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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
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
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
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
489
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)
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
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.
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
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
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
492
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
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
493
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],
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
494
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.
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
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
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
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
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
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
The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria
498
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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