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Journal of Earth Science and Engineering 5 (2015) 487-498 doi: 10.17265/2159-581X/2015.08.004 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria Nnamdi Enyereibe Ekeocha Department of Geology, University of Port Harcourt, Choba, PMB 5323, Rivers State, Nigeria Abstract: The mineralogical and engineering characteristics of Cretaceous and Tertiary shales in the lower Benue Trough were determined with a view to establishing how they affect civil engineering construction, with emphasis on road pavements in the area. Shale samples from the geologic formations of Imo, Enugu and Awgu shales were subjected to the following laboratory tests: clay mineral content, organic matter content, Cation Exchange Capacity and Plasticity according to methods specified by the British Standard Institute. The shales were classified based on Plasticity Index, liquid limit and Cation Exchange Capacity. The class of shales ranged from non-plastic to extremely plastic and low to high reactivity. The moisture content and plasticity values are related to the degree of weathering. The higher the weathering grade, the higher the moisture content and plasticity values. The organic matter content of the shales is generally low (0.2% to 11.2%) and influences the durability of the shales in an inverse manner. The clay mineral composition from x-ray diffraction consists of Illite-montmorillonite mixed layers, illite, and kaolinite. The illite-montmorillonite mixed layer clays are most prominent in road sections with most severe pavement failures. In contrast, sections with kaolinite as the dominant clay mineral experienced less severe and limited pavement failure. The contrasting engineering behaviour of these clay minerals is due to their structures. The study showed that the presence of clay minerals derived from underlying shales is a major contributory factor to the behaviour and performance of roads built over shale subgrades, that any effective remediation work must take cognizance of the amount and type of clay minerals present. Key words: Cation exchange capacity, illite, kaolinite, mineralogy, montmorillonite, plasticity. 1. Introduction Sections of the expressway that traverse the Cretaceous and Tertiary shales of the lower Benue Trough almost seasonally experience failure, and as a result cause serious traffic difficulties. The shales are essentially clayey materials [1] and break down in the presence of moisture. The clay mineral components of the shales are involved in cation exchange that brings about increased water adsorption and eventual deterioration in strength properties. These failures are more of an annual event and efforts towards rehabilitation have not yielded reasonable success. The shale formations traversed by the expressway include Imo, Awgu, and Enugu, with different ages Corresponding author: Nnamdi Enyereibe Ekeocha, Dr., research fields: geotechnics, water resources and environmental sustainability. and degree of weathering. This study addresses the dearth of data on the engineering properties and clay mineralogy of the shales with a view to formulating solutions to the re-occurring problem of widespread pavement failure associated with shales. The type of clay mineral in the shale is important as it determines the final breakdown product in conjunction with other environmental conditions. Some of the clay minerals will swell when wet and cause expansion of the rock mass, when exposed to rainfall. Low rainfall and alkaline environmental conditions favour smectite formation. Over time, due to dehydration arising from compaction, part of smectite alters to mica [2]. Soils that have significant clay mineral fraction, mechanical properties may be significantly modified by the soil structure. The attractive and repulsive D DAVID PUBLISHING

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Page 1: The Mineralogical and Engineering Characteristics of ... · The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria

Journal of Earth Science and Engineering 5 (2015) 487-498 doi: 10.17265/2159-581X/2015.08.004

The Mineralogical and Engineering Characteristics of

Cretaceous and Tertiary Shales in the Lower Benue

Trough, Nigeria

Nnamdi Enyereibe Ekeocha

Department of Geology, University of Port Harcourt, Choba, PMB 5323, Rivers State, Nigeria

Abstract: The mineralogical and engineering characteristics of Cretaceous and Tertiary shales in the lower Benue Trough were determined with a view to establishing how they affect civil engineering construction, with emphasis on road pavements in the area. Shale samples from the geologic formations of Imo, Enugu and Awgu shales were subjected to the following laboratory tests: clay mineral content, organic matter content, Cation Exchange Capacity and Plasticity according to methods specified by the British Standard Institute. The shales were classified based on Plasticity Index, liquid limit and Cation Exchange Capacity. The class of shales ranged from non-plastic to extremely plastic and low to high reactivity. The moisture content and plasticity values are related to the degree of weathering. The higher the weathering grade, the higher the moisture content and plasticity values. The organic matter content of the shales is generally low (0.2% to 11.2%) and influences the durability of the shales in an inverse manner. The clay mineral composition from x-ray diffraction consists of Illite-montmorillonite mixed layers, illite, and kaolinite. The illite-montmorillonite mixed layer clays are most prominent in road sections with most severe pavement failures. In contrast, sections with kaolinite as the dominant clay mineral experienced less severe and limited pavement failure. The contrasting engineering behaviour of these clay minerals is due to their structures. The study showed that the presence of clay minerals derived from underlying shales is a major contributory factor to the behaviour and performance of roads built over shale subgrades, that any effective remediation work must take cognizance of the amount and type of clay minerals present.

Key words: Cation exchange capacity, illite, kaolinite, mineralogy, montmorillonite, plasticity.

1. Introduction

Sections of the expressway that traverse the

Cretaceous and Tertiary shales of the lower Benue

Trough almost seasonally experience failure, and as a

result cause serious traffic difficulties. The shales are

essentially clayey materials [1] and break down in the

presence of moisture. The clay mineral components of

the shales are involved in cation exchange that brings

about increased water adsorption and eventual

deterioration in strength properties. These failures are

more of an annual event and efforts towards

rehabilitation have not yielded reasonable success.

The shale formations traversed by the expressway

include Imo, Awgu, and Enugu, with different ages

Corresponding author: Nnamdi Enyereibe Ekeocha, Dr.,

research fields: geotechnics, water resources and environmental sustainability.

and degree of weathering. This study addresses the

dearth of data on the engineering properties and clay

mineralogy of the shales with a view to formulating

solutions to the re-occurring problem of widespread

pavement failure associated with shales.

The type of clay mineral in the shale is important as

it determines the final breakdown product in

conjunction with other environmental conditions.

Some of the clay minerals will swell when wet and

cause expansion of the rock mass, when exposed to

rainfall. Low rainfall and alkaline environmental

conditions favour smectite formation. Over time, due

to dehydration arising from compaction, part of

smectite alters to mica [2].

Soils that have significant clay mineral fraction,

mechanical properties may be significantly modified

by the soil structure. The attractive and repulsive

D DAVID PUBLISHING

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488

forces that are associated with the clay minerals are

responsible for much of the real and apparent cohesion

in mineral particle systems and are mainly determined

by the type of clay mineral and the chemical

composition of the pore fluid. However, in practice,

while inter particle forces may affect the

microstructure of a sediment during sedimentation,

they have less significant effect on the subsequent

engineering behaviour [3].

Shale deterioration is caused by such factors as

structuring of water on clay surfaces, expansion

possibly due to osmotic pressures generated within the

rock and shales containing expansive clays e.g.

montmorillonite [4]. He further observed that shale

experiences volume change upon wetting and drying

in a manner related to the pattern of shrinkage of clays

which form the major constituent of shales. The clay

minerals and very fine-grained mica crystals in shales,

are oriented parallel with the bedding planes so that

the rock splits easily along these directions.

The structure of clay minerals impacts on the

peculiar characteristics of plasticity. Their structures

are based on composite layers built from components

with tetrahedrally and octahedrally coordinated

cations. Most of them occur as platy particles in

fine-grained aggregates which when mixed with water

yield materials, which have varying degrees of

plasticity.

The mineral kaolinite, which is the simplest clay

mineral in structure and purest in composition forms

by the hydrothermal alteration and superficial

weathering of feldspars by the action of water and

carbon dioxide [5, 6].

Kaolinite has low CEC (cation exchange capacity),

and this is believed to be partly due to ever-present

impurities, which hinder the determination of its true

values and the broken bonds at the edges of the flakes.

They transform quickly to more complex clays in the

presence of seawater [6]. Montmorillonites and illites

are the most important clay minerals in engineering

consideration. They are formed from structural units

comprising a central gibbsite octahedral sheet

sandwiched between two silicate sheets so that the tips

of the silica tetrahedral penetrate both the hydroxyl

layers of the gibbsite. The montmorillonite crystals

are formed by successive layers of these units, held

together by extremely weak bonding between oxygen

atoms in the adjacent units.

Illite is the most abundant clay mineral in sediments

but it is less obvious than kaolinite because it is

seldom present in crystals that can be seen with an

optical microscope [6]. It exhibits a cation exchange

capacity of between 100 and 400 meq/100g which

though greater than that for kaolinite, is considerably

less than those for halloysite, smectite and

vermiculite.

Montmorillonites, chief among the smectites [6],

are 2:1 phyllosilicates with a structure similar to that

of Illite [3]. They are not able to bond interlayer

cations with sufficient force to cause adjacent layers to

contract. The amount of interlayer water adsorbed

varies according to the type of smectite, the nature of

Table 1 Properties of some expansive clays [7].

Parameters Wyoming Texas Manitoba India Abakaliki Imo Fm

SiO2 58.0-64.0 63.5-64.9 63.72 66.05 68.2

Al2O3 18.0-21.0 9.26-9.32 19.86 13.13 19.6-23.7 20.0-24.0

Fe2O3 2.5-2.8 2.52-2.57 1.42 5.45 2.0-2.8 1.9-3.2

MgO 2.3-3.2 1.79-1.80 4.67 1.57 2.5-4.0.6 2.1-2.5

CaO 0.1-1.0 0.83-0.88 0.16 0.58 1.0-2.7 3.2-3.7

Na2O 1.5-2.7 4.03-4.06 0.77 0.09 1.4-4.2 1.3-1.8

K2O 0.2-0.4 1.20-1.28 0.26 1.40 0.1-0.9 0.3-0.5

FeO 0.2-0.4 NA 0.52 0.70 0.1-0.3 0.2-0.4

TiO 0.1-0.2 NA 0.12 0.17 0.1-0.2 0.1

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the interlayer cations and the physical conditions. On

heating, the interlayer water of smectites is lost mostly

between 100 and 250 oC however some attain the

temperature of about 300 oC when slow loss of

constitutional (OH) water begins. Rapid loss of (OH)

water takes place at about 500 oC and is complete at

about 750 oC.

Compositional variations through ionic or

isomorphous substitution within the clay mineral

crystal lattice (particularly, prevalent in

montmorillonite and vermiculites) of say trivalent

aluminum for quadrivalent silicon, can leave the

structural unit with a net negative charge. Substitution

also reduces the crystal size and alters its shape.

Exposed hydroxyl groups and broken surface bonds

can also lead to a net negative charge on the structural

unit. The presence of this net negative charge means

that soluble (also possibly insoluble) cations can be

attracted or adsorbed on to the surface of clay mineral

structural units without altering the basic structure of

the clay mineral. These cations can be exchanged for

other soluble cations if the ionic environment changes.

The most common soluble cations are those of sodium,

potassium, calcium, magnesium, hydrogen and

ammonium. There may also be some cases where net

positive charges caused by broken bonds at particle

surfaces can attract exchangeable anions, but these

have minor engineering significance. Cation exchange

capacity does, however, have major significance in

determining clay mineral properties, particularly the

facility with which they adsorb water.

1.1 Plasticity

This is the deformation causing permanent,

continuous strain that does not involve brittle failure

or significant change in total volume. In plastic

material, any stress above a critical value known as

the yield stress causes continuous, permanent strain.

The existence of a positive yield stress distinguishes

plastic behaviour from fluid flow. At stresses below

this value, the material is rigid-plastic if no

deformation occurs. It is important to note that earth

materials that possess low CEC (cation exchange

capacities) will have low water holding capacity and

by implication low plasticity. Sandy soils fall within

this description while the reversed state will have

clays as instances.

2. Geology of the Study Area

The study was carried out on the shales of the

Lower Benue Trough (Imo, Awgu, and Enugu), South

Eastern Nigeria. Samples of these shales were

collected from the area located within the

geographical coordinates of between 5°40′ and 6o25′

N and between 7°15′ and 8°23′ E (Fig. 1).

The geology of the area has been severally

described ([8-12] etc.), and is believed to be

associated with the tectonic activities that were

recorded during the Cenomanian. These tectonic

activities produced an uplift that had a NE-SW trend,

and were followed by the tectonic activities that took

place in Santonian times (i.e. the second tectonic

activity of the Lower Benue Trough), which resulted

in the folding and uplifting of the Abakaliki Sector of

the Trough and the subsidence of Anambra platform.

The latter event led to the formation of the Anambra

Basin, which constituted a major depocenter of clastic

sediments and deltaic sequences. In this part of the

Benue Trough, the stratigraphic succession begins

with the Abakaliki (Albian in age). The Abakaliki is

said to be about 3,000 m thick and lies unconformably

on an older basement complex [13]. The marine

Abakaliki is overlain with a transitional contact by the

Keana and Awe Formations. The Keana and Awe

Formations were deposited as (near) coastal sediments

during the Early Cenomanian regression. The Ezeaku

Formation lies conformably on the Keana and Awe

Formations. This formation was deposited during the

beginning of marine transgression in the Late

Cenomanian [14]. The age of the sediments in the

Basin ranges from Pre-Cretaceous to Recent with

Awgu shales (oldest formation in the Anambra basin)

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490

Fig. 1 Location map.

being deposited during the Coniacian times. It overlies

the Eze-Aku Group and its lateral equivalent, the

Agbani Sandstone. The Awgu Formation is made up

of bluish-grey to dark-black carbonaceous shales,

calcareous shales, shaley limestones, siltstones and

coal seams, suggesting rapid changes in the

depositional environments ([15], in Ref. [14]). The

erosion of the Abakaliki uplifted and folded belts

resulted in the development of a Proto-Niger Delta

sequence consisting of Enugu shale, Mamu, Ajali and

Nsukka Formations. The third and last depositional

cycle of the Lower Benue Trough started with a major

transgression that deposited the marine Imo shales in

the Anambra basin, during the Palaeocene Period.

This was followed by a regression that started during

the Eocene and continued to the present day with the

deposition of the sediments of the Tertiary Niger

Delta.

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491

3. Method of Study

Samples were collected from exposures of the

different shale formations. The samples, which showed

various levels of weathering ranging from slightly

weathered to moderately weathered, were taken to the

laboratory for the various kinds of analyses including

consistency tests (liquid limit, plastic limit), shrinkage

limit, CEC (cation exchange capacity), organic matter

content and XRD (x-ray diffraction). The methods of

analyses were in line with Ref. [16]; however, the

XRD was carried out as described below.

Samples of the shales were crushed to powder size

and soaked for a period of five days in a 40%

dispersant solution of Calgon (sodium

hexametaphosphate Na2PO4). The suspended

component was extracted using a 100 ml pipette and

transferred into the centrifuge, which was powered

and left on for 20 minutes after which the clay-sized

fraction was obtained for drying and preparation for

subsequent diffraction study. The different clay

mineral types present in the clay sized fractions were

determined by XRD method. The XRD patterns were

determined from thin clay films mounted on glass

slides. Four oriented slides were prepared from each

sample and subjected to XRD after air—drying,

glycolation, heating to 375 oC after glycolation and

heating to 550 oC after glycolation. The different clay

minerals were positively identified by the behaviour

of the peaks at the various pre-treatment. This

experiment was carried out on a PHILIPS high angle

diffractometer unit using nickel filter and a copper

cathode with a scanning speed of 10o per minute. The

clay minerals were estimated by comparing the

peaks/counts of specific diagnostic peaks of the

minerals with standard heights of equivalent peaks of

the pure minerals as established by Ref. [17].

4. Results Presentation, Interpretation and Discussion

The results of the various analyses carried out in the

laboratory are presented below.

4.1 Natural Moisture Content

The natural moisture content values recorded ranges

of between 16.6% and 46.8% at Awgu Shale, between

20% and 53% at Imo shale, between 4.7% and 21.6%

at Enugu Shale as shown in Table 2. There is a general

observation of greater moisture content and plasticity

in the younger formations than that in the older ones,

which is suggestive of the fact that the younger

formations tend to have greater proportion of clays.

This finding is in line with the assertion that older rocks

tend to contain a higher percentage of non-expansive

clay minerals [18]. The degree of weathering of the

various shale formations generally was influential on

the moisture content and plasticity of samples and this

is consistent with the assertion of Bell [20].

Table 2 Ranges of consistency values of the various shales [19].

Liquid limit (%)

Plastic limit (%)

Plasticity index (%)

Moisture content (%)

Consistency index IC

% Clay

Imo shale: moderately weathered No. of samples: 19

Minimum 23 18 1 22.9 0.1 5

Maximum 105 93 57 52.9 52 5

Mean 71.7 45.6 26.1 36.3 6.28 5

Enugu shale: highly weathered No. of samples: 15

Minimum 32 25 2 4.7 1.22 3

Maximum 59 42 32 21.6 10.3 5

Mean 46.9 33.3 14.2 12.6 4.11 4

Awgu shale: moderately weathered No. of samples: 27

Minimum 50 18 1 16.6 0.0 2

Maximum 109 67 76 46.8 10.3 8

Mean 76.9 35.5 41.8 28.4 5.76 4.3

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4.2 Atterberg’s Limits Test Results

Atterberg’s limits constitute one way of expressing

the consistency of a soil. The consistency of the

various shale samples as depicted by their liquid limit,

plastic limit and linear shrinkage is presented as follows.

4.3 Liquid Limit

The liquid limit ranges of the various shale units are

presented in Table 2. Awgu shale recorded liquid limits

range of 32% to 98% while the Imo shale recorded a

liquid limit range of 23% and 96%. The Enugu shale

recorded the values of between 32% and 59%. The

above result shows that liquid limit was generally high

at Imo shales with the highest value of 96%.

The results above suggest that the samples will

exhibit poor engineering qualities, being that they

show great tendencies to lose moisture that they

gained in the presence of water the moment they

experience dryness.

4.4 Plastic Limit

The plastic limit as recorded for the Awgu and

Enugu shales respectively ranged from 8-67% and

25-42% respectively while the Imo shale recorded a

range of 18-3%.

The highest value was recorded by the Imo shales,

just as was the case with the liquid limit.

4.5 PI (Plasticity Index)

The plasticity of clay soil is influenced by the

amount of its clay fraction and the type of clay

minerals present, since the amount of attracted water

held in a soil is influenced by clay minerals. As a

consequence, the index properties of clay deposits are

influenced by the principal minerals in the clay. This

agrees with Sabtan’s [2] assertion that the Hanadir

shale is the source of expansive soils in the area and

that the shale composition and its engineering

properties change abruptly in both horizontal and

vertical directions due to both the rock nature (grain

size, plasticity, mineralogy, and cementation) and

degree of weathering.

There is a general correlation between the clay

mineral composition of a deposit and its activity.

Kaolinitic and illitic clays are usually inactive, while

montmorillonitic clays range from inactive to active.

Generally, active clays have relatively high

water-holding capacity and a high cation exchange

capacity [20].

The PI, which is the difference between the liquid

limit and plastic limit, consequently recorded values

between 1-76% and 2-32% for the Awgu and Enugu

shales respectively and 1-57% at Imo Shales. Fig. 3

shows the plasticity plot of the various shale samples

on the Casagrande chart. The ranges of plasticity

index of the shale samples (Table 4) indicate that the

samples range from non-plastic to extremely plastic.

There is an observed trend that samples with higher

plastic limits recorded lower moisture content and

higher consistency index. It is also observed that the

higher the liquid limit, the higher the compression

index computed from liquid limit.

Table 3 Classification of the plasticity of the shales using liquid limit [19].

Shale identity Plasticity Range of liquid limit

Imo shale Low-extra high 23-96

Enugu shale Low-high 32-59

Awgu shale Low-extra high 32-98

Table 4 Range of PI & activity of the shales [19].

Shale identity Range of PI (%) Range of activity PI classification

Imo shale 1-57 0.08-0.2 Slightly-extremely plastic

Enugu shale 2-32 0.12-0.3 Slightly-highly plastic

Awgu shale 1-76 0.03-0.13 Slightly-extremely plastic

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4.6 Cation Exchange Capacity

CEC (cation exchange capacity) of a rock/soil is its

capacity to hold on to cations, which are positively

charged ions such as calcium (Ca2+), magnesium

(Mg2+), and potassium (K+), sodium (Na+), hydrogen

(H+), aluminium (Al3+), iron (Fe2+), manganese (Mn2+),

zinc (Zn2+) and copper (Cu2+). The cations are held by

the negatively charged clay and organic matter particles

in the soil through electrostatic forces (negative soil

particles attract the positive cations). The ranges of the

CEC values of the shales are: Imo shales 54 to 87

meq/100g, Awgu shale 45 to 88 meq/100g and Enugu

shale 49 and 84 meq/100g respectively. The Imo,

Enugu and Awgu Shales recorded very high CEC

values (Table 5). In terms of reactivity, it was

observed that samples that recorded very high CEC

had intermediate to high reactivity. These results agree

with the thoughts of Akpokodje (personal

communication) that the Imo, Awgu and Enugu shales

respectively have high reactivity.

From the results also, it was deduced that the

capacity to exchange cations reduced with reduction

in the grade of weathering, as areas with moderate

degree of weathering recorded very high CEC while

those with slight weathering degrees had low CEC.

The relationship is also thus defined with respect to

reactivity. Generally it was discovered that the higher

the CEC, the lower the organic matter content.

4.7 Organic Matter Content

The organic matter content of the shale samples

generally recorded low to medium loss on ignition

values of between 1.2% and 11.2% for the Imo shale,

0.2-8.0% for the Awgu shale while the Enugu shale

recorded values of between 2.5% and 9.3% (Table 6).

4.8 Mineralogical Characterization Using XRD

The XRD results show the presence of clay and non

clay minerals. The percentage composition of each

clay mineral was estimated from the XRD traces of

the samples. The clay minerals from the XRD traces

make up approximately 45% of the whole rock sample.

The approximate percentages of the various clay

minerals are: montmorillonite-illite mixed layer clays

15%, Illite 10%, kaolinite 10% and montmorillonite

10%. The non-clay minerals jointly contributed about

55% of the minerals of the study area and included

quartz and oxides of iron among others. Quartz

contributed about 5% of the non-clays. The different

shales exhibited different weathering degrees, e.g., the

Imo and Awgu shales were moderately weathered

while the Enugu shale was highly weathered

respectively.

The manner of clay mineral occurrence is in line

with the observed weathering pattern of the different

shale types, i.e., the more weathered the shale the

higher the concentration of clay minerals. This agrees

with the concept that clays form largely by the

chemical degradation of pre-existing minerals during

weathering [6, 21] and by the transformation of clay

minerals both during transportation and early burial

[22]. It is known that kaolinite is primarily associated

with the weathering or low temperature alteration of

feldspars, muscovite and other aluminium-rich

silicates usually acid rocks. It is important to also note

that the weathering of muscovite produces illite and

hydromuscovite which break down to form

montmorillonite and finally kaolinite via the loss of

potassium and increase of water and silica. Albite also

breaks down in the course of weathering to form

kaolinite.

It is also established that dominant clay minerals of

weathered volcanic rocks is smectite which commonly

swells when it comes in contact with water and this is

said to be major cause of engineering problems in the

Denver area [23]. From the foregoing, it is observed

that the clay mineral composition agrees with the

degree of weathering of the shales, i.e., the mostly

weathered samples recorded the strongest kaolinite

diffraction on the profile while the slightly weathered

showed illite composition. Also, in line with Ref. [5],

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Table 5 Ranges of CEC of the shale samples [19].

Shale identity CEC (meq/100g) CEC class Reactivity class

Lowest Highest Mean

Imo shale 54.332 81.491 68.5 Very high High

Enugu shale 49.528 83.095 68.2 Very high Intermediate-high

Awgu shale 45.532 87.054 68.2 Very high Intermediate-high

Table 6 Ranges of OMC (organic matter content) [19].

Shale identity Lowest Highest Mean

Imo shale 1.2 11.2 5.5

Enugu shale 2.5 9.3 5.8

Awgu shale 0.2 8.0 3.6

the occurrence of montmorillonite is associated with

high plasticity while illite is not as plastic, with a

plasticity index of 67% and in turn kaolinite is least

plastic with plasticity index of 21%. This thereby

shows that the plasticity reduces with the degree of

weathering.

The kaolinite group of minerals, which are results

of the breakdown of the original mineral under

varying environmental conditions such as weathering,

are the most stable, with many sheet stacking that

are difficult to dislodge due to the comparatively

strong hydrogen bonds [24]. Water therefore finds

it difficult to permeate the sheets to expand the unit

cells [23]. This behaviour accounts for the relative

stability observed in sections of the road that

recorded a predominance of kaolinites in comparison

with sections that had more of illite and the mixed

layer clays. The kaolinite peaks collapsed upon

heating to the temperature of 550 oC, resulting in

the absence of kaolinites from the heat treated

samples.

On the other hand, the structural arrangement of the

montmorillonite mineral is composed of units made of

two silica tetrahedral sheets with a central

alumina-octahedral sheet. The stacking nature of the

units results in a situation where neighbouring units

are adjacent oxygen layers of another, giving rise to a

weak bond between them. Water permeates the sheets

and as a consequence causes them to expand

significantly. This behaviour is responsible for the

high swelling and shrinkage characteristics of soils

containing considerable amount of montmorillonite

minerals. The illite clay mineral group has similar

structural arrangement as the montmorillonite group

except for the presence of potassium as the bonding

material between units which makes the group to

swell less. These assertions agree with the observation

that the areas of study that recorded relatively greater

road failure had more preponderance of

montmorillonite and illite minerals. The illites are

decomposed to form illite-smectite mixed layer clays,

while the mixed layer clays are absent where the illite

is relatively undecomposed. Locations that witnessed

complete weathering gave rise to the transformation of

illite to montmorillonite.

The finding of the influence of mineralogy on the

behaviour of the earth materials used in the road

construction is consistent with Ref. [2], who

established that the expansion of the soil in Tabuk is

mainly due to the presence of clay minerals (smectite

and illite) derived from shale. Also, in accordance

with Ref. [25], the illite dominated soils are associated

with low plasticity and consequently least susceptible

to deterioration on stauration; however, being derived

from shales, they are deficient in coarse particles that

are essential for mechanical stability. Figs. 2-7

(adapted from Ref. [19] show typical diffractograms

of the shale samples.

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495

C

B

A

Fig. 2 XRD of EN 1 [A = air treated, B = glycolated, C = heat treated].

C

B

A

Fig. 3 XRD of EN 2.

Illite

Illite

Illite/Smectite

Illite /Smectite

Illite /Smectite

Illite

Illite

Illite

Illite /Smectite

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496

C

B

A

Fig. 4 XRD of EN 3.

C

B

A

Fig. 5 XRD of EN 4.

Illite

Illite

Illite /Smectite

Illite

Illite /Smectite

Smectite

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497

C

B

A

Fig. 6 XRD of NK 1.

C

B

A

Fig. 7 XRD of NK 2.

Illite /Smectite

Illite /Smectite

Illite

Illite /Smectite

Montmorillonite

Montmorillonite

Montmorillonite

Montmorillonite

Illite

Illite

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5. Conclusions

From the foregoing, conclusion was drawn that:

The presence of clay minerals derived from

underlying shales is a major contributory factor to the

behaviour and performance of roads built over shale

subgrades;

Effective remediation work must take cognizance

of the amount and type of clay minerals present;

Results of this study are in agreement with other

studies elsewhere (Tabuk, Saudi Arabia, [2]).

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