area can be adjudged seismically stable. The

geologic sequence beneath the study area is composed of the topsoil, weathered layer, partlyweathered/fractured basement and fresh bedrock. The topsoil constitutes the layer within which normal

civil engineering foundation is founded. This layer is composed of sandy clay, clayey sand and laterite.Engineering competence of the topsoil can be qualitatively evaluated from layer resistivity and the

geotechnical parameter. The higher is the layer resistivity value, the higher the competence of a layer,hence from the point of view of resistivity value therefore; laterite is the most competent of the delineated

topsoil units, followed by clayey sand and sandy clay being the least competence.

The higher the geotechnical parameters of a soil, the lesser the competence of the soil as a foundation

material. The Federal Ministry Work and Housing (FMWH), 1972 recommend 35%, 50%, 30% and 20%

Maximum for the percentage finer, Liquid Limit, Plastic Limit and Plastic Index respectively and 8%

minimum of Linear Shrinkage for a good foundation material. The geotechnical properties of the topsoilare relatively good as most of soil samples taken within the topsoil fall within FMWH, 1972

recommendation. It can however be concluded that the subsoils on or within which engineering structures

will be founded within the study area are fairly competent.

CONCLUSIONS

The geophysical results revealed four geoelectric sequences within the study area which comprises of

topsoil, weathered layer, partly weathered /fracture basement and bedrock. The top soils are generallythin (< 2 m) and majorly composed of sandy clay/ clayey sand/ laterites while the weathered layer is

composed of clay/sandy clay. The geoelectric sections show depth of rock head of between 2 and 21 m.There is no evident of geological feature such as intensive fracture/fault within the bedrock which can

aid subsidence in the area. The geotechnical results show that the soils are generally of relatively lownatural moisture content. It has relatively low clay content as revealed by the percentage passing

0.075mm which are generally less or equal 35%. This was corroborate by the sandy nature of the Topsoilas revealed by the geophysical results. Since the Plastic Index of the soils within the area are less than

20%, the soil can be adjudged to be low to medium plasticity, hence, the soils are expected to exhibit lowto medium swelling potential. The linear shrinkage of the soils are greater than 8%, indicating active and

expensive nature of the soil.

The deduction from the above is that, the topsoil Formation may be rated as relatively good as a

foundation material. The foundation of the proposed civil structure can be hosted by this formation. Theuneven nature of this layer has to be considered in the design of the foundation.

REFERENCES

AASHO (1962): “Road Test – Report 5 (Pavement Research)”, Highway Research Board, Special Report

6IE Washington D.C.Adeleke, B. O. and Goh Cheng Leong, (1978): Certificate Physical and Human Geography, West African

Ed. Oxford University Press, Nigeria, Ibadan.Brink, A. B. A., Parridge, J. C. and Williams, A. A. B (1992): Soil Survey for Engineering, Claredon,

|Oxford.Federal Ministry of Works and Housing (1972): Highway Manual Part 1 Road Design, Federal Ministry of

Works and Housing, Lagos.


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Jegede, G. (2000): Effect of soil properties on pavement failure along the F209 highway at Ado-Ekiti,Southwestern Nigeria. Journal of Construction and Building Materials, vol. 14, pp. 311-315,

Kareem, 1997: Geological Map of Federal University of Technology, Akure: Unpublished M.Tech. Thesis,Dept. of Applied Geology, Federal University of Tech., Akure. 109pp

Rahaman, M.A., (1989): Review of the basement geology of southwestern Nigeria. In: Kogbe, C.A., (ed)Geology of Nigeria, Rock View (Nig.) Limited, Jos, Nigeria, pp. 39-56.

Table 1: Summary of the VES Interpretation Results.

VES ρ 1 (Ωm) ρ 2 (Ωm) ρ 3 (Ωm) ρ 4 (Ωm) h 1 (m) h 2 (m) H3 (m)

Curve

type

1 361.9 97.3 3786.6 0.7 2.8 H

2 381.8 49.6 ∞ 1 3 H

3 181.5 34.1 ∞ 1.5 1.9 H

4 228.3 65.7 ∞ 1.4 5.3 H

5 444.7 440.3 37.2 ∞ 1.0 2 2.8 QH

6 202.4 64 ∞ 1.9 4.4 H

7 340 170 3200 0.9 9 H

8 258.9 697.7 86.1 1500 1.2 1 6.8 KH

9 277.3 71.8 ∞ 2 4.8 H

10 297.3 62.6 843.6 1.4 4.1 H

11 776.4 305.3 856.2 ∞ 0.4 13.6 HA

12 230.8 444.9 1151.5 3232.6 1.4 13.2 18.8 A

13 170.2 1145.8 1745.3 1.9 3.9 A

14 319.4 229.7 1830.6 0.8 8.8 H

15 229 96.8 877.8 2 6.7 H

16 306.1 58.4 ∞ 2.9 4.9 H

17 192.1 95.4 ∞ 3.0 3.3 A

18 200.2 82 285.8 3630.9 0.6 1.1 9.6 HA

19 201.6 90.6 671.1 1.3 2.4 H

20 146 217.2 ∞ 0.3 17.6 A


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Table 2: Summary of the Geotechnical Results

Sample

No.

Natural

MoistureContent

(%)

Percentage

Passing0.075mm

(%)

Liquid

Limit(%)

Plastic

Limit(%)

Plastic

index(%)

LS (%) MDD

Kg/m3

OMC

(%)

1 7.81 41.88 32 27.10 4.9 9.60 1553 22.95

2 10.77 36.16 25.50 14.45 11.05 10.00 1868 14.00

3 9.56 45.32 32 21.40 10.60 8.60 1608 20.00

4 11.62 31.88 29.50 17.60 11.90 11.40 1770 15.29

5 15.90 34.60 30 20.90 9.10 8.60 1860 15.00

Fig.1: Geological Map of the Federal University of Technology, Akure.

(After Kareem, 1997)

21 253.8 336.7 41.7 8708.1 0.7 3.8 7.5 KH

22 185.4 88.6 ∞ 2.5 18.3 H

23 30.9 1967.1 81.7 3584 0.4 0.3 2.6 KH

24 222.8 170.5 3675.3 1 11.5 H

5 07' E

5 07' E

o

o

5 09' E o

5 09' E o

7 19' N o

7 19' N o

7 17' N o

7 17' N o

LEGEND

Porphyritic Granite Biotite Granite

Charnockitic rock Quartzite bands Gneiss Migmatite Stream Geological Boundar

250m

Study Area


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Fig. 2: Data Acquisition Map of the Study Area Showing the Vertical Electrical Sounding Stations and theGeotechnical Sampling Points

Fig. 3: Histogram of the Curve Type

0

2

4

6

8

10

12

14

16

H A QH KH HA

Curve Type

F r e q u e n c y ( U n i t )


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Fig. 4a: Typical „H‟ Type Sounding Curve

Fig. 4b: Typical „A‟ Type Sounding Curve

Fig. 4c: Typical „QH‟ Type Sounding Curve


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Fig. 4d: Typical „KH‟ Type Sounding Curve

Fig. 4e: Typical „HA‟ Type Sounding Curve

(a)

0

10 D e p t h ( m )

VES 4 VES 9 VES 16 VES 21

20m

10m

228

66

277 306337

254

72 58

42

SN

V V V V V

LEGEND

Topsoil

Weathered Layer

Bedrock

Laterite V V

V


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(b)

(c)

Fig. 5: Geoelectric Section along (a) S-N, (b) W-E and SE-NW Direction.

Fig. 6: Isoresistivity Map of the Topsoil.

0

10

20m

10m

D e p t h ( m )

LEGEND

Topsoil

Weathered Layer

Bedrock

VES 13 VES 14 VES 15 VES 16 VES 17 VES 18

Partly Weathered/Fractured Basement

W E

172

1146

1745

230

319

1831

229

97

306

58

192

95

82

286

200

3631878

LEGEND

Topsoil

Weathered Layer

Bedrock

D e p t h ( m )

0

10

20m

10m

VES 6 VES 8 VES 15 VES 23

202

64

698 259

86

1500

229

97

82

3584

878

259 31SE NW

VES Station

100ohm-m

200ohm-m

300ohm-m400ohm-m

500ohm-m

600ohm-m

700ohm-m

800ohm-m

900ohm-m

1000ohm-m

1100ohm-m

1200ohm-m

1300ohm-m

1400ohm-m

1500ohm-m

1600ohm-m

1700ohm-m

1800ohm-m

1900ohm-m

2000ohm-m

735870 735880 735890 735900 735910 735920 735930 735940 735950 735960 735970

807080

807090

807100

807110

807120

807130

807140

807150

0m 20m

300 Contour Line with Value


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Fig. 7: Isopach Map of the Topsoil.

Fig. 8a: Histogram of the Top Soil Resistivity

VES Station

735870 735880 735890 735900 735910 735920 735930 735940 735950 735960 735970

807080

807090

807100

807110

807120

807130

807140

807150

1.4 Contour Line with Value

0.3m

0.5m

0.7m

0.9m

1.1m

1.3m

1.5m

1.7m

1.9m

2.1m

2.3m

2.5m

2.7m

2.9m

0

2

4

6

8

10

12

14

16

0-200 200-400 400-600 600-800 800-1000 1000-1200 1200-1400 1400-1600 1600-1800 1800-2000

Topsoil Resistivity (ohm-m)

F r e q u e n

c y ( U n i t )


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Fig. 8b: Histogram of the Top Soil Thickness

Fig. 9: Isopach Map of the Overburden

0

2

4

6

8

10

12

0-1.0 1.0-2.0 2.0-3.0 3.0-4.0

Topsoil Thickness (m)

F r e q u e n c y ( U

n i t )

VES Station

735870 735880 735890 735900 735910 735920 735930 735940 735950 735960 735970

807080

807090

807100

807110

807120

807130

807140

807150

5.0 Contour Line with Value

1.0m

2.0m

3.0m

4.0m

5.0m

6.0m

7.0m

8.0m

9.0m

10.0m

11.0m

12.0m

13.0m

14.0m

15.0m

16.0m

17.0m

18.0m

19.0m

20.0m

21.0m


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Fig. 10: Histogram of the Overburden Thickness

0

2

4

6

8

10

12

14

0-5.0 5.0-10.0 10.0-15.0 15.0-20.0Thickness (m)

R e s i s t i v i t y ( o h m - m


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40

50

60

70

SILTCLAY SAND GRAVEL

COBBLESFine Medium Coarse Fine Medium Coarse Fine Medium Coarse

0.002 0.006 0.02 0.06 0.2 0.6 2 6 20 60 200

Particle Size (mm)

P e r c e n t a g e P a s s i n g ( % )

Fig. 11: Typical Grain Size Distribution Curve (Sample 1)

integration of geophysical and geotechnical investigations

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