Geotechnical Assessment of Road Failures in the Abakaliki Area, Southeastern Nigeria

Transcription

1 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: Geotechnical Assessment of Road Failures in the Abakaliki Area, Southeastern Nigeria Okechukwu P Aghamelu 1, M.Sc and Celestine O Okogbue 2, PhD 1 Department of Geology & Exploration Geophysics, Ebonyi State University, P. M. B. 053, Abakaliki, Ebonyi State, Nigeria. aghameluokeey@yahoo.co.uk 2 Department of Geology, University of Nigeria, Nsukka, Enugu State, Nigeria celokogbue@yahoo.com Abstract-- The impact of the Abakaliki Shale on the incessant road and highway failures in the Abakaliki area, southeastern Nigeria, has been investigated. Soil classification tests conducted on the shale samples which serve both as subgrade and aggregates showed that the shale classifies as A-7-6 and CH soils, according to the AASHTO and USCS classification systems respectively. A comparison of some of the tests results with the Nigerian Specifications and literature reveals that the Abakaliki Shale neither satisfies most requirements as fill and embankment, nor the requirements as sub-base course and base course material. The unsatisfactory behaviour of the shale as road material (as predictable of an A-7-6 or CH soil) is mainly due to its high plasticity, which simply indicates susceptibility to expansion on moisture influx, and high percentage of particles passing No. 200 British Standard sieve, which is significantly above the recommended limit. The shale, however, satisfies few specifications such as maximum dry density, optimum moisture content and soaked California Bearing Ratio, implying that it s appropriateness for use as road material could be improved by stabilization and prevention from excessive moisture influx. The results of this research will, to a great extent, assist the civil engineers in the area in designing and constructing roads and highways that can stand the taste of engineering time. Index Term-- Abakaliki Shale, Construction material, Geotechnical properties, Road Pavement failure, Nigerian specification. I. INTRODUCTION Road network is considered very vital in the economies of many nations, especially the developing ones, like Nigeria, that require(s) roads and highways for transportation of most goods and services. However, construction and maintenance of good road and highway networks in these parts of the world (i.e., the developing countries) are often problematic, and have resulted to economic setbacks. Causes of road failures in these areas include inadequacy of construction materials and poor quality of construction. In the Abakaliki district, southeastern Nigeria (Figure 1), shale is the predominant lithology and constitutes more than twothird (2/3) of the underlying Asu River Group sediments (Reyment, 1965). Other rocks which have been mapped in the area include sandstone, siltstone, limestone, pyroclastics and diorite (Reyment, 1965; Ofoegbu and Amajor, 1987; Obiora and Umeji, 2004). Owing to the abundance of the shale and high cost of haulage of the other alternative and often more suitable construction materials, the Abakaliki Shale is being utilized both as subgrade (i.e., in-situ) and as aggregates in most road and highway constructions. A survey of the roads and highways in the area has however revealed that these roads and highways suffer incessant failures. Shales have varied behaviour depending on the geotechnical and mineralogical properties. Some are hard and behave similarly as other hard rocks in civil engineering projects while others are soft and like most other soft rocks are inappropriate for use as aggregates or stones in construction projects. Soft rocks and expansive soils are most often associated with non-durability (Gamble, 1971; Ezeribe, 1994), foundation problems and structural failures (Holtz and Kovacs, 1982; Coduto, 1999; Punmia et al., 2005). This paper intends to ascertain, by means of geotechnical analyses, whether there are any relationships between the performance of the roads and highways and the properties of the materials on which and with which these roads were constructed. II. STUDY AREA DESCRIPTION Location Abakaliki, the capital territory of the present day Ebonyi State, southeast Nigeria, is located on latitude 06 19' north and longitude 06 19' east. It has quite a good network of roads connecting the city (as shown in Figure 1) with other suburbs in its outskirt. Among other things the administrative status and fertile agricultural soil of the surrounding towns are the most significant reasons for the rapid growth in population and socio-economic and spatial expansion as currently witnessed in the metropolis. Physiography and Climate The relief of the area is generally undulating and no location exceeds 400 m above-sea-level. Major relief structures are hills formed by the pyroclastic bodies. No trend has been established by previous research (Ofoegbu and Amajor, 1987) of these conical shaped hills and other residual hills that spread sporadically within the area. The predominant shale has favoured the low erodability of the lithology, resulting in absence or near absence of deep cut valleys and erosion channels. The major river that drains the area is the Ebonyi River and its tributaries; Udene and Iyiokwu Rivers. Both tributaries are perennial and usually overflow their banks at

2 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: the peak of the rains. Stunted trees and pockets of derelict woodland exist where the lithology has undergone high degree of laterization. Elsewhere, typical characteristics of the tropical rain forest are displayed; multitude of evergreen trees, climbing plants, parasitic plants that live on the other plants, and creepers. Two main seasons exist in the Abakaliki area, the dry season which lasts from November to March and the rainy season which begins in April and ends in October with a short period of reduced rains in August commonly referred to as August break. Temperature in the dry season ranges from 20 C to 38 C, and results in high evapotranspiration, while during the rainy season temperature ranges from 16 C to 28 C, with generally lower evapotranspiration. The average monthly rainfall ranges from 31mm in January to 270 mm in July, with the dry season experiencing much reduced volume of rainfall unlike the rainy season, which has high volume of rainfall. Average annual rainfall varies from 1,500 mm to 1,650 mm. These climatic conditions are responsible for the development of thick lateritic soils in the area. Geology The Abakaliki area is underlain by the southern Benue Trough. Most previous researchers, such as Wright (1968) and Burke et al. (1971), gave evidences to support the fact that the origin of the Benue Trough was closely associated with the breakup of western Gondwanaland during the separation of the African and South American Plates, and opening of the South Atlantic Ocean, in the early Cretaceous. The southern Benue Trough has stratigraphic record of deposits represented by sediments of three main marine depositional cycles, namely the Albian-Cenomanian, Turonian-Santonian, and Campano- Maastrichtian (Reyment, 1965; Ofoegbu, 1985; Ofoegbu and Amajor, 1987). Figure 2 gives the distributions of the Asu River Group and other major geological formations of the southern Benue Trough. Murat (1972) pointed out that the first marine transgression of the Benue Trough started around the middle Albian period, with the deposition of the Asu River Group (Abakaliki Shale Formation and other undifferentiated sediments) in the southern Benue Trough. The Asu River Group sediments are predominantly shales and localized development of sandstone, siltstone and limestone facies (Ofoegbu and Amajor, 1987) as well as extrusives and intrusives (Reyment, 1965; Tijani et al., 1996). The group has average thickness of about 2000m and rests unconformably on the Precambrian Basement (Benkhelil et al., 1989). The Abakaliki Shale Formation, which has an average thickness of about 500m, is dominantly shale, dark grey in colour (weathers to brownish material in the greater part of the formation), blocky, and non-micaceous in most locations. It is calcareous (calcite-cemented) and gives off effervescence on contact with dilute hydrochloric acid (Okogbue and Aghamelu, 2010). III. MATERIALS AND METHODS Classification and Mechanical property tests were conducted on a total of 10 (3 fresh and 7 samples collected from the subgrade materials of the studied roads. These samples are herein designated as S1to S10. Figure 1 shows the sampling points distribution, while Table 1 gives type of sample collected with respect to locations. The tests carried out on the shale samples included Atterberg limits (liquid, plastic limits), linear shrinkage and free swell. Others include specific gravity, natural moisture content, compaction, California Bearing Ratio (CBR) and shear strength. The sample preparations and laboratory testing for these geotechnical parameters followed British Standard Institution (BSI) methods of testing soil for civil engineering purposes. The liquid limit and plastic limit tests were carried out on airdried samples that passed mm (BSI No. 36) sieve; both tests then followed standard procedures specified by BSI 1377 (1990). Crushed shale samples that passed through mm (BSI No. 200) sieve and oven-dried at 105 C for 24 hours were utilized for the specific gravity tests. The tests were also carried out with the aid of a ml pycnometer bottle and distilled water with specific gravity of (distilled water temperature was 27 C), in accordance with testing method described by Lambe (1951). Laboratory procedure of this test attempted determination of the specific gravity of the shale excluding air and water contents. Natural moisture contents determination followed simple method outlined by Akroyd (1957). The laboratory compaction tests were limited to particles of shale that passed through mm BSI sieve and followed procedure specified by BSI 1377 (1990). CBR tests were performed on compacted samples in both unsoaked and soaked conditions, following the procedure of Bailey (1976). However, soaking was done overnight (24 hours) in a water-filled bathtub, as suggested by Okagbue and Ochulor (2007). Samples for the shear strength tests were prepared and tested following the BSI 1377 (1990) standard. The test type was the triaxial compression strength test (consolidated undrained). However, the triaxial cell for the tests was the 76 mm diameter type, hence, the tested samples were proportionately scaled down to 152 mm by 76 mm, height by diameter, respectively. Pressured water was also used for confining the samples inside the cell. Depths of sampling varied from 1.2m to 2.0m (see Table 1). The choice of the sampling points was guided by proximity to failed portions on the major roads. IV. RESULTS AND DISCUSSION Mechanical Analysis and Atterberg Limits The summary of the results of the laboratory tests carried out on the shale samples is presented in Table 2. Significant amounts of the particulate constituents of the shale samples are shown to be fines (percentage passing No. 200 BS sieve). Two samples (S1 and S7) contained insignificant amounts of sand-sized particles (both recorded 8% sand), while the rest of the samples showed sand less than 8%. All the samples showed medium to high values of both liquid limit (LL) and plasticity index (PI) when compared with shales from other parts of the Asu River, which have LL less than 40 and PI less than 20 (Ezeribe, 1994). This probably indicates that the shale contains clay mineral(s) of high plasticity. Obiora and Umeji (2004) had used XRD analyses to

3 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: identify occurrences of illite and smectite/illite mixed layer clay minerals in some shales of Asu River Group. The shale samples classified as A 7 6 (following AASHTO classification system) and CH (according to USCS classification system, see Figure 3). The samples, again, recorded appreciable linear shrinkage values (18-22%), at least higher than that of another Nigerian CH shale, Igumale Shale (a lateral equivalent of the Turonian Eze Aku Shale, see Figure 2), which has an average linear shrinkage value of 21 (see Table 3). Free swell values were also moderate to high (48-52%) in comparison with that of Igumale Shale (with an average value of 50). Compaction, California Bearing Ratio (CBR) and Specific gravity The results of the compaction and CBR tests are presented in Table 1. Comparatively, and as shown in Table 3, the compacted Abakaliki Shale achieved mean maximum dry density (MDD) and mean CBR (soaked) values significantly higher than that of Igumale Shale. The relatively high MDD could be attributed to high content of clay and calcite (specific gravity of clay minerals range from and calcite ). Interestingly, both shales (i.e., the Abakaliki and Igumale) have close mean values of specific gravity values of the samples (2.47 and 2.55, respectively). CBR has been correlated with pavement performance as well as used to establish design curves for pavement thickness (Sowers and Sowers, 1970; Mannering and Kilareski, 1998; Wignall et al., 1999). The recommended MDD, OMC and CBR standard, however, depends on specific use. For instance, the Nigerian Specification for Road and Bridge Materials (Nigeria Federal Ministry of Works and Housing, 1970) recommends that for a material to be used as generally as fills it should possess MDD > Mg/m 3, OMC < 18 % and soaked CBR values > 5%. A comparison of the results of the study and the Nigerian Specification for road materials is presented in Table 4. Unconfined Compressive Strength (UCS) The tested samples recorded reasonably high values of cohesion (c) with moderately low values of angle of shearing resistance ( ), in comparison with the strength parameters of shale sample collected elsewhere (at Ishiagu) but with the Asu River Group; shale samples from Ishiagu recorded average c and values of 25 kn/m 2 and 38 o (Ezeribe, 1994). High c could result from the calcareous nature of the shale (Blyth and de Freitas, 1984), while the low is attributable to the presence of expansive clay as reported by Obiora and Umeji (2004). Punmia et al. (2005) noted that shales that are predominated only by clays and are as well non-cemented, most often, record very low values of. V. CONCLUSIONS The scarcity of construction materials and wide coverage of the Abakaliki area by shale have resulted in the utilization of the Abakaliki Shale Formation in all components of the road and highway pavements (i.e. subgrade, subbase and base course). Results of the soil classification tests conducted on the samples show that the Abakaliki Shale classifies as a CH soil. Previous research (Sowers and Sowers, 1970) had noted that CH soils are associated with fair to poor compaction characteristics, very high compressibility and expansion, low drainage to impervious, fair stability value as fill material, poor to very poor value as pavement subgrade (when not subjected to frost), and are not suitable as base course for pavement. A comparison of the tests results of the Abakaliki Shale with the Nigerian Specification points to the fact that the shale neither satisfies all the specifications as fill and embankment, nor all the requirements as sub-base course and base course material. Table 5 reveals that despite having satisfactory values of MDD, OMC and strength parameters, the shale recorded natural moisture contents that suggested inappropriateness as an engineering material. More so, LL and PI values point to the fact that the clay content of the shale is expansive. These factors (unfavourable natural moisture content and predominance of expansive clays) are very likely to render the shale susceptibility to expansion on moisture influx, which would cause subgrade, subbase and base course fatigues and ultimately road pavement failure, especially in the peaks of rains (in May-July and September-October). Stabilization of the shale for use as road material is possible. This is owing to the fact that the OMC is generally low, the soaked CBR values are very satisfactory for use as general fill and embankment, cohesion as well as angle of internal friction. Such improvement will involve soil modification to reduce expansiveness and water proofing to prevent excessive moisture influx. A critical appraisal on the facts concerning the poor geotechnical quality of the shale as road material would assist the civil engineer develop appropriate road design, hence, nip the failure problems in the bud. ACKNOWLEDGEMENTS Peter Okanya is warmly acknowledged for his assistance during the laboratory analyses at the Material Laboratory of Marlun Construction Company Ltd, Enugu. Chukwudike Okeugo and Andrew Oha of the Department of Geology, University of Nigeria, Nsukka, digitized the figures used. They are warmly appreciated. REFERENCES [1] Reyment, R. A. (1965). Aspects of Geology of Nigeria. Ibadan University Press, Ibadan, 145 p. [2] Ofoegbu, C. O. and Amajor, L. C. (1987). A geochemical comparison of the pyroclastic rocks from Abakaliki and Ezillo, southeastern Benue Trough. Journal of Mining and Geology. Vol. 23, pp [3] Obiora, S. C. and Umeji, A. C. (2004). Petrographic evidence for regional burial metamorphism of the sedimentary rocks in the lower Benue Rift. Journal of African Earth Sciences. Vol. 38, pp [4] Gamble, J. C. (1971). Durability plasticity classification of shales and other argillaceous rocks. PhD Thesis, University of Illinois at Urbana-Champaign, 161 p.

4 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: [5] Ezeribe, I. E. (1994). The characterization of some Nigeria shales relative to their engineering uses. M.Sc Thesis, University of Nigeria, Nsukka, 133 p. [6] Holtz, R. D. and Kovacs, W. D. (1982). Introduction to Geotechnical engineering. Prentice-Hall, New Jersey, 733 p. [7] Coduto, D. P. (1999). Geotechnical engineering, principles and practices. Prentice-Hall, New Jersey, 759 p. [8] Punmia, B. C., Jain, A. K. and JAIN, A. K. (2005). Soil mechanics and foundations. 16 th ed. Laxmi, New Delhi. [9] Wright, J. B. (1968). South Atlantic continental drift and the Benue Trough. Tectonophysics. Vol. 6, pp [10] Burke, K., Dessauvagie, T. F. J. and Whiteman, A. J. (1971). Opening of the gulf of Guinea and geological history of the Benue Depression and Niger Delta. Nature and Physical Sciences. Vol. 233, pp [11] Ofoegbu, C. O. (1985). A review of the geology of the Benue Trough Nigeria. Journal of African Earth Sciences. Vol. 3, pp [12] Murat, R. C. (1972). Stratigraphy and paleogeography of the Cretaceous and lower Tertiary in Southern Nigeria. In: Dessauvagie, T. F. J. and Whiteman, A. J., (eds), Proceeding of the Conference on African Geology, Ibadan, pp [13] Tijani, M. N., Loehnert, E. P. and Uma, K. O. (1996). Origin of saline groundwaters in Ogoja area, lower Benue Trough, Nigeria. Journal of African Earth Sciences. Vol. 23, pp [14] Benkhelil, J., Guiraud, M., Ponsard, J. F. and Saugy, L. (1989). The Bornu Benue Trough, the Niger Delta and its offshore: Tectono-sedimentary reconstruction during the Cretaceous and Tertiary from geophysical data and geology. In: Kogbe, C.A. (ed.), Geology of Nigeria. 2 nd ed. Rock view, Jos, Nigeria. [15] Okogbue, C. O. and Aghamelu, O. P. (2010). Comparison of the geotechnical properties of crushed shales from southeastern Nigeria. Bulletin of Engineering Geology and the Environment. Vol. 69 (4), pp [16] British Standard Institution (BSI) 1377 (1990). Methods of testing soils for civil engineering purposes. British Standards Institution, London. [17] Lambe, T. W. (1951). Soil testing for engineers. Wiley, New York, pp 165. [18] Akyrod TN (1957). Laboratory testing in soil engineering. Soil Mechanics Ltd, London. pp 233. [19] Bailey MJ (1976). Degradation and other parameters related to the use of the shale in compacted embankments. Joint Highway Research Project No. 23, Purdue University and Indiana State Highway Commission, pp 209. [20] Okagbue, C. O. and Ochulor, O. H. (2007). The potential of cement-stabilized coal-reject as a construction material. Bulletin of Engineering Geology and Environment. Vol. 66, pp [21] Manasseh, J. and Olufemi, A. I. (2008). Effect of lime on some geotechnical properties of Igumale Shale. Electronic Journal of Geotechnical Engineering. Vol. 13. pp 1-12 [22] Sowers, G. B. and Sowers, G. E. (1970). Introductory soil mechanics and foundations. Macmillan, New York, 556 p. [23] Mannering, F. L. and Kilareski, N. P. (1998). Principles of highway engineering and traffic analysis. 2 nd ed, Wiley, New York, 340 p. [24] Wignall, A., Kendrick, P. S., Ancill, R. and Capson, M. (1999). Roadwork; theory and practice. 4 th ed. Butterworth-Heinemann, Oxford, 309 p. [25] Nigerian Federal Ministry of Works (1970). General specifications for roads and bridge works. Federal Government of Nigeria, Lagos. [26] Blyth, F. G. H. and de Freitas, M. H. (1984). A geology for engineers. 7 th ed. Arnold, London, 325 p. [27] Underwood, L. B. (1967). Classification and identification of shales. Journal of Soil Mechanics and Foundation, ASCE. Vol. 93(11), pp Table I Locations and types of shale samples used in the study Location* Designation Sample Type Sampling Depth (m) Presco Junction Hill Top Street, S1 S Mechanic Village S3 1.2 Ogoja Road, near St Theresa s S4 1.4 Abakaliki-Enugu Highway, near EBSUTH S5 Slightly weathered 1.6 Spera In Deo Junction S6 1.5 Mile 50, near Bishop s Court S7 Slightly weathered 1.2 Afikpo Road, near Iyiokwu River S8 2.0 Onwe Road, near Hope High School Ogoja Road, near Govt house *see Figure field locations S9 S10 Slightly weathered

5 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: Table II Results of the tests on the fresh and slightly weathered samples of Abakaliki Shale Bl-Bluish Br-Brownish Parameter S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 Grain Size Distribution Gravel (%) Sand (%) Fines (%) Liquid Limit Plastic Limit Plasticity Index Linear Shrinkage (%) AASHTO Classification USCS Classification Specific gravity Maximum Dry Density (Mg/m 3 ) Optimum Moisture Content (%) CBR, after 24 hrs soaking (%) % loss in CBR after 24 hr soaking Free Swell (%) Cohesion (kn/m 2 ) Angle of shear resistance ( o ) Colour Bl Bl Bl Bl Br Bl Br Bl Bl Br Natural moisture content

6 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: Table III Comparison of the properties the Abakaliki Shale with those of Igumale Shale. Parameter Igumale Shale* Abakaliki Shale Fines (%) Liquid limit Plastic limit Plasticity index Linear shrinkage AASHTO Classification A 7 6 A 7 6 USCS Classification CH CH Maximum Dry Density (Mg/m 3 ) Optimum Moisture Content (%) CBR, % (after 24 hrs soaking) Free swell (%) Specific gravity Colour Grey Bluish Brownish Natural moisture content (%) *data from Manasseh and Olufemi (2008). Table IV Comparison of results with Nigerian specification for road and bridge materials Properties of material Nigerian specifications a Abakaliki Shale Remarks General filling and embankment MDD (Mg/m 3 ) OMC (%) LL PI % Passing No. 200 (%) CBR (24 hrs soaked) BS (%) > < 18 < 40 < 20 < 35 > Poor to marginally suitable Sub-base course LL PI CBR (24 hrs soaked) at West African Standard and OMC (%) < 35 < 16 > Likely to be unsuitable Base course LL PI Unsoaked CBR at Modified AASHTO and OMC (%) % Passing sieve No. 200 (%) UCS (kn/m 2 ) < 30 < 13 > >103 ( a adapted from Okagbue and Ochulor, 2007) Most likely to be unsuitable

7 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: Laboratory test and in-situ observations Table V An engineering evaluation of some physical properties of the Abakaliki Shale. Physical properties Average range of values (Underwood, 1967) Unfavourable Favourable Abakaliki Shale Remarks Cohesive strength (kn/m 2 ) , Unfavourable Angle of internal friction ( ) Favourable Dry density (Mg/m 3 ) Favourable Natural Moisture content (%) Unfavourable Predominant clay minerals Montmorillonite, illite *data from Obiora and Umeji (2004) Kaolinite, chlorite *Montmorillonite, illite Unfavourable 6 o 24 N 8 o 00 E 8 o 12 E 6 o 24 N.Nkpume.Akpatakpa W N.Isieke.Idembia-Izzi E.Agalagu.Abofifia NIGERIA.Obeag u.ike Enyi. Abakaliki.Ngbo Agbaja S Umuaru Ezza.Umuaghara.Ntezi Aba S7 S1.Mgbabor S5 S9 S6 S10.JUJU HILL S2.ABAKALIKI S8 S4 S3.Ekperu Inyimagu.Ezzagu.Nchoke.Ndiechi Igbagu.Achara Unuhu.Ugbuloke.Agbaja Key.Ogharaugo Footpath.Ndigbo.Agu Akpu.Nkwagu Major Road Highway Sample locations City centre 6 o 14 N 0Km 8 o 00 E.Agalagu 4Km 8 o 12 E 6 o 14 N Fig. 1. Accessibility map of Abakaliki area showing sampled locations.

8 International Journal of Civil & Environmental Engineering IJCEE-IJENS Vol: 11 No: Benue Makurdi 9 00'E NIGERIA N 'N Enugu 2 4 Abakaliki 3 11 Niger Delta Sediments 10 Ameke Formation 9 Imo Shale 8 Nsukka Formation 7 Ajali Sandstone 6 Mamu Formation 5 Nkporo/Enugu Shale 4 Awgu Shale 3 Eze-Aku Group 2 Asu River Group 1 Basement Complex Niger 2 Major city River Study area 11 1 Fig. 2. Geological map of southeastern Nigeria showing the distribution of the Abakaliki Shale. 60 Low High 50 Legend Slightly weathered sample CH 40 sample U-line A-line PI 30 CL OH or MH CL-ML ML LL Fig. 3. Plots of the samples of the Abakaliki Shale in the plasticity chart.