CHAPTER ONE

1.0 INTRODUCTION

1.1 BACKGROUND OF STUDY

The stability of the foundation of a building, a bridge, an embankment or any other structure

built on soil depends on the strength and compressibility characteristics of the subsoil. The

field and laboratory investigations required to obtain the essential information on the subsoil

is called Soil Exploration or Soil Investigation. The purpose of proposed subsoil investigation

is to provide adequate information on subsurface and surface conditions for the foundations

and other structure for the proposed project leading to their economical safe designs

(Terzaghi, 1951; Bjerrum et al., 1960).

The success or failure of a foundation depends essentially on the reliability of the various soil

parameters obtained from the investigation and laboratory testing and used as in input into the

design of foundations. Information related to the local soil conditions is vital for risk

assessment and mitigation (Bjerrum et al., 1960).

The procedure for obtaining subsurface information is divided into two broad categories:

indirect and direct methods which include aerial photograph, topographic map interpretation

and study of existing geological reports, maps and soil survey. Direct methods consist of the

following: (a) geologic field reconnaissance, including the examination of insitu materials,

man- made structures, groundwater level and exploration of shafts, (b) application of modern

geophysical techniques for mapping subsurface structures, (c) boring, test pits, trenches and

shafts from which representative disturbed and/ or undisturbed samples of the insitu materials

may be obtained and analysed, (d) simple geotechnical field tests, such as the standard

penetration test (SPT), which can be correlated with other engineering parameters (Yangfang,

1991).

In recent years, several organizations and private individuals have been engaging in

infrastructural development but recent studies showed that many of them do not engage the

services of professionals in order to maximize profits; the effect being poor building

constructions which may ultimately lead to gradual or sudden collapse of such structures

(Oyedele and Olorode, 2010).

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Geophysical and geotechnical methods are mostly integrated to complement each other.

While geotechnical investigation of the soil is discrete, invasive and expensive; geophysical

investigation is continuous, non-invasive and cost-effective. Engineering geophysics can be

used to select borehole locations and can provide reliable information about the nature and

variability of the subsurface between existing boreholes. An isolated geologic structure such

as a limestone pinnacle might not be detected by a routine drilling program. An effective

geophysical survey, however, could detect the presence of the pinnacle and map the height

and aerial extent of the surveyed area (Sirles, 2006).

Some advantages of engineering geophysics are related to site accessibility, portability, and

operator safety. Geophysical equipment can often be deployed beneath bridges and power

lines, in heavily forested areas, at contaminated sites, in urban areas, on steeply dipping

slopes, marshy terrain, on pavement or rock, and in other areas that might not be easily

accessible to drill rigs or cone penetration test (CPT) rigs. Also, most surface based or

airborne geophysical tools are non-invasive and, unlike boring or trenching, leave little brunt

if any environmentally sensitive areas, on contaminated ground, or on private property

(Sirles, 2006).

In addition, geophysical surveys are generally considered less dangerous than drilling since

there are fewer risks associated with utility encounters and operations. Besides, geophysical

surveys can enable engineers to reduce the number of required boreholes. Engineering

geophysics is not intended to act as a substitute for boring and direct physical testing rather it

should complement a well-planned, cost-effective drilling and testing program, and provide a

volumetric image to the subsurface rather than a point measurement. Geophysicist are

encouraged to refer to borehole information and field geologic maps to constrain and verify

some geophysical interpretation (Sirles, 2006).

The goals of geotechnical and geophysical site characterization are to provide the

geotechnical engineer with sufficiently detailed information in order to plan, design,

construct and operate structures on or below the surface. Geophysical methods have several

important advantages compared with conventional geotechnical field investigation methods.

They can explore relatively large soil volumes, of which they can identify material properties,

material boundaries as well as variations in space and time. Many of the methods have

additional advantage of being non- destructive. However, a major limitation is that in most

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cases, the measured parameters need to be correlated with engineering properties, which is

not always straightforward and requires experience and judgement (Anderson, 2006)

In order to improve the reliability of geophysical investigations, it is advisable to combine

several methods and verify these by sampling and correlation with conventional geotechnical

field and laboratory methods. The application of the cone penetration test (CPT) for

geophysical site characterization opened new possibilities for geophysical site

characterization. The CPT has gained rapid acceptance and recognized as valuable in-situ

testing technique because of its speed, reliability, cost-effectiveness and excellent soil

profiling capability (Massarch, 1986).

In this study, electrical resistivity and geotechnical method were employed to delineate

competent layer to locate suitable foundation at proposed site in West Africa ENRG, KM3

Isheri- Igando road, Lagos.

1.2 STATEMENT OF PROBLEM

The site of investigation is an abandoned dumpsite at Isheri-Igando Road, Alimosho Local

Government Area of Lagos State. There is currently a proposal of erecting a building on the

land; and this calls for a thorough geophysical and geotechnical investigation in order to

unravel subsurface information for engineering purposes.

1.3 AIM AND OBJECTIVES

The aim of the investigation was to apply 1-D and 2-D electrical resistivity and geotechnical

methods in order to delineate the depth to competent layers for foundation of engineering

structures.

The objectives of the study are to:

(i) generate geoelectric layers of the subsurface in the area under investigation.

(ii) determine the lateral and vertical extent of the subsurface geologic materials in the

study area.

(iii) deduce the lithology of the subsurface from the cone penetration test data in the

study area.

(iv) correct the results of the VES, 2D imaging, CPT data and borehole log to obtain

the subsurface information within the study area.

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(v) recommend suitable foundation for the site under investigation

CHAPTER TWO

2.0 LITERATURE REVIEW

Akintorinwa and Adesoji (2009) carried out an integrated geophysical and geotechnical

investigations in evaluating an engineering site in the south eastern part of Nigeria. The

geophysical and geotechnical studies were conducted at a proposed station facility for

telecommunication at the site. The aim was to evaluate the subsoil conditions and electrical

properties of the soil which may have effect on the proposed mast switch system. The

geophysical investigation involved borehole drilling as well as cone penetration test which

carried out to provide controls on the geophysical interpretation. Four subsurface layers were

delineated within the study area, the top soil (mixture of sand, silt and clay), coarse sand,

clayey sand and sand. The findings correlate with the obtained geophysical soil layers. Based

on this study, it was recommended that the choice of foundation for the proposed structure

should be related to the consolidation settlement characteristics of the clayey material.

Kumari et al. (2009) carried out soil characterization using electrical resistivity tomography

and geotechnical investigations of two different sites in India. The site was proposed for

thermal power plants in Uttar Pradesh, India. Standard penetration test and dynamic

penetration tests were conducted at 28 points and two ERT profile. Resistivity values are

correlated with the soil matrix and grain size distribution. Linear relationship was presented

between transverse resistance derived from the ERT data and N-values obtained from

geotechnical tests at these sites. The determination of soil strength was found to be

economical using ERT, fast and efficient in comparison to the direct in-situ methods to

determine the soil strength for civil engineering purposes.

Fatoba et al. (2010) investigated the causes of the foundation failure of the PDS building in

the Mini-Campus of Iwoye Nigeria. It was carried out using the electrical resistivity imaging

method. The aim was to delineate the subsurface as a means of determining the cause (s) of

the foundation failure. Measurement involving Dipole-Dipole configuration and vertical

electrical sounding (VES were taken along four (4) traverses, using the Pasi Earth (16GL)

Resistivity meter. The result was presented as a pseudo-section, 2-D resistivity map and

geoelectric sections and interpreted with DIPRO software to provide both lateral and vertical

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information of the study area. Three geoelectric layers were delineated from the results; these

are the top soil (sandy clay), weathered layer (clayey sand) and fresh basement. The pseudo-

section and resistivity map suggest that clayey material constitute some part of the subsoil

materials. The building failure is manifested in form of cracks.

Oyedele and Olorede (2010) integrated resistivity method and cone penetration test to unravel

subsurface geological condition at Medina Estate, Gbagada, Lagos. It was observed that

several buildings had undergone differential settlement of various degree at the site. The

integration of the two methods in the study area revealed four geoelectric layers: cum

brownish clay, silty clay, hard clay, and sand respectively. A good correlation was found

between the thickness of clay layers delineated from the VES data and that of CPT data. It

was concluded that shallow foundation may not be possible except some form of soil

improvement is done.

Fathy et al. (2012) presented geotechnical assessment of groundwater conditions around a

tilted building in Cairo, Egypt using geophysical approaches. This study is attempting to

characterize the variations in the soil properties around the city Star shopping mall, in eastern

Cairo, where a large building has tilted over the past few years. This tilting may lead to

collapse of the whole building if it continues at the same rate. An integrated geophysical

investigation including 2D electrical resistivity tomography (ERT) was used to around the

affected building to help detect possible causes of deterioration. Integrating the

interpretations of the geophysical methods, provides a combined model that reflects lateral

and vertical variation in the soil properties. This variation becomes dramatic near the tilted

corner of the building.

Coker (2015) integrated geophysical and geotechnical techniques for site characterization at

School of Management Area, Lagos State Polytechnic, Ikorodu. Both techniques were used

to delineate the subsurface geology at the School of Management Area, Ikorodu, Lagos.

Based on the results of investigations, the main lithological unit consists of sandy clay and

sandy materials. It is concluded that the northern part of the study area consist of sandy clay,

a mechanically unstable soil formations which is capable of being inimical to building

structures and the southern part consist of the sand layer which is viewed as the only

competent geo-material for the foundation of engineering structures within the study area.

2.1 CONCEPT OF ELECTRICAL RESISTIVITY SURVEY

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Electrical resistivity of the soil can be considered as a proxy for spatial and temporal

variability of many other soil physical properties (structure, water content or fluid

composition). Because the method is non-destructive and very sensitive, it offers a very

attractive tool for describing the sub-surface properties without digging. It has been already

applied in various contexts like groundwater exploration, landfill and solute transfer

delineation, agronomical management by identifying areas of excessive compaction or soil

horizon thickness and bedrock depth, and at least assessing soil hydrological properties. The

surveys, depending on the area heterogeneities can be performed in one-, two- or three-

dimensions and also at different scales resolution from the centimetric scale to the regional

scale (Anatja et al., 2005). The integration of electrical resistivity with geotechnical

techniques have achieved a lot of advances due to the numerous studies that have been done

to access its methodology, advantages and limitations. Notable among the numerous works

on electrical resistivity which were used to delineate the various lithological units that

constitute the overburden (Onu et al., 2006). Electrical resistivity mapping is used for

detecting local relatively shallow inhomogeneities, geological mapping of fractures and

cavities (Olorunfemi and Meshida, 1987). For any engineering and geotechnical site

investigation in both sedimentary and areas underlain by crystalline basement complex rocks

drilling of exploratory boreholes are often embarked upon by most of construction and

consulting engineering firms to determine the depth to bedrock and the type of overburden

materials (Olayinka and Oyedele, 2001).

2.1.1 ELECTRICAL RESISTIVITIES OF GEOLOGICAL MATERIALS

The ground resistivity is related to various geological parameters such as the mineral and

fluid content, porosity and degree of water saturation in the rock (Loke, 1999). Generally,

most rock resistivities are roughly equal to that of pore fluids divided by the fractional

porosity. Archie’s law provides a closer approximation in most cases which is given by

equation 2.1:

ρr=a∅−m S−n ρw (Archie, 1942) (2.1)

where

∅ is the porosity,

S is the fraction of pores containing water

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ρw is the resistivity of water

a, m, and n are empirically determined constants i.e. (0.5 < a < 2.5, 1.3 < m < 2.5 and n ~ 2).

Resistivity values of common geological materials are given in Table 2.1 (Telford et al., 1990)

Table 2.1 Resistivity Values for Some Common Geological Formations (Telford et al., 1990).

Material Nominal resistivity (ῼm)Quartz 3 х 102 - 106

Granite 3 х 102 - 106

Granite (weather) 30 - 500Consolidated shale 20 – 2 х 103

Sandstones 200 - 5000Clays 50 - 200Boulder clay 1 - 102

Clay ( very dry) 15 - 35Gravel ( saturated) 50 - 150Lateritic soil 1400Dry sand soil 100Sand clay/ clayed sand 120 - 750Sand and gravel (saturated) 80 - 1050Mudstone 30 – 215Siltone 30 - 225Consolidated shale 20 - 120Sandstones 20 - 150

2.1.2 PRINCIPLES OF ELECTRICAL METHOD

The electrical resistivity survey is based on the principle that the earth material being tested

acts as a resistor in a circuit. After inducing an electrical current into the ground, we measure

the ability of that material to resist the current. Since various earth materials exhibit

characteristic resistivity values, they can be distinguished using this method. Ground

resistivity is measured by passing an electric current through the ground using two electrodes

(C1 and C2) and measuring the resultant potential using two or more potential electrodes (P1

and P2). Figure 2.1 illustrates this principle of operation. The depth of investigation is often

given as a function of the electrode spacing. That is to say that the greater the spacing

between the outer current electrodes, the deeper the electrical currents will flow in the Earth,

thus the greater the depth of exploration. Therefore, the depth of investigation is normally

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20% to 40% of the current electrode spacing depending on the structure of the Earth

resistivity. (Reynold, 1997)

Ohms law is generally used to calculate the resistance which is then multiplied by a

geometric factor (usually called a K factor) to calculate resistivity (MacDonald et al., 2002)

as shown in equations 2.2 - 2.4.

Assuming an electrically conductive body lends itself to the description of a one-dimensional

body (like a wire), the relationship between the current and potential distribution could be

described by Ohm’s law as expressed in equation 2.2:

V = IR (Reynold, 1997) (2.2)

Where; V = the potential difference (in volts),

I = current (in Amperes)

R = resistance (in ohms).

The resistance is therefore expressed in equation (2.3)

R=VI=ρ( L

A ) (2.3)

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Fig. 2.1: Schematic diagram illustrating basic concept of electrical resistivity measurement (Walton, 2010).

For an area, A(2π r) equation 2.1 could be rewritten in terms of Voltage, V as:

V= ρI2 πr (2.4)

Considering an electrode pair with current I at electrode C1, and –I at electrode C2 as shown

in Figure 2.1 above, the potential at any point is given by the algebraic sum of the individual

contributions. Hence,

V=V C1+V C 2=ρl( 12 πrC 1

−1

2 πr c2 ) (2.5)

Where; rC1 and rC2 = distances from the point between electrodes C1 and C2 respectively.

For the potential electrodes, P1 and P2 in Figure 2.1, the potential at any point is given as:

V = V P 1−¿ V p2= ρl( 1C 1 P 1−

1C 2 P1 +

1C 2 P 2−

1C 1 P 2 )¿ (2.6)

Where; Vp1 and Vp2 = potential at P1 and P2

C1 P1 = distance between C1 and P1

C1 P2 = distance between C1 and P2

When 1

2 π ( 1AM

− 1BM

+ 1BN

− 1AN )= 1

K , Equation becomes

ρ=KVI

=Rapp K (2.7)

where; ρ=resistivity (¿ohmmetre ), Rapp=apparent resistance(¿ ohm) and K = geometric

factor

The geometric factor, K varies for different electrode configurations. According to

Vogelslang. (1994), the geometric factor, K for the Wenner array is 2 π a . That of the

Slumberger array is given as πa

¿ and the dipole is given as

π n(n+1)(n+2) where a = electrode spacing, s = distance, n = dipole length factor

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2.1.3 MODE OF DEPLOYMENT OF ELECTRODES ARRAYS

The most commonly used configurations are the Wenner, Schlumberger and double-dipole

arrangements (Lowrie, 1997). In each configuration, the four electrodes are collinear but their

geometries and spacings are different. The generalized form is shown in Figure 2.2

Fig 2.2 The generalised form of the electrode configuration used in resistivity measurements (Herman, 2001)

WENNER ARRAY CONFIGURATION

Figure 2.3: Wenner array configuration ( Reynold, 1997)

In the Wenner configuration (Fig. 2.3), the current and potential electrode pairs have a

common mid-point and the distances between adjacent electrodes are equal, so that rA = RB

= a, and rB = RA = 2a. Inserting these values in equation 2.6 gives

ρ=2π VI

{( 1a− 1

2 a )−( 12 a

−1a)}

−1

(2.8)

ρ=2 πa VI (2.9)

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SCHLUMBERGER ARRAY CONFIGURATION

FIGURE 2.4: Schlumberger Array Configuration (Reynold, 1997)

In the Schlumberger configuration (Fig. 2.4) the current and potential pairs of electrodes

have a common mid-point, but the distances between adjacent electrodes differ. Let the

separations of the current and potential electrodes be L and a, respectively.Then r A=¿RB=(L−a2 )¿

and RA = r B=¿ L−a2

¿ . Substituting in the general formula, we get the resistivity as expressed in

the equation 2.10:

ρ=2π VI {( 2

L−a + 2L+a )−( 2

L+a− 2L−a )}

−1

(2.10)

ρ= π4

VI (L2−a2

a ) (2.11)

DIPOLE-DIPOLE ARRAY CONFIGURATION

In the double-dipole configuration (Fig. 2.5) the spacing of the electrodes in each pair is a,

while the distance between their mid-points is L, which is generally much larger than a. Note

that detection electrode D is defined as the potential electrode closer to current sink B. In this

case RA = rB = L, rA = L+a, and RB = L – a. The measured resistivity is expressed in

equations 2.12 and 2.13:

ρ=2π VI {( 1

L−1

L−a )−(1

L+a−1L )}

−1

(2.12)

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ρ=π VI ( L (L2−a2)

a2 ) (2.13)

Fig 2.5. Dipole – dipole Array Configuration (Reynold, 1997)

2.1.4 APPLICATION OF ELECTRICAL RESISTIVITY SURVEYING

Electrical resistivity surveys have been used for many decades in hydrogeological, mining

and geotechnical investigations. More recently, it has been used for environmental surveys

(Loke, 1999). Electrical resistivity techniques are either used in the profile mode (usually in

dipole-dipole surveys) to map lateral changes and detect near-vertical features (eg. fracture

zones) or in the sounding mode (usually in Schlumberger soundings) for determining depths

to geoelectric horizons (eg. depth to saline groundwater). Other common applications

include: estimating the depths to bedrock, to the water table or to other geoelectric

boundaries, delineation of aggregate deposits for quarry operations, measuring resistance for

electric grounding circuits or for cathodic protection and mapping of other geologic features.

2.1.5 LIMITATION OF ELECTRICAL RESISTIVITY SURVEY

Resistivity surveying is an efficient method for delineating shallow layered sequence or

vertical discontinuties involving changes of resistivity. It does however, suffer from a

number of limitation (Anatja et al., 2005)

(i) interpretation are ambigious. Consequently, independent geophysical and geological

controls are necessary to determinaste between valid alternatives interpretations of the

resistivity data.

(ii) Interpretation is limited to simple structural configuration. Any deviation from these

simple situation may be impossible to interprete.

(iii) Topography and the effect of new surface resistivity variation can mask the effect

deeper variations.

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(iv) The depth of penetration of the method is limited by maximum eletrical power that

can be introduced into the ground and by the physical difficulties of laying out long

length of cable. The pratical depth limit for most survey is about 1km

The susceptibility to interferance from nearby metal fences buried pipes and cable and other

metalliferous materials and the decrease in its effectiveness at a very low resistivity values

are some of its draw backs.

2.2 GEOTECHNICAL INVESTIGATION

Geotechnical investigations are performed by geotechnical engineers or engineering

geologists to obtain information on the physical properties of soil and rock around a site to

design earthworks and foundations for proposed structures and for repair of distress to

earthworks and structures caused by subsurface conditions. A geotechnical investigation will

include surface and subsurface exploration of a site. Sometimes, geophysical methods are

used to obtain data about sites. Subsurface exploration usually involves soil sampling and

laboratory tests of the soil samples retrieved.

To obtain information about the soil conditions below the surface, some form of subsurface

exploration is required. Methods of observing the soils below the surface, obtaining samples

and determining physical properties of the soils and rocks include test pits, trenching

(particularly for locating faults and slides planes), boring and insitu tests (Winterkorn et al.,

2006)

2.2.1 BASIC PRINCIPLE OF STATIC DUTCH CONE PENETRATION TEST

The cone penetration is a method used to determine the geotechnical engineering properties

of soils and delineating soil stratigraphy. It was initially developed in 1950s at the Dutch

Laboratory for Soil Mechanics in Delft to investigate soft soils. Based on this history, it has

also being called “Dutch cone test”. The early application of CPT mainly determined the soil

geotechnical property of bearing capacity. The original cone penetrometers involved sample

mechanical measurements of the total penetration resistance to pushing a tool with a conical

tip into the soil. Different methods were employed to separate the total measured resistance

into components generated by the conical tip (the tip friction) and friction generated by the

rod string. A friction sleeve was added to quantify this component of friction and aid in

determining soil cohesive strength in the 1960s ( Begemann et al., 1965).

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2.2.2 Mode of Operation of Cone Penetrometer Machine

The equipment is operated manually and has a base area of 1000 mm2 and an apex angle of

60°. With this arrangement, it was possible to measure the point resistance of the soil

encountered as the cone and the rods were driven through the soil. Measurements are read on

the attached gauge meter. The penetrometer readings were taken at interval of 250 mm and it

is presented in a graphical form. The tests were terminated when the machine had achieved

its maximum capacity and could no longer penetrate or when the anchorage were lifted. The

use and application of the static cone penetration test, CPT is being more and more frequently

considered for the insitu investigation of soils for engineering purposes. In this `test, a cone

on the end of a series of rods is pushed into the ground at a constant rate and continuous

measurements are made of the resistance to penetration of the cone defined in terms of cone

resistance, qc, and of a surface sleeve defined as local side sleeve friction, fs. (Murthy, 2002)

There are a variety of shapes and sizes of penetrometers being used for site investigation.

However, the one that is standard in most countries is the cone with an apex angle of 60° and

a base area of 10 cm2 with a friction sleeve having an area of 150 cm2. To obtain cone

resistance, qc and sleeve friction, fs a mechanical “friction jacket” cone developed in1953

(Begemann, 1969) shown in Fig. 2.7(a) can be advanced separately by means of sounding

rods pushed vertically into the soil at a constant rate of 2 cm/sec. Initially, the cone is pushed

through a distance of 5cm to measure qc and with further advancement of the cone, a flange

engages the friction jacket to measure both qc and fs. Subtracting qc from the latter reading

gives fs value at the corresponding depth. A further development is the “electric cone” in

which qc and fs can be measured independently and continuously with penetration by means

of load cells installed the body of the probe. Cone penetrometers that could also measure pore

water pressure (piezocone) were introduced in 1974. (Holden, 1974) with the filter element

placed close behind the cone as shown in Fig. 2.6(b).

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(a) Mechanical Cone (b) Electrical Cone tip

Figure 2.6: Different cone rod types (a) Mechanical Cone (b)Electrical Cone Tip by (Begemann, 1969)

Cone penetration resistance is obtained by diving the total force QC acting on the cone by the sbase area AC of the cone as shown in equation 2.13 (Begmann, 1953).

qc=Qc

A c

(Begmann, 1953) (2.14)

Fig.2.7: Detail of 60°/10cm2 piezocone (Begmann, 1953)

Local side friction is presented in equation 2.14 (Begmann, 1953)

Local side friction ( f c) = Qf

A f (2.15)

Where Qf =Qt−Qc = force required to push the friction jacket

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Qt=¿¿the total force required to push the cone and friction jacket together in the case of a

mechanically penetrometer.

A f=¿¿ surface area of friction jackect

Friction Ration, R f = F c

qc (2.16)

F c and qc are measured at the same depth and R f is expressed as a percentage. The friction

ratio is an important parameter for classifying soil.

2.2.3 SOIL CLASSIFICATION AND PROFILING

The major application of the CPT is for soil classification and description of soil strata

penetrated i.e. soil profiling as shown in Table 2.2. Typically, the cone resistance qc is high

in sandy soils and low in clayey soils and the friction ratio Rf is low in sandy soils and high

in clayey soils. It has been reported by many authors that the basic CPT parameters of cone

resistance qc, skin friction fs and friction ratio, R f may be used for soil classification. The

most popular and commonly used soil classification methods based on CPT data are probably

those proposed by Begemann (1969), Schmertmann (1977), Robertson (1990) and Fellenius

and Eslami (2000). The CPT soil classification charts or methods cannot be expected to

provide accurate predictions of soil type based on grain size distribution but provide a guide

to the mechanical characteristics of the soil, or the soil behavior. These CPT classification

methods may prove to be quite useful when applied in some soils different from those for

which they have been developed but differences may well be indicated in other locations

because of their empirical nature.

Table 2.2. Soil Classification based on friction ratio R f (Sangglerate, 1972)

R f( Cone resistance friction) Type of soil0 – 0.5 Loose gravel fill0.5 – 2.0 Sands or gravels2.0 – 5.0 Clay sand mixture and silts>5 Clays, peat

.

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2.2.4 ADVANTAGES OF CPT METHOD

The advantages of the CPT method as a soil investigation tool which makes it in many

casessuperior to other techniques include the following (Zein and Ismail, 1981):

(i) The test equipment can be easily and quickly mobilized to the site

(ii) The test is relatively quick, simple and economical The test results provide

information on soils in their undisturbed or natural conditions

(iii) The test provides a continuous record of data measurement for the whole investigate

soil depth.

(iv) The test provides repeatable and reliable data i.e. not operator dependent, and

(v) There are strong theoretical basis for CPT data interpretation

2.25 DISADVANTAGES OF CPT METHOD

(i) No soil samples could be retrieved during testing and

(ii) The penetration can be restricted in gravelly and highly cemented soil layers.

2.30 STANDARD PENETTRATION TEST

2.3.1 SHELL AND AUGER BORING

Detailed explanatory notes have been offered by several authors on the procedures of the

shell and auger drilling (Tomlinson, 1997; Murthy, 2002 and Das, 2010). The borehole was

executed using the light wire rope percussion rope shell and auger drilling technique with

Tripod rig equipped with the in-situ standard penetration test (SPT) accessories. Sampling

and in-situ testing were carried out progressively with the advancement of the borehole

through the over burden as follows:

1. Disturbed samples were taken within the sediments at regular intervals and at change

of strata as deemed necessary were selected so that they were as far as possible

representative of the materials encountered in the course of drilling the borehole.

Materials from the split spoon sampler used in the standard penetration test (SPT) and

cutting shoe of the 100mm sampler were also taken as disturbed samples.

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2. In the cohesive material, undisturbed samples were taken by driving a 100 mm

diameter sampler through a total distance of 450 mm. these samples were taken at

relevant depths.

3. Standard penetration tests (SPT) were carried out at 1.5m intervals to determine

penetration resistance in cohesionless strata. The tests involve obtaining the number

of blows (N values) producing the last 300 mm of penetration in connection with

overall 450 mm penetration test, by a 63.4 kg hammer having a free fall through 760

mm.

2.3.2 LABORATORY TESTING

A wide variety of laboratory test can be performed on soils to measure a wide variety of

properties. Some soil properties are intrinsic to the composition of the matrix and are not

affected by the sample disturbance, while other properties depend on the structure of the soil

as well as its composition, and can be effectively tested on relatively undisturbed samples.

Some soil tests measure direct properties of the soil, while others measured “Index

properties” which provide useful information about the soil without directly measuring the

property desired. Soil samples description was carried out in the laboratory using Atterbeg

Test and Grain size analysis.

ATTERBEG LIMITS TEST

This is carried out to determine the consistency of a soil. The test indicates the range of

plastic state (plasticity) is defined as the property of cohesive soils which possess the ability

to undergo changes of shape without rupture and other states. The different states through

which the soil sample passes through with decrease in the moisture content are predicted. The

water content corresponding to the transition from one state to another is termed as Atterbeg

Limits and the tests required determining the limits are the Atterbeg Limits Test.

GRAIN SIZE ANALYSIS

It involves classification of soil as Gravel, Sand, Silt and Clay. Soil particles which are

coarser than 0.075 mm are generally termed as cohesionless and the finer ones as Silt, Clay &

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Peat (Organic soil) are considered fine grained. In cohesionless soils, gravitational forces

determine the engineering characteristics. Whereas interparticle forces are predominant in the

case of fine grained soils. The dependence of the behavior of a soil mass on the size of

particles leads to soil classification according to their sizes. The physical separation of a

sample of soil by any method into two or more fractions, each containing only particles of

certain sizes, is termed Fractionation. The determination of the mass of material in fractions

containing only particles of certain sizes is termed Mechanical Analysis.

2.40 GEOLOGY OF SOUTH WESTERN NIGERIA

The geology of Nigeria (Fig. 2.8) is predominantly of both basement complex and

sedimentary environment. The basement complex is basically the crystalline igneous and

metamorphic rocks. The sedimentary rocks are composed of sediments of various earth

materials (Kogbe, 1976). Roughly 50 percent of the surface area of Nigeria is covered by

sedimentary rocks.

Fig. 2.8: Geological sketch map of Nigeria showing the major geological components; Basement, Younger Granites, and Sedimentary Basins (Adegoke, 1977)

2.4.1 DAHOMEY BASIN

20

The Dahomey Basin, also called the Dahomey Embayment or West Nigerian Basin in older

literature, extends from south-eastern Ghana in the West, through Southern Togo and

southern Benin Republic (formerly Dahomey) to Southwest Nigeria (the axis of the basin and

the thickest sediments occur slightly west of the border between Nigeria and Benin Republic.

The basin is bounded on the west by faults and other tectonic structures. The Benin Hinge

line, a major fault structure marking the western limit of the Niger delta basin, marks its

eastern limit. To the west of the Benin Hinge line is the Okitipupa Ridge (Adegoke, 1969).

The Tertiary sediments of the Dahomey basin thin out and are partially cut off from the

sediments of the Niger delta basin against this ridge of basement rocks. The basin’s offshore

limit is not well defined.

2.42 STRATIGRAPHY OF DAHOMEY BASIN

The oldest part of the sedimentary sequence is Maastrichtian onshore. Offshore, however,

considerably older sediments have been penetrated by boreholes (Billman, 1976). The oldest

sediments in the basin are non-fossiliferous, folded rocks of unknown thickness but pre-

Albian in age. The youngest strata are Pleistocene to Recent in age. The Cretaceous strata

have been assigned to the Abeokuta Group by Omatsola and Adegoke (1981) and subdivided

into three formations: Ise Formation (oldest), Afowo Formation, and Araromi Formation

(youngest). The stratigraphy is expressed in Figure. 2.10

Ise Formation: This is a sequence of continental sands, grits and siltstones with a basal

conglomerate overlying the Basement Complex. Interbedded kaolinitic clays occur in places.

Ise Formation attains a known maximum thickness of about 1860 metres.

Afowo Formation: This formation, according to Omatsola and Adegoke (1981) is equivalent

to the outcropping unit referred to in literature as the Abeokuta Formation. It is mainly

composed of coarse to medium-grained sandstones with variable, but thick, interbedded

shales, siltstones and clays, the shale component progressively increasing towards the top.

Its lower part is constituted by an alternation of brackish to marginal marine strata with well-

sorted, sub rounded clean, loose fluviatile sands. Intense pyritisation of some horizons is

common.

Araromi Formation: This formation is composed of fine to medium-grained sands at the

base, overlain by shale and siltstones with thin interbedded limestones and marls. Thin

21

lignitic bands are also common. The shales are light grey to black, mostly marine and with

very high organic content.

Ewekoro Formation: The Afowo Formation (old Abeokuta Formation) is, in parts of the

basin, conformably overlain by the Ewekoro Formation. In coastal boreholes and offshore, it

is not encountered (Reyment, 1965; Billman, 1976). There, it is replaced by the

predominantly shaley Imo Formation, which unconformably overlies the Afowo Formation.

Borehole studies indicate that the formation is lens-shaped, thinning out in all directions (and

eventually disappearing) from its maximum thickness of 34 metres at Ibeshe.

At its type locality (Ewekoro Limestone Quarry), the formation consists of 10 to 12.5 metres

of thinly bedded glauconitic and sandy limestone at the base, which then becomes massive,

grey and fossiliferous in the middle and fine-grained, marly and algal in the upper part. The

top, highly scoured layer consists of red, dense glauconitic, phosphatic and fossiliferous

limestone. Most authors date the formation Upper Paleocene. The faunal/floral assemblage

indicates that the formation was deposited in a shallow, nearshore (littoral to sublittoral)

fairly warm marine environment during a regressive phase (Adegoke, 1997).

Imo Formation: The Ewekoro Formation is unconformably overlain by the predominantly

shaly Imo Formation. Where the latter is missing, the Imo Shale lies directly,

unconformable, on the Afowo (old Abeokuta) Formation. Imo Formation consists of fine-

textured dark micromicaceous shale, locally silty with glauconitic marl and conglomerate at

the base. The greenish-grey variety of the shale encountered in the subsurface of most inland

areas of Western Nigeria and which in the Ewekoro quarry disconformably overlies the

Ewekoro Formation was named Akinbo Formation by Ogbe (1972).

Oshosun Formation: The shales of the Imo Formation grade into the overlying mudstones

and claystones of the Oshosun Formation. In its lower part, the formation is composed of

dull brown and brick red sandy mudstone and claystone with light grey and purplish mottling.

Thin pebble beds and coarse pebbly sandstones are locally interbedded. Rare inclusions of

phosphatic and glauconitic material occur, becoming more abundant and characteristic in the

middle part. Light grey arenaceous sediments are locally present near its top, constituting an

unevenly developed sequence to which Jones and Hockey (1964) applied the name Ilaro

Formation (Adegoke, 1977).

22

Fig. 2.9: Geological Map of Eastern Dahomey Basin (Modify after Billman, 1992

2.4.3 LOCAL GEOLOGY AND GEOMORPHOLOGY

Lagos belongs to the coastal plain sand formation which is made up of loose sediment

ranging from silt, clay and fine to coarse grained sand. The lithoral lagoonal deposits are

made up of clay, silt, and sands of coastal plains. The coastal belt varies in width from about

8 kilometres near the Republic of Benin border to 24 kilometres towards the eastern end of

the Lagos Lagoon (Jones and Hockey, 1964). The exposed rock unit in the area consists of

poorly sorted sands with lenses of clays. The sands are in part cross bedded and show

transitional to continental characteristics according to Jones and Hockey (1964), Omatshola

and Adegoke (1981), Agagu (1985), Enu (1990) and Nton (2001). The age Oligocene to

Recent was assigned to this formation on the basis of fauna contents.

The study area has a flat topographic surface typical of alluvial sediment and coastal ground,

Soft and very soft sediments which are usually formed when fine grained materials (fine

sands and silts) are deposited in a low energy environment (e.g. settle out of suspension in a

standing water body such as a lake or swamp). The area is well vegetated with grasses, reeds

and other plants that are peculiar to swamp or waterlogged regions.

2.4.4 ACCESSIBILITY OF THE STUDY AREA

The geographic location of the site is between 6.5°N and 3.25°E (Fig. 2.10). The study area is

accessible through Isheri – Igando express way beside solus waste management company.

The dumpsite has witnessed rehabilitation which consisted of reclamation of land,

construction of accessible road for ease of tipping, spreading and compaction of waste since

23

inception. report, Lagos State Waste Management Authority reported that a total of 469,

202.50 tons of municipal solid waste (MSW) was land filled in 2007 alone (Longe and

Balogun, 2010). It is accessible by tarred roads

2.4.5 GEOLOGY AND HYDROLOGY OF THE STUDY AREA

Lagos is basically a sedimentary area located within the Western Nigeria coastal zone, a zone

of coastal creeks and lagoons developed by barrier beaches associated with sand deposition.

The subsurface geology reveals two basic lithologies; clay and sand deposits. These deposits

may be interbedded in places with sandy clay or clayey sand and occasionally with vegetable

remains and peat (Ayolabi and Peters, 2005). It is identified that the geology is made up of

sedimentary rock mostly of alluvial deposits. These consist of loose and light grey sand

mixed variously with varying proportion of vegetation matter on the lowland; while the

reddish and brown loamy soil exists in the upland. The geology is underlain by interbedded

sands, gravelly sands, silts, and clays (Akoteyon, et al., 2011). The sub-surface is made up of

semi-permeable to impermeable material (Akoteyon et al., 2011).

Fig. 2.10: BASE MAP OF THE SURVEY AREA

24

CHAPTER THREE

3.0 METHODOLOGY

3.1 DATA ACQUISITION

The electrical resistivity survey using vertical electrical sounding (VES) through

Schlumberger array and 2D resistivity data via wenner array was carried out along three

traverses within the study area. A total of thirteen vertical electrical soundings were acquired

at predetermined distances along three 2D resistivity data. VES resistivity data are presented

in Appendix A

The data were acquired using the following equipment and accessories:

PASI Terameter (16 - GL model)

Four metal electrodes

Four hammers driving the electrodes into the ground

Four measuring tapes for measuring distances for the different electrode spacing

Global Positioning System (GPS) for finding the position and elevation of the survey

point

Four reels of electric cables

Power supply- 12V 60Ah battery

Field note to record the field data

The field data set up for the acquisition of electrical resistivity data using PASI Terameter is

shown in Plate 1.

Plate 1: The field set –up for the acquisition of electrical resistivity data using PASI

resistivity meter

25

3.1.1 VERTICAL ELECTRICAL SOUNDING

Four (4) electrodes were utilised in the survey, each is about 0.4 to 0.5 m long with springs to

fasten the current take out points on the cables to the electrodes and ensure firm or good

contact. The Hammers are used in driving the electrodes about half way into the subsurface.

The measuring tapes are each 200 m long and are used to measure and ensure equal distance

within the electrodes and inter profile spacing. The GPS (Geographic Information System)

used in the survey is a 12 channel handheld Garmin GPS. It is used to take the coordinates of

the investigated points and other noteworthy positions important to this study. The power

source used in this study is two 12V batteries of which one serves as a back- up battery. 1-D

resistivity data are shown in Appendix A.

3.1.2 2D ELECTRICAL RESISTIVITY SURVEY

Three (3) traverses were occupied in the study area and Wenner array was employed on each

traverse. An inter traverse spacing of 5m was maintained within each traverse at first, after

which we took readings for 10,15, 20 and 30 m spacing for CST. The traverse runs from

North to South of the study area while some run perpendicularly (East-West of the study

area). This traverse was set out to delineate relevant deeper subsurface structures which might

peradventure exists and also to provide engineering properties of the study area. 2D

resistivity data are shown in Appendix B.

3.1.3 DATA PROCESSING AND INTERPRETATION

After converting resistance to resistivity by multiplying with appropriate geometrical factors

for the Schlumberger array, the VES data were plotted on log-log graphs with apparent

resistivity (ρa) and half electrode separation (AB/2) values on the ordinate and abscissa

respectively. The resultant curves were interpreted qualitatively through visual inspection and

quantitatively through partial curve matching technique to generate the layered apparent

resistivities and thickness. The results were further iterated using WINRESIST computer

software. The 2D Wenner resistance data was converted to resistivity by multiplying it with

the appropriate geometrical factors of2 πa, where “a” is the spacing. The appropriate

resistivity values for the 2D data set were inverted for true subsurface resistivity using

DIPROWIN version 4.0 inversion software and the resulting estimated models presented and

interpreted accordingly. The electrical resistivity structure was presented in a colour coded

26

format or contour map of which the electrical resistivity of each colour was provided in a

colour scale bar. The horizontal scale bar on the 2D Electrical Resistivity structure is the

horizontal or lateral distance on the ground. The vertical scale bar provides the depth into the

surface. Both the vertical scale and horizontal scales are linear while the colour scale bar is

logarithmic.

The electrical resistivity variation along each profile was observed and relatively low

electrical resistivity was associated to the clay content of the subsurface. Since the electrical

resistivity of rocks are related to the size of the rock matrix, clayey sand is also associated to

low resistivity while sand are associated to relatively high resistivity value in the study area.

3.2 CONE PENETRATION TEST

Four Dutch Cone Penetrometer Test (Plate 2) was carried out each to the refusal of cone rod

penetration and anchor pulls. The equipment is operated manually and has a base area of

1000 mm2 and an apex angle of 60°. With this arrangement, it was possible to measure the

point resistance of the soil encountered as the cone and the rods were driven through the soil.

Measurements are read on the attached gauge meter. The penetrometer readings were taken at

interval of 250 mm and presented in a graphical form. The tests were terminated when the

machine had achieved its maximum capacity and could no longer penetrate or when the

anchorage were lifted. CPT data are presented in Appendix C.

Plate 2: CPT data acquisition process

3.3 BOREHOLE DATA

27

Borehole data within the study area was collected to correlate VES data, 2D data and CPT

data and also used to delineate competent layer within the subsurface for foundation type

recommendation.

CHAPTER FOUR

4.0 RESULTS AND DISCUSSION

4.1 RESULTS

VES curves and summary of interpreted VES results are presented in Appendix C. Borehole

log is presented in Figure 4.1. The geoelectric sections along the three traverses are presented

in Figures 4.2(a-c). The 2D resistivity structures are shown in Figures 4.3 (a-c). The CPT

plots are presented in Figures 4.3(a-d).

4.2 DISCUSSION

4.2.1 Geoelectric Sections

4.2.1.1 Geoelectric section along AA'

Figure 4.2 (a) shows the geoelectric section which comprises VES 1, 2, 3 and 4 along

traverse one. It has four geoelectric layers including; topsoil, clay/peat, sandy clay, clayey

sand and sand. The topsoil is characterized by resistivity values ranging from 75.2 – 155.3Ω

with layer thickness of 0.5 – 0.8 m. The second horizon depicts clay in VES 1 and VES 3

with resistivity value ranging from 22.3 – 28.7 Ωm and layer thickness of 1.2 – 1.4 m while

in VES 2 and VES 4, the geoelectric units is indicative of sandy clay having resistivity value

ranging between 53.6 – 63.7 Ωm and layer thickness of 1.0 m. The third geoelectric layer

VES 1, 3 and 4 connotes clay/peat having resistivity values and layer thickness ranging from

2.9 – 7.5 Ωm and 0.5 – 17.8 m respectively while in VES 2, the geoelectric unit indicates clay

with resistivity value of 13.9 Ωm and thickness of 11.9 m. The fourth stratum in VES 1 and

VES 3 denotes sand with resistivity values ranging from 110.3 – 461.3 Ωm, but the

thicknesses cannot be determined due to current termination within the region. The fourth

geoelectric layer in VES 2 and VES 4 represents clayey sand having resistivity value between

58.1 – 66.6 Ωm but the thicknesses cannot be determined due current termination.

28

Figure 4.1 presents borehole log generated from the borehole information of the study site.

The borehole information correlated well with the geologic formation delineated by the

resistivity survey. The competent layer was encountered at a depth of 18 m, this corresponds

with the depth at which a geoelectric layer symptomatic of sand was delineated in the

geoelectric section.

.

Figure 4.2 (a): Geoelectric Section along profile AA'

29

30

Figure 4.1: Borehole Log

4.2.1.2 Geoelectric section along BB'

Fig. 4.2(b) shows the geoelectric section generated from VES 1, 2, 3 and 4 along traverse

two. The first geoelectric layer is the topsoil with resistivity value ranges from 76 – 307.4

Ωm and thickness value between 0.5 – 0.7 m. The second layer depicts clay in VES 1, 2 and

3 with resistivity value ranging from 14.2 – 26.8 Ωm and thickness ranging from 1.4 – 1.7 m

while at VES 4, it depicts sandy clay with resistivity value of 57.4 Ωm and thickness of 1.5

m. The third horizon connote clay/peat in VES 1,2 and 3 with resistivity value ranging from

4.2 – 9.5 Ωm and thickness of 6.0 – 23.9 m while in VES 4, it depicts clay with resistivity

value of 10.1 Ωm and thickness of 3.8 m. The fourth layer is indicate of clayey sand with

resistivity value ranging from 34 – 83.9 Ωm in VES 1, 2 and 3. Their thicknesses could not

be determined due to current termination. VES 4 extend to fifth layer with the fourth layer

indicating clay/peat with resistivity value of 6.1 Ωm and thickness of 17.5 m.

The borehole information shows that a competent layer was encountered at a depth of 18 m in

the borehole. At this depth on the geoelectric section along traverse two, a horizon

corresponding to peat and sandy clay was indicated. This represents an incompetent layer.

31

32

Figure 4.1 (b): Geoelectric section along BB'

4.2.1.3 Geoelectric section along CC'

Fig. 4.2 (c) shows the geoelectric section generated from VES 1, 2, 3, 4 and 5 along traverse

3 with geoelectric layers including; topsoil, clay/peat, peat and clayey sand or sandy clay.

The first geoelectric layer has resistivity value ranging from 63.5 – 224.3 Ωm with thickness

between 0.5 – 0.8 m. This corresponds to topsoil. The second layer depicts clay in VES 1, 2,

3 and 4 with resistivity value ranging from 13.5 – 32.4 Ωm and thickness 2.2 – 2.9 m while in

VES 5, it depicts clay/peat with resistivity value of 4.1 Ωm and thickness of 17.1 m. The third

layer depicts clay/peat with resistivity value ranging from 4.1 – 9.2 Ωm and thickness

between 4.9 – 17.1 m. The fourth layer connote clayey sand in VES 3, 4 and 5 while the

thickness cannot be determined due to current termination. VES 1 and VES 2 extend to fifth

layer with fourth layer of VES 1 depicting clay/peat with resistivity value of 4.5 Ωm and

thickness of 16.5 m and fourth layer of VES 2 depicts peat with resistivity value of 1.6 Ωm

and thickness of 9.3 m. The fifth layer of VES 1 and VES 2 is indicative of clayey sand with

resistivity value ranging from 49.9 – 79.2 Ωm but the thicknesses could not be determined

due to current termination.

33

The borehole information shows that a competent layer was encountered at a depth of 18 m in

the borehole. At this depth on the geoelectric section along this traverse, a horizon

corresponding to peat and sandy clay was indicated. This represents an incompetent layer.

Figure 4.1 (c): Geoelectric section along CC'

34

4.2.2 Electric Resistivity Imaging

4.2.2.1 2D Resistivity Structure along Traverse 1

A total spread of 100m was surveyed and a depth of 25 m was probed with resistivity values

ranging from 6 -108 Ωm as shown in Figure 4.2 (a).At depth between 0-2 m, the subsurface

stratum is diagnostic of sandy material with resistivity value ranging from 53-108 Ωm with

the exception of lateral distance between 55-70 m where the geoelectric material connotes

clayey material. This is expected as this layer represent landfill materials. This layer is

underlain by a horizon of clayey/peaty material with resistivity value ranging from 6-13 Ωm

at depth of between 2- 12 m. This layer represent an incompetent layer which cannot support

building foundation. At depth 12 m and below, the geoelectric layer is predominantly sandy

clay with resistivity value ranging from 26-58 Ωm, this is a relatively competent layer as

indicated by the borehole information of the area. But at lateral distance between 70 m and

beyond, the subsurface material is indicative of clay/peat.

Fig. 4.2 (a): 2D Resistivity Structure along Traverse 1

35

PeatClay

Clayey Sand

Sand

4.2.2.2 2D Resistivity Structure along Traverse 2

A total spread of 100m was surveyed and a depth of 25 m was probed with resistivity values

ranging from 1 -54 Ωm as shown in Figure 4.2 (b). At depth between 0 – 2 m, the subsurface

stratum is a diagnostic of sandy clay material with resistivity value ranging from 45 – 55 Ωm

with exception of lateral distance between 45 – 60 m where the geoelectric material with

resistivity ranging from 10-23 Ωm. This is indicative of clayey material. This layer overlays a

horizon with resistivity value ranging from 1.2 – 8.0 Ωm at depth between 2- 10 m. This

layer is indicative clay/peat. At lateral distance between 15-32 m within the horizon, a pot of

highly conductive material with resistivity value between 1.2-2.8 Ωm which is indicative of

peat was mapped. At depth 15 m and below, a geoelectric layer with resistivity value ranging

from 3- 54 Ωm is mapped. This is dominantly clayey sand except at lateral distance of

between 15-35 m and 70-80 m with lower resistivity. This suggests sandy clay.

Figure 4.2 (b): 2D Resistivity Structure along Traverse 2

4.2.2.3 2D Resistivity Structure along Traverse 3

36

Peat

Clay

A total spread of 100m was surveyed and a depth of 25 m was probed with resistivity values

ranging from 4 -28 Ωm as shown in Figure 4.2 (c). At depth between 0-2 m, the subsurface

stratum is diagnostic of clayey material with resistivity value ranging from 9-29 Ωm. This

layer is underlain by a horizon of clayey/peaty material with resistivity value ranging from 6-

19 Ωm at lateral distance between 15 – 90 m. This layer represent an incompetent layer

which cannot support building foundation. At depth 5 m and below, the geoelectric layer is

predominantly peat with resistivity value ranging from 4.1 – 5.1 Ωm at lateral distance

between 15 – 43 m and vertically distinctive between 5 – 25 m. Below this stratum is

basically clay with resistivity value ranging from 12 – 15 Ωm at lateral distance between 50 –

90 m and vertical distance 15 – 25 m.

Figure 4.2 (c): 2D Resistivity Structure along Traverse 3

4.3 CONE PENETRATION PLOT

4.3.1 CPT PLOT 1

37

Fig 4.3(a) shows the plot of cone resistance value of CPT 1 recorded to a maximum depth of

3.75 m. At depth between 0 – 0.5 m, the cone resistance reading was uniform with cone

resistance value of 0 kg/cm2. This signifies an incompetent layer i.e. peat. At depth between

0.5 – 3.75 m, the cone resistance reading increases almost continuously with cone resistance

value ranging from 0 – 60 kg/ cm2. This is indicative of dump refuse and decomposed organic

materials as observed from the geotechnical borehole.

Coordinate: 06° 34' 312" N 003° 15' 229" E Elevation: 121ft

Figure 4.3 (a): A Graph of Depth (m) against Cone Resistance (kg/cm2) for CPT 1.

4.3.2 CPT PLOT 2

The cone resistance reading for CPT 2, 3 and 4 was recorded to a maximum depth of 4.25 m

as shown in Fig 4.3 (b, c and d), and all show similar pattern. At depth between 0 – 0.5 m,

the cone resistance reading was uniform with cone resistance value of 0 kg/cm2. This shows

38

that the cone end penetrates soft organic/dump refuse material. At depth between 0.5 – 4.25

m, the cone resistance reading increases with depth with cone resistance value ranging from 5

– 70 kg/ cm2. This signifies that the competency of the subsoil material increase with depth.

But from the information obtained from the borehole log, the subsoil material at this depth

shows dump refuse and decomposed organic materials. This increase in cone resistance value

with depth observed on the cone resistance readings might be due to compaction of the refuse

dump in the area.

Coordinate: 06° 34' 305" N 003° 15' 219" E Elevation: 124ft

Figure 4.3 (b): A Graph of Depth (m) against Cone Resistance (kg/cm2) for CPT 1.

CPT PLOT 3

Coordinate: 06° 34' 299" N 003° 15' 210" E Elevation: 129ft

39

Figure 4.3 (c): A Graph of Depth (m) against Cone Resistance (kg/cm2) for CPT 3

CPT PLOT 4

Coordinate: 06° 34' 297" N 003° 15' 227" E Elevation: 126ft

40

Figure 4.3 (d): A Graph of Depth (m) against Cone Resistance (kg/cm2) for CPT 4

4.4 CORRELATION OF VES, 2D RESISTIVITY IMAGING, CPT DATA AND

BOREHOLE LOG

41

The integration of the methods revealed similar soil layering consisting of topsoil, clay/peat,

sandy clay and sand.

The topsoil has resistivity value ranging from 63.5 - 307.4 ῼm and thickness between 0.5 -

0.8 m. The borehole reveal topsoil from the surface to about 0.75 m consisting of dump

refuse materials while the cone resistance reading ranges from 0 kg/cm2 to 10 kg/cm2 which

is mainly dump refuse materials.

The second layer consists of clay materials with resistivity value varying from 13.5 – 32.4

ῼm and the thicknesses ranging from 1.2 - 11.9 m. The borehole reveal landfill

materials/decomposed organic materials from 0.75 - 11.25 m with cone resistance reading

ranges from 5 - 70 kg/cm2. The cone penetration could not go beyond 4.25m before the

anchor pulls due to the buried dump refuse materials.

The third layer reflects clay/peat with resistivity value ranges from 2.6 – 9.2 ῼm and

thickness between 0.5 – 17.8 m. The borehole reveal at depth of 11.25 - 17.25 m is made of

firm to stiff brown lateritic clay across with NSPT value ranges from 7 – 13m.

The fourth layer ranges from medium fine to coarse sand in the borehole log at depth between

18 – 24 m but it is indicative of clayey sand in some of the VES points with resistivity value

ranging from 34.0 – 83.9 ῼm while others, connotes sand with resistivity value ranging from

110.3 – 461.3 ῼm and their thickness could not be determined due current termination within

zone.

CHAPTER FIVE

5.0 CONCLUSION AND RECOMMENDATION

42

5.1 CONCLUSION

An integrated geophysical and geotechnical survey was carried out at West Africa ENRG

KM 3, Isheri – Igando Road, Alimosho LGA of Lagos State, South Western -Nigeria in

order to characterise the engineering competency of the subsurface.

The geoelectric sections reveal four to five subsurface layers which correspond to topsoil,

clay, peat/clay, clayey sand and sand. The resistivity value of the topsoil varies from 63.5 -

307.4 ῼm while the thickness ranges from 0.5 – 0.8 m. The resistivity value of clay varies

from 13.5 - 32.4 ῼm with thickness varying from 1.2 – 11.9 m. The third layer is indicative

of clay/peat with resistivity value ranging from 2.6 – 9.2 Ωm and thickness varying from 0.5

– 17.8 m. The fourth layer is indicative of sand in VES 1 and VES 3 with resistivity value

ranging from 110.3 – 461.3 Ωm while in other VES data, the fourth layer is indicative of

clayey sand with resistivity value ranging from 34.0 – 83.9 Ωm but their thickness cannot not

be determined due to current termination within the zone. The results from the 2D resistivity

structures also reveal that the subsurface is composed of topsoil, clay, peat/clay, clayey sand

and sand. These correlate with the result of VES data acquired on the various traverses along

the study area.

The cone resistance reading for CPT data was recorded to a maximum depth of 4.25 m before

the 2.5 tons Dutch Cone Penetrometer anchor pulled out. At depth between 0 – 1.25 m, the

cone resistance reading was 5 kg/cm2 which passes through soft dark sandy clay with dump

refuse material. At depth between 1.25 – 4.25 m, the cone resistance reading ranged from 5 –

75 kg/ cm2 which is indicative of dump refuse and decomposed organic materials as observed

from in the geotechnical borehole.

From borehole data, the subsoil condition can be describe as strata of refuse dumps, clay and

sand. The depth from surface to 0.75m below is regarded as topsoil, 0.75 – 11.25 m is landfill

materials, 11.25 – 18.00 m is medium fine to coarse sand, 18 – 24 m is medium fine to coarse

sand and the depth from 24 m to the termination of the borehole at 30 m is dense fine to

coarse sand with occasional gravel.

5.2 RECOMMENDATION

43

As at the time of this survey, information about proposed load and shaft resistance was not

provided hence, settlement and safe working load cannot be determined. However, deep

foundation inform of piling is recommended to be placed at the depth 22 m beneath the

surface since both resistivity structure and boring data reflect competent layer at 18 m below.

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Akoteyon, I.S, Mbata, U.A and Olalude, G.A. (2011). Investigation of Heavy Metal

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Anatja, S., Isabelle, C., Alain, T., Ary, B. and Guy, R. (2005). Electrical resistivity in Soil

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49

APPENDIX A:

VES RESISTIVITY DATA

TABLE A.1: VES 1 – VES 6

50

TABLE A.2: VES 7 - 13

ElectrodeSeparationAB/2

K-factor

VES 7 VES 8 VES 9 VES 10 VES 11 VES 12 VES 13

1 6.28 15.2 37 11 20.1 8.4 7.2 10.32 25.12 1.5 3.1 1.4 1.3 0.889 0.936 23 56.54 0.31 0.884 0.35 0.288 0.251 0.272 0.5614 100.54 0.112 0.279 0.122 0.095 0.082 0.103 0.2546 226.2 0.035 0.074 0.031 0.026 0.023 0.035 0.0736 113.1 0.065 0.095 0.055 0.054 0.05 0.08 0.1589 254.47 0.044 0.028 0.015 0.018 0.019 0.024 0.064

12 452.4 0.012 0.019 0.01 0.01 0.011 0.009 0.04215 706.86 0.007 0.013 0.007 0.007 0.009 0.006 0.04415 353.45 0.01 0.027 0.019 0.02 0.013 0.01 0.02720 628.32 0.012 0.009 0.012 0.013 0.007 0.004 0.01725 981.75 0.007 0.008 0.009 0.009 0.005 0.003 0.01332 1608.5 0.005 0.004 0.008 0.007 0.005 0.0025 0.00540 2513.28 0.003 0.005 0.009 0.005 0.004 0.0023 0.00440 1005.31 0.004 0.007 0.021 0.006 0.008 0.0064 0.00750 1570.8 0.003 0.012 0.01 0.003 0.004 0.004 0.00465 2654.65 0.009 0.005 0.005 0.008 0.005 0.001 0.003

0.0034 0.003

51

ElectrodeSeparationAB/2

K-factor VES 1 VES 2 VES 3 VES 4 VES 5 VES 6

1 6.28 9.3 11.2 13 19.8 8.5 20.12 25.12 1.4 1.9 2.1 3.3 1.2 1.33 56.54 0.391 0.487 0.573 0.821 0.467 0.2884 100.54 0.17 0.112 0.234 0.291 0.162 0.0956 226.2 0.047 0.043 0.114 0.086 0.038 0.0266 113.1 0.086 0.079 0.353 0.16 0.079 0.0549 254.47 0.033 0.026 0.138 0.043 0.053 0.018

12 452.4 0.029 0.013 0.037 0.025 0.036 0.0115 706.86 0.026 0.01 0.04 0.029 0.077 0.00715 353.45 0.025 0.02 0.08 0.076 0.025 0.0220 628.32 0.018 0.018 0.023 0.039 0.011 0.01325 981.75 0.016 0.013 0.015 0.014 0.091 0.00932 1608.5 0.014 0.006 0.005 0.008 0.005 0.00740 2513.28 0.005 0.005 0.004 0.012 0.004 0.00540 1005.31 0.015 0.007 0.012 0.03 0.018 0.00650 1570.8 0.014 0.006 0.007 0.012 0.015 0.00365 2654.65 0.008 0.004 0.005 0.006 0.021 0.008

APPENDIX B:

2D RESISTIVITY DATA

TABLE C. 1: 2D data along traverse 1

52

Wenner traverse 1 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 1: a = 5C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)

0 5 10 15 0.747931 23.55 10 15 20 0.751114 23.6

10 15 20 25 0.728835 22.915 20 25 30 0.588797 18.520 25 30 35 0.684278 21.525 30 35 40 0.716104 22.530 35 40 45 0.681095 21.435 40 45 50 0.833864 26.240 45 50 55 0.601528 18.945 50 55 60 0.598345 18.850 55 60 65 0.480586 15.155 60 65 70 0.480586 15.160 65 70 75 0.432845 13.665 70 75 80 0.531509 16.770 75 80 85 0.970719 30.575 80 85 90 1.101209 34.680 85 90 95 2.399745 75.485 90 95 100 0.922979 29

Wenner traverse 1 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 1: a = 10C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)

0 10 20 30 0.177896 9.410 20 30 40 0.170326 920 30 40 50 0.174111 9.230 40 50 60 0.145723 7.740 50 60 70 0.181681 9.650 60 70 80 0.196821 10.460 70 80 90 0.183573 9.770 80 90 100 0.172218 9.180 90 100 110 0.164648 8.790 100 110 120 0.168433 8.9

100 110 120 130 0.123013 6.5110 120 130 140 0.177896 9.4120 130 140 150 0.244133 12.9130 140 150 160 0.283876 15140 150 160 170 0.230886 12.2

53

Wenner traverse 1 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 1: a = 20

C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 20 40 60 0.07638 9.620 40 60 80 0.08832 11.140 60 80 100 0.094685 11.960 80 100 120 0.086728 10.980 100 120 140 0.123329 15.5100 120 140 160 0.115372 14.5120 140 160 180 0.10105 12.7140 160 180 200 0.08832 35.4160 180 200 220 0.094685 108.1

54

Wenner traverse 1 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 1: a = 15

C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 15 30 45 0.601528 56.715 30 45 60 0.263102 24.830 45 60 75 0.200509 18.945 60 75 90 0.131551 12.460 75 90 105 0.159134 1575 90 105 120 0.098663 9.390 105 120 135 0.134734 12.7105 120 135 150 0.342669 32.3120 135 150 165 0.199448 18.8135 150 165 180 0.100785 9.5150 165 180 195 0.086993 8.2165 180 195 210 0.089115 8.4

Wenner traverse 1 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 1: a = 25

C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 25 50 75 0.077021 12.125 50 75 100 0.1623170 25.550 75 100 125 0.085933 13.575 100 125 150 0.085296 13.4100 125 150 175 0.122852 19.3125 150 175 200 0.276527 43.4

55

Wenner traverse 1 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 1: a = 30C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 30 60 90 0.046149 8.730 60 90 120 0.131021 24.760 90 120 150 0.1611787 30.5

TABLE C. 2: 2D along traverse 2

Wenner traverse 2 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 1: a = 5C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)

0 5 10 15 0.671547 21.15 10 15 20 0.318269 10

10 15 20 25 0.29599 9.315 20 25 30 0.257798 8.120 25 30 35 0.270528 8.525 30 35 40 0.337365 10.630 35 40 45 0.273711 8.635 40 45 50 0.611076 19.240 45 50 55 0.232336 7.345 50 55 60 0.690643 21.750 55 60 65 0.413749 1355 60 65 70 0.865691 27.260 65 70 75 0.862508 27.165 70 75 80 1.101209 34.670 75 80 85 2.399745 75.475 80 85 90 0.789306 24.880 85 90 95 1.600891 50.385 90 95 100 1.600891 50.3

Wenner traverse 2 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 2: a = 10C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)

0 10 20 30 0.401211 21.210 20 30 40 0.124905 6.620 30 40 50 0.02271 1.230 40 50 60 0.062453 3.340 50 60 70 0.09841 5.250 60 70 80 0.141938 7.560 70 80 90 0.102195 5.470 80 90 100 0.117335 6.280 90 100 110 0.130583 6.990 100 110 120 0.14383 7.6

100 110 120 130 0.107873 5.7110 120 130 140 0.228993 12.1120 130 140 150 1.038986 54.9130 140 150 160 0.283876 15140 150 160 170 0.230886 12.2

56

Wenner traverse 2 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 2: a = 15C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 15 30 45 0.05941 5.615 30 45 60 0.062593 5.930 45 60 75 0.068958 6.545 60 75 90 0.131551 12.460 75 90 105 0.076384 7.275 90 105 120 0.088054 8.390 105 120 135 0.381922 36105 120 135 150 0.08275 7.8120 135 150 165 0.09442 8.9135 150 165 180 0.182474 17.2150 165 180 195 0.322512 30.4165 180 195 210 0.117759 11.1

57

Wenner traverse 2 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 2: a = 20

C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 20 40 60 0.066041 8.320 40 60 80 0.080363 10.140 60 80 100 0.066041 8.360 80 100 120 0.061267 7.780 100 120 140 0.105824 13.3100 120 140 160 0.057288 7.2120 140 160 180 0.077976 9.8140 160 180 200 0.133673 16.8160 180 200 220 0.111394 14

Wenner traverse 2 Long 003° 15' 13.8" Lati 06° 34'16.7" Elev 33.7m

Traverse 2: a = 30C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 30 60 90 0.108211 20.430 60 90 120 0.074263 1460 90 120 150 0.073202 13.8

58

Wenner traverse 2 Long 003° 15' 12.6" Lati 06° 34'17.6" Elev 35.7m

Traverse 2: a = 25

C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 25 50 75 0.054742 8.625 50 75 100 0.075111 11.850 75 100 125 0.06429 10.175 100 125 150 0.078931 12.4100 125 150 175 0.070019 11125 150 175 200 0.085296 13.4

TABLE C. 3: 2D along traverse 3

Wenner traverse 3 Long 003° 15' 13.7" Lati 06° 34'19.5" Elev 33.5m

Traverse 3: a = 10C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)

0 10 20 30 0.097 6.110 20 30 40 0.078 4.920 30 40 50 0.12 7.530 40 50 60 0.132 8.340 50 60 70 0.086 5.450 60 70 80 0.088 5.560 70 80 90 0.08 5.070 80 90 100 0.09 5.7

59

Wenner traverse 3 Long 003° 15' 13.7" Lati 06° 34'19.5" Elev 33.5m

Traverse 3: a = 5C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)

0 5 10 15 0.908 28.55 10 15 20 0.494 15.5

10 15 20 25 0.611 19.215 20 25 30 0.501 15.720 25 30 35 0.385 12.125 30 35 40 0.243 7.630 35 40 45 0.291 9.135 40 45 50 0.297 9.340 45 50 55 0.317 10.045 50 55 60 0.364 11.450 55 60 65 0.314 9.955 60 65 70 0.398 12.560 65 70 75 0.383 12.065 70 75 80 0.456 14.370 75 80 85 0.452 14.275 80 85 90 0.529 16.680 85 90 95 0.619 19.485 90 95 100 0.6 18.9

80 90 100 110 0.097 6.190 100 110 120 0.11 6.9

100 110 120 130 0.117 7.4110 120 130 140 0.117 7.4120 130 140 150 0.118 7.4130 140 150 160 0.131 8.2140 150 160 170 0.192 12.1

Wenner traverse 3 Long 003° 15' 13.7" Lati 06° 34'19.5" Elev 33.5m

Traverse3: a = 15

C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 15 30 45 0.056 5.315 30 45 60 0.057 5.430 45 60 75 0.069 6.545 60 75 90 0.048 4.560 75 90 105 0.117 11.075 90 105 120 0.074 7.090 105 120 135 0.076 7.2105 120 135 150 0.087 8.2120 135 150 165 0.088 8.3135 150 165 180 0.083 7.8150 165 180 195 0.073 6.9165 180 195 210 0.095 9.0

Wenner traverse 3 Long 003° 15' 13.7" Lati 06° 34'19.5" Elev 33.5m Traverse 3: a = 20

60

C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 20 40 60 0.033 4.120 40 60 80 0.054 6.840 60 80 100 0.058 7.360 80 100 120 0.056 7.080 100 120 140 0.062 7.8100 120 140 160 0.067 8.4120 140 160 180 0.079 9.9140 160 180 200 0.062 7.8160 180 200 220 0.071 8.9

Wenner traverse 3 Long 003° 15' 13.7" Lati 06° 34'19.5" Elev 33.5m

Traverse 3: a = 25

C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 25 50 75 0.049 7.725 50 75 100 0.051 8.050 75 100 125 0.061 9.675 100 125 150 0.062 9.7100 125 150 175 0.058 9.1125 150 175 200 0.062 9.7

Wenner traverse 3 Long 003° 15' 13.7" Lati 06° 34'19.5" Elev 33.5m

Traverse 3: a = 30C 1 P 1 P 2 C 2 R(ῼ) ρ ¿ῼm)0 30 60 90 0.047 8.930 60 90 120 0.05 9.460 90 120 150 0.089 16.8

61

APPENDIX C: VES CURVES AND SUMMARY OF INTERPRETED VES DATA

62

Figure B. 1: Resistivity curve for VES 1

Figure B. 2: Resistivity curve for VES 2

Figure B. 3: Resistivity curve for VES 3

63

Figure B.4: Resistivity curve for VES 4

Figure B. 5: Resistivity Curve for VES 5

Figure B. 6: Resistivity Curve for VES 6

64

Figure B. 7: Resistivity Curve for VES 7

65

Figure B. 8: Resistivity Curve for VES 8

Figure B. 9: Resistivity Curve for VES 9

Figure B. 10: Resistivity Curve for VES 10

66

Figure B. 11: Resistivity Curve for VES 11

Figure B. 12: Resistivity Curve for VES 12

67

Figure B. 13:

Resistivity Curve for VES 13

68

VES NO RESISTIVITY (ohm-m)

THICKNESS (m)

DEPTH (m)

LITHOLOGY CURVE TYPE

VES 1

75.2 0.7 0.7 Topsoil

QH22.3 1.4 2.1 Clay7.5 17.8 19.9 Clay/Peat110.3 -------- --------- Sand

VES 2 155.3 0.7 0.7 Topsoil

QH63.7 1.0 1.7 Sandy Clay13.9 11.9 13.6 Clay66.6 ------- ------- Clayey Sand

VES 3 99.8 0.8 0.8 Topsoil

QH28.7 1.2 2.0 Clay2.9 0.5 2.5 Clay/Peat461.3 --------- -------- Sand

VES 4

107.4 0.5 0.5 Topsoil

QH53.6 1.0 1.5 Clayey Sand6.4 17.0 18.5 Clay/Peat58.1 ------- -------- Clayey Sand

VES 5

76.0 0.5 0.5 Topsoil

QH26.8 1.7 2.2 Clay9.5 6.0 8.2 Clay/Peat83.9 --------- --------- Clayey Sand

VES 6

252.7 0.5 0.5 Topsoil

QH14.2 1.7 2.3 Clay4.2 8.8 11.1 Clay/Peat34.0 --------- -------- Clayey Sand

VES 7

124.0 0.7 0.7 Topsoil

QH19.4 1.4 2.1 Clay4.6 23.9 26.0 Clay/Peat89.0 -------- --------- Clayey Sand

VES 8

307.4 0.6 0.6 Topsoil

QH57.4 1.5 2.1 Sandy Clay10.1 3.8 5.9 Clay6.1 17.5 23.4 Clay/Peat63.7 --------- -------- Clayey Sand

VES 9

91.2 0.5 0.5 Topsoil

QH32.4 2.9 3.4 Clay9.2 12.2 15.6 Clay/Peat4.5 16.5 32.1 Clay/Peat49.9 ------------ ----------- Clayey Sand

69

VES 10

63.5 0.6 0.6 Topsoil

QH13.8 2.2 2.8 Clay4.1 4.9 7.8 Clay/Peat1.6 9.3 17.0 Peat79.2 -------- -------- Clayey Sand

VES 11

96.1 0.6 0.6 Topsoil

QH13.5 1.6 2.1 Clay4.2 13.3 15.4 Peat42.3 -------- -------- Clayey Sand

VES 12

85.1 0.8 0.8 Topsoil

QH13.5 2.2 3.0 Clay2.6 5.5 8.5 Clay/Peat55.4 -------- -------- Clayey Sand

VES 13

224.3 0.6 0.6 TopsoilQH13.6 1.7 2.2 Clay

4.1 17.1 19.4 Clay/Peat41.7 -------- -------- Clayey Sand

70

APPENDIX D: CPT DATA

71

CPT DATA

CPT 1 0.25 0.5 0.75 FULL1 0 0 0 52 5 10 13 153 15 25 40 454 40 50 60

72

CPT 2 0.25 0.5 0.75 FULL1 0 2 2 52 5 10 15 153 20 18 25 304 32 40 45 455 70

CPT 3 0.25 0.5 0.75 FULL1 0 3 2 52 5 10 13 143 25 40 40 504 50 60 60 655 72

CPT 4 0.25 0.5 0.75 FULL1 0 0 2 52 5 10 10 203 45 40 55 454 71

73