Modern Ground Improvement Techniques

Arunai College of Engineering

MODERN GROUND IMPROMENT TECHNIQUES Presented by U.Vetrivel.B.E

12

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

different methods like dynamic compaction, blasting, vibro techniques ...etc.These 1. ABSTACT:

Ground improvement is the most imaginative

field

of

geotechnical

engineering. It is a field in which the

are briefly discussed in this paper.

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.

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

There are number of techniques available for improving the mechanical and

techniques are briefly discussed in this paper.

engineering properties of the soil. However, each technique has some limitations and suit

Finally, recent advancements in ground

improving

techniques

GEOTEXTILES,

using

ELECTRIC

TREATMENT METHODES are also briefly discussed in this paper. These techniques are abilities to get maximum improvement in

widely used in these days.

the soil conditions with minimum effort. Some of the important techniques are discussed in this paper. To improve the strength of the

2. INTRODUCTION:

Large

civil

engineering

soils, especially in case of granular type of

projects are being executed in all over

soils, COMPACTION METHODES are

the country in order to enhance the

found as best methods among all type of

infrastructure

techniques. Based on the mechanism applied

Infrastructure facilities have to be often

for compacting the soil, it is sub divided into

built at sites where the soil conditions

of

the

country.

are

not

ideal.

The

insitu

soil

characteristics of a construction site are



Treat the soil in place to improve its properties.

different from those desired, and almost

Rigid foundations such as piling

always far from ideal for a designed

present a solution but these are often

need.

expensive.

With

increased

urban

In

such

circumstances,

favorable

ground improvement using different

foundation conditions became depleted.

techniques offers a proved and economic

At times the civil engineer has been

solution. At present a variety of soil

forced to construct structures at site

improvement techniques are available

selected for reasons other than soil

for making soil to bear any type of

conditions. Thus it is increasingly

structure on it and also for mitigation of

important for the engineer to know the

seismic hazards. The costs of these

degree to which soil properties may be

methods vary widely and the conditions

improved or other alterations that can be

under which they can be used are

thought of for construction of an

influenced by nature and proximity of

intended structure at stipulated site.

structures and construction facilities.

development,

site

with

If unsuitable soil conditions are encountered at the site of a proposed structure,

one

of

the

following

four

3. GROUND TECHNIQUES:

procedures may be adopted to insure satisfactory performance of the structure. 





IMPROVEMENT

On the basis of mechanism by which they improve the engineering

By pass the unsuitable soil by means

properties of soil, the most of common

of deep foundations extending to a

of these can be divided into the

suitable bearing material.

following major categories. These are

Redesign the structure and it's



Densification techniques.

foundation for support by the poor



Reinforcement techniques.

soil. This procedure may not be



Stabilization techniques.

feasible or economical.



Miscellaneous methods

Remove the poor material and either treat it to improve and replace it (or) substitute for it with a suitable material.

Apart from the methods listed

above, there are some other simple

operations become more difficult when

methods like removal and replacement

the soil is very wet, even when the free

of soil. In this paper these are discussed

water

first before taking up above techniques.

generally avoided unless

3.1. REMOVAL AND REPLACEMENT OF SOIL:

necessary.

One of the oldest and simplest

pumped

out,

and

thus

are

absolutely

3.2. PRECOMPRESSION OF SOIL:

soil improvement methods is to simply

Another old and simple method

excavate the unsuitable soil and replace

of improving soils is to cover them with

them with compacted fill. This method is

a temporary surcharge fill as shown in

often used when the problem the soil is

figure.

that it is too loose. In that case, the same

precompression,

soils used to build the fill, except now it

surcharging. It is especially useful in soft

has a higher unit weight (because of

clayey and silty soils because the static

compaction) and thus has been better

weight of the fill causes them to

engineering

consolidate,

properties.

This

is

a

This

method

is

called

preloading,

thus

improving

or

both

common way to remediate problems

settlement and strength properties. Once

with collapsible soils.

the

desired

properties

have

been

obtained, the surcharge is removed and Removal also may be available

construction proceeds on improved site.

option when the excavated soils have other

Pre-compression

problems,

advantages

such

as

contamination

or

excessive organics, and need to hauled away. This method can be expensive



needed.

suitable disposal site for the excavated soils. 

Removal and replacement is

table.

Earthwork

conventional

special or proprietary equipment is

excavated. It also can be difficult to find a

water

only

following

which is readily available. No

imported soils to replace those that were

ground

requires

the

equipment earthmoving equipment,

because of the hauling costs and the need for

generally practical only above the

It

has

Any grading contractor can perform the work.



The results can be effectively monitored by using appropriate





instrumentation and ground level

particles

surveys.

configuration than they are packed

The method has a long track record

loosely. As a result, densification is one

of success.

of the most effective and commonly

The cost is comparatively low, so

used

long as soil for preloading is readily

characteristics. This can be approaches

available.

in following ways.

However,

there

also

of

in

a

dense

improving

soil

3.3.1 VIBRO TECHNIQUES:

The surcharge fill generally must

Vibro techniques use probes that

extend horizontally at least 10m beyond the perimeter of the planned



means

packed

are

disadvantages 

are

are vibrated through soil deposit in a

construction. This may not possible

grid pattern to densify the soil over the

for confined sites.

entire area of thickness of the deposit.

The transport of large quantities of soil onto the sites may not be

These are classified in to the following methods. These are

practical, or may have unacceptable environmental impacts (i.e., dust,

3.3.1.1VIBRO COMPACTION:

noise, traffic) on the adjacent areas. 

Vibro compaction is a method

The surcharge must remain in place

for compacting deep granular soils by

for months or years, thus delay in

repeatedly inserting a vibratory probe. It

construction.

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. 3.3 DENSIFICATION TECHNIQUES:

The vibratory energy is used to rearrange the granular particles in a denser state.

The

strength

and

stiffness of the soil is higher when the

Penetration of the vibro probe is

typically aided by water jetting at the tip of the probe.

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

The Vibro-Compaction Process

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 Some of advantages and disadvantages of this method are given below. 

that causes the soil surrounding the vibro

It is often an economical alternative to

deep

when

foundations,

considering

localized zone of temporary liquefaction

especially

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:

float to densify. Principle of the technique



most

to be an effective and economical

effective in clear granular soils

alternative to preloading, foundation

with the fine contents less than

piling, deep vibratory compaction, and

20% and clay contents below

soil undercutting and replacement

Vibro

flotation

is

3%. 

Vibro flotation has been used successfully to density soils to deep [this of up to 115 ft.]

Dynamic Compaction is normally used under the following circumstances:

3.3.2. DYNAMIC COMPACTION:



To increase in-situ density and in this

way

improve

capacity

the

and

bearing

consolidation

characteristics of soils (or waste materials) to allow conventional foundation

and

surface

bed

construction to be carried out. The technique typically improves the insitu

soils

such

that

allowable

bearing pressures of up to 250 Kpa can

be

used

with

foundation

settlements of the order of 10 to 20

Dynamic compaction is a ground improvement process for compacting

mm. 

To increase in-situ density and in

and strengthening loose or soft soils to

this

support buildings, roadways, and other

permeability

heavy

liquefaction potential

construction.

The

method

involves the systematic dropping of heavy weights, 100 to 400kN, from a

way

improve

in-situ

and/or

reduce

What soils are suitable? Most soil types can be

height of 5 to 30m, in a pattern designed

improved, including silts and some

to remedy poor soil conditions at the

clays. The most commonly treated soils

proposed building site. In soft ground

are old fills and granular virgin soils.

areas, dynamic compaction has proved

Soils below the water table are routinely

treated. However, careful control has to

produces strong vibrations that

be used to allow dissipation of excess

may damage near by structures

pore pressures created during the weight

or produce significant ground

dropping.

movements. 3.3.4. COMPACTION GROUTOING:

3.3. 3. BLASTING: Compaction grouting Blasting

densification

uses displacement to improve ground

involves the detonation of multiple

conditions.

A

very

viscous

(low

explosive charges vertically spaced at 10

mobility) aggregate is pumped in stages,

to 20 ft apart in drilled or jetted bore

forming grout bulbs, which displace and

holes. The bore holes are usually spaced

densify the surrounding soils.

between 15 to 50 ft apart and back filled A

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

Blasting is most effective in loose sands that contain less than

Although

blasting

is

quite

economical, it is limited by several

considerations,

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 pressuredensification. Pump pressure transmitted through low mobility grout, produces compaction by displacing soil at depth until resisted by the weight of over lying

as



Fine grained soils with sufficient permeability to allow excess water to dissipate best suits for

20% silt and less than 5% clay. 

soil

soils.

desired degree of densification. 

consistency

it

compaction grouting.



It has also been used successfully

the process of vibroflotation) In the

in a wide variety of soils and

Frankie method, a steel casing initially

fills.

closed at the bottom by a gravel plug is driven to the desired depth by an internal

3.4.

REINFORCEMENT

TECHNIQUES:

driven beyond the bottom of the casing

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

structural

hammer. At that depth part of the plug is

may

materials,

such

consist as

of

steel,

concrete or timber and geomaterials such

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:

as densified gravel. Granular soils can be improved 3.4.1. STONE COLUMNS:

by the installation of compaction piles.

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

embankments consolidation

structures by

(by

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

and

seismic performance of a soil by three

accelerating

different mechanisms. First the flexural

allowing

radial

strength of piles themselves provides

drainage) and introducing columns of

resistance

stronger material.

(reinforcement). Second, the vibrations

Stone columns can be installed in a variety of ways. (They may be constructed by introducing gravel during

to

soil

movement

and displacements produced by their installation cause densification. Finally, the installation process increases the

lateral stress in the soil surrounding the piles.

Grouting techniques involve of cementitious materials into voids of the soil or into fractures in the soil so that

Compaction

piles

generally

densify the soil with in a distance of 7 to

the particle structure of the majority of the soil remains intact.

12 pile diameters and consequently are usually installed in a grid pattern.

Mixing

techniques

introduce

Between compaction piles a relative

cementitious materials by physically

density of up to 75% to 80% are usually

mixing them with the soil, completely

achieved. Improvement can be obtained

disturbing the particle structure of the

with reasonable economy to depth of

soil. Grouting and mixing techniques

about 60ft.

tend to be expensive but can often be accomplished with minimal settlement

3.4.3 DRILLED INCLUSIONS: Structural reinforcing elements

or vibration. 3.5.1.PERMEATION GROUTING:

can also be installed in the ground by drilling or auguring. Drilled shafts, some

Permeation grouting involves the

times with very large diameters, have

injection of low viscosity liquid grout

been used to stabilize many slopes.

into the voids of the soil without disturbing the soil structure. Particulate

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

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.

effective manner. Grout 3.5 GROUTING TECHNIQUES:

AND MIXING

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

3.5.2. JET GROUTING:

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

In Jet grouting the soil is mixed

space in the voids between soil particles, reducing the tendency for densification.

with cement grout injected horizontally under high pressure in a previously



Stopping leaks in

drilled bore hole.

below-grade structures 

Jet grouting uses a special pipe Stopping leaks in

below-grade utilities 

equipped with horizontal jets that inject grout into the soil at high pressure. The

Excavating support

of non-corrosive soils

pipes are first inserted to the desired depth, then they are raised and rotated



Strengthening

soil mass to accept new loads

of

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.

Improve shear strength. Increase bearing strength. Decrease settlement. Soil and chemicals are mixed either 3.6.

STABILIZATON

USING

ADMIXTURES:

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 SOIL STABILIZATION: It is the process of improving the engineering properties of

these admixtures as stabilizers are discussed below.

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

3.6.2.1. LIME STABILIZATION: This is done by adding lime to soil. It is useful for stabilization of clayed soils. When lime

Mechanical stabilization

reacts with soil, there is exchange of cations

Chemical stabilization

in the adsorbed water layer and a decrease in plasticity of soil occurs .The resulting material is more friable than the original

3.6.1.MECHANICAL STABILIZATION: Mechanical stabilization is the process of

clay and is therefore more suitable as sub grade.

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

This method is not effective for sandy soils. However these soils can be stabilized in combination with clay, fly

3.6.2. CHEMICAL STABILIZATION: Chemical stabilization is the form of lime,

ash or other pozzolanic materials, which serve hydraulically reactive in gradients.

cement, fly ash and the combination of the above is widely used in soil stabilization to Reduce the permeability of the soil.

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

fabrics manufactured products and others

mixing soil and cement is known as soil

such

cement .The mix becomes hard and durable

polypropylene and polyvinylchloride, nylon,

structural material as the cement hydrates

fiber glass and various mixtures of these.

and develops strength.

These are having permeabilities comparable

as

polyester

,polyethylene,

in range from coarse gravel to fine sand.

The soil cement is quite weather resistant and strong. It is commonly used

Geotextiles have been used in a

for stabilizing sandy and other low

variety of civil engineering works. Thus

plasticity soils. Cement interacts with the

in the selection of a proper geotextile,

silt and clay fractions and reduced their

due importance has to be given to the

affinity for water .It reduces the swelling

major function that the geotextile is

characteristics of the soil .

intended to perform. These are majorly used as follows. 1. They acts as separators between

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

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

and

pozzolanic in nature. Fly ash reacts actively

2. They act as filter. When the

with hydrated lime and hence is used in

silt laden turbid water passes through the

combination with lime as a stabilizer. A

geotextile, the silt particles are prevented

mixture of about 10 to 35 % of fly ash and 2

from movement by the geotextile.

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

3. Geotextiles themselves function as a drain because they have a high water transporting capacity than that of the surrounding material.

adequate quantity of water. 4. They serve as REINFOREMENT in soil since they are a good in tensile 3.7. GEOTEXTILES: Soil conditions can be improved in an excellent manner by using geo textiles. Geotextiles are porous

strength.

3.7.

ELECTRO

OSMASIS

AND

2.

Mainly

soil

improvement

ELCTRO CHEMICAL HARDENING

techniques can be divided in to

METHOD:

four

broad

categories;

Densification

technique,

be used to increase the shear strength

Reinforcement

technique,

and reduce the compressibility of soft

grouting or mixing technique and

clayey

stabilization technique.

The electroosmasis process can

and

silty

foundation.

By

soils

beneath

introducing

an

3.

electrolyte such as calcium chloride at

most

the anode, the base exchange reaction

soil

tendency of granular soils to

formation of ferric hydroxides which

densify

bind the soil particles together. However

when

subjected

to

vibrations. However there is a

because cost of electric power and

possibility of damaging adjacent

wastage of electrodes, electroosmasis without

used

densification techniques relay on

soil is increased, resulting in the

or

commonly

improvement technique. Most

between the iron anode and surrounding

with

Densification is probably the

structures and pipelines due to

electrochemical

application of this technique.

hardening can be considered only for special situations where the alternative

4.

of piling cannot be adopted.

Reinforcement

techniques

introduce discrete inclusions that stiffen and strengthen a soil

4. CONCLUSION: 1.

deposit. The high stiffness and

Unfavorable soil conditions can

strength of the inclusions also

frequently be improved using soil

tend

improvement

imposed on the weaker material

variety

of

techniques. soil

A

reduce

the

stresses

between the inclusions.

improvement

techniques have been developed.

to

5.

Grouting techniques involve the

However a suitable technique has

injection

to

to

materials into the voids of the

necessity of the structure and

soil or into fractures of the soil,

economy.

so that the particle structure of

be

adopt

according

of

cementitious

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 Engineering” L.Kramar.

Earthquake by

Steven

.