ABSTRACT
The geotechnical and geochemical properties of lateritic soil developed on migmatite
along Ogbomosho – Ilorin road, south-western Nigeria was studied. Soil samples were collected from seven (7) different laterite profiles along the road cuts and analyzed in the laboratory so as to determine the geotechnical, geochemical and petro-graphical characteristics of each horizon within the profile and their suitability as construction and/or foundation material. The geotechnical tests conducted according to the British standard procedures (1377) include: index test and performance test. Field geological mapping shows that the parent rock from which the in – situ lateritic soil was derived is a migmatite with an average soil profile depth of 4.2m. The principal mineral compositions of the migmatite are quartz (40%), biotite (25%), feldspar (21%), hornblende (6%), and muscovite (5%). The moisture content obtained range between 3.1 and 14.2%,Specific gravity value range between 2.50 and 2.74, bulk density and dry density range between 1.44 and 1.77g/cm3 and 1.31 and1.62g/cm3 with an average value of 1.56 and 1.44g/cm3 respectively which indicate that the soil was produced through in – situ weathering of granitic rocks rich in felsic mineral. The results of the Atterberg consistency limit range between 3.3 and 14% (PL), 40 and 58% (LL), 17.7 and 35% (PI) with an average value of 47.2%, 49% and 8.1% respectively. Similarly plots on the plasticity chart shows that the soil falls above A- line corresponding to CL zone which is classified as inorganic clay of moderate plasticity based on Unified Soil Classification System (USCS). The grain size above 0.075 mm revealed fine to coarse grained sand.The OMC and MDD have an average value of 13.08% and 1.76kg/m3 and the CBR value for soaked and unsoaked ranging between 17 and 66% and 42 – 74% with an average of 57.58% and 34.8% which indicate that the soil can be used for sub – base and subgrade in pavement construction of roads. The value obtained from angle friction (ϕ) and cohesion (C) range between 22 and 32° and 10 and 30KN/m2 with an average value of 27.04 and 21.5KN/m2 which indicate that the soils can be used as material in the construction of fill road embankment. The x-ray fluorescence analysis shows that the major oxides include SiO2, Fe2O3, Al2O3, K2O, Na2O, TiO2, and MgO while SiO2, Fe2O3 and Al2O3 constitute 70–80% of the oxides. The research reveals that with the exception of Horizon A and C of the profile the geotechnical and geochemical characteristics of the lateritic soil derived from migmatite conform to Nigerian Federal Ministry of Works guidelines on road construction materials and compare favourably with the results of other authors. Sectional failures might have resulted from inappropriate material handling and/or equipment inadequacies.
CHAPTER ONE
1.0 INTRODUCTION
1.1 Background to the Study
Lateritic soils are soils that are composed almost entirely of iron and aluminium oxides; they are usually reddish in colour and are the least soluble product of rock weathering in tropical climates (Plummer, McGeary and Carlson, 2001). They are formed in regions of high temperature and abundant rainfall, where the soils are highly leached. Under such conditions, weathering is deep and intense. Tropical weathering (laterization) is a prolonged process which produces a wide variety in the thickness, grade, chemistry and mineralogy of soils. The leaching of rocks by percolating rain water during wet season results to solution containing the leached ions which form soluble salt compounds by capillary action, during the dry season these salts are brought to the surface and wash away during the following wet season.
Gidigasu (1972) divided the stages of laterite formation into three, namely:
(a) Stage of decomposition:- This involves the physical and chemical breakdown of rock forming mineral to yield simple ionic compounds.
(b) Stage of laterization:- This involves the leaching away of the soluble bases such as the oxides of sodium and calcium, leaving an enrichment of insoluble bases such as the oxides and hydroxides of magnesium, titanium, iron and aluminium. The process of leaching involves acid dissolving the host mineral lattice, hydrolysis and precipitation of insoluble oxides and sulphates of aluminium, iron, and silica under intense temperature condition of humid sub-tropical climate.
(c) Stage of desiccation:- This involves dehydration of insoluble bases.
Lateritic soils are very important in the construction industries and activities as construction material and foundation support for engineering structures. Although studies of the engineering properties of lateritic profile began some decades ago, but there is still paucity of geotechnical and geochemical data. This research tends to focus on the influence of position of horizon within the soil profile and the resultant geotechnical and geochemical properties of soils developed over migmatite and also examine their suitability as construction and /or foundation material.
1.2 Statement of the Problem
Ogbomosho – Ilorin New road was commissioned in 2009 but already showing some signs of failure in some sections. The non durability and cracks on Ogbomosho – Ilorin road are due to the misuse of construction material among other factors. Engineering structures are found in all construction works involving the use of lateritic soil however, not all lateritic soil can be assumed to be suitable for use as construction materials. Their classification test must be performed to ascertain their nature before usage. In Nigeria soils excavated almost everywhere is often used directly for various construction purposes without thorough investigation of their geotechnical properties to ascertain their suitability which give rise to failure in roads. It is therefore reasonable to ensure that the structures are in top conditions at all times. One of the most important ways of attaining this is by ensuring that the soils used in the construction meet design standards and specifications. The geotechnical properties of soil that do not meet design standard and specification can be improved. The commonly used methods are index test and performance test. For this research, index and performance tests are adopted.
1.3 Justification
Road failure is prevalent in Nigeria and the trend is a cause of concern to both the users and the road maintenance authorities as they have economic disadvantages. In this regard, it is necessary that research be initiated to determine the possible causes of such road failures; and a case study was carried out on Ogbomosho – Ilorin Highway, South- western Nigeria.
1.4 Scope of Work and Limitations
The scope of the research constitutes desk work on the available secondary data collection. Geological mapping of highway cut slope at Eyenkorin, Lasoju, Ote, Gambari, Ladoke Akintola University of Technology (LAUTECH) Junction, field studies and sampling of soil and rock. Geotechnical and geochemical laboratory test/analysis was carried out on disturbed soil samples. Thin sectioning of rock samples was done. Inability to sample several of the exposed soil profiles along the study area because of cost implication was a major limitation to this research.
1.5 Description of the Study Area
The study area is located along Ogbomosho-Ilorin road with Ogbomosho in Oyo state to the south and Ilorin in Kwara State to the north with emphasis on places like Lasoju, Ote, Gbede, Abduka, Gambari, Egbeda, Akanbi and LAUTECH junction (Figure 1.1). The lateritic soils found in this area develop on many rock types in different sub-climate and drainage environment.
1.5.1 Location, Extent and Accessibility
The area is situated in the transitional zone between the forest and savannah region of south-western Nigeria. The Ogbomosho – Ilorin new road is situated on latitude 8˚ 26′ N 4˚ 24′ E and 8˚ 9′ N and 4˚ 37′ E. The road trends on NNE – SSW with Ilorin in the north and Ogbomosho to the south.
1.5.2 Topography and Drainage
The topography of the study area is slightly undulating with rounded low hills, occasional often elongated ridges indicating the characteristics residue setting of a typical basement terrain with an average height ranging between 180 and 360m above sea level. The area is drained mainly by river Asa, with Elekunkun, Awe, Idandan, Oshin, Moshi, and Ero being some of the tributaries forming a dendritic drainage pattern and generally flowing northward to the River Niger.
The dendritic drainage type is due to the loose nature of the top soil whereby water flows in all directions (Fig 1.2).The channels of these smaller streams are dry for many months especially from November to May.
1.5.3 Climate and Vegetation
The climatic condition is humid tropic, characterized by alternating wet and dry season with total annual rainfall of 1000 to1200mm which is good for the formation of laterite Alao (1983). The wet season occurs between April and October and dry season from November to March. The vegetation is guinea savannah and characterized by scanty trees and green grasses. However, due to the persistent human activities, the natural vegetation has been destroyed which give rise to the savannah type of vegetation (Ige, Ogunsanwo, and Inyang, 2011).
1.5.4 Geological Setting of Study Area
The study area is a typical Precambrian Basement Complex terrain with an elevation of about 394m above sea level. The rocks of this Basement include granite gneiss, migmatite, biotite gneiss, porphyroblastic gneiss, pegmatite and quartzite of which migmatite is the dominant rock type. The superfacial deposit within the Basement Complex terrain varies in thickness from 4m to 8m and are mostly clayey loamy topsoil and dark sandy soil, usually less than 2m thick followed by reddish brown laterite soil in most cases.
1.6 Aim and Objectives
This research project is aimed at studying the influence of position of the lateritic horizons on the geotechnical and geochemical properties within the profiles developed over migmatite.
Objectives:-
The major objectives of the study were as follows
I. To study the geology of the study area.
II. To determine the geotechnical, geochemical and petrographical characteristics of each soil horizon within the profile. III. To determine the suitability of each horizon as construction and/or foundation material
This material content is developed to serve as a GUIDE for students to conduct academic research
GEOTECHNICAL AND GEOCHEMICAL PROPERTIES OF LATERITIC SOIL ON MIGMATITE ALONG OGBOMOSHO – ILORIN HIGHWAY SOUTH-WESTERN NIGERIA
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