IMPROVEMENT OF BEARING CAPACITY OF LATERITIC SOIL FOR PAVEMENT SUBGRADE BY ADDITION OF ROCK FLOUR

ABSTRACT

This study evaluated the improvement of bearing capacity of lateritic soil for pavement subgrade by addition of rock flour stabilized with 0 – 12% rock flour by dry weight of soil at incremental rate of 3% and compacted using British Standard Light (BSL), West Africa Standard (WAS) and British Standard Heavy (BSH) compactive efforts. Results show that the lateritic soil sample used for this study is classified as A-7-6 according to American Association of State Highway and Transportation Officials (AASHTO). The formulated mixtures from the A-7-6 soil and rock flour showed an improvement in the index properties of the mixtures with increasing rock flour. The Liquid limit and Plasticity index reduced from 42 – 32% and 29.30 – 13.48% respectively as rock flour increased from 0 – 12%, while plasticity index of A-7-6 soil with rock flour showed considerable reduction in plasticity indices of mixtures with increasing content rock flour content. 1.802, 1.820 and 1.870g/cm³ were obtained as MDD values and 12.40, 11.90 and 11.60% as OMC values for the natural soil using BSL, WAS and BSH compaction efforts respectively. Highest set of values were obtained at 9% addition of rock flour. BSH gave the highest UCS value of 250.89kN/m², while BSL and WAS gave 180.8 and 218.12kN/m² respectively. Generally, a progressive improvement in soaked and unsoaked CBR values were observed for the stabilized specimen with increasing rock flour content. The least CBR value for soaked and unsoaked conditions were observed at 3 % addition of rock flour. For soaked condition at 3% addition of rock flour, the soaked CBR values obtained for BSL, WAS and BSH were 21.15, 25.93 and 28.21% respectively, while 37.88, 41.74 and 42.73% were obtained for unsoaked CBR in the same other of energy level adopted. In terms of consistency limits, the mixtures did not achieve the required threshold values for subgrade specified as LL < 35 and PI < 12% in local codes suggesting the use of higher rock flour contents to enhance these parameters. As regards to soaked and unsoaked CBR, results from the three energy levels adopted that is; BSL, WAS and BSH, meets the minimum requirements of 10% for flexible pavement subgrade according to NGS, (1997).   To optimize their structural strength for subgrade application, the mixtures should be compacted to 100% of the relative densities.

CHAPTER ONE

1.0     INTRODUCTION

1.1       Background to the Study

Natural soils vary in its properties and are mostly heterogeneous. These properties changes at different depth in the ground because of several reasons which include depositional environment, physical environment and extent of weathering (Elkateb et al., 2003; Lumb, 1974; Jones et al., 2002). Soil has wide range of variation in its structure, texture and composition at different depositions, which influences its index, engineering and geotechnical properties. Upon initial deposition. Soil undergo continuous modification due to external stresses, chemical reactions, weathering (Uzielli et al., 2006). The inherent properties of some soil types limits their application in some engineering projects. Therefore, the need to modify these properties to make it fit for specific engineering purpose becomes inevitable. Nigeria, being a country that depends mainly on land transportation of goods and person requires functional, serviceable and durable roads. Several  deformations and  failures seen  on Nigeria roads are due to structural failure of the road component, overloading beyond the design load, poor construction, among others (Ndefo, 2012).

AASHTO (1986) classified soil with respect to their behaviour as subgrade material into seven groups, which are A-1, through A-7. Groups A-1, A-2 and A-7 are further classified into A-1-a, A-1-b, A-2-4, A-2-5, A-2-6, A-2-7 and A-7-5, A-7-6 respectively. A-7 soils are Silt-clay materials having more than 35 % of total sample passing No. 200 sieve with Liquid Limit of 41 minimum and Plasticity index of 11 minimum. This class of material is largely available in Nigeria but are challenging to geotechnical engineer in its application for road construction. A-7-6 soil are generally clayey according to AASHTO (1986) classification, this limits its application for road construction because of its poor strength and susceptibility to volumetric changes on exposure to moisture. This class of soil are readily available across the country. The challenge of instability of clay is as a result of swelling nature of expansive clay material thereby making the soil unsuitable material for construction in foundation of buildings, highway, railway or any other engineering structures (Ogunribido and Abiola, 2015). Several methods have been adopted in the past to stabilize A-7-6 soil so that it meets certain criteria. Several research works have been done using different methods of soil stabilization. Some of the methods have shown to be efficient and effective and at varying cost, while sourcing the stabilizing agent have rendered some of the findings impracticable.

Modern constructions involve high speed road and rail networks that require highly stable retaining and foundation systems (Reddy et al., 2017). Silt and clay are predominant in natural soils having high content of plastic fines which are responsible for large scale of deformation under traffic loads when such soil is in saturated state and their subsequent settlement leads to road failures (Satyanarayana and Pradeep, 2013). The deplorable nature of Nigerian roads is mostly caused by failure of the subgrade, subbase or base course, other possible cause is overloading of the pavement beyond design considerations (Afolayan and Abidoye, 2017: Ndefo, 2012). In addition, potholes, pavement surface wash, depressions of roadway, block and longitudinal cracks, drainage collapse are also responsible for road failure in Nigeria. Structurally stable road with adequate structural components is critical to durability and functionality of road.

Developed and developing countries produces rock flour as an industrial waste while processing coarse aggregate from rock from crusher plants. Rock flour can also be obtained as an effluent while drilling through rock. Rock flour as the name implies is in powder form, having angular constituent particles. Rock flour is a stable material at different degree of moisture content, they contain mineral such as quartz, silica and feldspar (Reddy and Moorthy, 2002). It has varying applications in its use for infrastructure developments such as a fill material in Highway construction, retaining material without reinforcement (Satyanarayana and Pradeep, 2013). Rock flour was found to be a good stabilizing material for lateritic soil when used with Ordinary Portland Cement to stabilize lateritic soil (Ogunribido and Abiola, 2015).

1.2       Statement of the Research Problem

A-7-6 soil though a poor material for road construction it is abundantly available across Nigeria (Itafe, 2020). There is need to effectively utilize this grade of lateritic soil by improving its, geotechnical and engineering properties. Huge cost, time lag and high rate of wear on construction equipment is associated with hauling material through long distances, which also influences construction duration. Considering high availability and poor nature of this grade of lateritic soil, the need to find ways of stabilizing this poor grade of soil with local additives to meet requirements for road subgrade becomes inevitable (Amadi, 2010; Okunade, 2010; Mohammed and Alhaji, 2015). It is therefore necessary that the properties of this soil grade is improved with rock flour. Rock flour being an industrial waste in quarry cannot be rendered an absolute waste therefore, it is intended to check its suitability as replacement for cement, lime and other additive in soil treatment. It is expected that Rock flour used as soil additive should have high tendency of reducing construction cost hence its cheaper when compared with cement or lime.

1.3      Aim and Objectives of the Study

The aim of this research work is to evaluate the improvement of bearing capacity of lateritic soil for pavement subgrade by addition of rock flour. To achieve the aim, the objectives of this work are:

i.      To determine the Atterberg limits i.e., Liquid Limit, Plastic limit and plasticity index of the natural soil samples and samples stabilized with varying percentages of rock flour.

ii.      To determine the California Bering Ratio, (CBR) value, Unconfined Compressive strength (UCS) value of the natural soil sample and samples stabilized with varying percentages of rock flour.

iii.      To determine the durability of samples stabilized with varying percentages of rock flour.

1.4      Scope of the Study

Rock flour and lateritic soil were sourced in Chancahaga and Agaie local government areas respectively in Niger state, Nigeria. The laboratory tests were carried out at the Civil Engineering laboratory of Federal University of Technology, Minna. All tests were carried out in accordance with procedure outline in BS 1377 (1990) for natural samples and BS 1924 1990, for the stabilized specimens using 3 – 12% of rock flour by dry weight of natural soil, at incremental rate of 3%.  All tests on the natural soil and stabilized samples using three energy levels that is; BSL, BSH and WAS, following procedures outlined in BS 1924-2: (1990) and NGS (1997) respectively.

1.5     Justification of the Study

Land transportation is mostly used for commuting persons and goods in Nigeria through over 200,000 km stretch of road (Ndefo, 2012). The need to construct functional and durable roads founded on strong and stable subgrade through 923,768 km² land mass become necessary irrespective of soil class which construction Engineers come across. This research work sought to provide alternative to the already known additive such as cement, lime and iron tailings among others for the improvement of poor soil bearing capacity. In addition, this study hope to provide an efficient utilization of quarry dust which is regarded as industrial waste in quarries. To provide an alternative to stabilization of A-7 soils and other fair to poor soils such as A-4, A-5 and A-6. In general, the results of this study would provide an easier and cheaper technique for improving bearing capacity of A-7-6 soil.

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