CHAPTER 2

 

LITERATURE REVIEW

 

Expansive soil are clays that have the
tendency to swell and soften when their moisture content is increased, or
shrink and dry cracked when their moisture content is decreased. Soils
containing the clay mineral montmorillonite generally exhibit these properties.
The mica-like groups , which includes illites and vermicullites , can be
expansive , but generally does not cause significant problems. Problems
associated with expansive soil which located in many regions China and
particularly in the northwest of Hubei province are well known. During the last
few decades damage due to swelling action has been observed clearly in these
regions in the forms of cracking and breakup of pavement, building foundations
, embankments and irrigation system (Zhang Ji-ru and CAO Xing , 2002).

 

            The
term expansive soil applies to soil that have the power to swell when their
moisture content is allowed to extent. The moisture could come back from rain,
flooding , leaking water or sewer lines, or from the reduction in surface
evapotranspiration when a section is roofed by a building or pavement. Several
correlations are helpful in identifying doubtless expansive soils. It is going
to even be doable to spot them visually. Visually indications embody (Wayne et
al, 1984):

·               
Deep and Wide
shrinkage cracks occurring throughout dry periods.

·               
Soil is rock-hard
when dry, however terribly sticky and soft when wet.

·               
Damages on the
encompassing structures because of enlargement of soils.

 

 

Origin and characteristics of Expansive soil

“BLACK COTTON” soils are the name given
usually as a Tropicana expansive soils. This name is considered possess
originated in India because of many locations of those soils are often and
favourite to growing of cotton. These particular soils are formed from the
sedimentary rocks and basaltic.  Even
though , various native terms are used, all expansive soils have same feature
for engineer to test it.

Expansive
soil is composed of soil particles, water, and gas that exist in pores between
the soil particles. The pores between soil particles can be seen as a kind of
microfissures, and the visible macroscopic cracks are caused by cracking of
these microfissures on load. The essence of expansive soil cracking is the
external manifestation of uneven volume shrinkage and deformation. The stronger
the volume’s uneven shrinkage is, the higher the development degree of crack
will be. While the soil shrinkage is closely related to its own initial state,
the external environmental state and so on,

The soil’s initial states affecting expansive
soil’s cracks development factors include the montmorillonite content, the
moisture content, the structural morphology, the compaction degree, Poisson’s
ratio, and the content of organic salt. Compared with other clay minerals, the
montmorillonite has stronger hydrophilic characteristic, can form thicker
hydrated film, and can provide greater shrinkage space during the drying and
dehydration process. Therefore, the higher the montmorillonite content is in
the expansive soil, the easier the crack development is and the greater the
development width is. Under conditions of high water content, the clay
particles are coated with a thick hydrated film, the space between particles is
larger; in the process of dehydration and drying, the free water is gradually
evaporated, the space between particles becomes smaller, meanwhile air goes
into the soil constantly, the bound water in the hydrated film is gradually
released to become free water and as a result the film becomes thinner, under
the roles of suction and lubrication of free water, the soil particles are
rearranged and gradually move closer to each other, pores become smaller gradually,
which will be exhibited as volume shrinkage deformation macroscopically, and
cracks will appear when the tensile stress of a weak zone reaches the ultimate
tensile strength. Seeing from the soil’s microstructure morphology,
laminar-flow structure is conducive to the generation of expansive soil cracks,
has good pores connectivity, and is helpful for moisture migration and cracks
cut-through after expansion. Poisson’s ratio reflects the relationship between
vertical and horizontal deformations. A high Poisson ratio of soil means that
the soil is influenced more by vertical deformation than by lateral
deformation, it is softer, the contact lubrication between the soil particles
is better, the mobility is better, and it is easier for the particles to slip
and dislocate; therefore, volumetric strain is not prone to occur in the soil
body 13. Under high degree of compaction, the expansive soil’s
structure is dense, the connection between the particles is compact, the injury
degree of the original structure is small, the volume content of free water is
low, the infiltration ability is not good, and the matrix suction is strong
when the surface soil loses water loss and shrink; therefore, it is not easy
for the soil body to crack. The expansive soil contains a relatively large
amount of soluble organic salt, which is dissolved when meeting water, and then
number of pores inside the expansive soil structure increase, the infiltration
coefficient becomes larger, which is helpful for soil evaporation and diffusion
of water vapor; as a result, the soil is prone to crack.

Montmorillonite may be 3 layered mineral having
one octahedral alumna sheet sandwiched between 2 silica sheets to present a
two: 1 lattice structure as shown in (Figure 2.1). The bonds are weak, and
dipolar molecules of water will enter between the sheets inflicting them to
expand readily. Therefore, soils containing substantial amount of
montmorillonite can exhibit high shrinkage and swelling characteristics within
the soil.

Figure 2.1 : Model of layer of montmorillonite.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fly ash

Fly ash by itself has little cementatious value but in
the presence of moisture it reacts chemically and forms cementatious compounds
and attributes to the improvement of strength and compressibility characteristics
of soils. It has a long history of use as an engineering material and has been successfully
employed in geotechnical applications.

 

Effect of Flyash on expansive soil was studied by
Erdal Cokca, Flyash

consists
of often hollow spheres of silicon, aluminium and iron oxides and unoxidized
carbon. Thereare two major classes of flyash, class C and class F. The former
is produced from burning anthracite or bituminous coal and the latter is
produced from burning lignite and sub bituminous coal. Both the classes of fly
ash are puzzolans, which are defined as siliceous and aluminous materials. Thus
Fly ash can provide an array of divalent and trivalent cations (Ca2+,Al3+,Fe3+etc)
under ionized conditions that can promote flocculation of dispersed clay
particles. Thus expansive soils can be potentially stabilized effectively by
cation exchange using flyash. He carried out investigations using Soma Flyash and
Tuncbilek flyash and added it to expansive soil at 0-25%. Specimens with flyash
were cured for 7days and 28 days after which they were subjected to Oedometer
free swell tests. And his experimental findings confirmed that the plasticity
index, activity and swelling potential of the samples decreased with increasing
percent stabilizer and curing time and the optimum content of flyash in decreasing
the swell potential was found to be 20%. The changes in the physical properties
and swelling potential is a result of additional silt size particles to some
extent and due to chemical reactions that cause immediate flocculation of clay
particles and the time dependent puzzolanic and self hardening properties of
flyash and he concluded that both high –calcium and low calcium class C fly
ashes can be recommended as effective stabilizing agents for improvement for
improvement of expansive soils. (Erdal Cokca, 2001)

 

Studied the effect of two types of fly ashes Raichur
fly ash (Class F) and Neyveli fly ash (Class C) on the CBR characteristics of
the black cotton soil. The fly ash content was increased from 0 to 100%.
Generally the CBR/strength is contributed by its cohesion and friction. The CBR
of BC soil, which consists of predominantly of finer particles, is contributed
by cohesion. The CBR of fly ash, which consists predominantly of coarser
particles, is contributed by its frictional component. The low CBR of BC soil
is attributed to the inherent low strength, which is due to the dominance of
clay fraction. The addition of fly ash to BC soil increases the CBR of the mix
up to the first optimum level due to the frictional resistance from fly ash in
addition to the cohesion from BC soil. Further addition of fly ash beyond the
optimum level causes a decrease up to 60% and then up to the

second
optimum level there is an increase. Thus the variation of CBR of fly ash-BC
soil mixes can be attributed to the relative contribution of frictional or
cohesive resistance from fly ash or BC soil, respectively. In Neyveli fly ash
also there is an increase of strength with the increase in the fly ash content,
here there will be additional puzzolonic reaction forming cementitious
compounds resulting in good binding between BC soil and fly ash particles (Pandian
et.al, 2002).

 

A similar study was carried out by Phanikumar and
Sharma and the effect of fly ash on engineering properties of expansive soil
through an experimental programme. The effect on parameters like free swell
index (FSI), swell potential, swelling pressure, plasticity, compaction,
strength and hydraulic conductivity of expansive soil was studied. The ash blended
expansive soil with flyash contents of 0, 5, 10,15 and 20% on a dry weight
basis and they inferred that increase in flyash content reduces plasticity characteristics
and the FSI was reduced by about 50% by the addition of 20% fly ash. The
hydraulic conductivity of expansive soils mixed with flyash decreases with an
increase in flyash content, due to the increase in maximum dry unit weight with
an increase in flyash content. When the flyash content increases there is a
decrease in the optimum moisture content and the maximum dry unit weight
increases. The effect of fly ash is akin to the increased compactive effort.
Hence the expansive soil is rendered more stable. The undrained shear strength
of the expansive soil blended with flyash increases with the increase in the
ash content (Phanikumar and Sharma, 2004).

 

LABORATORY TESTS

Following
laboratory tests have been carried out as per IS: 2720. The tests were carried
out both on natural soil and stabilized soil with fly ash collected from Ennore
Thermal Power Plant.

(i) Grain Size
Analysis

(ii) Atterberg
Limit Test

(iii) Proctor
Compaction Test

(iv) Unconfined
Compression Test

(v)
Permeability Test