The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Benue Trough, Nigeria

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1 Journal of Earth Science and Engineering 5 (2015) doi: / X/ D DAVID PUBLISHING 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 -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

2 488 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower 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. 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 [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 SiO Al 2 O Fe 2 O MgO CaO Na 2 O K 2 O FeO NA TiO NA

3 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower 489 the interlayer cations and the physical conditions. On heating, the interlayer water of smectites is lost mostly between 100 and 250 o C however some attain the temperature of about 300 o C when slow loss of constitutional (OH) water begins. Rapid loss of (OH) water takes place at about 500 o C and is complete at about 750 o C. 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 6 o 25 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)

4 490 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower 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.

5 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower 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 Na 2 PO 4 ). 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 o C after glycolation and heating to 550 o C 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 10 o 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 (%) (%) Imo shale: moderately weathered No. of samples: 19 Plasticity index (%) Moisture content (%) Consistency index IC Minimum Maximum Mean Enugu shale: highly weathered No. of samples: 15 Minimum Maximum Mean Awgu shale: moderately weathered No. of samples: 27 Minimum Maximum Mean % Clay

6 492 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower 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 Enugu shale Low-high Awgu shale Low-extra high Table 4 Range of PI & activity of the shales [19]. Shale identity Range of PI (%) Range of activity PI classification Imo shale Slightly-extremely plastic Enugu shale Slightly-highly plastic Awgu shale Slightly-extremely plastic

7 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower 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 (Ca 2+ ), magnesium (Mg 2+ ), and potassium (K + ), sodium (Na + ), hydrogen (H + ), aluminium (Al 3+ ), iron (Fe 2+ ), manganese (Mn 2+ ), zinc (Zn 2+ ) and copper (Cu 2+ ). 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, % 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%, 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],

8 494 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower Table 5 Ranges of CEC of the shale samples [19]. Shale identity CEC (meq/100g) CEC class Reactivity class Lowest Highest Mean Imo shale Very high High Enugu shale Very high Intermediate-high Awgu shale Very high Intermediate-high Table 6 Ranges of OMC (organic matter content) [19]. Shale identity Lowest Highest Mean Imo shale Enugu shale Awgu shale 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 o C, 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.

9 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower 495 /Smectite C /Smectite B /Smectite Fig. 2 XRD of EN 1 [A = air treated, B = glycolated, C = heat treated]. A C /Smectite B Fig. 3 XRD of EN 2. A

10 496 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower C B /Smectite Fig. 4 XRD of EN 3. A Smectite C B /Smectite Fig. 5 XRD of EN 4. A

11 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower 497 /Smectite /Smectite C Montmorillonite B /Smectite Fig. 6 XRD of NK 1. A Montmorillonite C Montmorillonite B Montmorillonite Fig. 7 XRD of NK 2. A

12 498 The Mineralogical and Engineering Characteristics of Cretaceous and Tertiary Shales in the Lower 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]). References [1] O Flaherty, C. A Highway Engineering. Edward Arnold London 2: 95. [2] Sabtan, A. A Geotechnical Properties of Expansive Clay Shale in Tabuk, Saudi Arabia. Journal of Asian Earth Sciences 25: [3] Attewell, P. B., and Farmer, I. W Principles of Engineering Geology. London: Chapman and Hall, 16-29, [4] Hudec, P. P Durability of Shales in Embankments and Backfills. Proc of 14 th Annual Forum on the Geology of Industrial Minerals. The University of the State of New York, Bull (436): [5] Garg, S. K Soil Mechanics and Foundation Engineering, 7th ed. New Delhi: Khanna Publishers, [6] Selley, R. C Applied Sedimentology. (Online) (Accessed October 1, 2011). premiumdownloadn3.jsp?ref=300def. [7] Okogbue, C. O., and Ene, G. E Geochemical and Geotechnical Characteristics and the Potential for Use in Drilling Mud, of Some Clay Bodies in Southeastern Nigeria. Journal of Mining & Geology 44 (2): [8] Reyment, R. A Aspects of Geology of Nigeria. Ibadan University Press, 184. [9] Short, K. C. and Stauble, A. J Outline of the Geology of the Niger Delta. Geol. en. Mijnboaw. 50: [10] Burke, K. C., Dessauvagie, T. F. J., and Whiteman, J. A Geological History of the Benue Valley and Adjacent areas. In African Geology, edited by Dessauvagie, T. F. J., and Whiteman, A. J. Nigeria: University of Ibadan, [11] Kogbe, C. A. ed Geology of Nigeria. Paris: Rock view International, [12] Agagu., O. K., Fayose, E. A., and Petters, S. W Stratigraphy and Sedimentation in the Senonian Anambra Basin of Eastern Nigerian. Nig. Journ. Mining and Geology 22. [13] Likkason, O. K., Ajayi, C. O., Shemang, E. M., and Dike, E. F. C Indication of Fault Expressions from Filtered and Werner Deconvolution of Aeromagnetic Data of the Middle. Journal of Mining & Geology 41 (2): [14] Obaje, N. G., Ukpabi, E. J., and Funtua, I. I Micrite Maceral Evidence of Hydrocarbon Generation in Cretaceous Coal Measures of the Middle Benue Trough, Nigeria. Journal of Mining & Geology 35 (1): [15] Obaje, N. G., Ligouis, B., and Abaa, S. I Petrographic Composition and Depositional Environments of Cretaceous Coals and Coal Measures in the Middle Benue Trough of Nigeria. International Journal of Coal Geology 26: [16] British Standard Institution (BS, 1377) Methods of Testing for Soils for Civil Engineering Purposes. British London: Standard Institution. [17] Joint Committee on Powder Diffraction Standards (JCPDS) Mineral Powder Diffraction File 1 & 11. USA: International Centre for Diffraction Data, [18] Patrick, D. M., and Snethen, D. R An Occurrence and Distribution Survey of Expansive Materials in the United States by Physiographic Areas. Federal Highway Administrative Report No FHWA-RD [19] Ekeocha, N. E The Engineering Geological and Mineralogical Properties of Cretaceous and Tertiary Shales in the Lower Benue Trough, South Eastern Nigeria. Ph.D. Thesis, University of Port Harcourt, Nigeria. [20] Bell, F. G Engineering Geology, 2nd Ed. London: Elsevier, [21] Wilson, M. J The Origin and Formation of Clay Minerals in Soils: Past, Present and Future Perspectives. Clay Miner. 34: [22] Keller, W. D Environmental Aspects of Clay Minerals. Journ. Sedimentary Petrol. 40: [23] Murthy, V. N. S Textbook of Soil Mechanics and Foundation Engineering. New Delhi, India: CBS Publishers & Distributors, , [24] Nton, M. E., and Elueze, A. A Compositional Characteristics and Industrial Assessment of Sedimentary Clay Bodies in Part of Eastern Dahomey Basin, Southwestern Nigeria. Journal of Mining and Geology 41 (2): [25] Akpokodje, E. G The Engineering Geology of Arid Zone Soils and Their Stabilization. Ph.D. Thesis, University of New South Wales, Australia. [26] Cornell University Agronomy Fact Sheet #22: CEC (Cation Exchange Capacity).