THE EFFECT OF WOOD ASH AND LIME ON THE GEOTECHNICAL PROPERTIES OF EXPANSIVE SOILS

ABSTRACT

This work assessed the combine effect of wood ash and lime on the geotechnical properties of expansive soils. The wood ash used was a waste product from bakery industry while the lime was of industrial grade. The soil was sampled from the location of the study area where engineering structures have been damaged by the expansive soil that underlain the area. The chemical composition of the wood ash and the mineralogical composition of the soil were determined using X-ray fluorescence (XRF) and X-ray diffraction (XRD) method respectively while the geotechnical properties such as grain size analysis, Atterberg limits, linear shrinkage, free swell index, optimum moisture content, maximum dry density, and unconfined compressive shear strength were carried out on the natural soil and that of the soil with varying proportion of wood ash and lime. The XRD results revealed that the soil contains appreciable amount of expansive clay minerals; vermiculite, smectites, and mixed layer clays, while the XRF result revealed that the wood ash has substantial amount of calcium oxide with high pH value and can be used as soil stabilizing additives. The natural soil was classified as highly plastic inorganic soil with high swell potential, high expansivity, and high activity. The addition of wood ash reduced the activity and linear shrinkage of the soil up to acceptable standards but failed to improve significantly the other geotechnical properties of the soil such as free swell, compaction properties and shear strength, but on addition of lime to the optimum wood ash-soil admixture lead to drastic reduction in the free swell potential, improvement in the compaction properties and increase in strength value which also improved drastically after 28days of curing the treated samples. The optimum lime-wood ash-soil admixture was 4-18-78 in weight percentage. It was therefore concluded that high plastic inorganic soils can successfully be stabilized for use in pavement construction with the combined effect of wood ash and lime, which will not only reduce the cost of carrying out engineering projects, but also reduces the environmental problems associated with indiscriminate disposal of wood ash.

CHAPTER ONE

INTRODUCTION

Some civil engineering structures built in Agwu and Agbani towns of Enugu state, south-eastern Nigeria were observed to have developed cracks, heaved or have totally failed. These geotechnical problems were as result of the expansive nature of the soils in the area caused by the presence of clay minerals illite, vamiculite and montmorillonite (Uduji et al., 1994), which have the ability to absorb water in-between their crystals and layers when wet and loose them when dry (Popescu, 1986; Taylor and Smith, 1986), in such soils resulting to swelling and shrinking. This volume change occurs near the ground surface where the soil profile is subjected to seasonal changes (Frilund and Rahardjo, 1993) and resulting to cracking of the civil  engineering structures like roads, houses, pipeline or bridges etc the soil support and eventual complete failure of the structures if the cracks are not properly checked (Popescu, 1979; Driscoll, 1983; Okogbue, 1990).  On a world-wide basis, argillaceous sediments make up about 60% of the stratigraphical column, with clay minerals accounting for up to two-thirds of the constituents (Taylor and Smith, 1986). As such, the damages caused on a world-wide scale have been estimated to worth billions of dollars as reported in Reece (1980), Holtz (1983), Wray and Mayer (2004) and Wyoming multi hazard mitigation plan draft (2011). In South-eastern Nigeria alone, damages worth millions of naira annually was reported by Uduji et al. (1994), and due to the increase in population/urbanization, there has been increase in construction of civil engineering activities such as buildings, roads, bridges and pipelines which starts failing after few years of construction. In other to maintain the stability and durability of these structures, there is need to improve qualities of the soil supporting such structures.

Different methods have been used to combat the problems associated with expansive soils. These include removal and replacement of expansive soils (Craig, 1992), design of foundations to withstand cracking and heaving (Chen, 1975; Lee D and Ian, 2012), mechanical modification of the expansive soils like compaction, pre wetting, and pressure injection (Arora, 2008) and  also chemical stabilization of the expansive soils (Okogbue and Onyenubi, 1998; Al-Rawas et al., 2002; Seco et al., 2011).

Mechanical stabilization though is quite effective in stabilizing expansive soils, but in most cases is very expensive and therefore increases the cost of carrying out engineering projects. Also, the engineering properties of chemically stabilized soils using lime improved considerably (Anifowose, 1989; Al-Rawas et al., 2002; Ismaiel, 2006; Buhler and Cereto, 2006; Rao et al., 2012) and have been found to be more durable than mechanically stabilized soils. This is because, lime has high amount of calcium oxide (CaO) which undergoes cation exchange, flocculation and time depending pozzolanic reactions with the clay minerals in the soil (Terzaghi and Peck, 1967; Winterkorn and Pamukcu, 1991; Show et al., 2003; Mitchell and Soga 2005; and Savapulliah, 2006). But conventional source of lime, quick lime and Portland cement are also expensive which also increases the cost of carrying out engineering projects. These have made researchers to look for alternative source of chemical stabilizers that will improve the engineering properties of expansive soils and yet reduce the cost of carrying out engineering projects.

Some industrial wastes have been observed to contain CaO in appreciable quantity. Wastes like marble dust (Okogbue and Onyenubi, 1998; Baser, 2009; Agrawal and Gupta, 2011), limestone waste dust (Okogbue and Yakubu, 1997), and pyroclastic dusts (Ene and Okogbue, 2009) have been used to improve the engineering properties of expansive soils.

Ashes a by-product of combustion of organic materials have also been used to stabilize expansive soil. Cokca (2001), Kumar and Sharma (2001), Ji-ru and Xing (2002) and Bhuvaneshwari (2005) used fly ash a by-product of coal power plant which is considered a waste material to stabilize expansive soil and the results shows improvement in the engineering properties of the stabilized soil. Brooks (2009) and Rao et al.(2012) also reported reduction in swelling ability and strength gain of expansive soil stabilized with rice husk ash, while Okogbue (2007) reported an improvement in the grain size, reduction in the plasticity and maximum dry density of an expansive soil stabilized with wood ash.

Though the use of unconventional lime stabilizing material such as limestone ash, marble dust, fly ash, rice husk ash and wood ash improved the engineering properties of expansive soils, but the results obtained shows that the improved soils did not meet the required geotechnical standards especially for road construction purposes unlike those stabilized with conventional lime material, lime and Portland cement (Okogbue and Yakubu, 1997; Okobue, 2007; Bulher and Cerato, 2006).

In other to improve the quality of these unconventional lime stabilizing materials and also to reduce the cost of carrying out engineering operations on expansive soils, researchers have tried to determine the combined effect of lime with other unconventional stabilizing materials on the engineering properties of expansive soils. The reports show much improvement in the engineering properties of the soils stabilized with lime and fly ash (Ismaiel, 2006; Malhotra and Naval, 2013), cement and fly ash (Amu et al., 2012) and with lime and rice husk ash (Brooks, 2009; Rao et al., 2012) than the ones stabilized with only lime, cement, wood ash, fly ash, rise husk ash, marble dust and other unconventional lime stabilizing material.

In Nigeria, due to the inadequate supply of electricity, the use of wood as a source of energy in bakeries, restaurants, homes and other factories that depend on wood as their source of energy are in the increase and these generates enormous amount of wood ash which are mostly disposed indiscriminately to the environment (Kersten et al., 1998; Imam et al., 2007; Babayemi and Dauda, 2009). The environmental and health implications associated with indiscriminate dumping of wood ash to the environment have been highlighted by Pitman (2005), Poykoi et al. (2005), and Pasquini (2006).

The objectives of this research work is to determine the combined effect of wood ash which is a waste product from bakery and industrial quick lime (CaO) on the engineering properties of expansive soils; to determine the optimum lime-wood ash admixture for stabilization; and also to determine the durability of the stabilized soils and evaluate the environmental and economic importance of the possible use of wood ash as a stabilizing material for pavement constructions.

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