Geophysical Investigation of Road Failure Using Electrical Resistivity Imaging Method.
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.
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
INTRODUCTION
The incessant incidence of pavement failure of road structure is becoming alarming and has become a common phenomenon in many parts of Nigeria. These failures have been attributed to a number of factors such as inadequate information about the soil and the incompetence of these subsurface geologic materials. Failures are not limited to any particular geologic setting. Failures have been recorded on crystalline, basement, complex rocks and sedimentary formations.
The state of Nigerian roads had remained poor for a number of reasons. The number one problem is poor quality roads, resulting from faulty designs, lack of gutters and very thin coatings that are easily washed away by floods and hardly withstand heavy traffic. Secondly, funding of road maintenance has been grossly inadequate. From 1999 to 2002 in retrospect, less than 10 per cent of the funding request made by the Federal Ministry of Works and Housing (FMW&H, 2002) for road maintenance was appropriated by the Federal Government of Nigeria (CBN, 2002).
Even at this, only about 53.5 per cent of the appropriation was released. These were the collections from tollgates across the country – N569.29 million, N742.72 million and N779.84 million in 2000, 2001 and 2002 respectively (FMW&H, 2002, CBN, 2002). For each year, tollgates collections alone were much higher than the total funds released for road maintenance. Third is the excessive use of the road network, given the undeveloped state of waterways and the poor state of the railways, which are alternative transport modes.
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