SOIL STABILIZATIONAbstract
A stabilized soil has increased strength. It has greater bearing
capacity, and decreased water sensitivity, which diminishes volume
change during wet/dry cycles. Soil stabilization is used to provide
a firm base or sub-base for all types of paved are as, to improve
foundation conditions, and as a lining for ditches and banked earth
work s. The need for stabilization, the uses of stabilization and
the various agents used for stabilization and methods used for
stabilization with the application is discussed. 1.0
INTRODUCTION
Stabilization in a broad sense incorporates the various methods
employed for modifying the properties of a soil toprove its
engineering performance. Stabilization is being used for a variety
of engineering works, the most common application being in the
construction of road and airfield pavements, where the main
objective is to increase the strength or stability of soil and to
reduce the construction cost by making best use of locally
available materials.
1.1 HISTORY
It is discovered from archaeological excavations that during
Indus civilization the art of burning bricks and utensils made of
earth was made perfect to a very high degree. It is rather doubtful
that they purposely stabilised by adding sand and/or lime, although
the chemical analysis of this soil shows that it contains sand and
lime in adequate quantities and of proper proportions.
Burning of bricks and utensils prepared of soil is in itself a
method of stabilisation, which concludes that the art of soil
stabilisation was perfected by Indians from ancient time.
The soil could be stabilised by adding fine powder of coarse
sand, stones and rock.
The floor of burnt brick is called surkhi. The Indians knew that
addition of surkhi to the soil is one of the ways to stabilise it.
Addition of ash of burnt coal is also another method of stabilising
soils.
It seems that a number of methods of soil stabilisation as given
in treatise of Silpa Sastra were developed by Indians and were used
successfully for various purposes, before the advent of Western
knowledge in India. The lime stabilised soil used as plaster to the
wall is the best plaster known as yet.1.2 DEFINITION
Natural soil is both a complex and variable material. Yet
because of its universal availability and its low cost winning it
offers great opportunities for skilful use as an engineering
material. Not uncommonly, however the soil at any particular
locality is unsuited, wholly or partially, to the requirements of
the construction engineer. A basic decision must therefore be made
whether to:Accept the site material as it is and design to
standards sufficient to meet the restrictions imposed by its
existing quality.Remove the site material and replace with a
superior material.Alter the properties of existing soil so as to
create a new site material capable of better meeting the
requirements of the task in hand.The latter choice, the alteration
of soil properties to meet specific engineering requirements is
known as Soil stabilization.Improving an on-site soils engineering
properties is referred to as either soil modification or soil
stabilization.
The term modification implies a minor change in the properties
of a soil, while stabilisation means that the engineering
properties of the soil have been changed enough to allow field
construction to take place.
Soil stabilization aims at improving soil strength and
increasing resistance to softening by water through bonding the
soil particles together, water proofing the particles or
combination of the two. Usually, the technology provides an
alternative provision structural solution to a practical
problem.1.3 NEED OF STABILIZATION:
The load-bearing capacity of the soil helps the engineer to
design the foundation to support the deign loading. It is desirable
from an engineering standpoint to build upon a foundation of ideal
and consistent density. Thus, the goal of soil stabilization is to
provide a solid, stable foundation.
When the available soil is not suitable enough for construction
then the soil can be used by manipulating its composition by adding
suitable stabilizers.
Stabilizing enhances the given property of the soil type.
Increase Tensile and Shear strength.
Reduce shrinkage.
1.4 USES OF STABILIZATION:
It is used for many purposes and it increases different soil
properties as listed below:
It is used to reduce the permeability and compressibility of the
soil, soil mass in earth structures and to increase its shear
strength.
It is used to increase the bearing capacity of foundation
soils.
It is used to improve the natural soils for the construction of
highway and airfields.
It is also used to make an area trafficable within a short
period of time for emergency purposes.
It is used to increase the unit weight of the soil.
It helps to decrease the void ratio of the soil.
It increases the shear strength of the soil.
2.0 SOILS2.1 IDENTIFICATION OF SOILSTABLE 1:TYPES OF SOILS
S.no.Type of SoilCharacteristicSuitability
1GravelSmall pieces of stone varying from the size of a pea to
that of an egg. If you soak what you think is gravel for 24 hours
in a bucket of water, and if it disintegrates, it is not
gravel.alone is of no use for mud wall building - the tiny lumps of
stone have nothing to bind them together.
2SandSimilar small pieces of stone (usually quartz), which are
small than a pea but each grain, are visible to
the eye.similar to gravel, it is of no use for wall making by
itself - but if mixed with clay, i.e. sandy clays or
clayey sands, it is the ideal mud wall building soil.
3Silt The same as sand except that it has been ground so finely
that you cannot see individual grains.by itself is also no good for
building walls. It will hold together but is not strong.
Furthermore, it will
not compact so it is also of no use for pressed blocks or rammed
earthwork.
4ClaySoils that stick when wet - but very hard when completely
dry. Some of these clays shrink when they
dry and expand when wet, but there are also clays, which do not
shrink at all.can be rammed or compressed but in drying out they
often shrink. During the monsoon they get damp
and expand again and crack form.
5Organic SoilSoil mainly composed of rotting, decomposing
organic matters such as leaves, plants add
vegetable matter. It is spongy when wet, usually smells of
decaying matter, is dark in colour and usually
damp.are mainly useless for wall building. A reliable rule is
that if a soil as good for growing plants
in, it is not good for building walls with.
2.2 TESTING OF SOILS
There are two kinds of tests: Field tests
Colour tests
Touch and smell test
Biscuit test
Hand wash test
Cigar test
Adhesion test
Lab tests
Sieve test
Sedimentation test
COLOUR TEST Procedure
Observe the colour of soil.
Interpretation
Deep yellow, orange and red, ranging to deep browns indicate
iron content which is good as building mud.
Greyish or dull brown, ranging to dirty white indicates more
clay.
Dull brown with slightly greenish colour indicates organic
soil.ADHESION TEST Procedure
Make ball out of wet soil.
Pierce a knife into it and remove.
Observer the knife after removing.
Interpretation
If little soil sticks on the knife then it has more silt.
If lot of soil sticks on the knife then it has more clay.
If the knife is clean after removal than the soil has more
sand.
BISCUIT TEST Procedure
Make a smooth paste from the soil removing all gravels.
Mould it into a biscuit of 3cm diameter and 1cm height.
Leave it to dry and observer for shrinkages or cracks.
Break the biscuit to noting how hard it is.
Interpretation
If biscuit cracks or leaves gap from the mould then it contains
more clay.
If its very hard to break then soil contains more clay.
If it breaks easily and can be crumpled between finger then it
has good sand-clay proportion.
If breaks and reduce to powder then the soil has more sand or
silt.
(Fig.1). Biscuit test
TOUCH & SMELL Procedure
Rub small quantity of dry soil on palm to feel its texture.
Moisten the soil and rub again.
Interpretation
Soil that feels course when dry but sticky when wet contains
lumps of clay.
Soil that feels course when dry but gritty when wet contains
sand.
Soil that feels course when dry but little gritty when wet
contains silt.
If the wet soil gives off musty smell then it contains organic
matter. HAND WASH TEST Procedure
Play with wet soil till your hands get thoroughly dirty.
Wash your hands to see how difficult it is to clean.
Interpretation
If hands get cleaned quickly, then soil contains more sand.
If it takes little time to clean and feels like flour then soil
contains more silt.
If it feels soapy or slippery and takes time to clean then soil
contains more clay.
(Fig.2). Touch/Smell/Wash text
CIGAR TEST Procedure
Make a smooth paste from the soil removing all gravels.
Roll it on palm to make a cigar.
Slowly push it outside your palm.
Measure the length at which it breaks.
Interpretation
Length below 5cm - too much sand.
Length above 15cm - too much clay.
Length between 5cm to 15cm - good mixture of sand and clay.
(Fig.3) Cigar Test
SIEVE TEST Procedure
Pass soil from series of standard sieves set on top of on
another with finest sieve at bottom.
Observer the soil collected in each sieve.
Interpretation
Silt will be collected in lowermost sieve.
Gravels will be collected on top.
Sand and lumps of clay will be collected in intermediate
sieves
SEDIMENTATION TEST Procedure
Take a transparent cylindrical bottle or jar of 1Lt.
Capacity.
Fill it with soil and water.
Shake well and allow it to settle for 30 min.
Interpretation
Coarse gravels will be settled at bottom, followed by sand, silt
and clay on top.
Measuring the layers will give us the approximate proportions of
each content.
3.0 METHODS OF SOIL STABILIZATION TECHNIQUES:It must also be
recognized that stabilization not necessarily a magic wand by which
every soil property is changed for the better. Correct usage
demands a clear recognition of which soil properties must be
upgraded, and this specific engineering requirement is an important
element in the decision whether or not to stabilize. Properties of
soil may be altered in many ways, among which are included
chemical, thermal, mechanical and other means.The chief properties
of a soil with which the construction engineer is concerned are:
volume stability, strength, permeability, and durability.Methods of
stabilization may be grouped under two main types:1. Chemical or
additive addition of cement, lime, bituminous or other chemical
agents
2. Mechanical most common form, it is the physical compaction of
the soil
3.1 Chemical Stabilisation
One method of improving the engineering properties of soil is by
adding chemicals or other materials to improve the existing soil.
This technique is cost effective : for example, the cost,
transportation, and processing of a stabilizing agent or additive
such as soil cement or lime to treat an in-place soil material will
probably more economical than importing for the same thickness of
base course.
Additives can be chemical, meaning that the addictive reacts
with or changes the chemical properties of the soil, thereby
upgrading its engineering properties.
Additives can also be mechanical, meaning that upon addition to
the parent soil their own load bearing properties bolster the
engineering characteristics of the parent soil.
Placing the wrong kind or wrong amount of additive or improperly
incorporating the additive into the soil can have devastating
results on the success of the project.In order to properly
implement this technique, an engineer must have:
A clear idea of the desired result
An understanding of the type(s) of soil and their
characteristics on site
An understanding of the use of the additive(s), how they react
with the soil type and other additives and how they interact with
surrounding environment
An understanding of and means of incorporating (mixing) the
additive
An understanding of how the resulting engineered soil will
perform.3.1.1 TYPES OF SOIL STABILIZERSThere are many kinds of
additives available. Not all additives work for all soil types and
a single additive will perform quite differently with different
soil types. Generally, an additive may be used to act as a binder,
alter the effect of moisture, increase the soil density or
neutralise the harmful effects of a substance in the soil.
Following are some of the most widely used additives and their
application:
Portland cement
Lime
Fly ash
Calcium chloride
Bitumen
Chemical or Bio-remediation
Various other indigenous stabilisers include
Straw
Plant Juices
Gum Arabic
Sugar Or Molasses
Cow Dung
Animal Urine
Tannic Acid
OilPortland cement
It is a mechanical additive, used for soil modification which
improves the soil quality or soil stabilization which is to convert
the soil to a solid cement mass
Amount of cement used will dictate whether modification or
stabilization has occurred
Nearly all types of soil can benefit from the strength gained by
cement stabilization
Best results have occurred when used with well-graded fines that
posses enough fines to produce a floating aggregate matrix
Lime
It is a chemical additive, has been used as stabilizing agents
in soils for centuries
It reacts well with medium, moderately fine and fine-grained
clay soils.
In clay soils, main benefit from lime stabilization is reduction
of the soilss plasticity, by reducing the soils water content, it
becomes more rigid
It increases the strength and workability of soil and its
ability to swell
It is very important to achieve proper gradation, by breaking up
the clay into small sized particles, and allow lime to introduce
homogeneously and properly react with the clay
Lime can be applied to dry soil, but in populated or dust blown
areas, lime is mixed with water to form slurry
Curing time is 3 to 7 days to allow lime to react with soil,
surface of soil is wetted periodically
Fly ash
It is a chemical additive, consisting mainly of silicon and
aluminium compounds, is a by-product of the combustion of coal
It can be missed with lime and water to stabilize granular
materials with few lines, producing a hard, cement-like mass.
It acts as a pozzolan and/or filler product to reduce air
voids
Common application is as part of lime-cement-flyash (LCF) to
coarse-grained soils that possess little or no fine grains
As it is essentially a waste product, it is very inexpensive
Calcium chloride
It is a chemical additive, has the ability to absorb moisture
from the air until it liquefies into a solution.
Presence lowers freezing temperature of the moisture present in
the soil, so it is a proven for cold-climate application
If the water in soil cant freeze, there is less soil movement,
and become more stable
Also works as a binder, making the soil easier to compact and
reducing dustBitumen
It is a mechanical additive, that occurs naturally or as a
by-product of petroleum distillation
The black pitch used to make asphalt is bitumen
Asphalt cement, cutback asphalt, tar and asphalt emulsions are
used
Soil type, construction method and weather are all factors in
choosing the type of bitumen to use as additive
Use of bitumen lead to fewer weather-related delays during
construction and makes compaction easier and more consistent
Chemical or Bio remediation
Petroleum hydrocarbons, lead, PCBs, solvents, pesticides and
other hazardous natural and man-made substances which come as the
resultant of industries often contaminated the soil
Even contaminated real estate is valuable pollution is
undesirable efforts are made to return the contaminated soil to an
acceptable condition for human habitation
Goal is to convert hazardous substances into inert ones and to
prevent them from spreading or leaching
Type of additive depends on the contaminant and environment
Chemical additives are often proprietary chemical cocktails, but
the science is well understood and quite effective at neutralizing
hazardous substances
Bio- remediation is typically done by introduction of natural
means like bacteria or insects that eat contaminants and convert
them to natural substances.3.2 MECHANICAL STABILIZATION
Mechanical Soil stabilization refers to either compaction or the
introduction of fibrous and other non-biodegradable reinforcement
to the soil. This practise does not require chemical change of the
soil, although it is common to use both mechanical and chemical
means to achieve specified stabilization.3.2.1 METHODS OF
MECHANICAL STABILIZATION
Compaction Soil Reinforcement
Addition of Graded Aggregate Materials
Mechanical Remediation
CompactionCompaction typically employs a heavy weight to
increase soil density by applying pressure from above. Machines are
often used for this purpose, large soil compactors with vibrating
steel drums efficiently apply pressure to the soil, increasing its
density to meet engineering requirements. Operators of the machines
must be careful not to over-compact the soil, for too much pressure
can result in crushed aggregates that lose their engineering
properties.
Soil ReinforcementSoil problems are sometimes remedied by
utilizing engineered or non-engineered mechanical solutions.
Geo-textiles and engineered plastic mesh are designed to trap soils
and help control erosion, moisture conditions and soil
permeability. Larger aggregates, such as gravel, stones and
boulders are often employed where additional mass and rigidity can
prevent unwanted soil migration or improve load-bearing
properties.
Addition of Graded Aggregate Materials
A common method of improving the engineering characteristics of
a soil is to add certain aggregates that lend desirable attributes
to the soil, such as increased strength or decreased plasticity.
This method provides material economy, improves support
capabilities of the sub grade, and furnishes a working platform for
the remaining structure.
Mechanical RemediationTraditionally, mechanical remediation has
been the acceped practice for dealing with soil contamination. This
is a technique where contaminated soil is physically removed and
relocated to a designated hazardous waste facility far from centres
of human population. In recent times, however, chemical and bio
remediation have proven to be a better solution, both economically
and environmentally. It is often cheaper to solve tha problem where
it exists rather than relocate the problem somewhere else and
possibly need to deal with it again in the future.4.0 STABILISATION
PROCESSBoth new construction and rehabilitation projects are
candidates for soil stabilization. While the precise stabilization
procedures will vary depending on many factors including location,
environment, time requirements, budget, available machinery and
weather the following process is generally practiced:
Assessment and testing
Site preparation
Introduce additives
Mixing
Compacting and shaping / trimming
Curing
Assessment and Testing
The soils of the site are thoroughly tested to determine the
existing conditions. Based on analysis of existing conditions,
additives are selected and specified. Generally, a target chemical
percentage by weight and a design mix depth are defined for the
sub-base contractor. The selected additives are ssebsequently mixed
with soil samples and allowed to cure. The cured sample is then
tested to ensure that the additives will produce the desired
results.Site Preparation
The existing materials on site, including existing pavement if
it is being reclaimed, is pulverised utilizing a rotary mixer. Any
additional aggregates or base materials are introduced at this
time. The material is brought to the optimal moisture content by
drying overly wet soil or adding water to overly dry soil. The
grade is shaped if necessary to obtain the specified material
depth.Introduce Additives
Cement, lime or fly ash can be applied dry or wet. When applied
dry, it is typically spread at a required amount per square yard
(meter) or station utilizing a cyclone spreader or other device.
When lime is applied as slurry, it is either spread with a tanker
truck or through the rotary mixers on-board water spray system.
Calcium chloride is usually applied by a tanker truck equipped with
a spray bar.
Bituminous additives are typically added utilizing an on-boar
emulsion spray system on a rotary mixer. It can also be sprayed on
the surface, but this method requires several applications and
additional mixing.Mixing To fully incorporate the additives with
the soil, a rotary mixer makes several mixing passes until the
materials are homogenous and well-graded. It is crucial that the
rotary mixer maintains optimal mixing depth, as mixing too shallow
or too deep will create undesireable proportions of soil and
additive. Inappropriate proportions of soil and additive will
decrease the load-bearing properties of the cured layer. Some
projects require multiple layers of treated and compacted soil.
When applying cement and fly ash, it is important to finish mixing
as soon as possible due to the quick-setting characteristics of the
additives.
Compaction and Shaping/TrimmingCompaction usually follows
immediately after mixing, especially when the additive is cement or
fly ash. Some bituminous additives require a delay between mixing
and compaction to allow for certain chemical changes to occur.
Compaction is accomplished through several passes using
different machines. Initial compaction is begun utilizing a
vibratory pad foot compactor. The surface is then shaped and
trimmed to remove pad marks and provide a more suitable profile.
Intermediate compaction follows utilizing a pneumatic compactor,
which provides a certain kneading action that further increases
soil density. A tandem drum roller is used on the finishing pass to
provide a smooth surface. A final shaping gives the material a
smooth finish and a proper crown and grade.
Curing
Sufficient curing will allow the additive to fully achieve its
engineering potential. For cement, lime and fly ash stabilization,
weather and moisture are critical factors, as the curing can have a
direct bearing on the strength of the stabilised base.
Bituminous-stabilized bases often require a final membrane of
medium-curing cutback asphalt or slow-curing emulsified asphalt as
a moisture seal. Generally, a minimum of seven days are required to
ensure proper curing. During the curing period, samples taken from
the stabilized base will reveal when the moisture content is
appropriate for surfacing.
5.0 Factors Affecting the Strength of Stabilized Soil Presence
of organic matters, sulphates, sulphides and carbon dioxide in the
stabilized soils may contribute to undesirable strength of
stabilized materials (Netterberg and Paige-Green, 1984, Sherwood,
1993).
5.3.1 Organic Matter In many cases, the top layers of most soil
constitute large amount of organic matters. However, in well
drained soils organic matter may extend to a depth of 1.5 m
(Sherwood, 1993). Soil organic matters react with hydration product
e.g. calcium hydroxide (Ca(OH)2) resulting into low pH value. The
resulting low pH value may retard the hydration process and affect
the hardening of stabilized soils making it difficult or impossible
to compact.
5.3.2 Sulphates The use of calcium-based stabilizer in
sulphate-rich soils causes the stabilized sulphate rich soil in the
presence of excess moisture to react and form calcium
sulphoaluminate (ettringite) and or thamausite, the product which
occupy a greater volume than the combined volume of reactants.
However, excess water to one initially present during the time of
mixing may be required to dissolve sulphate in order to allow the
reaction to proceed (Little and Nair, 2009; Sherwood, 1993).
5.3.3 Sulphides In many of waste materials and industrial
by-product, sulphides in form of iron pyrites (FeS2) may be
present. Oxidation of FeS2 will produce sulphuric acid, which in
the presence of calcium carbonate, may react to form gypsum
(hydrated calcium sulphate) according to the reactions (i) and (ii)
below
i. 2FeS2 + 2H2O +7O2= 2FeSO4 + 2H2SO4
ii. CaCO3 + H2SO4 + H2O = CaSO4.2 H2O + CO2
The hydrated sulphate so formed, and in the presence of excess
water may attack the stabilized material in a similar way as
sulphate (Sherwood, 1993). Even so, gypsum can also be found in
natural soil (Little and Nair, 2009).
5.3.4 Compaction In practice, the effect of addition of binder
to the density of soil is of significant importance. Stabilized
mixture has lower maximum dry density than that of unstabilized
soil for a given degree of compaction. The optimum moisture content
increases with increasing binders (Sherwood, 1993). In cement
stabilized soils, hydration process takes place immediately after
cement comes into contact with water. This process involves
hardening of soil mix which means that it is necessary to compact
the soil mix as soon as possible. Any delay in compaction may
result in hardening of stabilized soil mass and therefore extra
compaction effort may be required to bring the same effect. That
may lead to serious bond breakage and hence loss of strength.
Stabilized clay soils are more likely to be affected than other
soils (Figure 1) due to alteration of plasticity properties of
clays (Sherwood, 1993). In contrary to cement, delay in compaction
for lime-stabilized soils may have some advantages. Lime stabilized
soil require mellowing period to allow lime to diffuse through the
soil thus producing maximum effects on plasticity. After this
period, lime stabilized soil may be remixed and given its final
compaction resulting into remarkable strength than otherwise
(Sherwood, 1993).
5.3.5 Moisture Content In stabilized soils, enough moisture
content is essential not only for hydration process to proceed but
also for efficient compaction. Fully hydrated cement takes up about
20% of its own weight of water from the surrounding (Sherwood,
1993); on other hand, Quicklime (CaO) takes up about 32% of its own
weight of water from the surrounding (Roger et al, 1993; Sherwood,
1993). Insufficient moisture content will cause binders to compete
with soils in order to gain these amounts of moisture. For soils
with great soil-water affinity (such as clay, peat and organic
soils), the hydration process may be retarded due to insufficient
moisture content, which will ultimately affect the final
strength.
5.3.6 Temperature Pozzolanic reaction is sensitive to changes in
temperature. In the field, temperature varies continuously
throughout the day. Pozzolanic reactions between binders and soil
particles will slow down at low temperature and result into lower
strength of the stabilized mass. In cold regions, it may be
advisable to stabilize the soil during the warm season (Sherwood,
1993; Maher et al, 1994).
5.3.7 Freeze-Thaw and Dry-Wet Effect Stabilized soils cannot
withstand freeze-thaw cycles. Therefore, in the field, it may be
necessary to protect the stabilized soils against frost damage
(Maher et al, 2003; Al-tabbaa and Evans, 1998). Shrinkage forces in
stabilized soil will depend on the chemical reactions of the
binder. Cement stabilized soil are susceptible to frequent dry-wet
cycles due to diurnal changes in temperature which may give rise to
stresses within a stabilized soil and, therefore, should be
protected from such effects (Sherwood, 1993; Maher et al, 2003).6.0
APPLICATIONSFoundationsThere are three basic soil conditions that
pose particularity serious problems for architects, engineers and
building contractors. First is the swelling and shrinkage movements
of expansive clays; secondly, the occurrences of settlement or
densification from load bearing forces; and, thirdly, the influence
of moisture on the soil and building structure. Individually any
one of these soil behaviors would create tremendous economic damage
to a building structure.
The chemical stabilization process addresses these three basic
soil concerns in several meaningful ways including: reduction of
shrink/swell potential and plasticity on expansive clays, increased
load bearing support as measured by unconfined compressive
strength, and reduction of the treated soils permeability, making
it less susceptible to water infiltration.
New Pavements
Pavements, especially flexible pavements, are constantly under
changing conditions, thus they are inherently unstable. Water
infiltration weakens the underlying soil condition and variable
loading moves those conditions throughout the pavement structure.
Asphaltic concrete pavements are constantly under the debilitating
effects of oxidation and the actions of water stripping the
asphaltic binder from the aggregate structure.
The use of chemical stabilization in roadway design speaks
directly to these issues of long-term life-cycle stability.
Pavement Rehab
There is a solution to deteriorated pavements! A perpetual
pavement foundation can be achieved by the in-place recycling of
the existing pavement materials and stabilizing them with cement.
This process, known as Full Depth Rehabilitation (FDR), provides
significant cost savings, a sustainable allocation of resources,
and the structural enhancement required for long-term performance.
Cement stabilized bases have provided economical and long lasting
pavement foundations for more than 70 years.Bio-soils pads
The processing and removal of biosolids from waste management
and composting facilities has traditionally been a major
operational concern. Most large processing centers require removal
of compost or sludge by large heavy equipment. This heavy loading
creates a high fatigue factor on the native subgrade soils, causing
the processing table to become weak and unstable under repetitive
loading. This issue, along with the added concern of bio-solids
commingling with the underlying soft soils during harvesting, lead
to additional processing cost.Depending upon the type of soil,
stabilization can be accomplished with quicklime, lime-pozzolans
blends, and Portland cement. This method of stabilization is
conducted under a controlled environment to provide a consistent
and uniform mat structure. The stabilized mat creates a harden
surface that allows for many years of maintenance access for
compost and sludge processing and removal. Environmental
remediation
Chemical Stabilization/Solidification (S/S) of soils
contaminated with hazardous waste is a tried and proven chemical
remediation technology. Both the technology and its acceptance has
progressed dramatically over a number of years as a simple, cost
effective and flexible treatment method for remediation of soils
and recycling them back to usable land applications.Soil
stabilization/solidification (S/S) is a process that immobilizes
contaminants, mitigating the risk of exposure and potential harm to
human health and the environment. Cement or lime is mixed with
impacted soil and hardens to form a soil-cement matrix that
encapsulates the impacted materials. The process is performed on
site with soil in-place or on adjacent mixing tables.Site
winterization
Construction sites are susceptible to rain delays that cost both
time and money. When winter hits, project managers turn to the only
proven method for site winterization, the process of chemically
treating the surface soils to provide a high-strength and low-
permeable cementitious barrier.This type of treatment ensures
immediate access to construction sites after a storm event, while
eliminating fatigue rutting from repetitive loading. By reducing
the permeability of the native soil, the treatment process reduces
the susceptibility of the subgrade to saturation. An added benefit
of this type of soil modification is the vast improvement of the
subgrade strength characteristics and decreased potential for
shrink/swell fluctuation of any clayey material. Since the
construction process requires heavy loading from construction
equipment, the subgrade soils are required to carry loads far
greater than their design intended.Winterization is the changing of
soil behavior, principally through the reduction of excess
moisture, in order to expedite construction. Winterization is
commonly performed on subgrade and sub-base materials in order to
expedite compaction and subsequent paving.When free water is
encountered, an evaluation should be made to determine if water is
infiltrating from an outside source. If the flow of water is
continuous, dewatering will be required prior to any treatment.
Dewatering should extend to at least 12 inches below the bottom of
the treatment zone to reduce wicking of water. If it is determined
that the water is only perched, areas containing any standing water
should be pumped prior to treatment.Treating before winter rains
hit typically is the most economical choice. Pre-winter treatments
require a lower percentage of stabilizing reagent and a shallower
depth of treatment. This process creates an impervious liner at the
surface grade that prevents winter rains from saturating the
underlying soils.Water resources
The use of chemical stabilization in Water Resource projects has
increased considerably over the last 30 years. Chemical
Stabilization in the form of Soil-Cement or Soil-Lime has been a
main focus of the U.S. Bureau of Reclamation (USBR) in the
construction of dams and other water resource applications.The
first use of Soil-Cement stabilization for slope protection was a
test section constructed by USBR at Bonny reservoir in eastern
Colorado in 1951. Observation of the performance of this test
section for the first 10-year period of service indicated excellent
performance of the stabilized section which was subject to harsh
wave action and repeated cycles of freezing and thawing. This
successful application lead to the conclusion that the use of
chemical stabilization for slope protection, levee and dam cores,
impervious liners, and maintenance accessibility was feasible based
on both economical and service life considerations.
The key factor that accounts for a successful chemical
stabilization project is careful predetermination of engineering
control factors in the laboratory and implementation and
verification of those results during construction.Other
applications like:
Staging and Storing Hardscape
When the need for a native hardscape surface is required for
expanded storage needs or temporary event staging, the process of
soil stabilization address the performance needs of heavy loading,
extended durability, dust control, and storm water runoff. By
stabilizing the existing soils, a temporary or permanent mat
structure is developed that can handle repetitive heavy loading,
while maintaining a durable low dust exposed wearing
surface.Synthetic Sports Field
The new synthetic sports fields are popular for their high
durability and low maintenance features. One of the major
installation needs of synthetic fields is an appropriate drainage
system. These turf systems allow water to permeate through the
surface to be collected by either a blanket or manifold drainage
system. A stabilized mat structure under a blanket drain system or
integrated into a manifold system allows for an impervious,
high-strength soil structure that is maintained even when saturated
over time.Equestrian Facilities
Equestrian facilities, used for a wide variety of horse-
training functions, require a stable non-yielding substructure
under the loose cutting surface materials. By stabilizing the
underlying soils, a great reduction in maintenance is achieved,
since the underlying soil can no longer contaminate the loose
wearing surface materials. The stabilized section also allows for
all weather use, because the stabilized section is impervious to
water and will not loose strength or grade over repetitive
use.Hard-court Stability
Tennis, basketball, and other hardcourt surfaces require a high
degree of stability, since any grade variations that may develop
would be magnified under these applications. If courts are built on
clay soils, grades will move as underlying soils shrink and swell
with moisture fluctuation. All soils types would benefit from the
long-term benefits of an underlying stabilized section, including
reduction of water infiltration, reflective shrinkage cracking, and
un-controlled grade fluctuation.Extreme Conditions
When extreme conditions are encountered, it's important to
prevent costly overruns from consuming contingent dollars at the
onset of project construction. Extreme conditions may be the result
of intrusion into the site water table, encountering of bay mud or
dredging materials, or a condition that requires added structural
support from the existing unsuitable soils. HSI has experienced the
most extreme site conditions and presents constructability plans
that resolve these issues in the most cost-effective manner.7.0
LATEST TRENDSRapid urban and industrial growth demands more land
for further development. In order to meet this demand land
reclamation and utilization of unsuitable and environmentally
affected lands have been taken up. These, hitherto useless lands
for construction have been converted to be useful ones by adopting
one or more ground improvement techniques. The field of ground
improvement techniques has been recognized as an important and
rapidly expanding one.7.1 Vibro-compaction
Vibro-compaction, sometimes referred to as Vibrofloation, is the
rearrangement of soil particles into a denser configuration by the
use of powerful depth vibration. Vibrocompaction is a ground
improvement process for densifying loose sands to create stable
foundation soils. The principle behind vibrocompaction is simple.
The combined action of vibration and water saturation by jetting
rearranges loose sand grains into a more compact state.
Vibrocompaction is performed with specially-designed vibrating
probes. Both horizontal and vertical modes of vibration have been
used in the past. The vibrators used consist of torpedo-shaped
probes 12 to 16 inches in diameter which vibrates at frequencies
typically in the range of 30 to 50 Hz. The probe is first inserted
into the ground by both jetting and vibration. After the probe
reaches the required depth of compaction, granular material,
usually sand, is added from the ground surface to fill the void
space created by the vibrator. A compacted radial zone of granular
material is created.
(Fig.4) Vibro-compaction
APPLICATIONS: Reduction of foundation settlements.
Reduction of risk of liquefaction due to seismic activity.
Permit construction on granular fills.
7.2 Vaccum consolidation
Vacuum Consolidation is an effective means for improvement of
saturated soft soils. The soil site is covered with an airtight
membrane and vacuum is created underneath it by using dual venture
and vacuum pump. The technology can provide an equivalent
pre-loading of about 4.5m high conventional surcharge fill.
Vacuum-assisted consolidation preloads the soil by reducing the
pore pressure while maintaining a constant total stress.
(Fig.4) Vaccum consolidation
APPLICATIONS: Replace standard pre-loading techniques
eliminating the risk of failure.
Combine with a water pre-loading in scare fill area. The method
is used to build large developments on thick compressible soil.
Combine with embankment pre-load using the increased
stability
7.3Preloading
Preloading has been used for many years without change in the
method or application to improve soil properties. Preloading or
pre-compression is the process of placing additional vertical
stress on a compressible soil to remove pore water over time. The
pore water dissipation reduces the total volume causing
settlement.Surchargingis an economical method for ground
improvement. However, the consolidation of the soils is time
dependent, delaying construction projects making it a non-feasible
alternative.
The soils treated are Organic silt, Varved silts and clays, soft
clay, Dredged material The design considerations which should be
made are bearing capacity, Slope stability, Degree of
consolidation.(Fig.5) PreloadingAPPLICATIONS: Reduce
post-construction
Settlement
Reduce secondary compression.
Densification
Improve bearing capacity
7.4 Heating
Heating or vitrifaction breaks the soil particle down to form a
crystalline or glass product. It uses electrical current to heat
the soil and modify the physical characteristics of the soil.
Heating soils permanently alters the properties of the soil.
Depending on the soil, temperatures can range between 300 and 1000
degree Celsius. The impact on adjacent structures and utilities
should be considered when heating is used.
(Fig.6)Heating
APPLICATIONS: Immobilization of radioactive or contaminated
soil
Densification and stabilization
7.5 Ground freezing
Ground freezing is the use of refrigeration to convert in-situ
pore water to ice. The ice then acts as a cement or glue, bonding
together adjacent particles of soil or blocks of rock to increase
their combined strength and make them impervious.The ground
freezing considerations areThermal analysis,Refrigeration system
geometry, Thermal properties of soil and rock,freezing rates,
Energy requirements, Coolant/ refrigerant distribution system
analysis.
(Fig.7) Ground freezingGROUNDFREEZING APPLICATIONS: Temporary
underpinning
Temporary support for an excavation
Prevention of groundwater flow into excavated area
Temporary slope stabilization
Temporary containment of toxic/hazardous waste contamination
7.6 Vibro-replacement stone columns
Vibro-Replacement extends the range of soils that can be
improved by vibratory techniques to include cohesive soils.
Reinforcement of the soil with compacted granular columns or stone
columns is accomplished by the top-feed method. The important
Vibro-replacement stone columns are Ground conditions, Relative
density, Degree of saturation, Permeation.(Fig.8)
Vibro-replacement
PRINCIPLES OF VIBRO-REPLACEMENT:The stone columns and
intervening soil form and integrated foundation support system
having low compressibility and improved load bearing capacity. In
cohesive soils, excess pore water pressure is readily dissipated by
the stone columns and for this reason, reduced settlements occur at
a faster rate than is normally the case with cohesive soils.
There are different types of installation methodswhich can be
broadly classified in the following manner:
Wet top feed method
Dry bottom feed method
Offshore bottom feed method
VIBRO-REPLACEMENT APPLICATIONS: Reduction offoundation
settlement Improve bearing capacity/reduce footing size
requirements
Reduction of the risk of liquefaction due to seismic
activity
Slope stabilization
Permit construction on fills
Permit shallow footing construction
7.7 Micro pilesMicro-piles are small diameter piles (up to 300
mm), with the capability of sustaining high loads (compressive
loads of over 5000 KN).The drilling equipment and methods allows
micro piles to be drilled through virtually every ground
conditions, natural and artificial, with minimal vibration,
disturbances and noise, at any angle below horizontal. The
equipment can be further adapted to operate in locations with low
headroom and severely restricted access.
(Fig.9) Micro piles
(Fig.10) Enlarged section of Micro piles
APPLICATIONS: For Structural Support and stability
Foundation for new structures
Repair / Replacement of existing foundations
Arresting / Prevention of movement
Embankment, slope and landslide stabilization
Soil strengthening and protection7.8 Grouting
routing is the injection of pumpable materials into a soil or
rock formation to change the physical characteristics of the
formation.Grouting selection considerations are Site specific
requirement, Soil type, Soil groutability, Porosity.Grouting can be
prevented byCollapse of granular soils, Settlement under adjacent
foundations, Utilities damage, Day lighting.Grouting can
provideIncreased soil strength and rigidity, reduced ground
movement, Predictable degree of improvement.DESIGN STEPS: Identify
underground construction problem.
Establish objectives of grouting program.
Perform special geotechnical study.
Develop initial grouting program.
Develop performance prediction.
Compare with other solutions.
Refine design and prepare specifications.
GROUTING TECHNIQUES:The various injection grouting techniques
used by grouting contractors for ground improvement / ground
modification can be summarized as follows:
Permeation
Compaction Grouting:
Claquage
Jet GroutingJET GROUTING:Jet grouting is a general term used by
grouting contractors to describe various construction techniques
used for ground modification or ground improvement. Grouting
contractors use ultra high-pressure fluids or binders that are
injected into the soils at high velocities. These binders break up
the soil structure completely and mix the soil particles in-situ to
create a homogeneous mass, which in turn solidifies. This ground
modification / ground improvement of the soil plays an important
role in the fields of foundation stability, particularly in the
treatment of load bearing soils under new and existing buildings;
in the in-depth impermeabilization of water bearing soils; in
tunnel construction; and to mitigate the movement of impacted soils
and groundwater.
(Fig.11) Jet Grouting
(Fig.12) Jet Grouting stages
7.9 MECHANICALLY STABILIZED EARTH STRUCTURES:(Fig.13) Section of
Mechanical stabilizationA segmental, precast facing mechanically
stabilized earth wall employs metallic (strip or bar mat) or
geosynthetic (geogrid or geotextile) reinforcement that is
connected to a precast concrete or prefabricated metal facing panel
to create a reinforced soil mass.
PRINCIPLES: The reinforcement is placed in horizontal layers
between successive layers of granular soil backfill. Each layer of
backfill consists of one or more compacted lifts.
A free draining, non plastic backfill soil is required to ensure
adequate performance of the wall system.
For walls reinforced with metallic strips, load is transferred
from the backfill soil to the strip reinforcement by shear along
the interface.
For walls with ribbed strips, bar mats, or grid reinforcement,
load is similarly transferred but an additional component of
strength is obtained through the passive resistance on the
transverse members of the reinforcement.
Facing panels are typically square, rectangular, hexagonal or
cruciform in shape and are up to 4.5m ^2 in area.
MSEW- Mechanically Stabilized Earth Walls, when the face batter
is generally steeper than 70 degrees.
RSS- Reinforced Soil Slopes, when the face batter is
shallower.
APPLICATIONS: RSS structures are cost effective alternatives for
new construction where the cost of embankment fill, right-of-way,
and other consideration may make a steeper slope desirable.
Another use of reinforcement in engineered slopes is to improve
compaction at the edges of a slope to decrease the tendency for
surface sloughing.
DESIGN:Current practice consists of determining the geometric
reinforcement to prevent internal and external failure using limit
equilibrium of analysis.
7.10 SOIL NAILING:The fundamental concept of soil nailing
consists of reinforcing the ground by passive inclusions, closely
spaced, to create in-situ soil and restrain its displacements. The
basic design consists of transferring the resisting tensile forces
generated in the inclusions into the ground through the friction
mobilized at the interfaces.(Fig.14) Soil Nailing
APPLICATIONS: Stabilization of railroad and highway cut
slopes
Excavation retaining structures in urban areas for high-rise
building and underground facilities
Tunnel portals in steep and unstable stratified slopes
Construction and retrofitting of bridge abutments with complex
boundaries involving wall support under piled foundations
8.0 SUMMARY AND CONCLUSIONCurrent knowledge permits the
conclusion that soil stabilizing agents, including cement, asphalt,
lime and other chemicals as well, can serve many useful purposes.
Similarly, the various mechanical methods of soil stabilization
goes a long way and in a more permanent situation towards ground
improvement. In general, these purposes are to: increase soil
strength or bearing capacity, minimize soil compressibility and/or
the flow of migration of subsurface moisture, prevents erosion from
surface water, provide a stable working platform for construction,
aid in the mechanical compaction of soils and reduce the expansive
property of soil.