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CHAPTER   ONE

1.1    INTRODUCTION

Residual soils are products of the in-situ physical and chemical weathering of bedrock, and are commonly situated above the groundwater table. They are often unsaturated in nature and possess negative pore-water pressures or matric suctions relative to the atmospheric conditions that contribute to the shear strength of soils (Rahardjo et al., 1995). Residual soils are the soils formed in place by the weathering of the rock at the location, with little or no movement of the individual soil particles (Lambe and Whitman, 1969).  They tend to be characterized by angular to subangular particles, mineralogy similar to parent rock, and the presence of large angular fragments within the overall soil mass. Among the factors influencing the rate of weathering and the nature of the products of weathering are climate (temperature and rainfall), time, and type of parent rock, vegetation, drainage, and bacterial activity.

Temperature and other factors have been favorable to the development of significant thicknesses of residual soils in many parts of the world, particularly Southern Asia, Africa, Southeastern North America, Central America, the Islands of the Caribbean, and South America. From this distribution, residual soils tend to be more abundant in humid and warm regions that are favorable to chemical weathering of rock, and have sufficient vegetation to keep the weathering products (residual soils) from being easily transported as sediments (Lambe and Whitman, 1969). All engineering structures are expectedly founded on residual soils and all construction works involve the use of residual soils but not all residual soils however can be assumed to be suitable for use for construction purposes. Foundation studies usually provide subsurface information that normally assist civil engineers in the design of foundation of civil engineering structures. A foundation is an integral part of a structure and its stability depends upon the stability of the supporting soil.

The foundation must be stable against shear failure of the supporting soil and must not settle beyond a tolerable limit to avoid damage to the structure. The statistics of failures such as road, buildings, dams and bridges throughout the nation has increased geometrically. The need for pre-foundation studies has therefore become very imperative so as to prevent loss of valuable lives and properties that always accompany such failures. Several factors have been found responsible for the failures of these structures which include geological, geomorphological, geotechnical, construction methods, usage and maintenance. The geological factors include nature of soil (laterite) and near surface geological sequence, geological structures like fractures, faults, joints, and cavities etc, existence of ancient stream channels and shear zones, position of soil in soil profile, and degree of weathering. Engineering geological mapping is a tool used in the development and planning of civil and mining works and its use had increased in recent years.

This tool helped in the planning of Nigeria’s capital city, Abuja, the Federal Capital Territory Nigeria (Malomo et al., 1983). The study area is a fast-growing Federal University of Technology which has only built up 45% of her land area. The area is covered by residual soils. Due to the rapid development on the campus following FUTA Campus Master Plan 2013-2022 (Figure 1.1), there has been increasing number of building projects going on . Classification and strength tests must be performed on the underlying soils to ascertain their nature and to which construction works they are well suited for (Ogunsanwo, 1989). Strength tests such as compaction, consolidation, California Bearing Ratio and triaxial shear strength were carried out for stability (bearing capacity) and determination of the magnitude and rate of settlement of the study area.

1.2    AIM AND OBJECTIVES 

This research sets out to investigate the strength characteristics of the residual soils derived from basement rocks in the Federal University of Technology, Akure Campus, Southwestern Nigeria. It will also establish their behavior and suitability as foundation and construction materials for any civil engineering structures. The specific objectives are to: 

(i)     determine suitability of residual soils on FUTA campus for construction purposes; 

(ii)    establish contribution of local geology to the strength of the soils; 

(iii)   estimate the ultimate bearing capacities of the soils and determine the rate and magnitude of settlement of the soils; 

(iv)   predict engineering behavior of soils in the study area as foundation materials for civil engineering structures; and 

(v)    establish empirical relations between the geotechnical parameters of residual soils in the study area.

1.3   EXPECTED CONTRIBUTIONS TO KNOWLEDGE 

This study is expected to provide the following: 

(i)     insight to the influence of local geology on the strength characteristics of residual soils; and 

(ii)    information on soil properties required in planning, designing and the construction of civil engineering structures on the university campus.

1.4   LOCATION OF THE STUDY AREA 

The study area is a university community, the Federal University of Technology Akure,  located along Akure-Ilesha expressway, in southwestern  Nigeria. (Figure 1.2). It lies between Latitudes 7o17’0’’N – 7o19’0’’N and Longitudes 5o7’0’’E- 5o9’0’’E.

1.5   CLIMATE 

The study area is characterized by humid tropical climate of the West African Monsoonal type with distinct wet and dry seasons. Annual rainfall reaches mean value of about 1350mm coupled with high temperatures reaching a peak of about 32oC around February and a threshold of about 21oC around August. Relative humidity ranges from about 70% around January to about 90% in July.

1.6   VEGETATION 

Vegetation in the study area is of regrowth rainforest type which consists of grasses and scattered trees but had in many parts been modified with human activity such as buildings, roads and land cultivation etc.

1.7   SOILS 

The surface soils in the study area are composed largely of residual soils which are weathering products of the basement rocks. The soils are reddish to brownish in color, having medium to coarse-grained mineral matters with some clayey materials. They are characterized by mottled and sticky features in some locations. The soils of the study area resemble laterites in nature.

1.8   TOPOGRAPHY

The topography is undulating with the presence of depressions at the southern part and fairly level plain at the northern part of the study area. There are some ridges with their trends in the generally north-south direction. In some places, all the basement rocks except the charnockites occur as inselbergs which stand out against relatively low-lying surroundings (Figure 1.2).

1.9   DRAINAGE

The study area is well-drained and the streams draining the area flow southward.  The stream directions are largely controlled by the trend of the rocks and joints (Figure 1.2). Most of the stream channels shows a meandering pattern while some flow in a straight pattern of about 300m which can be suggested as fracture-controlled drainage system. The drainage is of medium density which is as a result of influence of climate, infiltration capacity and the vegetation cover.