GEOPHYSICAL AND GEOTECHNICAL DETERMINATION OF THE CAUSES OFROAD FAILURES (A CASE STUDY OF IKUN TO AFO) IN ONDO STATE, SOUTHWESTERN NIGERIA

GEOPHYSICAL AND GEOTECHNICAL DETERMINATION OF THE CAUSES OFROAD FAILURES (A CASE STUDY OF IKUN TO AFO) IN ONDO STATE, SOUTHWESTERN NIGERIA
CHAPTER ONE
INTRODUCTION

1.1 PREAMBLE
Soil and rock are materials widely used for construction purposes. The qualities of these construction materials are essential in their choice for various civil works. These qualities are evaluated by the geotechnical test of the soil materials. Therefore engineering geological properties of the subsoil that constitute sub grade material along Ikun to Afo Akoko were evaluated in order to determine their suitability for the civil engineering works. Roads of various categories in Nigeria have shown signs of failure in the form of cracking, rutting, deformation and peeling. In some cases, these signs of distress are visible within a short period of commissioning the road, yet the causes have not been fully established. Some researchers (Arumala and Akpokodje, 1987; Jegede, 2004; Osadebe and Omange, 2005) and agencies (National Emergency Agency , NEMA and Federal Roads Maintenance Agency, FERMA) have attempted to link these failures to poor construction materials, soil properties, poor workmanship, high traffic volume, lack of maintenance etc., but geological considerations, the design and construction have not been fully addressed

1.2 LOCATION AND ACCESSIBILITY
The study area is located in Akoko North East Local Government Area of Ondo State, South Western Nigeria. It lies with the Nigeria basement complex. It is located between Latitudes 070331N and 070401N and Longitudes 05°45 E 05048 91 East. The elevation varies from 218m to 239m above the sea level. It is easily accessible through roads mainly untarred road networks and footpaths and covers estimate distance of 15 km.

1.3 AIM AND OBJECTIVES
The major aim of the project is to determine the geotechnical and geophysical properties of subgrade soil materials taken along Ikun to Afo Akoko highway (15 km distance) in Ondo State Southwestern Nigeria in order to reassess their usefulness as soil in civil engineering for road construction.
Below are the objectives of carrying out the geophysical and geotechnical determination of failed road.
(1) To investigate the engineering geological properties of the sub-grade lateritic soil for construction purpose, provide great assistance to engineers, and industrial developer who might be employing the soil for various projects.
(2) To determine the nature of the near surface materials, whether there will be needed to import fill materials during construction of the road.
(3) To delineate subsurface geologic sequence and to know the synthesis of both field and laboratory testing of the disturbed soil samples collected from the established localities.

1.4 TOPOGRAPHY AND DRAINAGE PATTERN
The area is mostly undulating to uplands and lowlands (Rahaman, 1976).The drainage pattern of the study area is the combination of both trellis and dendrite type of drainage. They show the resistance of the underlying crystalline basement rock and displaced structural features. Stream and rivers are developed during major joint direction and foliation trend with generally straight courses. Most of the streams are seasonal (Figures. 1.1 and 1.2).

Figure 1.1: Drainage map of the Study Area. (Source: Department of Earth sciences, AAUA.)

Figure 1:2. Topographic map of Ikun – Afo showing the contour lines
(Modified, after independent mapping 2016)

1.5 ROAD AND ROAD FAILURE
Roads are paths established over land for the passage of vehicles, people and animals. Roads provide dependable pathway for moving people and goods from one place to another. They range in quality from foot paths to concrete-paved multilane highways (Microsoft Encarta, 2008). Sitwell, (1999) explained that there are different types of roads, ranging from multilane freeways and express ways to two-way country roads. Roads can be classified into three broad categories: highways, urban or city streets, and rural roads. Road failures could be define as a discontinuity in road network resulting in cracks, potholes, bulges ravelling, edges failure, polished aggregate, patching and depressions Meshida, (2006). A road network is supposed to be a continuous stretch of asphalt lay for a smooth ride or drive but visible cracks, potholes, bulges and depressions may punctuate such smooth ride and this punctuation is smooth ride is generally regarded as road failure. The majority of expressway failure in the tropics can be traced to geotechnical factors [Gidigasu (1976), Meshida (1987), Madebors (1983) and Adeyemi (1992)]. These authors observed that expressway failure often occur when the pavement is built on Saprolite instead of lateritic profiles. These failures occur most in the form of waviness, pitting and cutting, Sequel to the loss of lives and properties as a result of expressway failure, it is pertinent that geotechnical investigation of the failed sections of the road as well as the stable section, be carried out. This will however enhance future designing and construction of durable roads.

1.6 CAUSES OF ROAD FAILURE:
Serious site investigation and laboratory experiment work confirm that the stability of the road pavement depends on a number of factors which include the type of soil and aggregate used, design techniques, construction procedures, age of pavement and more importantly the geology, soil type, climate and drainage conditions. Different investigations carried out, showing clearly, that roads failed due to one or combination of the following factors; Settlement, poor drainage system, changes in moisture level, inadequate compaction, and low pavement thickness, Selection of poor materials for construction or, change in strength of the subgrade (due to action of traffic with time). Other causes include the use of wrong materials for subgrade and as fill materials, poor quality control during construction, the present of a significant quantity of clay and silty in the road shoulder, age, and high precipitation, the effects of geologic structure, alteration of soils and rock strength by water in physiochemical changes and uneven temperature

1.7 SOIL FORMATION
Soils are formed by the disintegration of rocks. The disintegrated or weathered material may either be found deposited at its own place of origin, or may get transported by agents of denudation like water, wind, etc. before deposition. Moreover depending upon whether the sediments are transported by water, ice, or wind, the soils are called alluvial, glacial, or Aeolian, respectively (Garg, 2009).The three stages involved in the formation of soil are: weathering, transportation and depositions
Weathering: Is the process of the breaking down of rocks. Weathering breaks down rocks that are either stationary or moving (Plummer and Carson, 2008). There are different types of weathering: mechanical weathering, chemical weathering and biological weathering. Cracks in the rock are enlarging gradually by frost action or plant growth, and as a result, more surfaces are exposed to attack by chemical agents such as water.
Transportation: After a rock fragment is picked up (eroded), it is transported. Transportation is the movement of eroded particles by agents such as rivers, wave’s glaciers, or wind. Weathering process continue during transportation
Deposition: When transported material settles or comes to rest, deposition occurs. Sediment is deposited when running water, glacial, ice, waves or wind loses energy and can no longer transport its load.
1.7.1: Factors Affecting Soil Formation
The rate of soil formation is controlled by rainfall, temperature, slope, and to some extent, the type of rock that weathers to form soil. High temperature and abundant rainfall speed up soil formation, but in most places, a fully developed soil that can support plant growth takes hundreds or thousands of years to form (Plummer and Carlson, 2008). It would seem that the properties of a soil should be determined by the rock (parent material) from which it formed. This is partly true and these factors are:
1.7.1.1: Parent Material
The character of soil depends partly on a parent material from which it develops. The parent material is a source of weather mineral matter that makes up most of soil. A soil developing on weathering granite will be sandy, as sand-sized particles of quart and feldspar are released from the granite.
1.7.1.2: Slope
The slope of the land surface provides an important control on the formation of soil. Engineers need to know the soil tend to be thin or nonexistent on steep slope where gravity keeps water and soil particles moving downhill.
1.7.1.3: Climate
Climate is perhaps the most influential factor affecting soil thickness and character. The same parent material in the same topography will form significantly different soil types under different climatic conditions. Temperature and precipitation determine whether chemical or mechanical weathering processes will dominate and strongly influence the rate and death of weathering.
1.8 CLIMATE AND VEGETATION
The study area belong to the tropical rain forest region and can be described as good and support agricultural activities due to the texture of the soil and nature of vegetation in the area. Rainy season occur between March and October while the dry season lasts from November to February. The mean annual rainfall is approximately 2000mm and the temperature in the area varies between 280C and 380C with high temperature between February and April (Olayinka, 2005). The vegetation of the area is in relation to climate, soil, elevation and human impact on the environment. The vegetation comprises of tall trees with buttress roots, evergreen and broad leaves and numerous species of trees like Mahogany, Iroko, African walnut which encourages timber business. There are also light forest and savannah orchard bush. Many parts of the study area are covered by secondary vegetation due to intense agricultural activities.

1.9 PREVIOUS WORK
Many researchers have investigated the geotechnical properties of lateritic soils in Southwestern Nigeria with considerable attention on their stability as high foundation and construction materials.
Adeyemi (2003) proposed a criterion for rating a probable field performance of lateritic soil and position in pavement based upon aggregate as well as specific gravity and water absorption tests.
Adeyemi et al., (2003), while investigating some geotechnical properties of some granite – gneiss – derived lateritic soil from Ile-Ife, South Western Nigeria were stabilized with between 0 and 20% by weight of cement, this was with a view to determining the influence of cement on some engineering properties on soil.
Ola (1983) noted that laterite soils responded more favourably to cement stabilization than temperate soils. He recommended cement stabilization for road construction using lateritic soil materials.
Aguda (1982), also investigated the possible causes of high pavement failure in Oyo State. In his study, a detailed evaluation of pavement and slope failure that occur along cut sections suggested the causes to be due to inadequate provision drainage among other factors.
Adeyemi and Oyeyemi (2000), while investigating the geotechnical basis for the failure of the Lagos-Ibadan expressway, Southwest Nigeria, indicated that significant differences need not exist between geotechnical properties of soil below stable zones and unstable sections of flexible highway pavement in the tropics.
Ogunsanwo (2000), in his work on geotechnical properties of some soils in southern-western Nigeria, noticed tremendous depreciation in the shear strengths and compressibility of soil upon inundations (by carrying out shear strengths and consolidation tests) thus reducing the bearing capacity drastically and leading to foundation failures. The reduction in cohesion is as much as between 28kpa to 437kpa while the angle of internal friction decreased from 21 to 34 to 19 and 31.
Despite the numerous contribution and researches that have been carried out on engineering of soil there is still needed for more work to be done. This will be helpful in the construction of stable roads and as in the area of bricks.

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