Ground Improvement

Ground Improvement-Applications and Quality Control

Raju ,V. R.Managing Director

e-mail: [email protected]

Keller Far East, Singapore

ABSTRACT

This paper presents an overview of various ground improvement techniques available and discusses factors

influencing the choice of technique. It then briefly describes relevant quality control procedures for common

techniques. This is followed by some applications of these techniques to different types of structure as well as

different soil conditions. Structures and facilities that have utilized ground improvement include roads and highways,

railways, ports and airports, land reclamations, storage tanks, chemical plants, tunnels and residential buildings.

The basis for choosing the particular technique for the project is explored, be it time, cost, technical performance

or environmental considerations. In addition, the quality control procedures adopted for various techniques are

explored. The paper will show that ground improvement is often the ideal foundation solution for such structures.

Indian Geotechnical Conference – 2010, GEOtrendz

December 16–18, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTION

Infrastructure projects such as highways, railways, airports

and harbours, cover large areas of land, sometimes over tens

of kilometers. This often leads to highly variable soil

conditions for the same project. Almost invariably, projects

such as railways or highways encounter problematic soils. In

addition, a large portion of infrastructure and building work

is in coastal regions, where soils typically have low strengths

and are highly compressible. Construction in increasingly

urban environments means that sites with poor soil conditions

and even landfills are being utilized for various structures and

facilities. This construction activity on poor soils often leads

to the necessity for ground improvement prior to start of

construction. The type of ground improvement required

depends very much on the type of structure to be built (and its

sensitivity to ground movement), the type of soil being treated

(and its short and long term behaviour) and the types of tools

and materials available. However, it is not sufficient to merely

select the appropriate ground improvement method. One must

also ensure that the work is done to an acceptable standard of

quality. This paper presents an overview of the more common

ground improvement methods in use and quality control

procedures that need to be adopted. Specific projects are

described to illustrate the various techniques and quality

control procedures.

2. TYPES OF GROUND IMPROVEMENT

Ground Improvement refers to any technique or process

that improves the engineering properties of the treated soil

mass. Usually, the properties modified are shear strength,

stiffness and permeability. Ground improvement is usually

done based on the following principles:

• Consolidation (e.g. prefabricated vertical drains &

surcharge, vacuum consolidation, stone columns)

• Chemical Modification (e.g. deep soil mixing, jet

grouting, injection grouting)

• Densification (e.g. vibro compaction, dynamic

compaction, compaction grouting)

• Reinforcement (e.g. stone columns, geosynthetic

reinforcement)

Some techniques improve the ground by a combination

of mechanisms. For example, compaction grouting not only

densifies the in-situ soils, but also forms high-strength,

high stiffness grout bulbs that reinforce the ground. Stone

columns installed in silty sands reinforce the ground,

densify the in-situ soils and function as large drainage

elements.

Consolidation Methods

In consolidation methods, the essential elements are (a) the

introduction of very permeable elements to shorten drainage

paths and (b) a means of increasing the stress that the soil

matrix experiences. In the PVD + Surcharge method, the

drainage elements are thin plastic drains, about 100 mm x 5

mm in cross section. These prefabricated vertical drains are

installed at fairly close spacings- from about 1 m to 2 m apart,

on a triangular or square grid. A soil pre-load is placed to

increase the total stress on the soil, leading to a temporary

122 V. R. Raju

increase in excess pore pressures. As the excess pore pressures

dissipate, consolidation occurs. The principle is that by pre-

loading the soil, much or all of the settlement that would occur

under the final structural load can be “forced” to occur prior

to construction. Therefore, little or no settlements will occur

during the service life of the structure. Often, a load higher

than the final structural load is placed as a pre-load, to reduce

the consolidation time. With the PVDs installed, the total

compression of the ground under the pre-load is not altered,

but the consolidation process is accelerated.

In vacuum consolidation, the load is applied by

suction, rather than by a physical pre-load. This has the

advantage of maintaining better internal stability of the

soil mass (no sudden increase in excess pore pressures).

Although pressures measured at the pumps can be -80 to

-90 kPa, losses in the system and differences in the

groundwater level and pump inlet, means that the

practical negative pressure that can be maintained in the

soil is usually between -50 to - 70 kPa. For this reason,

vacuum pressure is often combined with a physical pre-

load.

Stone columns, typically 1.0m in diameter, function

as large drainage elements. However as the columns act

primarily to reinforce (i.e. strengthen and stiffen) the

ground, a pre-load higher than the final load is seldom

necessary.

Chemical Modification

Chemical modification relies on the introduction of a

chemical binder to alter the physical properties of the soil

mass. Typical chemical binders include lime, cement and

fly ash. Often, the objective is to improve the strength and

stiffness of the soil. In some cases, the objective is to reduce

permeability. Ground improvement by chemical

modification is usually classified according to the means

by which the binder is introduced into the soil matrix.

Broadly, these categories are:

• Grouting- The voids in the soil matrix are filled with a

chemical such as sodium silicate or Portland cement.

The voids can simply be the pore spaces between sand

grains, or fissures within a limestone formation. Often,

the objective of grouting is to reduce the overall

permeability of the soil/ rock mass.

• Fracture Grouting- In this group of techniques, the

binder is injected under pressure resulting in controlled

fracturing of the soil rather than permeation of the soil

matrix. This technique is used mainly to lift structures

(on the surface or even buried) or to compensate for

settlement or volume losses. Hence it is also referred to

as Compensation Grouting. There is some overall

strength gain and reduction in permeability, but this is

not usually the primary purpose.

• In-situ Soil Mixing- The soil grains are mixed with a

binder, such as cement or lime using a mechanical tool.

The binder can be introduced as a slurry or dry powder.

The binder cures over time and the strength and stiffness

increases. When applied to sands, permeability is

reduced.

• Jet Grouting- The soil grains are eroded by a high

pressure fluid jet, and mixed with a fluid binder

(typically cement grout). Typically columns are formed,

with significantly increased strength and stiffness. In

the case of sands, the permeability is significantly

reduced. Because of the jet’s ability to form a good

connection with the neighbouring column, jet grouting

is often used to form base slabs for groundwater control.

Densification

While we apply the term consolidation to fine grained soils

such as clays, densification methods are used to reduce the

pore spaces between the particles of coarse grained soils

such as sands or gravels. To some extent, silts can also be

densified. The primary means of densifying sands and

gravels is to use a “shear wave” of energy to induce

rearrangement of the soil grains. The energy can be applied

at the ground surface (e.g. dynamic compaction, rapid

impact compaction) or at depth (e.g. blast densification,

vibro compaction, Mueller resonance compaction).

Intermediate soils such as silts do not respond as well

to wave energy. Densification of soils such as silty sands

usually involve the displacement and hence compaction of

soil mass. For example, stone columns installed by a depth

vibrator displace the silty sands laterally. Together with

the intense vibrations produced by the tool, the soil

surrounding the column is densified. Compaction grouting

involves the introduction of a very stiff grout bulb, injected

slowly and at a carefully chosen pumping pressure. The

slow, radial displacement of the soil results in increased

density of the surrounding soil mass.

Densification results in an increase of the internal angle

of friction and stiffness. The improved soil has a higher

bearing capacity, shows reduced settlements and improved

resistance to liquefaction.

Reinforcement

Reinforcement methods introduce a material foreign to the

in-situ soil matr ix to help “carry” the loads. The

reinforcement can be in the vertical direction (e.g. stone

columns) or horizontal (e.g. geotextiles, geogrids). The

relative stiffness between the reinforcing element and the

in-situ soil will determine the extent to which the loads are

shared. Stone columns act together with the in-situ soils

and in the process share the load because of their ability to

bulge (Greenwood, 1991). Very stiff elements relative to

the in-situ soil tend to carry most of the loads and behave

Ground Improvement- Applications and Quality Control 123

in a more rigid or pile-like fashion (e.g. vibro concrete

columns, deep soil mixing columns, rigid inclusions),

(Sondermann & Wehr, 2004). An interesting type of

reinforcing element is the “mixed-modulus column”

(sometimes referred to as CMM). The lower portion of the

column is a concrete rigid inclusion, while the uppermost

section (usually 1.5 m to 2.5 m) is a conventional stone

column. The stone column head eliminates the risk of

punching failure of overlying floor slab, and allows it to be

designed as a regular non-suspended slab. For a more

detailed description and comparison of the various

techniques, the reader is referred to Holtz et al (2001),

Kirsch & Sondermann (2003) and Moseley & Kirsch

(2004). Some of the more common ground improvement

methods are described in Table 1.

3. CHOICE OF TECHNIQUE

As can be seen from Section 3 above, several techniques

are available for ground improvement and the choice of

the appropriate one is important. The following section

gives a few factors to consider.

Suitability of the Method

Some methods lend themselves naturally to certain soils.

Vibro compaction of reclaimed sand fills is a good example.

In reclamation fills, the sands are relatively clean, and

therefore the method is very fast and economical, even to

large depths. Some methods and soils do not go well

together. Deep soil mixing of peaty soils is one such

example. A large quantity of cement or other binder may

be required to achieve the desired strength, if at all possible.

Technical Compliance

This is usually verified by design calculations to check for

sufficient bearing capacity, factor of safety against slope

failure or that the magnitude of settlements (total and

differential) etc. are within limits. Some structures such as

earth embankments and storage tanks are able to tolerate

settlements in the order of decimetres during the

construction stage. Therefore “soft” techniques relying on

consolidation are often suitable. Other structures such as

industrial plants require solutions which do not allow

settlements more than a few centimetres. In such cases

“hard/rigid” solutions such as DSM in clays, densification

of sands or some form of preload (to preclude long term

settlements) are required.

Availability of QA/QC Methods

The availability of methods to ensure that quality is ensured

during and after construction is important. Pre and post-

improvement testing by penetration methods (e.g. CPT),

sampling etc. are essential. In addition, real-time

monitoring during the improvement process using

automated data loggers to inform the operator of what is

happening below ground are very helpful to ensure quality.

In addition, the data loggers can be used to provide a

printout of the construction process, a so called “birth

certificate” of the improvement point for daily review by

the Engineer. (More details are given in the next section).

Availability of Material

Ground improvement uses a range of materials, some

natural (e.g. stone) and some manufactured (e.g. cement,

geotextiles). The availability of these materials will

influence the choice of technique. Malaysia for example

has several soft soil deposits in the coastal regions with

nearby hilly terrain. The hilly terrain makes stone easily

available and has led to extensive use of stone columns to

treat the soft coastal soils.

Time

Methods which require long consolidation periods will

obviously not be suitable for fast track projects. Installation/

construction time is also important. Nowadays however,

modern high-production machinery allows a significant

reduction in construction time. For example, the use of the

“twin” configurations in Vibro or DSM equipment or the

use of computerised cranes to drop the pounder in dynamic

compaction have very significantly increased production

rates.

Cost

Assuming that the solution satisfies technical requirements,

cost often becomes the deciding factor. Methods which use

less or cheaper added material are of course cheaper.

However if the “cost” of time or the risk of non-performance

are added, then other apparently expensive solutions

become economical.

Convenience

Solutions which do not require other additional measures

such as the placement of a large preload, or excavations

(as in excavate and replace methods) are more convenient

and practical.

Protection of the Environment

Methods which produce spoil are of course not desirable.

In-situ treatment methods which do not remove the soil or

discharge excess cement/binder are preferred. For example,

stone columns installed by the “dry” method only displace

the in-situ soil. In contrast, the “wet” method of column

installation flushes out some of the soil. For this reason,

the “dry” method is often preferable. Similarly in-situ soil

mixing would be preferable to jet grouting where possible.

Another criterion would be the influence of the method on

sensitive structures nearby.

4. QUALITY CONTROL PROCEDURES

Quality control procedures are important firstly to assure

124 V. R. Raju

Technique Soil Type Geotechnical

Problems

Basic

Principle(s) Comments

PVD + Surcharge Typical grid spacing = 1m to 2m

Vacuum Consolidation

Soft clays, silts

Large long term

consolidation settlements,

Low bearing capacity

Drainage/

Consolidation Typical grid spacing = 1m to 2m

The “surcharge” load is applied by a combination of physical surcharge and

negative pressure.

Vibro Compaction (or Vibroflotation)

Energy input is at depth.

Typical grid spacing = 2.5m to 5m Maximum depth = 65m

Dynamic Compaction

Loose

sands,

gravels

Excessive

settlement, Creep,

Liquefaction

Densification

Energy input is at the ground surface. Typical grid spacing = 4m to 6m

Practical (& Economical) = 10m

Vibro Stone Columns

Typical area replacement ratios = 15% –30%

Maximum depth = 45m

(Typical depth = 5m to 20m)

Sand Compaction Piles

Loose sands, silts,

clays

Excessive settlement, Low

bearing capacity,

Liquefaction.

Reinforcement, Drainage/

Consolidation,

Densification

Typical area replacement ratios = 40% –70%

Mixed- Modulus

Columns

Loose sands, silts,

clays


bearing

capacity

Reinforcement

Similar settlement characteristics to Rigid Inclusions, but with no “punching effect” at

the floor slab. Have been installed to depths of

30m.

Dynamic Replacement Silts, Clays

Excessive

settlement, Low bearing

capacity.

Reinforcement, Densification

Typical column diameter = 2.5m Typical column spacing = 5m

Typical depth = Approx. 5–6m

Deep Soil Mixing Sands,

silts, clays


bearing capacity

Chemical

modification

Typical column diameter = 0.8m–2.5m

Maximum depth = 40m Some techniques (e.g. JACSMAN) combine

mechanical mixing and jetting to form

columns

Jet Grouting Sands,

silts, clays

Excessive settlement, low

bearing

capacity, high permeability (in

sands)

Chemical

modification

Typical column diameter = 1.5m to 4m,

depending on jetting system and soil.

Typical depth = 10m to 30m

Injection Grouting Sands, silts High

permeability

Chemical

modification

In soils, the binder is often injected via a tube-

a-manchette giving rise to the name TAM

grouting. Binder choice is determined by soil

type.

Compaction Grouting Sands, silty

sands

Excessive settlement,

Liquefaction

potential

Densification,

reinforcement

Typical spacing between points = 1– 4m

Grout bulb diameter = 0.5–1m Has also been used to lift structures.

Table 1: Some Features of Various Ground Improvement Techniques


the client that the product he receives is of a high standard,

secondly to prevent costly re-work for the contractor and

most importantly to ensure public safety. Generally, quality

control is applied pre-construction, during construction and

post-construction. Various standards can be used to aid in

the fomulation of good contract specifications and quality

control procedures. Some typical quality control procedures

for common ground improvement techniques are described

below.

Vibro Stone Columns

For Vibro Stone Columns, it is essential to ensure that

columns are built to the right depth, to the right diameter

and are properly compacted. Computerized monitoring of

the penetration depth of the vibrator easily ensures that the

design depth is reached. Sensors within the depth vibrator

can readily measure the amperage drawn by the motor,

giving an indication of the compaction effort of the depth

vibrator. IS 15284 (Part 1): 2003 gives guidelines on the

estimation of the column diameter based on fill

consumption. In the case of dry bottom-feed stone columns

(See Raju & Sondermann, 2005), even the location of each

charge of stone along the depth of the column may be

determined from the record of depth vs. amperage. Post-

construction, load tests are routinely performed as a quality

control measure. As we will see from a later example, there

are situations where load tests are not practical, and we

have to rely on the other quality control methods. Another

useful general standard for stone column construction and

testing is EN 14731:2005.

Vibro Compaction (Vibro-flotation)

Once the suitability of the soil for Vibro Compaction is

determined by detailed soil investigation, the actual

compaction should be carefully monitored. Sometimes, a

field trial is required to confirm the compaction parameters,

particularly the grid-spacing to be used. With the

compaction parameters fixed, it then falls on the site team

to ensure that the sand is densified in a systematic,

disciplined manner. Computerized recording of the time

and date of compaction, point number, depth of compaction

and amperage drawn by the depth vibrator greatly assists

the crane operators and engineers in eliminating human

errors. However, even with well-chosen compaction

parameters and meticulous execution, re-compaction is

sometimes necessary given the natural variability of the

ground. Post-compaction electric cone penetration testing

(CPT) is perhaps the most practical tool in determining if

the target degree of compaction has been met. Should the

CPT result be unsatisfactory, re-compaction of the particular

zone can be performed. Using Standard Penetration Tests

(SPT) as a quality control test for Vibro Compaction is

undesireable as in a granular soil, borehole disturbance

and operator errors tend to give erratic results. Also,

compared to CPTs, drilling boreholes to perform SPTs are

slow and laborious.

Chemical/ Cement-Based Techniques

For cement based techniques (grouting, deep soil mixing

and jet grouting), quality control is focussed on the careful

control of the materials that are used, as well as the mixing

or pumping process. The grout mix is tested for its density

(often using a hydrometer) and viscosity (often using a

Marsh cone). In the case of silica gels for permeation

grouting, setting times are crucial for the success of the

grouting program, and are carefully tested also. The

installation process is also carefully monitored. Key

parameters such as flow-rate, pressure and total volume

injected are carefully monitored, almost always

automatically. In the case of deep soil mixing on jet

grouting, parameters such as rotation speed and withdrawal

rate are also important. Post-construction testing is of some

value, especially in instances where re-injection is possible.

For permeation grouting, post-construction testing can take

the form of pumping tests. In the case of deep soil mixing

or jet grouting, coring and UCS testing is often performed.

However, for deep soil mixing or jet grouting, it is far more

desirable to “get it right the first time” as re-work is very

difficult and sometimes impossible. For further details on

grouting works, the reader is referred to Raju & Yee (2006)

and Semprich & Stadler (2003). EN 14679: 2005 gives

guidance on the construction and quality control of deep

soil mixing.

Other Techniques

Some common techniques such as prefabricated vertical

drains (PVD) have standards that govern construction and

testing (e.g. IS 15284 (Part 2): 2004; EN 15237: 2007).

Other techniques like Dynamic Compaction or Rigid

Inclusions rely on generally accepted industrial practises,

which are then written into detailed contract specifications.

Because ground improvement techniques work on different

principles and are constructed in a variety of ways, the key

features to be checked vary from technique to technique.

Therefore it is vital that contract specifications for ground

improvement be drafted specifically for the technique. For

example, one cannot simply apply piling specifications to

Vibro stone columns or DSM columns simply because they

are vertical element within the ground.

5. APPLICATIONS

Highways & Roads

Roads on Hydraulic Sand Fill (Jurong Island,

Singapore)

Jurong Island is a petrochemical centre, housing petroleum

storage tanks, petrochemical plants and other related

126 V. R. Raju

facilities. It was formed by joining together 7 small islands

by reclamation. Reclamation was done using sandfill. This

means that certain facilities may be built on the original

islands, while others are built on the in-filled channels

between islands. The Banyan Region of Jurong Island

however is fully reclaimed. It is home to a VLCC jetty,

several large storage facilities including Universal Terminal

and Helios Terminal, in addition to other chemical plants.

To serve these facilities, a network of roads has been

constructed by JTC Corporation. In general, the ground

consists of 20 to 30 m of sandfill, sometimes followed by a

thin layer of marine clay. Underlying the sandfill and

marine clay (if any) are stiff Jurong Formation residual

soils. Typically sandstone or mudstone is encountered at

30 to 40 m depth.

In order to ensure minimal settlement in the reclaimed

land, JTC Corporation has specified that if any significant

layer of marine clay is present, it is to be treated with PVDs

and surcharge. The sandfill is to be densified using Vibro

compaction. The design traffic load is 30 kN/m2. The

sequence of improvement is typically as follows:

• Install PVDs into marine clay layer

• Complete Vibro compaction of sand layer

• Place soil surcharge and maintain it for required

consolidation time

• Remove soil surcharge and continue with road

construction.

The specifications are as follows:

For Clay Layers: No more than 100 mm future

settlements and a minimum of 90 % consolidation

For reclaimed sandfill:

Depth (m) Cone Resistance

(MPa)* Relative Density (%)*

0- 2 8 80

2- 8 12 70 > 8 17 70

*Whichever is lower

Because the soft clay layers are not common in the

Banyan Region, PVD’s and surcharge have seldom been

necessary. The design concept is shown in Fig. 1. In the

Banyan Region, over 500,000 m2 of road and drainage

reserve have been improved in this way. Where PVDs have

been unnecessary (absence of significant layer of marine

clay), this scheme has proved to be very quick. For example,

a single Vibro compaction rig is able to densify 300-400

m2 per day, for compaction depths of 20-25 m. It is common

for areas of 3,000 m2 to be completed in 3-4 days and handed

over within a 7-10 days of completion, including testing

by CPT. Typically, 1 post-compaction CPT is performed

every 1,500 m2. In Fig. 2, sample pre-compaction CPT

result is compared with the post-compaction test in the same

area. Usually, Vibro compaction work proceeds for merely

2-4 weeks before the rest of the road works commence.

Fig. 1: Ground Improvement Concept for Roads

Fig. 2: Sample Pre and Post Compaction CPTs on Banyan

Road, Jurong Island

Railways

Railway Embankment Built on Soft Silts and Clays

(Double Tracking Project, Malaysia)

The Malaysian government has been upgrading the railway

network in the country. Certain sections of the existing

network are to be expanded to a double track, for high speed

electric trains. As the new line had more stringent

restrictions on gradients, in general, embankment heights

were raised.

The railway line passes through areas that have

seen extensive tin mining activity in the past. Soils

encountered were highly variable, with a mixture of

loose sands, very soft silts and very soft clays as deep

as 24 m. Cone tip resistance (qc) values in the very

soft silts and clays were often between 150 and 250

kPa. The performance criteria laid down by the railway

authorities are as follows:


• Differential settlement of not more than 10 mm in 10m

• Total settlement of not more than 25 mm in the first 6

months of operation

• Factor of safety against slip failure of the embankment

greater than 1.5

Various foundation and ground improvement methods

were implemented for the track which travelled over a wide

variety of ground conditions.

• Where the soil was stiff, no improvement was done

except for surface preparation prior to embankment

construction

• Where the soft soils were 2 m to 3 m thick, they were

simply excavated out and replaced with fill and

compacted

• Where deeper soft soils were encountered, Prefabricated

Vertical Drains (PVD) + Surcharge were normally used.

• Vibro stone columns were specified under the following

circumstances;

(i) Where embankment heights (and therefore

loadings) were anticipated to result in higher than

acceptable settlements

(ii) Where site constraints did not permit excavation

& replacement (e.g. close proximity to existing

live track or high water table)

(iii) Where shorter construction time was required, and

therefore a long pre-loading period with PVDs was

not acceptable

More details of the Vibro stone column works are given

below.

• Where necessary, piles were driven and the railway

embankment was constructed on a deck.

• For an 800 m stretch of embankment near the town of

Serendeh, Dry Deep Soil Mixing was also employed.

Columns were installed to a depth of 14 m. Geotextile

reinforcement was placed over the relatively rigid DSM

columns prior to embankment construction to better

assist in the transfer of the embankment load to the

column.

Vibro stone columns were installed to depths of 8 m to

18 m to support embankments with heights from 2 m to 11

m. The design concept is shown in Fig. 3. Work was often

carried out very close (2 m) to the existing track, with no

disruption to train operations. In addition to the upgrading

of the existing track, Vibro stone columns works were

carried out to support embankments and reinforced-earth

walls for Road-Over-Rail Bridges. The RE walls were up

to 12 m high. For this project, over 1,000,000 m2 of ground

were improved using Vibro stone columns.

The Ipoh – Rawang section has been completed and

presently, the Ipoh-Padang Besar and the Seremban to

Gemas sections of the double-tracking project are ongoing,

with extensive use of Vibro stone columns, PVDs and

surcharge as well as Remove-and-Replace techniques.

Soft

Clay

Stone columns

Fig. 3: Vibro Stone Column Design Concept & Installation for

Railway Embankments

A typical single-column load-test result is shown in

Fig. 4 below.

Fig. 4: Sample Single-column Load Test

Ports

Access Roads and Hardstanding Pavements on Soft

Clay (Pipavav, India)

Pipavav Shipyard Limited (PSL) is currently developing

an integrated shipbuilding facility in Pipavav. The area

being developed is 85 hectares in total. The construction

of the shipyard includes the construction of a block making

facility for hull blocks, the installation of a ship lift facility

and the conversion of an existing wet basin into a 651 m x

65 m dry dock. Geotechnical and foundation activities

include Vibro stone column works, bored cast in-situ piles

and diaphragm walls. The ground generally consists of

murram fill followed by soft marine clay and then weathered

rock.

128 V. R. Raju

As part of the facility, a 2.5 km x 14 m approach road

needed to be built between the block making facility and

the hardstanding pavement (900 m x 25 m) next to the dry

dock. At both these areas, Vibro stone columns were

installed to an average depth of 12 m, on a 2.5 m triangular

grid spacing. A total of 144, 000 linear meters of stone

columns were installed. Fig. 5 shows installation in

progress.

Fig. 5: Installation of Vibro Stone Columns at Pipavav

Shipyard

Quality control procedures were adopted from pre-

costruction to post-construction. During construction,

computerised monitoring of installation parameters was

performed. Parameters were displayed to the operator in

his cabin. In addition, printouts were generated in the

operators cabin in real-time, ensuring rapid review of the

construction process for each column. A sample printout

from the project is shown in Fig. 6 below.

Vibro stone column work is also ongoing for the

Offshore Yard area. A load-settlement curve from a single-

column load test is shown in Fig. 7 below. At the design

load, the measured settlement was 5.38 mm, below the

allowable settlement of 12 mm.

Fig. 6: Sample Printout from Pipavav Vibro Stone Column

Installation

Fig. 7: Load- Settlement Graph for Single Column Load Test

Storage Tanks & Industrial Plants

Concrete LNG Tanks on Silty Sand (Hazira, India)

In 2002, ground improvement works were carried out for

Shell India, for 2 nos. LNG tanks in Hazira, India. The

tanks were 84 m in diameter, with a filling level of 35 m.

The soils consisted of 16 m of silty sand and fill, overlying

alternating layers of dense sand and stiff clay.

Fig. 8: Hazira LNG Tanks in Operation

The ground improvement design required the

maximum permissible settlement along a radial line from

the periphery to the center of the tank to be limited to 1:300,

based on BS 7777 (Part 3, 1993). In addition, the ground

improvement was to be designed against a Safe Shutdown

Earthquake (SSE) level of a = 0.25 g and an Operating

Base Earthquake (OBE) level of a = 0.10 g.

Vibro stone columns were chosen as an alternative to

the conventional piling method because of significant

savings offered in cost and time. A significant technical

advantage of the ground improvement solution was its

ability to resist lateral loads generated by earthquakes and

also to mitigate liquefaction. The lateral resistance is

mobilized by base friction between tank foundation and

improved ground. (It is to be noted that concrete piles are

rather inefficient in resisting lateral loads.) Because of

liquefaction considerations, the zone of improvement was

105 m in diameter for each tank, versus a tank diameter of

84 m. Stone column installation works were completed in


3 months using 2 Vibro Replacement rigs. Fig. 8 shows a

picture of the tanks in operation.

Sewage Treatment Plant on Marine Clay and Old

Landfill

The Jelutong Sewage Treatment Plant in Penang, Malaysia

was built in 2008, to cater to a population of 1.2 million.

The main structures in the plant are 12 sequential batch

reactors of size 80 m x 60 m x 7 m tall. These are large

concrete tanks on a raft, imposing a spread load on the

ground. Other auxiliary structures are gas storage tanks,

sludge holding tanks and compound walls. All these

structures impose spread loads on the ground and lend

themselves to foundation systems using ground

improvement.

Soils at this location are soft marine clays to a depth

of 10m followed by stiff sandy slits and sandy clays to depths

of about 50m followed by very dense to hard silty sands. In

addition, the central portion of the plant was a former

landfill with household waste to a depth ranging between

2 m and 5 m.

Plant specifications required that the total settlements

be less than 75mm and differential settlements be less than

1:360. A driven pile solution was considered, but this would

have meant driving piles to about 50 m depth. Ground

improvement to treat the upper rubbish fill and the soft

clay to 10 m depth proved to be an exceptionally economical

and quick solution.

Fig. 9: Plan View of the Jelutong Sewage Treatment Plant

Showing Various Structures and Treatment Schemes

It was important to penetrate and displace the

unpredictable and highly organic rubbish fill and also to

not rely on any long term support from the rubbish (for

obvious reasons of long term decay, etc.). This was achieved

by using Vibro concrete columns (VCC) to about 10 m depth

(see Fig. 9). The other sequential batch reactors, not affected

by the rubbish fill, were founded on Cement columns (or

Deep Soil Mixing columns). The sludge holding tanks were

built on Vibro stone columns and the substation was built

on cement mixed piles. The Vibro concrete columns and

Cement columns offer “rigid” performance with settlements

in the treated layers of less than 25 mm. The Vibro stone

columns, although “flexible”, nonetheless provided

foundations with expected settlements of less than 75 mm.

As part of the quality control program, detailed pre-

construction soil investigation was performed. Quality

control for the DSM columns, VCC and Vibro stone

columns were by means of materials testing prior to

construction, automatic monitoring and recording of

construction parameters as well as post-construction load

tests.

Under operating load conditions, settlements measured

over a period of 10 months ranged between 5mm and 20

mm for both Cement columns and VCC. A more

comprehensive description of the ground improvement

works carried out on this project and measured settlement

data can be found in the Yee et al (2009).

Olefins Plant on Hydraulic Sandfill (Jurong Island,

Singapore)

Process plants have frequently been founded on

improved ground. The Singapore Olefins Plant in Jurong

Island was constructed in 1998. Vibro Compaction was

carried out for the foundation for the process plant (reactors,

piping, etc.) where the underlying soils were reclaimed

sands. Varying intensities of treatment were adopted to meet

the different requirements in specifications. In general

however, settlements of the steel structures under working

loads had to be restricted to 25-50 mm. Factors of safety

against bearing capacity was to be higher than 2.0. Fig. 10

shows a picture of the Singapore Olefins Plant in operation.

Fig. 10: SOP1 in Operation

As part of the ExxonMobil’s Singapore Parallel Train

project, Vibro Compaction was also carried out for the new

Olefins plant, called “SOP 2”, adjacent to the existing plant.

Vibro Compaction works were completed at the end of 2008

and construction of the plant is ongoing. Fig. 11 shows the

compaction work in progress. Quality control was by means

of detailed pre-compaction site investigation (using CPTs),

automatic recording of compaction parameters and post-

compaction CPTs. During the ground improvement works

for SOP 2, a certain portion of the reclamation fill was

found to contain a large amount of decomposed wood. (This

was not detectable by CPTs.) This would pose an

unpredictable and hence unacceptable long-term settlement

risk. For this reason, that portion of the new plant utilized

driven spun-piles as the foundation system for the heavy

structures.

130 V. R. Raju

Fig. 11: Vibro Compaction in Progress for “SOP 2”

(Nov 2008). The Operational SOP 1 can also be seen

Other plants that have used ground improvement

include the Shell Malampaya Onshore Gas Plant and the

CAPCO- PTA project (See Raju & Sondermann, 2005). In

the Middle East, the Jebel Ali Power Plant, Sharjah Power

& Desalination Plant, Dubai Aluminum Plant and Al-

Khobar Power & Desalination Plant have all been built

using Vibro compaction, Vibro stone columns or a

combination of the two.

Underground Construction

Ground Anchors for New Delhi Metro Project

The Delhi Metro Rail Corporation project (DMRC)

connects the Indira Gandhi International Airport and New

Delhi Railway Station with an exclusive Airport Metro

Express Line. As a part of this project, an underground

metro station and multi-level car parking facility was

planned near the New Delhi Railway station. The site in

general consisted of silty clay, with the Quartzite bedrock

varying from 5 m to 20 m deep.

Fig. 12: Typical Ground Anchor

The site had to be excavated to a maximum depth of

19 m to facilitate the construction activities for the station

building and underground parking facility. The excavation

was supported by a Soldier pile – Anchor & Strut system.

The soldier pile walls were embedded 0.5 m into the

Quartzite bedrock. At most locations, two levels of anchors

were designed, at 2.5 m and 8 m below ground level. To

facilitate the removal of anchor strands after construction

of the intended wall, U-Turn retrievable ground anchors

were installed. For such anchors, the steel strands are

covered with a PVC jacket, turned over a U-loop (U-turn

saddle) at the bottom and connected to a reinforcement

rod. Fig. 12 shows a typical Ground Anchor.

A hydraulic rotary drill rig (Casagrande C6) was used

for inclined drilling (30 deg to the horizontal, to a maximum

length of 22 m) and simultaneous installation of the casing.

The anchors consisted of 7-ply 12.7 mm diameter strands

conforming to IS 14268: 2005, with an ultimate tensile

strength of about 187 kN. Primary and secondary grouting

were performed after washing the borehole. As a part of

quality control procedures, operating parameters such as

flow rate, grout pressure, total grout volume, etc. were

recorded at site for each anchor. Fig. 13 shows a drilling in

progress for the second level of anchors. The anchors were

designed to withstand a working load of 60 T and were

tested at 1.1 times the working load (66 T).

Fig. 13: Drilling using a Casagrande C6 for the Second

Level of Anchors

6. CONCLUSIONS

Ground improvement has developed into a sophisticated

tool to support foundations for a wide variety of structures.

Properly applied, i.e. after giving due to consideration to

the nature of the ground being improved and the type and

sensitivity of the structures being built, ground

improvement often reduces directs costs and saves time. In

Asia, it has been extensively used for the construction of a

wide range of infrastructure and building facilities.

However, careful attention must be paid to quality control

procedures. The focus should not only be on testing after

construction (load testing, etc.), but also on careful

supervision while the work is in progress. Full advantage

should be taken of modern automatic monitoring and

recording of key installation parameters.

ACKNOWLEDGEMENTS

The author would like to thank Jonathan Daramalinggam

and G.T. Senthilnath for their contributions to this paper.

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Documents

Ground Improvement