INDEX:
1. ABSTRACT
2. INTRODUCTION
3. GROUND IMPROVEMENT TECHNICS
3.1REMOVAL AND REPLACEMENT OF SOIL
3.2 PRECOMPRESSION OF SOIL
3.3DENSIFICATION TECHNQUES
3.3.1 VIBRO TECHNICS
3.3.1.1 VIBRO COMPACTION
3.3.1.2 VIBROFLOTATION
3.3.2 DYNAMIC COMPACTION
3.3.3 BLASTING
3.3.4 COMPACTION GROUTING
3.4 REINFORCEMENT TECHNIQUES
3.4.1 STONE COLUMNS
3.4.2 COMPACTION PILES
3.4.3 DRILLED INCLUSIONS
3.5 GROUTING TECHNIQUES
3.5.1 PERMEATION GROUTING
3.5.2. JET GROUTING
3.6 STABILIZATION USING ADMIXTURES
3.6.1 MECHANICAL STABILIZATION
3.6.2 CHEMICAL STABILIZATION
3.6.2.1 CEMENT STABILIZATION
3.6.2.2 LIME STABILIZATION
3.6.2.3 FLY ASH STABILIZATION
3.7 GEO TEXTILES
3.8 ELECTROOSMASIS METHOD
4. CONCLUSION
5.BIBILOGRAPHY
1. ABSTACT:
Ground improvement is the
most imaginative field of geotechnical
engineering. It is a field in which the
engineer forces the ground to adopt the
project's requirements, by altering the
natural state of the soil, instead of having to
alter the design in response to the ground's
natural limitations. The results usually
include saving in construction cost and
reduction of implementation time.
There are number of techniques
available for improving the mechanical and
engineering properties of the soil. However,
each technique has some limitations and suit
abilities to get maximum improvement in
the soil conditions with minimum effort.
Some of the important techniques are
discussed in this paper.
To improve the strength of the
soils, especially in case of granular type of
soils, COMPACTION METHODES are
found as best methods among all type of
techniques. Based on the mechanism applied
for compacting the soil, it is sub divided into
different methods like dynamic compaction,
blasting, vibro techniques ...etc.These
are briefly discussed in this paper.
When there are some
limitations encountered for applying the
above technique, grouting techniques,
stabilization of soil using different
admixtures can be adopted effectively
which can bring variations in the soil
conditions. The various types of above
techniques are briefly discussed in this
paper.
Finally, recent advancements
in ground improving techniques using
GEOTEXTILES, ELECTRIC
TREATMENT METHODES are also
briefly discussed in this paper. These
techniques are widely used in these days.
2. INTRODUCTION:
Large civil engineering
projects are being executed in all over
the country in order to enhance the
infrastructure of the country.
Infrastructure facilities have to be often
built at sites where the soil conditions
are not ideal. The insitu soil
characteristics of a construction site are
different from those desired, and almost
always far from ideal for a designed
need. With increased urban
development, site with favorable
foundation conditions became depleted.
At times the civil engineer has been
forced to construct structures at site
selected for reasons other than soil
conditions. Thus it is increasingly
important for the engineer to know the
degree to which soil properties may be
improved or other alterations that can be
thought of for construction of an
intended structure at stipulated site.
If unsuitable soil conditions are
encountered at the site of a proposed
structure, one of the following four
procedures may be adopted to insure
satisfactory performance of the structure.
By pass the unsuitable soil by
means of deep foundations
extending to a suitable bearing
material.
Redesign the structure and it's
foundation for support by the poor
soil. This procedure may not be
feasible or economical.
Remove the poor material and either
treat it to improve and replace it (or)
substitute for it with a suitable
material.
Treat the soil in place to improve its
properties.
Rigid foundations such as piling
present a solution but these are often
expensive. In such circumstances,
ground improvement using different
techniques offers a proved and economic
solution. At present a variety of soil
improvement techniques are available
for making soil to bear any type of
structure on it and also for mitigation of
seismic hazards. The costs of these
methods vary widely and the conditions
under which they can be used are
influenced by nature and proximity of
structures and construction facilities.
3. GROUND IMPROVEMENT TECHNIQUES:
On the basis of mechanism by
which they improve the engineering
properties of soil, the most of common
of these can be divided into the
following major categories. These are
Densification techniques.
Reinforcement techniques.
Stabilization techniques.
Miscellaneous methods
Apart from the methods listed
above, there are some other simple
methods like removal and replacement
of soil. In this paper these are discussed
first before taking up above techniques.
3.1. REMOVAL AND REPLACEMENT OF SOIL:
One of the oldest and simplest
soil improvement methods is to simply
excavate the unsuitable soil and replace
them with compacted fill. This method
is often used when the problem the soil
is that it is too loose. In that case, the
same soils used to build the fill, except
now it has a higher unit weight (because
of compaction) and thus has been better
engineering properties. This is a
common way to remediate problems
with collapsible soils.
Removal also may be available
option when the excavated soils have other
problems, such as contamination or
excessive organics, and need to hauled
away. This method can be expensive
because of the hauling costs and the need for
imported soils to replace those that were
excavated. It also can be difficult to find a
suitable disposal site for the excavated soils.
Removal and replacement is
generally practical only above the
ground water table. Earthwork
operations become more difficult when
the soil is very wet, even when the free
water pumped out, and thus are
generally avoided unless absolutely
necessary.
3.2. PRECOMPRESSION OF SOIL:
Another old and simple method
of improving soils is to cover them with
a temporary surcharge fill as shown in
figure. This method is called
precompression, preloading, or
surcharging. It is especially useful in
soft clayey and silty soils because the
static weight of the fill causes them to
consolidate, thus improving both
settlement and strength properties. Once
the desired properties have been
obtained, the surcharge is removed and
construction proceeds on improved site.
Pre-compression has the following
advantages
It requires only conventional
equipment earthmoving equipment,
which is readily available. No
special or proprietary equipment is
needed.
Any grading contractor can perform
the work.
The results can be effectively
monitored by using appropriate
instrumentation and ground level
surveys.
The method has a long track record
of success.
The cost is comparatively low, so
long as soil for preloading is readily
available.
However, there also are
disadvantages
The surcharge fill generally must
extend horizontally at least 10m
beyond the perimeter of the planned
construction. This may not possible
for confined sites.
The transport of large quantities of
soil onto the sites may not be
practical, or may have unacceptable
environmental impacts (i.e., dust,
noise, traffic) on the adjacent areas.
The surcharge must remain in place
for months or years, thus delay in
construction.
3.3 DENSIFICATION TECHNIQUES:
The strength and
stiffness of the soil is higher when the
particles are packed in a dense
configuration than they are packed
loosely. As a result, densification is one
of the most effective and commonly
used means of improving soil
characteristics. This can be approaches
in following ways.
3.3.1 VIBRO TECHNIQUES:
Vibro techniques use probes that
are vibrated through soil deposit in a
grid pattern to densify the soil over the
entire area of thickness of the deposit.
These are classified in to the following
methods. These are
3.3.1.1VIBRO COMPACTION:
Vibro compaction is a method
for compacting deep granular soils by
repeatedly inserting a vibratory probe. It
is also known as VIBRO
DENSIFICATION.
By inserting depth vibrators, the
vibrations are produced by rotating a
heavy eccentric weight with the help of
an electrical motor with in the vibrator.
The vibratory energy is used to
rearrange the granular particles in a
denser state. Penetration of the vibro
probe is typically aided by water jetting
at the tip of the probe.
The Vibro-Compaction Process
Some of advantages and disadvantages of
this method are given below.
It is often an economical alternative
to deep foundations, especially
when considering the added
liquefaction protection in seismic
areas.
It is most effective in granular soils
It cannot be used in cohesive soils
3.3.1.2. VIBRO FLOTATION:
In vibro flotation a torpedo like
probe (the vibro float) suspended by a
crane is used to density a soil
deposit.Vibro floats usually 12 to 18
inch in diameter and about10 to 16 ft
long, contain weights mounted
eccentrically on a central shaft driven by
electric or hydraulic power.
The vibro float is initially
lowered to the bottom of the deposit by a
combination of vibration and water or
air jetting through ports in its pointed
nose cone. The vibro float is then
incrementally with drawn in 2 to 3 ft
intervals at an over all rate of about 1ft /
min while still vibrating. Water may be
jetted through ports in the upper part of
the vibro float to loosen the soil above
the vibro float temporarily and aid in its
with drawl. The vibrations produce a
localized zone of temporary liquefaction
that causes the soil surrounding the vibro
float to densify.
Principle of the technique
Vibro flotation is most
effective in clear granular soils
with the fine contents less than
20% and clay contents below
3%.
Vibro flotation has been used
successfully to density soils to
deep [this of up to 115 ft.]
3.3.2. DYNAMIC COMPACTION:
Dynamic compaction is a ground
improvement process for compacting
and strengthening loose or soft soils to
support buildings, roadways, and other
heavy construction. The method
involves the systematic dropping of
heavy weights, 100 to 400kN, from a
height of 5 to 30m, in a pattern designed
to remedy poor soil conditions at the
proposed building site. In soft ground
areas, dynamic compaction has proved
to be an effective and economical
alternative to preloading, foundation
piling, deep vibratory compaction, and
soil undercutting and replacement
Dynamic Compaction is normally used
under the following circumstances:
To increase in-situ density and in
this way improve the bearing
capacity and consolidation
characteristics of soils (or waste
materials) to allow conventional
foundation and surface bed
construction to be carried out. The
technique typically improves the in-
situ soils such that allowable
bearing pressures of up to 250 Kpa
can be used with foundation
settlements of the order of 10 to 20
mm.
To increase in-situ density and in
this way improve in-situ
permeability and/or reduce
liquefaction potential
What soils are suitable?
Most soil types can be
improved, including silts and some
clays. The most commonly treated soils
are old fills and granular virgin soils.
Soils below the water table are routinely
treated. However, careful control has to
be used to allow dissipation of excess
pore pressures created during the weight
dropping.
3.3. 3. BLASTING:
Blasting densification
involves the detonation of multiple
explosive charges vertically spaced at 10
to 20 ft apart in drilled or jetted bore
holes. The bore holes are usually spaced
between 15 to 50 ft apart and back filled
prior to detonation. The efficiency of
densification process can be increased
by detonating the charges at different
elevations at small time delays.
Immediately after detonation, the ground
surface rises & gas & water are expelled
from fractures. The ground surface then
settles as the excess gas & water
pressure dissipates. Two or three rounds
of blasting are often used to achieve the
desired degree of densification.
Blasting is most effective in
loose sands that contain less than
20% silt and less than 5% clay.
Although blasting is quite
economical, it is limited by
several considerations, as it
produces strong vibrations that
may damage near by structures
or produce significant ground
movements.
3.3.4. COMPACTION
GROUTOING:
Compaction grouting
uses displacement to improve ground
conditions. A very viscous (low
mobility) aggregate is pumped in stages,
forming grout bulbs, which displace and
densify the surrounding soils.
A consistency soil
cement paste is injected under pressure
in to the soil mass, consolidating, and
there by densifying surrounding soils in
place. The injected ground mass
occupies void space created by pressure-
densification. Pump pressure transmitted
through low mobility grout, produces
compaction by displacing soil at depth
until resisted by the weight of over lying
soils.
Fine grained soils with sufficient
permeability to allow excess
water to dissipate best suits for
compaction grouting.
It has also been used successfully
in a wide variety of soils and
fills.
3.4. REINFORCEMENT
TECHNIQUES:
In some cases it is possible to
improve the strength and stiffness of a
existing soil deposit by installing
discrete inclusions that reinforce the soil.
These inclusions may consist of
structural materials, such as steel,
concrete or timber and geomaterials
such as densified gravel.
3.4.1. STONE COLUMNS:
Soils deposits can be improved
by the installation of dense columns of
gravel known as stone columns. They
may be used in both fine and coarse
grained soils. In fine-grained soils, stone
columns are used to increase the shear
strength beneath structures and
embankments by accelerating
consolidation (by allowing radial
drainage) and introducing columns of
stronger material.
Stone columns can be installed in
a variety of ways. (They may be
constructed by introducing gravel during
the process of vibroflotation) In the
Frankie method, a steel casing initially
closed at the bottom by a gravel plug is
driven to the desired depth by an internal
hammer. At that depth part of the plug is
driven beyond the bottom of the casing
to form a bulb of gravel. Additional
gravel is then added and compacted as
the casing is with drawn. The diameter
of the resulting stone column depends on
the stiffness and compressibility of the
surrounded soil
3.4.2. COMPACTION PILES:
Granular soils can be improved
by the installation of compaction piles.
Compaction piles are displacement
piles , usually prestressed concrete or
timber, that are driven into a loose sand
or gravel deposit in a grid pattern and
left there.
Compaction piles improve the
seismic performance of a soil by three
different mechanisms. First the flexural
strength of piles themselves provides
resistance to soil movement
(reinforcement). Second, the vibrations
and displacements produced by their
installation cause densification. Finally,
the installation process increases the
lateral stress in the soil surrounding the
piles.
Compaction piles generally
densify the soil with in a distance of 7 to
12 pile diameters and consequently are
usually installed in a grid pattern.
Between compaction piles a relative
density of up to 75% to 80% are usually
achieved. Improvement can be obtained
with reasonable economy to depth of
about 60ft.
3.4.3 DRILLED INCLUSIONS:
Structural reinforcing elements
can also be installed in the ground by
drilling or auguring. Drilled shafts, some
times with very large diameters, have
been used to stabilize many slopes.
Soil nails, tie backs, micro piles
have been used for this purpose. The
installation of such drilled inclusions can
be quite difficult. However in the loose
granular soils that contribute to increase
the strength of the soil in a every
effective manner.
3.5 GROUTING AND MIXING
TECHNIQUES:
Grouting techniques involve of
cementitious materials into voids of the
soil or into fractures in the soil so that
the particle structure of the majority of
the soil remains intact.
Mixing techniques introduce
cementitious materials by physically
mixing them with the soil, completely
disturbing the particle structure of the
soil. Grouting and mixing techniques
tend to be expensive but can often be
accomplished with minimal settlement
or vibration.
3.5.1.PERMEATION GROUTING:
Permeation grouting involves the
injection of low viscosity liquid grout
into the voids of the soil without
disturbing the soil structure. Particulate
grouts (i.e., aqueous suspensions of
cement, fly ash, bentonite, micro fine
cement or some combination there of) or
chemical grouts (e.g., silica & lignin
gels, or phenolic & acrylic resins) may
be used.
Grout pipes are typically
installed in a grid pattern at spacing of 4
to 8 feet. The grout may be injected in
different ways. In ‘stage grouting’, a
boring is advanced a short distance
before grout is injected through the end
of the drill rod. After the grout sets up,
the boring is advanced another short
distance and grouted again. This process
continues until grout has been placed to
the desired depth.
Permeation grouting produces
soil improvement by two mechanisms.
First the grout tends to strengthen the
contacts between individual soil grains,
there by producing a soil skeleton that is
stronger and stiffer than that of the un
grouted soil. Second, the grout takes up
space in the voids between soil particles,
reducing the tendency for densification.
Stopping leaks in below-
grade structures
Stopping leaks in below-
grade utilities
Excavating support of non-
corrosive soils
Strengthening of soil mass
to accept new loads
3.5.2. JET GROUTING:
:
In Jet grouting the soil is mixed
with cement grout injected horizontally
under high pressure in a previously
drilled bore hole.
Jet grouting uses a special pipe
equipped with horizontal jets that inject
grout into the soil at high pressure. The
pipes are first inserted to the desired
depth, then they are raised and rotated
while the injection is in progress, thus
forming a column of treated soil.
Because of high pressure, this
method is usable on a wide range of
soil types.
3.6. STABILIZATON USING
ADMIXTURES:
SOIL STABILIZATION: It is the process
of improving the engineering properties of
soil by mixing some binding agents thus
binding the soil particles .In a broader sense
it also includes compaction, pre
consolidation and many more such process.
Soil stabilization is classified as
Mechanical
stabilization
Chemical
stabilization
3.6.1.MECHANICAL STABILIZATION:
Mechanical stabilization is the process of
improving the properties of soil by changing
its gradation. Two (or) more types of natural
soils mixed to obtain composite which is
suspension to any of its components
3.6.2. CHEMICAL STABILIZATION:
Chemical stabilization is the form of lime,
cement, fly ash and the combination of the
above is widely used in soil stabilization to
Reduce the
permeability of the soil.
Improve shear
strength.
Increase bearing
strength.
Decrease
settlement.
Soil and chemicals are mixed either
mechanically in place or by bath
process .the optimum benefit of using these
agents in stabilization must be determined
by laboratory testing. The general principle
of these admixtures as stabilizers are
discussed below.
3.6.2.1. LIME STABILIZATION: This is
done by adding lime to soil. It is useful for
stabilization of clayed soils. When lime
reacts with soil, there is exchange of cations
in the adsorbed water layer and a decrease in
plasticity of soil occurs .The resulting
material is more friable than the original
clay and is therefore more suitable as sub
grade.
This method is not effective for
sandy soils. However these soils can be
stabilized in combination with clay, fly
ash or other pozzolanic materials, which
serve hydraulically reactive in gradients.
3.6.2.2. CEMENT STABILIZATION:
Cement stabilization is done by mixing
pulverized soil and Portland cement with
water and compacting the mix to attain a
strong material .The material obtain by
mixing soil and cement is known as soil
cement .The mix becomes hard and durable
structural material as the cement hydrates
and develops strength.
The soil cement is quite weather
resistant and strong. It is commonly used
for stabilizing sandy and other low
plasticity soils. Cement interacts with
the silt and clay fractions and reduced
their affinity for water .It reduces the
swelling characteristics of the soil .
3.6.2.3. FLY ASH STABILIZATION: Fly
ash is a by product of the pulverized coal
combustion process. Fly ash has silica,
alumina and various oxides and alkalis as its
constituents .It is fine grained and
pozzolanic in nature. Fly ash reacts actively
with hydrated lime and hence is used in
combination with lime as a stabilizer. A
mixture of about 10 to 35 % of fly ash and 2
to 10 % of lime forms as effective stabilizer
for stabilization of highway bases and sub
bases .Soil-lime-fly ash mixes are
compacted under controlled condition with
adequate quantity of water.
3.7. GEOTEXTILES: Soil conditions can
be improved in an excellent manner by
using geo textiles. Geotextiles are porous
fabrics manufactured products and others
such as polyester ,polyethylene,
polypropylene and polyvinylchloride, nylon,
fiber glass and various mixtures of these.
These are having permeabilities comparable
in range from coarse gravel to fine sand.
Geotextiles have been used in a
variety of civil engineering works. Thus
in the selection of a proper geotextile,
due importance has to be given to the
major function that the geotextile is
intended to perform. These are majorly
used as follows.
1. They acts as separators between
two layers of soils having a large difference
in particle size to prevent migration of small
size particles into the voids of large size
particles
2. They act as filter. When the
silt laden turbid water passes through the
geotextile, the silt particles are prevented
from movement by the geotextile.
3. Geotextiles themselves function
as a drain because they have a high water
transporting capacity than that of the
surrounding material.
4. They serve as REINFOREMENT
in soil since they are a good in tensile
strength.
3.7. ELECTRO OSMASIS AND
ELCTRO CHEMICAL HARDENING
METHOD:
The electroosmasis process can
be used to increase the shear strength
and reduce the compressibility of soft
clayey and silty soils beneath
foundation. By introducing an
electrolyte such as calcium chloride at
the anode, the base exchange reaction
between the iron anode and surrounding
soil is increased, resulting in the
formation of ferric hydroxides which
bind the soil particles together. However
because cost of electric power and
wastage of electrodes, electroosmasis
with or without electrochemical
hardening can be considered only for
special situations where the alternative
of piling cannot be adopted.
4. CONCLUSION:
1. Unfavorable soil conditions can
frequently be improved using
soil improvement techniques. A
variety of soil improvement
techniques have been developed.
However a suitable technique has
to be adopt according to
necessity of the structure and
economy.
2. Mainly soil improvement
techniques can be divided in to
four broad categories;
Densification technique,
Reinforcement technique,
grouting or mixing technique and
stabilization technique.
3. Densification is probably the
most commonly used soil
improvement technique. Most
densification techniques relay on
tendency of granular soils to
densify when subjected to
vibrations. However there is a
possibility of damaging adjacent
structures and pipelines due to
application of this technique.
4. Reinforcement techniques introduce
discrete inclusions that stiffen
and strengthen a soil deposit. The
high stiffness and strength of the
inclusions also tend to reduce the
stresses imposed on the weaker
material between the inclusions.
5. Grouting techniques involve the
injection of cementitious
materials into the voids of the
soil or into fractures of the soil,
so that the particle structure of
the majority of soil remains
inject. In permeation grouting,
very low viscosity grouts are
injected intothe voids of the soil
with out disturbing the soil
structure. In intrusion grouting,
thicker and more viscous grouts
are injected under pressure to
cause controlled fracturing of the
soil.
6. Now a days, geotextiles are
extensively used for improving
the soil conditions. These has
multiple applications as they act
as filters, reinforcement,
separations..etc.
5. BIBILOGRAPHY:
1. “Geotechnical Engineering
Principles & Practices” by Donald
P.Coduto
2. “Foundation Design &
Cinstruction “by M.J.Tomlinson.
3. “Geotechnical Engineering” by
Purshotham raj
4. “Geotechnical Earthquake
Engineering” by Steven
L.Kramar.
.