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1.1 Preamble
Geotechnical engineers are often faced with the problem of constructing road
beds on soil which do not have adequate strength to support the wheel loads imposed
upon them either in construction or during the service life of the pavement. It is
sometimes necessary to treat these soils, termed expansive soils to give a stable subgrade
or working platforms for the construction of the pavement. Treatment of soil are
generally classified into two processes: soil modification or soil stabilization. The
purpose of sub-grade modification is to provide a working platform for construction
equipment, while sub-grade stabilization is meant to increase the strength and durability
of the soil.
1.1.1 Black cotton soil
Expansive soil can be found almost any where in the world but they are
basically confined to semi arid regions of the tropical /temperate zones. These areas are
usually characterized by marked dry and wet seasons with low rainfall, poor drainage
and exceedingly great heat. The precipitation is such that the annual evaporation
exceeds the precipitation (Chen, 1988).
Two groups of parent rock materials have been associated with the formation of
expansive soils. The first group comprises sedimentary rock of volcanic origin, which
can be found in North America, South Africa and Israel (Ola, 1978). The second group
of parent materials are basic igneous rocks found in India, Nigeria and South Western
U.S.A (Plait, 1953). Tropical black clays are formed by weathering (Morin, 1971).
Black cotton soil shows high shrink – swell characteristic with surface cracks opening
during the dry season which are more than 50 millimetre or more wide and several
millimetre deep. The cracks close during the wet season and uneven soil surface is
produced by irregular swelling and heaving. Such soils are especially troublesome as
pavement sub-grades or under shallow foundation. The amount of potential volume
change is dependent on the mineralogical compositions and clay minerals types. This
means that the soils which contain more montmorillonite have higher potential volume
change than those that have less of the minerals (Ola, 1983).
The ability of black cotton soil to absorb or release water when wet or dry,
respectively, gives the soil its low bearing capacity under wet conditions and cracking
under dry conditions. When engineering structure is associated with an black cotton, it
experiences either settlement or heave depending on the stress level and the swelling
pressure. Because of its soft, highly plastic and cohesive nature, mobility of wheeled
vehicles, compaction difficulty and non-provision of satisfactory sub-grade support for
subsequent pavement construction operations, problems in construction are always
encountered. The combined effects of insufficient shear strength of the soil and the
development of excessive deformation under the applied wheel loads give rise to these
1.2 Ordinary Portland Cement
Cement can be described as a material with adhesive and cohesive properties
which make it capable of binding minerals fragments into a compact whole (Ejeh,
2007). Ordinary Portland Cement is composed of four main oxides(CaO, SiO2, Al2O3
and Fe2O3) and the variation in the relative proportions of these four main oxides give
rise to different types of cement with some little addition of some other oxide. The
oxides composition that form the main compound of ordinary Portland cement are
tricalcium silicate (3CaO, SiO2, C3S), dicalcium silicate (2CaO.SiO2, C2S), tricalcium
aluminate (3CaO. Al2O3, C3A) and tetracalcium aluminoferrite (4CaO.Al2O3.Fe2O3,
C4A). . The two calcium silicate (C2S, C3S) which are the most stable of these
compound, form 70 to 80 percent of constituents of cement and contribute most to the
physical properties of the mix.
The degree of fineness or specific surface area of the clinker grinding also
contributes highly to the various types of cement. The finer the grinding, the more rapid
is the rate of hydration. Ordinary portland cement has a minimum specific surface of
250 cm2 /g.
Hydration is the chemical combination of cement and water producing a very
hard and strong medium for soil particles and is accompanied by the liberation of heat.
The rate of hydration depends on the relative properties of silicate and aluminate
compound, the cement fineness and the ambient conditions (temperature and moisture).
1.3 Locust Bean Waste Ash
It is commonly accepted today in soil improvement work to examine the effect
of local materials considered to be waste or residue and that may be economically
viable for construction purpose (Osinubi 2000). One of the local materials been
considered is the ash obtained from the combustion of the waste from the locust bean
fruit, It has three components namely: the seed commonly used to produce local
seasoning for food, the powder and the husk. The husk is the waste product and when
burnt in open air will eventually result into locust bean waste ash (LBWA).
The locust bean tree is common in our environment and it grows to about 15 m
in height and has dark evergreen pinnate leaves. The fruit is brown, leathering pod
about 10 to 30 cm long. The seed are edible but the waste product litter our
environment with corresponding negative environmental impact. The treatment of soil
with locust been waste ash could be a viable way to ameliorating this social menace.
1.4 Statement of the Problem
Black cotton soil is an expansive soil characterized by excessive volume
change, expansive tendencies and low bearing capacities when wet (Chen, 1988,
Nelson and Miller, 1992; Warren and Kirby, 2004). These soils have high clay content
and expand when wet and shrink when dry; these cause serious damages to structures
founded on these soil. For example it has been estimated that in the United States of
America damages to buildings founded on expansive soils are well over 15 billion US
Also in the north eastern part of Nigeria, where the soil is predominant, it is
definite that road and other Civil engineering infrastructure being erected on these soil
will pose great challenges to geotechnical engineers, since avoiding it or finding a
suitable replacement is not feasible. It therefore becomes necessary for the geotechnical
engineer to look for means of improving the geotechnical properties of the soil in-situ,
at a cheaper and efficient means.
Locust bean waste ash is the ash obtained from combustion of the waste from
the locust bean. The availability of this waste is high and could be beneficially used for
the improvement of this troublesome soil and rid the environment of the waste.
1.5 Justification for the Study
The significance of a good soil to a geotechnical engineer cannot be over
emphasized, especially in the area of construction, where the stronger the soil the
lesser the cost to erect any structure on the soil. For years, geotechnical engineers have
been concerned with improving the characteristics of poor quality soils and have
therefore been researching into various methods of doing so. One of such is to use the
abundantly available natural waste materials (e.g. locust bean waste ash) in
combination with cement to make a poor quality, extensively deposited black cotton
soil a better engineering soil. Hence information on the improved workability of such a
soil makes this research worthwhile.
1.6 Aim and Objectives of the Study
This study was aimed at establishing the effect of locust bean waste ash on
cement modified black cotton soil. This was achieved through the following specific
a) Determination of the properties of the natural soil.
b) Modification of black cotton soil using known quantities of cement and
locust bean waste ash as admixture when compacted with the British
Standard light (BSL), West African Standard (WAS) and British Standard
heavy (BSH) conpaction energies.
c) Evaluation of the effect of locust bean waste ash on the cement modified
black cotton soil using particle size distribution, Atterberg limits,
compaction tests and shear strength parameters.
d) Determination of the optimum mix ratio of cement – locust bean waste ash
for optimum workability of black cotton soil.
1.7 Scope of Research
The study involved modification of black cotton soil with cement using locust
bean waste ash as the admixture. The modification of black cotton soil with cement
served as control and locust bean waste ash was introduced as an admixture. The
Atterberg limit, particles size analysis, compaction and shear strength parameters of the
soil – cement – LBWA modified samples shall be evalutated to determine the extent of

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