2. Stabilization
Modification of soils can be done by mechanical addition
of granular materials or chemical compounds such as
cement, lime, bitumen and cacl2.
The purpose of mixing these additives is to
1. Increase strength
2. Reduce deformability.
3. Control shrinkage & deformability.
4. Reduce erodability.
5. Increase durability
3. Stabilization
Mechanical stabilization:
Mechanical stabilization covers the rearrangement of soil
particles and improvement of soil gradation.
The aggregate should be correctly proportioned before lying
and should have sufficient mechanical strength.
In order to attain adequate mechanical stability, it is
necessary to have a well proportioned coarse material
containing some clay binder.
Maximum density is obtained in soils which have a particle
size distribution which can be approximated by the following
expression (Fuller and Thompson).
4. Stabilization
Mechanical stabilization:
% passing any sieve = 100√ (d/D)
d= aperture size of the sieve.
D= size of the largest particle.
To obtain adequate cohesion, particle size less than
0.075mm is necessary.
Surface PI=4 - 9% LL=25%
Base PI=6% LL=35%
Good compaction can be obtained if it is used by traffic for
few months before surfacing is applied.
5. Mechanical stabilization
Mechanical stabilization can be done by…
Addition of binding materials: The aggregate material have to be
selected to give the densest mixture by minimize the amount of
voids. The binder fine material is intended to give cohesion to the
mixture.
Addition of any material to reduce permeability: In order to
reduce the permeability of a given soil, it is common practice to
add sodium montomorillonite (bentonite). It is also possible to
reduce the permeability by addition of a suitable locally available
fine grained soil.
Removal of fines from gravel: In order to use the gravel for the
purpose of pavements, the presence of fines should be less. The
easiest way of removal of fines from gravel is by washing.
6. I. Portland cement stabilization
Binding of soil particles together referred to as
stabilization by cementing. Cement and soil blended
material is referred to as soil-cement. In this the cement
is reacts with the siliceous soil to cement the particles
together.
In soil cement mostly coarse grained soils are cemented
and proportion of fine grained soil cementation is less.
The physical properties of soil cement depends on the
nature of the soil treated, type, amount of cement and
cure conditions. This is mostly used in base of roads and
air fields.
7. I. Portland cement stabilization
Nature of soil:
Since organic matter reduces the strength of soil-cement,
inorganic soils can be stabilized using cement. About 2% of
organic matter is considered upper limit. Lime or cacl2 is
sometimes added to stabilize the soil. Calcium ions are most
desirable for ease of cement stabilization.
Soils with following limits can be economically stabilized.
Maximum size 75mm
Soil passing 4.5mm > 50%
0.425mm > 15%
75μ < 50%
PI < 8% and LL < 40%
In general the best results are obtained with well graded soils
having less than 50% of its particles finer than 0.075mm &
PI<20%
8. I. Portland cement stabilization
Amount of cement:
Cement content varying from 5-20% is satisfactory for
stabilization.
Following amounts are usually required
for gravel, cement level of 5-10% by weight.
for Sand, cement level of 7-12%
for Silts, cement level of 12-15%
for Clays, cement level of 12-20%
A given increase in the cement content with the more
clayey soils produced a smaller increase in compressive
strength than with sands.
An increase in strength is obtained with increasing
cement content.
9. I. Portland cement stabilization
Mixing:
More uniform soil-cement water mixture provides strong
and durable soil-cement. The continued mixing should be
only up to the optimal level. Further continued mixing lead
to segregation of components. It is observed that mixtures
made in the laboratory have high strength compared to
the similar mixes made in the field (about 50-70%).
Moisture content:
The moisture content is governed by the soil type and
method of compaction.
Compaction conditions:
In natural soils, having same cement content and amount
of compaction, the greatest strength is obtained for the
one compacted at approximately the optimum moisture
content.
10. I. Portland cement stabilization
Age and Curing:
The compressive strength of soil-cement increases with
age. Like concrete, damp environment is desirable for
curing. Soil-cement cures rapidly with increase in
temperature.
Admixtures for soil-cement:
In order to accelerate the setting time and to improve
the properties of soil-cement, lime or cacl2 can be added.
These chemicals permits reduction in the amount of
cement required to treat the soil.
11. II. Bitumen stabilization:
Bitumen stabilization:
Bituminous materials: Bitumen, asphalt, Tar.
Bituminous materials stabilize the soil either by binding
the particles (takes place in cohesion less soils) together
or to protect the soil from deleterious effects of water
(water proofing in cohesive soils) or both.
Most of the bitumen stabilization has been done with
asphalt and is referred to as soil-asphalt.
As the straight run asphalt (produced from vacuum
distillation process) has low viscosity and low softening
temperature, it is commonly used in soil stabilization.
12. II. Bitumen stabilization:
Asphalt can not be directly added to the soil because of
its high viscosity. Its fluidity can be increased by
1. Heating
2. Emulsifying in water (emulsions)
3. Cut back with some solvents like gasoline.
Soil asphalt is mostly used for highway and airfields.
13. II. Bitumen stabilization:
Nature of soil:
Organic matter of acid origin is harmful to soil-asphalt. It is
difficult to handle plastic clays because of mixing problem.
All inorganic soils can be stabilized and the following
requirements yield the best results.
1. maximum particle size should be less than one third of the
compacted soil layer
2. particles finer than 4.75mm are greater than 50%
3. 35-100% particles finer than 0.42mm
4. Greater than 10% but less than 50% particles finer than
0.075mm
5. Liquid limit less than 40%, Plasticity index less than 18%
14. II. Bitumen stabilization:
Amount of Asphalt:
Increase in asphalt gives better results. In fine grained soils addition
of asphalt does not increase the strength but improves the water
proofing property and thereby forms better stabilized soil. Asphalt
should be added optimally otherwise results in sticky mixture which
can not be properly compacted.
Mixing:
A thorough mixing of additives with soil yields better stabilized soil.
Compaction conditions:
Density of mixture is depends on the volatiles content, amount and
type of compaction. In general lower the volatile content higher the
strength.
15. II. Bitumen stabilization:
Cure conditions:
The strength of soil asphalt is inversely proportional to the
volatile content. The longer the period of cure and warmer the
temperature of cure, the greater the volatiles lost. The longer
the period of immersion, greater the water pick up.
The sequence of operation is as follows:
1. Pulverization of soil 2. addition of water
3. Adding mixing of bitumen.
4. Aeration.
5. Compaction 6. Finishing
7. Aeration and curing. 8. Application of surface cover.
16. III. Chemical Stabilization:
Chemical Stabilization:
Chemical stabilization primarily consists of bonding the soil
particles with a cementing agent which is produced by a
chemical reaction with soil. Chemical as a secondary additive
increases the effectiveness of cement.
17. Lime Stabilization:
Lime can be used alone or in combination with other
admixtures like flyash, cement or bitumen.
Two types of chemical reactions take place when lime is
added to wet soil. First one occurring immediately by ion
exchange of calcium for the ions naturally carried by the soil.
The second reaction takes considerable time in a cementing
action which involves the reaction between the calcium
from the lime with the available reactive aluminum or silica
from the soil.
Plasticity of the soil, density and strength are changed by
addition of lime to soil. Addition of lime reduces the
plasticity of the soils and the soil can be handled easily.
18. Lime Stabilization:
Construction Procedure:
Same as employed for soil-cement but it will take more time.
Normal sequence for lime stabilized base is
1. Scarify the base.
2. Pulverization of soil
3. Spread the lime
4. Mixing the lime and soil.
5. Addition of water to bring to OMC
6. Mixing the lime and soil.
7. Shape the stabilized base.
8. Cure and keep moist and traffic free for at least 5 days.
9. Add wearing surface.
19. Calcium chloride (CaCl2):
CaCl2 is an inorganic salt.
Physical properties of CaCl2:
CaCl2 is hygroscopic (attracts and absorbs water form
atmosphere)
highly soluble (59.9g dissolve in 100ml of water at 00c)
deliquescence (absorbs moisture from the atmosphere
until it dissolves in the absorbed water and forms a
solution)
Higher surface tension.
Lower freezing point than water (-510c)
20. Calcium chloride (CaCl2):
Effect on soil properties:
Sodium ions (Na+) present around the negatively
charged clay platelets and they are replaced by ca+2
and the thickness of diffused double layer is reduced.
This causes lower plasticity and increases strength.
CaCl2 also reduces inter granular repulsion and
strengthens molecular bonds between particles.
Fig: shows compaction properties of gravelly clay with
and without CaCl2. since CaCl2 has high surface tension,
reaches high density at low water content.
21.
22. Calcium chloride (CaCl2):
CaCl2 reduces evaporative water losses from soils. This
character facilitates moisture control during construction
and helps in the control of dust generated on unpaved
roads.
CaCl2 as a secondary additive can also benefits in
cement or lime stabilization by increasing early strength
values. Only 0.5 to 1.5% of CaCl2 may be needed for
this purpose.
23. Sodium Silicate Stabilization:
Sodium silicate can be used in soil stabilization both as
additives to stabilizers and as primary stabilizers.
Sodium silicate reduces the plasticity index of clay.
Additions up to 2% by mass have been considered
effective in road engineering. Silicates derive their benefits
because of their cementitious components.
24. Gypsum Stabilization (CaSO4.2H2O):
Gypsum alone is not effective as soil stabilizer for
engineering purposes.
However it enhances the stabilizing properties of lime by
accelerating its reaction with the soil.
Addition of gypsum to soil can limit clay swelling and
dispersion and thus improve soil structural stability by
means of cation-exchange effects
