Permeability Characteristic of Some Sub-Grade Soils Along Part of the Sagamu-Ore Highway, Southwestern, Nigeria

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1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 4(4): Scholarlink Research Institute Journals, 2013 (ISSN: ) jeteas.scholarlinkresearch.org Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 4(4): (ISSN: ) Permeability Characteristic of Some Sub-Grade Soils Along Part of the Sagamu-Ore Highway, Southwestern, Nigeria Bayewu, O. O. Olufemi, S. T. and Adewoye, A. O. Department of Earth Sciences, Olabisi Onabanjo University, Ago-Iwoye, Nigeria. Department of Earth Sciences, Ladoke Akintola University of Technology, Nigeria. Corresponding Author: Bayewu, O. O. Olufemi Abstract Lateritic soils along a section of Sagamu-Ore highway were accessed for their suitability as sub-grade soils with the aim to determine the geotechnical basis for the observed state of the highway pavement failure. The work was carried out in two major stages; field sampling and laboratory analysis. Field sampling involved the study of physical and geological settings of the area, sample collection which include collection of ten bulk samples and twenty undisturbed samples from trial pits, description and preparation for laboratory tests. Laboratory analyses involved permeability test, grain size distribution analysis, consistency limit test, compaction test, linear shrinkage and clay mineralogical analysis. Petrographic analysis of parent rocks (granite, granite gneiss and porphyritic granite) samples were also carried out by studying the representative parent rock samples collected. The results showed that all the sub-grade soils belong to group A 6 of the American Association of State Highways and Transportation Officials (AASHTO) classification system they are thus fair to poor as subgrade materials. The average value of the linear shrinkage is 7.8%, thus, the studied soils are good highway soils. The permeability coefficient determined for sub-grade soils range between 2.70x10-6 mm/s to 2.32x10-4 mm/s for stable locations while for unstable locations they range between 4 to 14 percent and 4.50x10-6 mm/s to 9.17x10-5 mm/s respectively. Soils below stable locations generally have higher amounts of kaolinite than those below unstable sections, thus an indication that stable locations are better drained than unstable ones. The stability of the pavement in stable locations is thus due to good drainage in the locations. Therefore, the practice of designing road pavement without adequate drainage system should be abolished to reduce or prevent contact between water and subgrade soils. Keywords: Lateritic soils, AASHTO classification, permeability coefficient and linear shrinkage. INTRODUCTION From time immemorial numerous efforts have been made by Nigerian government in the development of roads and highways. However, after few years of construction or even before attaining the design age, roads usually fail. Sometimes they served as death traps. Some factors that may be accountable for road failures include: misuse of road by motorist, over usage and poor construction (Ajayi, 1985). It is therefore imperative to study the characteristics of soils such as grain size distribution and plasticity, permeability characteristics, consistency limit, linear shrinkage, compaction characteristics at Optimum Moisture Content (OMC) and Maximum Dry Density (MDD), mineralogical characteristics and so on. These will help to know the suitability of soils for sub-grade materials. Pavement may be of two types; flexible pavement and rigid pavement. Flexible pavements are those sections which cannot withstand the appreciable tensile stress, whereas, rigid pavements are those roads covering which can withstand appreciable 581 tensile stress. Failure of flexible highway pavement is a common phenomenon in most parts of the tropical world. Such failures can occur in form of pitting, rutting and waviness (Gidigasu, 1972). Apparently, a rigid pavement can span over small depressions in the sub-grade without itself getting subsided in them. While on the other hand, a flexible pavement changes itself, along with the change in sub-grade. That is why bituminous road have lots of ups and downs, without the cracking of the riding surface. In the past, most highway projects have been based on comparison of the geotechnical and engineering properties of soils in stable and unstable locations with the view to accounting for the stability of the stable locations and instability of failed sections. This work was meant to ascertain whether or not the stability of road can be correlated with permeability characteristics of subgrade soils based on recommended standard for subgrade soils in the tropics.

2 LOCATION AND GEOLOGY OF THE STUDY AREA The study area (figure 1) is located along Sagamu- Ore highway which falls within the southwestern Nigeria. It lies within the longitude I 59.1 II E- 004 o 12 l 12.8 ll E and latitude 06 o 47 l 26.0 ll N- 06 o 45 l 47.9 II N and falls in the humid tropical region. It has an averagely high temperature of c throughout the year with a mean minimum temperature of 21 0 C. There are two distinct seasons within the continental masses with an alternative wet and dry season limestone and shale. The top soil is brown with lateritic sandy clay. The sandy constituent that is fine to medium grained increase towards the base and gravelly. The shale overly the limestone unit with highly laminated plastic and greyish. The shale unit is often calcareous towards the limestone bed. The limestone bed is crystalline and shelly. It is often described as pack stone and wackestone base on their textural relationship. The limestone within the limestone-shale interbed is also crystalline some are often fragmented and contain abundant trace fossils Figure 1: The location map of the study area showing the sampling points The wet season occurs between March and October. Dry season commences from late November and continues until the end of February the following year and relative humidity is averagely high throughout the year. The predominant and natural vegetation of the area can be grouped into forest vegetation. The area is well drained with the main rivers which include River Yemoji that runs from the north eastern part of Ijebu-Ode to the south, forming a confluence with River Owa. As noted by Adeyemi (1992) that surface morphology, land form features such as relief and drainage system depend to a large extent on the differences in the composition of rocks and the frequency of joints and features of rocks. The drainage pattern of the area is dendritic. All the rivers together with Main River Osun and Shasha flowing from the northern part, empty their contents into the Lagos lagoon which is at the southern extremity of the study area. The study area falls within the southwestern Basement Complex of Nigeria and sedimentary terrain of Dahomey Basin (figure 2). The predominant rocks are basement rocks and are found at the eastern and central part of the study area. These are porphyritic granites, granites and granite gneiss. The eastern part is occupied by part of Dahomey Basin and the rocks found include sandstone, 582 Figure 2: Geological map of the Study Area METHODOLOGY This work was carried out in two major stages namely: field investigation and the laboratory analyses. The field investigation includes the study of physical and geological settings of the area, samples collection, description and preparation for laboratory tests. The laboratory analysis involved pre-test samples preparation, geo-technical test, and petrographic analysis. Thirty samples were collected in all; twenty (20) undisturbed and ten (10) bulk samples. For the undisturbed samples, the core-cutter was used to collect two samples each from ten (10) different locations (five stable and five unstable locations) at about a minimum of one kilometre apart. The weathered materials from the parent rock which weathered at insitu were also collected at different locations, no visible rock samples were cited along the highway. The undisturbed samples collected with the aid of core-cutter were waxed and put in polythene bags to prevent the exchange of moisture content between the soils and the atmosphere. The samples were used for permeability test. The disturbed samples were prepared for laboratory analyses to drive out the in-

3 situ moisture content. This is because air-dried samples are ideal for most geotechnical investigations (Ogunsanwo, 2002, Malomo, 1983). Part of air-dried disturbed samples was sieved using sieve with mesh size 0.063μm. Soils that pass through the sieve were used for classification tests; linear shrinkage, compaction at Modified AASTHO level, liquid and plastic limit. The unsieved part of the disturbed sample was used for the grain size analysis. The sets of laboratory analyses employed in this work can be broadly categorized as; petrographic analysis and engineering test. The Petrographic analysis was basically used to determine the detailed textural and mineralogical characteristics of the rock samples in relation to the derived soils. The photomicrographs were studied accordingly to identify various mineral components of the rock samples especially their clay mineral contents. In engineering tests, detailed geotechnical characteristics of the representative soil samples were investigated by carrying out; the compaction at modified AASHTO level, permeability, grain size analysis, atterberg limit, linear shrinkage and clay mineralogical analysis. RESULTS AND DISCUSSIONS The petrographic analysis of the parent rocks (porphyritic granite, granite and granite gneiss) were carried out in order to know the mineralogical composition, the relative abundance of the minerals and texture of the rock studied. This is because the properties of residual soils are closely associated to those of the parent rocks (Adeyemi, 1992). The major minerals observed in the rocks are quartz, plagioclase feldspars and micas. The summary of the modal analyses of minerals present in the thin section of rock samples are shown in table 1. Rocks rich in quartz will weather to predominantly coarse-grained minerals (sand and gravel) particles, abundance of feldspar will lead to fine-grained clay soils while abundance of mica will produce soils that are rich in silt-size particles (Gidigasu, 1976, Adeyemi and Oyeyemi, 2001). Gidigasu (1976) explains that a soil that is rich in flaky minerals will make compaction of such soils difficult as a result of spring action. The summary of grain size distribution parameters of the study soil samples are shown in table 2. In this table, the soil particles spread evenly across the grain size. This is an indication that the soil samples contain a wide range of grain sizes and can therefore be described as being well-graded. This was also verified by the values obtained from the computation of the co-efficient of uniformity C u for the various grain sizes. It is generally believed that a well graded soil makes a good subgrade material (Mesida, 1963, Gidigasu, 1974). The samples contain high amount of fine grained particles (silt size) in relation to other particle size fractions which are probably derived from the micas and feldspars that constitute the bulk of the minerals of the parent rock Table 1: Modal Composition of the study rock samples Minerals % I II III Quartz Microcline Plagioclase feldspar Biotite Muscovite Others Total Rock names Porphyritic Granite Granite Granite Gneiss The moderate to low amount of clay size fraction might be due to high degree of laterization that may lead to increase in its crushing strength (Adeyemi et al., 1990). The lower the amount of fines in a soil, the better it is for engineering purposes. Soils that are largely made up of fines (clay and silt) are likely to have poorer geotechnical properties as highway subgrade than soils that constitute coarse particles (Gidigasu, 1974, 1976). This explains the failure observed in some unstable locations. Stable location five (SL5) has a greater tendency of failing but was probably prevented due to good drainage facilities installed. (Table 2). Table 3 shows the permeability and co-efficient of uniformity determination using grain sizes. The permeability values obtained are very low in both samples from stable and unstable locations. These low values are not good for subgrade soils because such soils can retain water for a fairly long time, a situation that can result in loss in strength. The fact that part of the location is stable does not mean that is going to remain like that if necessary preventive measure such adequate drainage facilities are not taken. C u = D 60 / D 10 C u - is the Co-efficient of Uniformity D 10 is the effective size Where: C u > 5 indicates a well graded soil C u < 3 indicates a uniform soil Note: D 60 and D 10 are obtained from the grading curve and it corresponds to the percentage passing at 60 percent and 10 percent respectively. K = CD 2 50 K = permeability in mm/sec, C = Mickinlay s constant which is approximately , D 50 = maximum diameter of the smallest 50 percent of the sample. 583

4 Table 2: Grain Size Distribution of the study soil sample Gravel Coarse Sand Medium Fine Sand Silt % Clay % Amount of fines % % Sand % % USL USL USL USL USL SL SL SL SL SL Table 3: Permeability and Co-efficient of Uniformity Determination Using Grain Sizes (estimated from Mickinlay s and Hazen s formulae) D 60 D 10 C u D 50 D 2 50 K (mm/sec) Grading Type (mm) (mm) USL x 10-5 Well Graded USL x USL x 10-6 Well Graded USL x 10-5 Well Graded USL x 10-5 Well Graded SL x 10-6 Well Graded SL x 10-5 Well Graded SL x SL x 10-5 Well Graded SL x 10-6 Well Graded The difference between the amount of fines in compacted and uncompacted samples are not significant. This means the soils are mechanically stable and can be compacted at the West African level. The summary of the consistency limits and classification tests are presented in tables 4-6 while figure 3 presents the Casagrande chart classification of the soils. The soils have liquid limit ranging between 26.5 percent percent and plasticity index between 9.95 percent 18.95percent. The plasticity index for the soils is less than 25 which is the maximum recommended value for subgrade tropical soils (Madedor, 1963). In 1970, the Federal Ministry of Works for Road and Bridges (FMWRB) gave the specification of 40 percent maximum and 20 percent maximum for liquid limit and plasticity index respectively for laterite of highway subgrade materials. Most of the values obtained for plasticity index and liquid limit agrees with the specification except for few liquid limit values obtained from the unstable locations which exceed the stated values. The Casagrande chart indicates low to medium plasticity. This explains why some locations do not prove problematic after construction had been carried out (Terzaghi, 1958). Table 4: Casagrande chart Classification Liquid limit % Plastic limit % Plasticity Index % Plasticity type Soil Type USL Medium Inorganic USL Medium Inorganic USL Medium Inorganic USL Medium Inorganic USL Medium Inorganic SL Medium Inorganic SL Low Inorganic SL Low Inorganic SL Medium Inorganic SL Medium Inorganic 585

5 Table 5: Soil Classification With Respect To USC Classification System % passing sieve no. 200 Liquid limit % UCS Major Divisions Rating as Subgrade USL Fine grained silty soils Fair to poor USL Fine grained clayey soils Fair to poor USL Fine grained clayey soils Fair to poor USL Fine grained clayey soils Fair to poor USL Fine grained silty soils Fair to poor SL Fine grained clayey soils Fair to poor SL Fine grained clayey soils Fair SL Fine grained clayey soils Fair SL Fine grained clayey soils Fair to poor SL Fine grained clayey soils Fair to poor Table 6: Soil Classification With Respect To AASHTO (1993) Classification System No. % passing sieve no. 200 Liquid limit % Plastic limit % Plasticity Index % AASHTO Classification Rating as Subgrade Material USL A-6 Fair to poor USL A-6 Fair to poor USL A-6 Fair to poor USL A-6 Fair to poor USL A-6 Fair to poor SL A-6 Fair to poor SL A-6 Fair to poor SL A-6 Fair to poor SL A-6 Fair to poor SL A-6 Fair to poor Fig. 8: Casagrande chart classification of subgrade soils The values of the linear shrinkage obtained are presented in table 7 below. Madedor (1983) suggested that soils with linear shrinkage less than 10 percent would not pose any field compaction problem and will therefore be good for highway subgrade soils. The values obtained ranges from 4 percent to 14 percent with an average value of 7.8 percent. Thus, the studied soils are good highway soils. Most of the linear shrinkage values of soils below unstable locations exceed the normal requirement, hence its instability. On the other hand, most values obtained from the stable locations are within the limit of the set values, hence its stability. 585 Table 7: Linear Shrinkage of the studied soil samples location Initial length (mm) Final length (mm) ( %) USL % USL % USL % USL % USL % SL % SL % SL % SL % SL % The soils were compacted at the modified AASHTO level of compaction to determine the compaction level for the highway subgrade materials. From the results in Table 8, the maximum dry density (MDD) ranges from kg/m 3 to kg/m 3 with optimum moisture content (OMC) ranging from percent to percent at Modified AASHTO level. The higher the MDD, the lower the OMC, the better the soil for engineering purpose(s). Table 8: samples Compaction Parameters of studied soil No. Modified ASSHTO Level of Compaction Subgrade Foundation Materials OMC % MDD (kg/m 3 ) USL Poor USL Fair USL Fair USL Good USL Good SL Poor SL Fair SL Good SL Fair SL Poor

6 The activity of the soil samples can be calculated from the relationship between the plasticity index and percentage of clay in a particular sample by weight. A = PL/(% of clay-sized fraction by weight) Where: A = activity PL = plastic limit Table 9 shows the calculated activity of the soils from their percentage of clay by weight and plasticity index respectively. The plastic properties of the soils result from the adsorbed water that surrounds the clay particles. This is directly related to the type of clay minerals and their amount in the soil which in turn affect the liquid limit and plastic limit (Skempton, 1953). On the basis of these results the soils exhibit activity ranging from and for unstable and stable locations respectively. It can be inferred from the results that the dominant clay minerals are Kaolinite-Illite. Table 9: Table of Activity of Studied soil samples Clay content Plasticity Activity location (%) index USL USL USL USL USL SL SL SL SL SL Table 10 shows the co-efficient of permeability of the remoulded samples from the studied area. The values of permeability co-efficients range from 4.50 x 10-6 mm/sec to 2.32 x 10-6 mm/sec. The permeability coefficient are generally low and they are typical of those of impervious soils (silt and clay). The potholes observed in the failed zones are probably going to increase with time due to low permeability coefficient values determined. The permeability values obtained using the constant head permeameter shows significant agreement with the ones obtained from grain size fractions. In both cases, the permeability values are generally low. Table 10: Summary of the Permeability characteristic Using constant Head permeameter No. Permeability coefficient K (mm/sec) USL x10-5 USL x10-6 USL x10-6 USL x10-6 USL x10-6 SL x10-6 SL x10-5 SL x10-5 SL x10-4 SL x Based on the percentage in composition, kaolinite and quartz have been proved to be abundant clay mineral in the studied soils. The soils contain small amount of Illite while the amount of montmorillonite are insignificant. Alexander and Cardy (1964) studied the lateritic soils in southwestern Nigeria and concluded that majority of lateritic soils in southwestern Nigeria contain large amount of Kaolinite and Quartz. The amount of Kaolinite in stable locations base on t-test analysis is higher than that of unstable locations and these may account for stability in stable locations. Adeyemi (1992) studied highway geotechnical properties of laterized soils in Southwestern Nigeria and concluded that the higher the amount of Kaolinite-Illite ratio in the soil, the more stable would be the pavement placed on it. CONCLUSIONS The results of the various tests performed on rock and soil samples obtained from the study area have given enough information to arrive at the following conclusions: 1. Petrographic studies of the rocks reveal predominant rock like quartz, feldspars and micas. Such rocks often weather to produce well-graded soils. Quartz has been found to be higher than feldspars and micas while other minerals are minor. 2. The soils below the pavement in the study locations have high amount of fines which make them to fall into A-6 of the AASHTO classification system and are thus fair to poor. The difference between the amount of fines in compacted and uncompacted samples is not significant. 3. The plasticity index and linear shrinkage of soil samples from both locations are typical of good subgrade soils. 4. The values of the permeability co-efficient of the soils are typical of impervious clayey soils with very low transmitting capacity. 5. The permeability values obtained using the constant head permeameter shows slight agreement with the ones estimated from grain size parameters. In both cases, the permeability values are generally low. These low values are not good for subgrade soils because they retain water which will weaken the soils/ pavement. 6. Most of the values obtained for plasticity index and liquid limit agrees with the Federal Ministry of Work for Road and Bridges (FMWRB) specification except for few liquid limit values obtained for samples from the unstable locations which exceed the stated values. 7. Most of the linear shrinkage values of samples from the unstable location exceed the normal requirement, hence its instability. On the other hand, most values obtained from the stable

7 location are within the limit of the set values, hence its stability. 8. Soils largely made up of fines (clay and silt) are likely to have poorer geotechnical properties as highway subgrade than soils that are rich in coarse particles. This explains the failure observed in some unstable locations. Stable location five (SL5) has a greater tendency of failing but was probably prevented due to good drainage facilities installed. REFERENCES AASHTO, (1993): Classification of soil and soil aggregate mixture for highway Construction purposes. Highway material, Vol. 1. Alexander, L.T. and Cady, J.G. (1963): Genesis and hardening of laterites soils. US Dept Agric. Bulletin No. pp Mesida, E.A. (1963): The influence of geological factors on the engineering properties of some southwestern residual lateritic soils as highway construction materials. Ogunsanwo, O. (2002): Effect of induration on the geotechnical properties of some soils from part of southwestern Nigeria. Jour. Min. Geol. Vol 38 (1) Practice A.A. Nalkema Publishers, Rotterdam, pp Skempton, A.W (1953) The colloidal Activity of clays proceedings, 3 rd international conference on Soil Mechanics and Foundation Engineering, London, Vol.1, Terzaghi, K, (1958): Design and performance of the Sasuniwa Dam. Institute of civil Engineering, London 9: pp Ajayi, L.A (1985): Laterite and lateritic soils of Nigeria geotechnical practice in Nigeria, ISSMFE Golden Jubilee commemorative volume NGA, Lagos 7-15 pp. Adeyemi, G.O; Ojo, A and Omidiran, M.O (1990): Relationship between some index properties and crushing strength of three south western Nigeria lateritic clay deposits nig Jour Res, Vol, 2, pp Adeyemi, G.O (1992): Highway Geotechnical properties of laterized residual soils in the Ajebo- Ishara geological transition zone of southwestern Nigeria, unpublished PhD (geology) Thesis Obafemi Awolowo University, Ile-Ife, Nigeria. Adeyemi, G.O and Oyeyemi, F. (2001): Geological basis for the failure of the sections of the Lagos- Ibadan Expressway, southwestern Nigeria, Bull. Eng. Geol. Env, Vol Gidigasu, M.D.(1972): Mode of formation and geotechnical characteristics of laterite materials of Ghana in relation to soil factor. Engineering Geology 6, pp Gidigasu, M.D.(1974): Degree of weathering in the identification of laterite materials for engineering purposes. Engineering geology, Amsterdam, S (3): pp. Gidigasu, M.D. (1976): Laterite soil engineering Elsevier Amsterdam, 554pp. Malomo, S. (1983): An investigation of the peculiar characteristics of lateritic soils from Southern Nigeria. Bulletin of the International Association of Engineering Geology, Paris, 28,