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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
Excessive settlement, Low
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
Excessive settlement, Low
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
Ground Improvement- Applications and Quality Control 125
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:
Ground Improvement- Applications and Quality Control 127
• 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
Ground Improvement- Applications and Quality Control 129
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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