SOIL STABILIZATION METHODS AND MATERIALS

TABLE OF CONTENTS

PREFACE

INTRODUCTION

COMPONENTS OF STABILIZATION

Soils

Stabilizing Agents

Cement

Lime

Fly–Ash

Blast Furnace Slags

Pozzolanas

Factors Affecting the Strength of Stabilized Soil

Organic Matter

Sulphates

Sulphides

Compaction

Moisture Content

Temperature

Freeze-Thaw and Dry-Wet Effects

STABILIZATION METHODS

In–Situ Stabilization

Deep Mixing Method

Quality Control and Quality Assurance

Applications

Mass Stabilization

Ex-Situ Stabilization

REFERENCES

INTRODUCTION

Site feasibility study for geotechnical projects is of far most beneficial before a project

can take off.

Site survey usually takes place before the design process begins in order to understand the characteristics of subsoil upon which the decision on location of the project can be made.

The following geotechnical design criteria have to be considered during site selection.

Design load and function of the structure.

Type of foundation to be used.

Bearing capacity of subsoil.

In the past, the third bullet played a major in decision making on site selection.

Once the bearing capacity of the soil was poor, the following were options:

Change the design to suit site condition.

Remove and replace the in situ soil.

Abandon the site.

Abandoned sites due to undesirable soil bearing capacities dramatically increased, and the outcome of this was the scarcity of land and increased demand for natural resources.

Affected areas include those which were susceptible to liquefaction and those covered

with soft clay and organic soils.

Other areas were those in a landslide and contaminated

land.

However, in most geotechnical projects, it is not possible to obtain a construction site that will meet the design requirements without ground modification.

The current practice is to modify the engineering properties of the native problematic soils to meet the

design specifications.

Nowadays, soils such as, soft clays and organic soils can be

improved to the civil engineering requirements.

This state of the art review focuses on soil stabilization method which is one of the several methods of soil improvement.

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.

The simplest stabilization

processes are compaction and drainage (if water drains out of wet soil it becomes stronger).

The other process is by improving gradation of particle size and further improvement can be achieved by adding binders to the weak soils.

Soil stabilization can be accomplished by several methods.

All these methods fall into

two broad categories namely;

mechanical stabilization

Under this category, soil stabilization can be achieved through physical process

by altering the physical nature of native soil particles by either induced vibration or compaction or by incorporating other physical properties such as barriers and nailing.

Mechanical stabilization is not the main subject of this review and will not be further discussed.

chemical stabilization

Under this category, soil stabilization depends mainly on chemical reactions between

stabilizer (cementitious material) and soil minerals (pozzolanic materials) to achieve the desired effect.

A chemical stabilization method is the fundamental of this review and, therefore, throughout the rest of this report, the term soil stabilization will mean

chemical stabilization.

Through soil stabilization, unbound materials can be stabilized with cementitious

materials (cement, lime, fly ash, bitumen or combination of these).

The stabilized soil materials have a higher strength, lower permeability and lower compressibility than the native soil.

The method can be achieved in two ways, namely;

(1) in situ stabilization

(2) ex-situ stabilization.

Note that, stabilization not necessary a magic wand by which every soil properties can be improved for better.

The decision to technological usage depends on which soil properties have to be modified.

The chief properties of soil which are of interest to engineers are volume stability, strength, compressibility, permeability and durability.

For a successful stabilization, a

laboratory tests followed by field tests may be required in order to determine the engineering and environmental properties.

Laboratory tests although may produce higher strength than corresponding material from the field, but will help to assess the effectiveness of stabilized materials in the field.

Results from the laboratory tests, will enhance the knowledge on the choice of binders and amounts.

COMPONENTS OF STABILIZATION

Soil stabilization involves the use of stabilizing agents (binder materials) in weak soils to improve its geotechnical properties such as compressibility, strength, permeability and durability.

The components of stabilization technology include soils and or soil minerals and stabilizing agent or binders (cementitious materials).

Soils

Most of stabilization has to be undertaken in soft soils (silty, clayey peat or organic soils)

in order to achieve desirable engineering properties.

According to Sherwood fine grained granular materials are the easiest to stabilize due to their large surface area in

relation to their particle diameter.

A clay soil compared to others has a large surface area

due to flat and elongated particle shapes.

On the other hand, silty materials can be sensitive to small change in moisture and, therefore, may prove difficult during stabilization. Peat soils and organic soils are rich in water content of up to about 2000%, high porosity and high organic content.

The consistency of peat soil

can vary from muddy to fibrous, and in most cases, the deposit is shallow, but in worst cases, it can extend to several meters below the surface.

Organic soils have high exchange capacity;

it can hinder the hydration process by retaining the calcium ions liberated during the

hydration of calcium silicate and calcium aluminate in the cement to satisfy the exchange

capacity.

In such soils, successful stabilization has to depend on the proper selection of binder and amount of binder added.

Stabilizing Agents

These are hydraulic (primary binders) or non-hydraulic (secondary binders) materials that when in contact with water or in the presence of pozzolanic minerals reacts with water to

form cementitious composite materials.

The commonly used binders are:

cement

lime

fly ash

blast furnace slag

Cement

Cement is the oldest binding agent since the invention of soil stabilization technology in

1960’s.

It may be considered as primary stabilizing agent or hydraulic binder because it can be used alone to bring about the stabilizing action required.

Cement reaction is not dependent on soil minerals, and the key role is its reaction with water that may be available in any soil.

This can be the reason why cement is used to stabilize a wide range of soils.

Numerous types of cement are available in the market; these are ordinary Portland cement, blast furnace cement, sulfate resistant cement and high alumina cement.

Usually the choice of cement

depends on type of soil to be treated and desired final strength.

Hydration process is a process under which cement reaction takes place.

The process starts when cement is mixed with water and other components for a desired application resulting into hardening phenomena.

The hardening (setting) of cement will enclose soil as glue, but it will not change the structure of soil.

The hydration reaction is slow proceeding from the surface of the cement grains and the centre of the grains may remain unhydrated.

Cement hydration is a complex process with a complex series of unknown chemical reactions.

However, this process can be affected by

presence of foreign matters or impurities

water-cement ratio

curing temperature

presence of additives

specific surface of the mixture.

Depending on factor(s) involved, the ultimate effect on setting and gain in strength of

cement stabilized soil may vary.

Therefore, this should be taken into account during mix

design in order to achieve the desired strength.

Calcium silicates, C3S and C2S are the two main cementitious properties of ordinary Portland cement responsible for strength

development.

Calcium hydroxide is another hydration product of Portland cement that further reacts with pozzolanic materials available in stabilized soil to produce further cementitious material.

Normally the amount of cement used is small but sufficient to improve the engineering properties of the soil and further improved cation exchange of clay.

Cement stabilized soils have the following improved properties:

• decreased cohesiveness (Plasticity)

• decreased volume expansion or compressibility

• increased strength.

Lime

Lime provides an economical way of soil stabilization.

Lime modification describes an

increase in strength brought by cation exchange capacity rather than cementing effect brought by pozzolanic reaction.

In soil modification, as clay particles flocculates, transforms natural plate like clays particles into needle like interlocking

metalline structures.

Clay soils turn drier and less susceptible to water content changes.

Lime stabilization may refer to pozzolanic reaction in which

pozzolana materials reacts with lime in presence of water to produce cementitious compounds.

The effect can be brought by either quicklime, CaO or hydrated lime, Ca (OH)2. Slurry lime also can be used in dry soils conditions where water may be required to achieve effective compaction (Hicks, 2002).

Quicklime is the most commonly used lime; the followings are the advantages of quicklime over hydrated lime.

- higher available free lime content per unit mass

- denser than hydrated lime (less storage space is required) and less dust

- generates heat which accelerate strength gain and large reduction in moisture

content according to the reaction equation below

o ( ) (65 / ) CaO + H2O → Ca OH 2 + Heat kJ mol .

Quicklime when mixed with wet soils, immediately takes up to 32% of its own weight

of water from the surrounding soil to form hydrated lime; the generated heat accompanied by this reaction will further cause loss of water due to evaporation which in turn results into increased plastic limit of soil i.e. drying out and absorption.

The effect can be explained from Figure 1 for soil at a moisture content of 35% and plastic limit 25%. Addition of 2% lime will change the plastic limit to 40% so that the moisture content of the soil will be 5% below plastic limit instead of 10% above plastic limit. investigated the decrease in plasticity as brought about in first instance by cation exchange in which

cations of sodium and hydrogen are replaced by calcium ions for which the clay mineral

has a greater water affinity.

Even in soils (e.g. calcareous soils) where, clay may be

saturated with calcium ions, addition of lime will increase pH and hence increase the

exchange capacity.

Like cement, lime when reacts with wet clay minerals result into increased pH which favors solubility of siliceous and aluminous compounds.

These compounds react with calcium to form calcium silica and calcium alumina hydrates, a cementitious product similar to those of cement paste.

Natural pozzolanas materials

containing silica and alumina (e.g. clay minerals, pulverized fly ash, PFA, blast furnace

slag) have great potential to react with lime.

Lime stabilizations technology is mostly widely used in geotechnical and environmental applications.

Some of applications include encapsulation of contaminants,

rendering of backfill (e.g. wet cohesive soil), highway capping, slope stabilization and foundation improvement such as in use of lime pile or lime-stabilized soil columns.

However, presence of sulphur and organic materials may

inhibit the lime stabilization process.

Sulphate (e.g. gypsum) will react with lime and

swell, which may have effect on soil strength.

Fly–Ash

Fly ash is a byproduct of coal fired electric power generation facilities; it has little

cementitious properties compared to lime and cement.

Most of the fly ashes belong to

secondary binders; these binders cannot produce the desired effect on their own.

However, in the presence of a small amount of activator, it can react chemically to form

cementitious compound that contributes to improved strength of soft soil.

Fly ashes are readily available, cheaper and environmental friendly.

There are two main classes of fly ashes; class C and class F.

Class C fly ashes are

produced from burning subbituminous coal; it has high cementing properties because of high content of free CaO.

Class C from lignite has the highest CaO (above 30%) resulting in self-cementing characteristics.

Class F fly ashes are produced by burning anthracite and bituminous coal; it has low self-cementing properties due to limited

Figure 1: Effect of the addition of the lime on plasticity properties of London clay (Sherwood, 1993)

amount of free CaO available for flocculation of clay minerals and thus require addition

of activators such as lime or cement. The reduction of swell potential achieved in fly

ashes treated soil relates to mechanical bonding rather than ionic exchange with clay

minerals (Mackiewicz and Ferguson, 2005). However, soil fly ash stabilization has the

following limitations (White, 2005):

Soil to be stabilized shall have less moisture content; therefore, dewatering may be

required.

- Soil-fly ash mixture cured below zero and then soaked in water are highly

susceptible to slaking and strength loss

- Sulfur contents can form expansive minerals in soil-fly ash mixture, which reduces

the long term strength and durability.

2.2.4 Blast Furnace Slags

These are the by-product in pig iron production. The chemical compositions are similar to

that of cement. It is however, not cementitious compound by itself, but it possesses latent

hydraulic properties which upon addition of lime or alkaline material the hydraulic

properties can develop (Sherwood, 1993; Åhnberg et al, 2003). Depending on cooling

system, Sherwood (1993) itemized slag in three forms, namely:

- Air-cooled slag

Hot slag after leaving the blast furnace may be slowly cooled in open air,

resulting into crystallized slag which can be crushed and used as aggregate

Granulated (merit 5000) or Pelletised slag

Quenching (i.e. sudden cooling with water or air) of hot slag may result into

formation of vitrified slag. The granulated blast furnace slag or Merit 5000

(commonly known in Sweden) is a result of use of water during quenching

process, while, the use of air in the process of quenching may result into

formation of pelletised slag.

- Expanded slag

Under certain conditions, steam produced during cooling of hot slag may give

rise to expanded slag.

2.2.5 Pozzolanas

Pozzolanas are siliceous and aluminous materials, which in itself possess little or no

cementitious value, but will, in finely divided form and in the presence of moisture,

chemically react with calcium hydroxide at ordinary temperature to form compounds

possessing cementitious properties (ASTM 595). Clay minerals such as kaolinite,

montmorillonite, mica and illite are pozzolanic in nature. Artificial pozzolanas such as

ashes are products obtained by heat treatment of natural materials containing pozzolanas

such as clays, shales and certain silicious rocks. Plants when burnt, silica taken from soils

as nutrients remains behind in the ashes contributing to pozzolanic element. Rice husk

ash and rice straw and bagasse are rich in silica and make an excellent pozzolana

(Sherwood, 1993).

2.3 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 PaigeGreen, 1984, Sherwood, 1993).

2.3.1 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.

2.3.2 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).

2.3.3 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).

2.3.4 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).

Figure 3: Dry density versus time elapsed since the end of mixing of two material stabilized with 10%

cement (Sherwood, 1993)

2.3.5 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 soilwater 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.

2.3.6 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).

2.3.7 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; Altabbaa 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).

STABILIZATION METHODS

3.1 In–Situ Stabilization

The method involves on site soil improvement by applying stabilizing agent without

removing the bulk soil. This technology offer benefit of improving soils for deep

foundations, shallow foundations and contaminated sites. Planning of the design mix

involves the selection and assessment of engineering properties of stabilized soil and

improved ground. The purpose is to determine the dimensions of improved ground on the

basis of appropriate stability and settlement analyses to satisfy the functional

requirements of the supported structure (Keller Inc.). The technology can be

accomplished by injection into soils a cementitious material such cement and lime in dry

or wet forms. The choice to either use dry or wet deep mixing methods depend among

other things; the in-situ soil conditions, in situ moisture contents, effectiveness of binders

to be used, and the nature of construction to be founded. Depending on the depth of

treatment, the in situ stabilization may be regarded as either deep mixing method or mass

stabilization.

3.1.1 Deep Mixing Method

The deep mixing method involves the stabilization of soils at large depth. It is an in situ

ground modification technology in which a wet or dry binder is injected into the ground

and blended with in situ soft soils (clay, peat or organic soils) by mechanical or rotary

mixing tool (Porbaha et al, 2005; EuroSoilStab, 2002). Depending on applications, the

following patterns may be produced (Figure 4); single patterns, block patterns, panel

pattern or stabilized grid pattern (EuroSoilStab, 2002). Note that, the aim is to produce

the stabilized soil mass which may interact with natural soil and not, to produce too

stiffly stabilized soil mass like a rigid pile which may independently carry out the design

load. The increased strength and stiffness of stabilized soil should not, therefore, prevent

an effective interaction and load distribution between the stabilized soil and natural soil

(EuroSoilStab, 2002). Thus the design load should be distributed and carried out partly

by natural soil and partly by stabilized soil mass (column).

Wet Mixing

Applications of wet deep mixing involve binder turned into slurry form, which is then

injected into the soil through the nozzles located at the end of the soil auger (Massarsch

and Topolnicki, 2005). The mixing tool comprise of drilling rod, transverse beams and a

drill end with head. There are some modifications to suit the need and applications. For

instance, the Trench cutting Re-mixing deep method (TRD) developed by circa Japan, in

1993 provides an effective tool for construction of continuous cutoff wall without the

need for open trench. The method uses a crawler-mounted, chainsaw-like mixing tool to

blend in-situ soil with cementitious binder to create the soil-cement wall. It further

consists of a fixed post on which cutting, scratching teeth ride on a rotating chain and

injection ports deliver grout into treatment zone. Wall depths up to 45 m having width

between 0.5 m and 0.9 m are achievable. The wall quality for groundwater barrier is high

with permeability between 1 x 10-6 and 1 x 10-8 cm/s (www.HaywardBaker.com). Similar

to TRD, in 1994, Germany developed the FMI (Misch-Injektionsverfahren) machine. The

FMI machine has a special cutting arm (trencher), along which cutting blades are rotated

by two chain system. The cutting arm can be inclined up to 80 degrees and is dragged

through the soil behind the power unit (Stocker and Seidel, 2005). Like TRD, the soil is

not excavated, but mixed with binder which is supplied in slurry form through injection

pipes and outlets mounted along the cutting arm (Figure 5).

Dry Mixing

Dry mixing (DM) method is clean, quiet with very low vibration and produces no spoil

for disposal (Hayward Baker Inc). It has for many years extensively used in Northern

Europe and Japan. The method involves the use of dry binders injected into the soil and

thoroughly mixed with moist soil (Figure 13). The soil is premixed using specialized tool

during downward penetration, until it reaches the desired depth. During withdrawal of the

mixing tool, dry binder are then injected and mixed with premixed soil leaving behind a

moist soil mix column. In Scandinavians countries and Sweden in particular, this method

is referred to as Lime Cement Column (LCC), whereas, in Italy, the method is termed as

Trevimix and in Japan, the same technology is called dry jet mixing (DJM) (Bruce et al,

1996; Yasui and Yokozawa, 2005).

A typical DM machine consists of track mounted installation rig and a drill motor. Binder

is fed into compressed air through the hose into mixing shaft to the outlet of mixing shaft

into the ground (Figure 13). Powdery binders under compressed air are injected into soft

ground without processing into slurry form. Blade rotates creating a cavity in the soil in

which air and binders fill in during withdrawal. During construction, the most efficient

sequence is to work the stabilizing machine within its operational radius as much as

possible (EuroSoilStab, 2002). Figure 9 and Figure 10 shows the construction principle

and detail of mixing blade (Yasui and Yokozawa, 2005; EuroSoilStab, 2002). The native

soil is thoroughly mixed with this compressed binder resulting into hardened column

within the ground; the column size up to 1.5 meter diameter may be achieved with a

maximum depth up to 40 m (EuroSoilStab, 2002). In sensitive soils, penetration induced

vibration may cause loss of soil strength; therefore, it may be required to inject some of

the binder into the ground during penetration. A wide range of strength can be obtained

from low strength to high strength through regulation of amount of binder. High

improved ratio can be achieved by overlapping mixing or interlocking the column. This

method has wide application from embankment stabilization, slope protection to

foundation improvement and liquefaction mitigation (Yasui and Yokozawa, 2005). It

should be noted that, the effectiveness of the method depends on percentage moisture of

the soil. Thus, the method is not effective in sandy layer with low water content of less

than 30% (Nozu, 2005).

Quality Control and Quality Assurance

In deep mixing methods, automatic quality control and quality assurance (QC and QA)

has to be implemented. A variety of installed monitoring instruments in the mixing

machine and binder feeder may help to control column positions, mixing proportional,

binder quantity, penetration and retrival speed (Stocker and Seidel, 2005; Yasui and

Yokozawa, 2005) (Figure 14).

Applications

For geotechnical and environmentally purposes, the typical application of deep mixing

methods can be grouped into two main categories:

 Non-structural purposes

o Ground cutoff wall

o Dewatering wall

o Containment of contaminants

o Secondary containment

 Structural purposes

o Deep and shallow foundation

o Tunnel and Retaining wall (stabilization of cuts and open excavation)

Porbaha et al (2005) categorized deep mixing application into six main applications,

namely;

• Hydraulic barrier systems

• Retaining wall systems

• Foundation support systems

• Excavation support systems

• Liquefaction/Seismic mitigation systems

• Environmental remediation systems

Foundation support systems

Application of deep mixing methods in foundation engineering includes foundation

systems to heavy machinery, highway embankment, storage tanks, dome silo and rail

systems (Figure 15-Figure 17) for both shallow and deep foundations.

Hydraulic barrier support systems

The application of deep mixing in hydraulic structures, aims at control of flood, seepage

and piping through the installation of cut-off wall systems. Figure 18 shows the

application of TRD in construction of a groundwater barrier at Herbert Hoover dike in

USA

Retaining wall systems

A free standing wall (Figure 19) can be constructed using DM which may be used to

retain soil behind it. These retaining walls (Gravity or reinforced wall systems) are most

useful for river front, sea wall for Ports and Harbor, secant walls, water bulkhead and

open excavations.

Excavation support systems

Under this category, the applications include the construction of support to open

excavations and underground constructions such as braced excavations, building

excavation, cut and cover tunnel, trenches for railway tracks. All of these may use deep

mixing to construct retaining wall for maintaining the open excavations (Figure 20).

Seismic and Liquefaction mitigation systems

Application of deep mixing may include seismic retrofit of dam foundation, alleviation of

lateral spreading, liquefaction mitigation of culvert foundation and river banks,

strengthening around an excavation and levee, stabilization of dune deposits, see Figure

24–Figure 25 (Porbaha et al, 2005; Yasui and Yokozawa, 2005). The main objective of

stabilization in such application is to reduce pore water pressures, to increase the shear

strength of the soils that could liquefy (EuroSoilStab, 2002) and or to minimize the

propagation of waves in the super-and substructure of the infrastructure systems, see

Figure 21–Figure 23 (Holm et al, 2002).

Mass Stabilization

Mass stabilization is a shallow to deep stabilization method in which the entire volume of

soft soil can be stabilized to a prescribed depth (Figure 26). The technique is relatively

new and is highly suited for the stabilization of high moisture content such as clay, silty,

organic soils and contaminated sediments (EuroSoilStab, 2002; Hayward Baker Inc).

Mass stabilization offers a cost effective solution to ground improvement in site

remediation especially with a huge amount of contaminants and high water content.

Remediation of most deposits of contaminated dredged sediments, organic soils and

waste sludge usually make use mass stabilization method (Keller, 32-01E). The method

provides an alternative to traditional method of soil improvement such as removal and

replaces techniques.

The blending of the soil mass may be achieved by either use of excavator mounted

mixing tool with unique shuttles pneumatically delivering the binder to the head of the

mixing tool and into the mix zone (Figure 29) or by self-injection of binder into a rotating

auger or mixing head and the soil (Figure 28). The mixer rotates and simultaneously

moves vertically and horizontally while mixing the soil block. The diameter of mixing

tool normally lies between 600 mm to 800 mm, with rotation speed between 80 and 100

rpm. Usually, the soil is stabilized in a sequence of a block which is defined as the

operating range of the machine. The typical range correspond to 8 to 10 m

2 in plan and

1.5 to 3 m in depth (i.e. 2 m wide x 5 m long x 3 m deep) with production rate between

200 and 300 m3 of stabilized soft soil per shift (Figure 27). The amount of binder is

typically in the range of 200 to 400 kg/m3 (EuroSoilStab, 2002).

In Nordic countries the amount of binder is in a typical range of 150 and 250 kg/m3

, and

the targeted shear strength is 50 kPa (Massarsch and Topolnicki, 2005). The method has

advanced to include use of rapid cement as a binder in stabilization of contaminated

dredged material at Port Hamina and shoreline of Helsinki, Finland, where stabilized

contaminated dredged materials deposited between embankments created new areas

(Andersson et al, 2001). Prior to initial set of the stabilized mass, a geo-membrane

separator have to be placed on top of stabilized soil on which a selected granular base

course material lies. These fill materials compresses the freshly stabilized mass forcing

out all air pocket that may have formed during mixing (Hayward Baker Inc; Massarsch

and Topolnicki, 2005; EuroSoilStab, 2002). According to EuroSoilStab (2002), deep

stabilization method compared to other methods of stabilization (Figure 30) has the

following main advantages:

- economic and flexible

- saving of materials and energy

- rapidity in improved engineering properties of the soil

- can be flexible linked with other structures and with the surroundings (no harmful

settlement differences)

Ex-Situ Stabilization

The technology involves dislodging of the soils and or sediments from the original

position and moves to other place for the purpose of amendment. These can be

encountered in dredging of river channel and Ports. The main objectives of dredging can

be either for amending the contaminated sediments to reduce toxicity and mobility or to

maintain or deepen navigation channels for the safe passage of ships and boats (US EPA,

2004). Offsite treatment of the sediment can be done in confined disposal facilities (CDF)

and then be used or disposed at designated site. Method of removal, means of

transportation, availability of treatment location, disposal site or demand for reuse is key

factors to consider when planning for ex-situ stabilization (Miller and Miller, 2007;

PIANC, 2009). Treatment of sediments in CDF falls under ex-situ mass stabilization

method, which can be accomplished in several ways depending on natural of sediments

and water contents (Figure 31).

Search This Blog

Environmental studies