Geophysical Investigation of Road Failure using Electrical Resistivity Imaging Method, A Case Study of Uhiele Opoji Road Edo State

ABSTRACT

A shallow geophysical investigation for road surface failure using 2D electrical resistivity imaging profiling was conducted to produce an approximate model of the subsurface resistivity. This study was done with the aim of revealing the horizontal and vertical geological discontinuities using electrical resistivity, an intrinsic property of all materials. Probable zones of untimely failure along the road are then investigated by variation in resistivity. Four traverses were established on the road with one parallel to the road segment. The Electrical Resistivity Imaging (ERI) Profiling involving the Wenner array 2D Imaging was adopted for the resistivity survey. The Pseudosection results revealed that the road structure is founded on a near homogeneous substratum indicating that the road is situated on a better geology than those from previous studies done in this geologic environment. The apparent resistivity values for all the profiles ranged from 273.94 Ωm – 3566.7 Ωm for profile I; 1561.2 Ωm – 4062.4 Ωm for profile II; 714.36 Ωm – 3856.4 Ωm for profile III and 700.06 Ωm – 3994.65 Ωm for profile IV. Apparent Resistivity values of Area Studied ranged from 273.94Ωm to 4062.4Ωm with an average of 2168.17Ωm which characteristically placed the studied area in a sedimentary basin with the presence of clay- sandstone intercalation. The low apparent resistivity values between 273.93 Ωm to 979.64 Ωm were obtained alternately with high apparent resistivity values along all the profiles investigated and were also obtained where there were major cracks, accentuated by water ingress. Geotechnically, it was inferred that highly resistive soils, usually at most times correlate with competent zones and the low resistive zones correlate to incompetent soils. The engineering properties of soils using Weaver’s rippability rating chart (seismic velocity and hardness) of soils, showed that the low resistive soils were incompetent for engineering structures like road pavement etc. The causes of road pavement failure on the studied road was found to be majorly as a result of a combination of clayey topsoil/sub grade soils, water-logged sands due to ingress with characteristically low resistivity values and thin pavement thereby unable to withstand pressure exerted on the road.

TABLE OF CONTENTS

Title page - - - - - - - - - - i

Certification - - - - - - - - - - ii

Approval page - - - - - - - - - iii

Dedication - - - - - - - - - - iv

Acknowledgement - - - - - - - - - v

Abstract - - - - - - - - - - vi

Table of contents - - - - - - - - - vii

List of plate - - - - - - - - - - x

List of tables - - - - - - - - - - xi

List of figures - - - - - - - - - - xii

CHAPTER ONE

1.0 Introduction - - - - - - - - - 1

1.1 Sand and sandstones - - - - - - - - 8

1.2 Roadbed clays- - - - - - - - - - 11

1.3 Drainage- - - - - - - - - - - 14

1.4 Study area description - - - - - - - - 17

1.4.1 Location- - - - - - - - - 17

1.4.2 Physiography and climate- - - - - - - 18

1.5 Geology of study area - - - - - - - 20

1.6 Statement of the problem - - - - - - - - 23

1.7 Purpose of study - - - - - - - - - 24

CHAPTER TWO

2.0 Geophysical methods- - - - - - - - - 27

2.1 The Principle of Electromagnetic method- - - - - - 30

2.1.1 Theoretical considerations- - - - - - - 32

2.1.2 Limitations of electromagnetic method- - - - - 33

2.1.3 Very low frequency (VLF) - Electromagnetic method- - 33

2.1.4 VLF-EM Application - - - - - - - 35

2.1.5 VLF-EM Advantage - - - - - - 36

2.1.6 VLF-EM Disadvantage - - - - - - - 36

2.1.7 VLF-EM Limitations - - - - - - - 37

2.2 Electrical Surveying methods - - - - - - - 37

2.2.1 Resistivity methods - - - - - - - 38

2.2.1.1Vertical electrical methods (VES) - - - - 39

2.2.1.2 Constant separation Techniques (CST) - - - 39

2.2.2 Induced polarization - - - - - - - 40

2.2.3 Self potential (SP) method - - - - - - 41

2.2.4 Theoretical considerations - - - - - - 42

2.2.5 Ohm’s law- - - - - - - - - 43

2.2.6 Poisson equation- - - - - - - - 45

2.2.7 Operating principle - - - - - - - 46

2.2.8 Wenner Array Method - - - - - - - 47

2.2.9 Electric profiling or Areal Method - - - - - 49

2.2.10 Advantage - - - - - - - - 49

2.2.11 Limitations- - - - - - - - - 49

2.2.12 Electrical Resistivity Imaging - - - - - - 50

2.3 Factors responsible for resistivity of soil or rock - - - - - 51

2.4 Typical resistivity values for different types of subsurface materials - - 52

CHAPTER THREE

3.0 Research Methodology - - - - - - - - 54

3.1 Data presentation - - - - - - - - - 54

3.1.1 Collection procedures - - - - - - - 54

3.1.2 Data acquisition - - - - - - - - 57

3.2 Interpretation and discussion - - - - - - 69

3.2.1 2D electrical resistivity imaging (ERI) method- - - - 69

3.3 Discussions - - - - - - - - - 86

3.4 Correlation between geophysical data and conventional engineering properties of soils 91

3.5 Resistivity of rippable soils - - - - - - - 95

3.6 Effect of clay on resistivity - - - - - - - 99

CHAPTER FOUR

4.1 Conclusion - - - - - - - - - 101

4.2 Conclusions from geophysical study - - - - - - - 104

4.3 Recommendations- - - - - - - - - 106

References - - - - - - - - - - 107