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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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.
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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