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CHAPTER 13
MODIFICATION BY ADMIXTURES
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Terminology, Con!r"#!ion Te#$ni%"e, &n'
Ty(i#&l UeThis chapter deals with modifying soils by the mechanical
addition of granular materials or chemical compounds such as
cement, lime, bitumen, and calcium chloride.
The purpose:Increase strength
Reduce deformability
Provide volume stability
Reduce permeability
Reduce erodibility
Increase durability
Control variability
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Improving the engineering properties of a soil by admixtures is often referred
to as soil stabiliation.
!oil additives may help to:
"nhance subgrade or subbase properties in order to reduce the re#uired
overall pavement thic$ness.
Improve trafficability on construction sites
Prepare the ground for shallow foundations
!tabilie slopes by improving the soil%s shear strength
Reduce erosion by surface runoff or internal seepage &piping'
Construct emban$ments
(orm load)bearing columns &in situ'
Improve the wor$ability of borrow materials
Reduce traffic)generated dust
Contain haardous wastes
Rehabilitate polluted or mined ground
*ifferent ways of mixing additives with soil are illustrated in (igure +.+.
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Fig"re 13)1 E*&m(le o+ ro"n' Mo'i+i#&!ion -i!$ A'mi*!"re
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!tabiliation with admixtures is distinguished from
grouting by the fact that the in situ soil is at least
partially remolded during construction process. The additive is usually in the form of a pulveried solid
or thic$ slurry, rather than a viscous li#uid as in
grouting.
In-ecting grout increases the density of a soil by fillingits existing voidsor by displacing and compressing the
surrounding ground.
In contrast, mixing soil with additives li$e lime oftencreates additional voids nevertheless, chemical and
physical reactions still ensure increase ground strength
and reduced compressibility.
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Ty(e o+ A'mi*!"re . T$eir E++e#! on Soil
Pro(er!ie
The most common artificial additives are: Portland cement
/ime
0itumen and tar
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r&n"l&r A'mi*!"re
n
D
dp
=+11
where:
p 2 percentage passing the sieve with aperture d
* 2 maximum particle sie
n 2 exponent
(or most soils used in pavement construction, densest particle
pac$ing is achieved when 3n4 is in the range of 1.56 to 1.6well
graded soil.
The strength coarse)grained materials is largely related to their
density, which in turn depends significantly on the particle sie
distribution. 7aximum density is obtained in soils which have a
particle sie distribution which can be approximated by
&899!R9, +;
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Por!l&n' Cemen!
!oil with cement admixtures are generally termed
cement)stabilied or cement)treated. 7itchell &+=
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"ngineering 0enefits of Cement !tabiliation
L
fm
SSE =
@here:
!f 2 the strength of the soil mixed in the field
!/2 the strength of the soil mixed in the laboratory
>ood construction procedures may result in efficiencies in excess
of ;1A.
Typical cement contents in soil stabiliation range from ? to +1A.
7a-or gains in the cement treatment of soils are:Increased strength B stiffness
0etter volume stabilityIncreased durability
The mixing efficiency "m :
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/ime
/ime is used in the form of #uic$lime, Ca, or hydratedlime, Ca&D'?. 9 third version of lime, CaC,, is used
for agricultural purposes only.
Ion CaEE 7gEE 8aE FaE.
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Soil 0 /ime C&O2 Re!ion
!hort)term reactions include hydration &for #uic$lime'
and flocculation &ion exchange'. /onger)term reactions are cementation and carbonation.
Dydration: This drying action is particularly beneficial inthe treatment of moist clays.
(locculation: @hen lime is mixed with clay, sodium and
other cations &E ion' adsorbed to the clay mineral
surfaces are exchanged with calcium./ime causes clayto coagulate, aggregate, or flocculate. The clay%s
plasticity is reduced, ma$ing it more easily wor$able and
potentially increasing its strength and stiffness.
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Cementation: The clay)lime reaction removes silica
from the clay mineral lattice to form products not
unli$e those of cement hydration. Cementation is the
main contributor to the strength of the stabilied soil.
Carbonation: Reaction of lime with carbon dioxide
&C?' in the open air or in voids of the ground forms
a relatively wea$ cementing agent.
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Engineering Bene+i! o+ /ime S!&ili4&!ion
1) Im(ro5e' -or6&ili!y7
o.
@or$ability is improved because flocculation ma$esthe clay more friable.
o.The plasticity decreases, mainly because of an increase
in the plastic limit.
o.The // may increase or decrease, depending on the
type of soil &(ig. +.5'.
o./ime increases the optimum water content for
compaction. The maximum dry density achieved with aparticular compactive effort is reduced &(ig. +.6'. The
compaction curve for lime)treated clay is flatter, which
ma$es moisture control less critical and reduces the
variability of the density produced.
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Engineering Bene+i! o+ /ime S!&ili4&!ion
o In the first few hours after mixing, lime additives
cause a steady increase in strength, but a slower ratethan cement. The need for compaction immediately
after mixing is less critical for lime than cement &(ig.
+.
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Engineering Bene+i! o+ /ime S!&ili4&!ion
8) In#re&e S!reng!$7
/ime increases the strength of a clayey soil, typicallydemonstrated in term of unconfined compressive strength
or C0R test results.
3) In#re&e 9ol"me S!&ili!y7
Improved volume stability means reduce shrin$age andswell upon drying and wetting, respectively. 9 typical
result of a shrin$age and swelling test is shown in (ig.
+.=.
:) In#re&e (erme&ili!y7
It could mean better drainage and less pore pressure
buildup under load.
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C&l#i"m C$lori'e
Calcium Chloride &CaCl?' has been used in highway
construction and maintenance since early ?1thcentury.
Today, CaCl?still has a variety of uses, but it isprobably
most appreciated as a dust palliative on highly traffic$ed
unpaved roads, such as haul roads in mining and onlarge earth)moving pro-ects.
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Fly A$
(ly ash is a solid waste product created by the
combustion of coal.
nly +6 to ?1A of fly ash produced in the G!9 in +;