Electrical Resistivity Investigation for Topsoil Thickness, Competence

and Corrosivity Evaluation: A Case Study from Ladoke Akintola

University of Technology, Ogbomoso, Nigeria

O. G Bayowa 1

and N. S. Olayiwola 2

1, 2 Department of Earth Sciences, Ladoke Akintola University of Technology, P.M.B 4000, Ogbomoso,

Nigeria.

Abstract. This study investigated the application of electrical resistivity on topsoil thickness, competence

and corrosivity to determine the suitability of soils in the study area for construction of buildings. The paucity

of data on topsoil competence necessitated the investigation of the suitability of soils to improve on civil

engineering designs. Vertical Electrical Resistivity (VES) data were acquired and interpreted quantitatively

to obtain the topsoil resistivities values which were used to categorize the topsoil in the area into different

competence and corrosivity zones by constructing the isopach and isoresistivity maps. The isopach and

isoresistivity maps showed 0.3 – 2.1 m soil thickness. The topsoil quality varied between highly competent

( 750 ohm-m) and moderately competent (107-347 ohm-m). Non corrosive soils ( 200 ohm-m) and mildly

corrosive soil (100-200 ohm-m) had underlain the area. The study concluded that the study area was

underlain by highly competent to competent and practically non-corrosive to mildly corrosive soils.

Keywords: Electrical Resistivity, Isopach, Soil Competence, Soil Corrosivity, Nigeria.

1. Introduction

One important concept in civil engineering practice is competence of the earth materials employed in

construction processes. However, topsoil thickness also plays an important role in foundation design.

Competence (or strength) of any geological material is influenced by several factors such as the mineralogy,

the character of the particle contact and the agent of weathering [1]. Every civil engineering structure is

seated on geological earth materials, it is imperative to conduct pre-construction investigation of the

subsurface of the proposed structures to ascertain the strength and the fitness of the host earth materials as

well as the timed post-construction monitoring of such structure to ensure its integrity. [2] Correlated ranges

of apparent resistivity values with subsoil competence as presented in Table 1. Civil engineering construction

such as buried pipes is susceptible to corrosion and subsequent failure if the host soil medium is corrosive or

aggressive. The formation of corrosion cells which can lead to severe corrosion failure in civil engineering

structures is known to be associated with low resistivity or high conductivity (Table 2). Low electrical

resistivity is indicative of good electrical conducting path arising from reduced aeration, increased electrolyte

saturation or high concentration dissolved salts in soils. Electrical resistivity method is the most common

technique used for such purpose [3]-[8]. Generally, the higher the resistivity of the soil, the lower the risk of

corrosion [9].

The study area is Ladoke Akintola University of Technology (LAUTECH) Campus, Ogbomoso,

Southwestern Nigeria. The Campus lies within Latitudes 40 15′ and 4

0 16′ and Longitudes 8

0 9′ and 8

0 10′

(Fig. 1). As the population of the students and staff increases, there is the need for expansion of

Corresponding author. Tel.: +2348030820291.

E-mail addresses: oyebayowa@yahoo.com, obayowa@lautech.edu.ng.

2015 2nd International Conference on Geological and Civil Engineering IPCBEE vol. 80 (2015) © (2015) IACSIT Press, Singapore

DOI: 10.7763/IPCBEE. 2015. V80. 11

52

infrastructures such as buildings. This has necessitated soil/subsoil investigations for competence and soil

corrosivity within the university community.

Table 1: Soil Competence Rating [2]

Apparent Resistivity (Ωm) Lithology Competence Rating

˂100 Clay Incompetent

Sandy clay Moderately Competent

Clayey sand Competent

˃750 Sand/Laterite/Bedrock Highly Competent

Table 2: Soil Corrosivity Rating [10]

Soil Resistivity (Ωm) Corrosivity Rating

˃200 Essentiallynon-corrosive

100 - 200 Mildly Corrosive

50 - 100 Moderately Corrosive

30 - 50 Corrosive

10 - 30 Highly Corrosive

˂10 Extremely Corrosive

Fig. 1. Map of the Study Area showing the VES Distribution.

The area is underlain by the Basement Complex Rock of Southwestern Nigeria [11], [12]. However, the

study area is devoid of rock outcrops which make it difficult to have complete description of the underlying

geology. Samples of grey gneiss have been extracted from few boreholes and hand dug wells constructed

around the campus. Its vegetation is tropical rainforest type.

2. Methodology

In the study, seventy (70) Vertical Electrical Sounding (VES) data were acquired in the study area (Fig.

1). Schlumberger Array was adopted while the half current electrode spacing (AB/2 (m)) was varied between

1m to a maximum of 100 m. The VES data acquired were interpreted quantitatively to obtain the soil

parameters (resistivities and thicknesses). The final soil parameters (resistivities and thicknesses) were used

to generate isopach and isoresistivity maps. The maps were used to categorize the study area into different

soil thickness, competence and corrosivity zones using different thicknesses and resistivities range. 53

3. Results and Discussion

3.1. Isopach Map of Soils Fig. 2 presents the isopach map of topsoil in the study area. The topsoil constitutes the layer within

which civil engineering structure(s) could be grounded. The map shows the thickness distribution of the

topsoil which varies from 0.3 to 2.1 m. Thickness value range between 0.9 and 1.5 m was observed in most

part of the area. Thickness range of between 0.3 and 0.9 m was observed around the north-east, east and part

of southwest and northwest. The western, southeastern and part of the northern part of the area is

characterized by thickness range of between 1.5 and 2.1 m.

Fig. 2. Isopach Map of the Topsoil showing different Thickness Ranges.

3.2. Isoresistivity Map of Soils Fig. 3 shows the isoresistivity map of the soil. The figure shows that the resistivity value of the topsoil in

the study area mostly varies from 170 – 2586 ohm-m typical of sandy clay, clayey sand and laterite. The

northeast, southeast and southwestern parts of the study area are characterized by relatively high resistivity

values greater than 1000 ohm-m.

Fig. 3. Isoresistivity Map of Soils in the Study Area.

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3.3. Soil Competence and Corrosivity Evaluation

Using [2], soil resistivity-competence classification, the soil resistivity values were used to classify the

study area into different competence zones (Fig. 4). The competence of the soil within the study area can be

described as highly competent, competent and moderately competent. High competent soil is observed

around the centre part of the study area with resistivity value ˃ ohm-m. The resistivity values indicate

that the area is underlain essentially by laterite. Northeastern, Southeastern and Northwestern part of the

study is underlain by competence soil with resistivity value ranges between 396 ohm-m and 674 ohm-m.

Northeastern part of the area is underlain by moderately competent material. The zone has soil resistivity

values range from 107ohm-m and 347ohm-m.

Fig. 4. Isoresistivity Map of the Topsoil showing different Competence Zones.

Fig. 5 shows the soil (within the upper 0-3 m) corrosivity map developed from Table 2 which delineates

the study area into different corrosivity zones. Mildly corrosive soil is observed in the extreme Northern part

of the study area. The resistivity values between 100 and 200 ohm-m. Practically non corrosive soils

dominated the Western, Southern, and Eastern part of the study area. The locations are underlain by material

with resistivity values ˃2 ohm-m. Mildly corrosive soil underlay small portion of the Northern part of the

study area. It is however evident that large portion of the soil in the area is essentially underlain by non

corrosive soils.

Fig. 5. Corrosivity Map of the soil in the Study Area.

4. Conclusion

It has been established in this study that the electrical resistivity method is useful in determining the soil

thickness, competence and corrosivity. The soil thickness in the study area ranged between 0.3 to 2.1 m. The

isopach and isoresistivity maps of the study area were used to categorize the area into different soil 55

competence and corrosivity zones. The soil conditions in the area was categorized as moderately competent,

competent and highly competent based on the soil resistivity ranged from 107-347 ohm-m, 396-674 ohm-m

and 750 ohm-m. Practically non-corrosive soils with resistivity values 200 ohm-m underlain greater part

of the study area while the mildly corrosive areas with resistivity value ranged from 100-200 ohm-m was

observed in the extreme north and eastern part of the area. The study concluded that the study area was

underlain by highly competent to competent and practically non-corrosive to mildly corrosive soils.

5. References

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[10] http://www.corrosion-doctors.org/cp/soil-resist.htm

[11] Rahaman, M.A. (1988). Recent advances in the study of the Basement Complex of Nigeria. In: Oluyide, P.O.,

Mbonu, W.C., Ogezi, A.E., Egbuniwe, I.G., Ajibade, A.C. and Umeji A.C. (eds.). Precambrian Geology of Nigeria,

G.S.N., pp. 11-41.

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