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In the last two decades, the use of
ground improvement with stone columns
on local soils to improve their capacity has
greatly increased throughout the world.
Typically, stone columns consist of vertical
reinforcement introduced by constructing
a column of densely-packed stones to
partially or fully replace the local weak soil.
The construction can either be by wet or
dry method. The placement of stone
columns in a specific grid pattern allows
the soil mass to behave like a homogenous
layer with improved density and stiffness.
This process yields enhancement of load
bearing capacity and minimizes the
settlements of the treated ground com-
pared to the untreated ground.
Stone columns act as a drainage path
allowing for rapid consolidation, which in
turn improves the strength and defor-
mation characteristics of the ground at a
much faster rate. Stone columns con-
structed using vibro techniques allow
displacement rather than replacement of
the weak soil, which then leads to further
improvement of displaced weak soil by
faster dissipation of construction pore
water pressure. Also, the improved
drainage capabilities of the stone-column
treated ground provide a much better
resistance to liquefaction of the sur-
rounding soil. The resistance to lique-
faction is achieved by densification of
surrounding weak soil and also by the
increased capacity for dissipation of excess
pore water pressure.
FEATURE ARTICLE
DEEP FOUNDATIONS • SEPT/OCT 2016 • 77
AUTHORS Anirudhan I. V., Geotechnical Solutions, Madan Kumar Annam and Hari Krishna Y, Keller India
Ground improvement refers to a technique
that improves the engineering properties of
a weak soil mass such as shear strength,
stiffness and permeability. Ground
improvement has been developed into a
sophisticated tool to support foundations
for a wide variety of structures. It often
reduces direct costs and saves construction
time if properly applied, i.e., after giving
due consideration to the nature of the
ground being improved and the type and
sensitivity of the structures being built.
Ground Improvement Techniques for a Residential Complex
Sequence of installation of dry vibro stone columns
78 • DEEP FOUNDATIONS • SEPT/OCT 2016 DEEP FOUNDATIONS • SEPT/OCT 2016 • 79
immediately below the existing ground
level. The high structural loads planned for
the complex would induce large settle-
ments of about 220 mm (8.7 in), with the
top 6 to 8 m (19.7 to 26.2 ft) of soil
inducing about 115 mm (4.5 in). As a
result, driven cast-in-situ piles bearing in
the hard clay layers 25 m (82 ft) below were
selected. However, a residential neighbor-
hood nearby strongly resisted pile driving
activities, mainly due to environmental
issues including noise pollution, ground
vibrations and the carbon footprint. This
called for a rethinking of the foundation
sys tem; the feas ib i l i ty o f ground
improvement was therefore assessed.
The developer contacted Keller India to
design and construct the ground improve-
ment works. Considering the project
boundary conditions, vibro replacement
(stone columns with dry bottom feed
method) was selected as a viable method for
soil improvement and a full raft foundation
supported by the treated ground as the
alternative foundation system. The selected
method of ground improvement addressed
the environmental issues raised at the pro-
ject site. In this method, the stone columns
are installed by displacement technique
(without removing any soil) so the site
would be comparatively clean. In addition,
benefits including economizing the foun-
dation cost and optimizing construction
time proved invaluable for the project.
Vibro TechniquesVibro replacement is an accepted method
for soil improvement in which columns of
coarse grained material are installed in the
soil by means of vibrators. Performance of
this composite system consisting of stone
columns and weak soil mass can be
established theoretically and also be
verified by full-size field plate load tests.
Contrary to vibro compaction, which
densifies noncohesive soils through
vibration, vibro replacement improves
cohesive and noncohesive soils by
reinforcing the weak soil with load bearing
columns of well-compacted, coarse-
grained material. When the weak soil is
fully replaced with a well-compacted
coarser material, there is no complexity in
understanding its improved load carrying
capacity and corresponding deformations.
But, when the weak soil is partially replaced
and displaced by the introduction of these
stiffer reinforcing elements at regular grid
patterns, response of this modified ground
becomes complex. There are ways for
arriving at an equivalent stiffness matrix of
a system that replaces some part with a
material of larger stiffness. Similarly, there
are ways and means to establish the
modified density and stiffness when the
entire soil mass is densified. But when the
improvement is attributed to both
displacement and replacement, the
quantification of improvement is difficult
to determine. Considerable efforts such as
large-scale load tests can only prove the
effectiveness of the installed stone
columns. An improvement factor is
established to compare how much stone
columns increase the stiffness of the soil
they are installed in. It can be calculated
based on the area replacement ratio and the
reinforcing material used for the stone
columns.
The deformation modulus of the
composite system is another one of the
basic inputs for finalizing the design of
stone columns. However,
the reality is that in many
p r a c t i c a l c a s e s t h e
reinforcing effect of stone
columns installed by
vibro replacement is
c o m b i n e d w i t h t h e
densifying effect of vibro
compaction, i.e., the
insta l la t ion of s tone
columns densifies the soil
between grids increasing
its K0 (coefficient of earth
pressure at rest) and Kp
(coefficient of passive
earth pressure). In such
cases, the densification of
the soil has to be evaluated on the basis of
original soil data and correspondingly the
design of vibro replacement can be
modified as needed to achieve particular
improved site conditions.
Dry Vibro Stone Column MethodologyA custom-built machine called the Vibrocat
was developed by Keller for installation of
vibro stone columns without using water.
The Vibrocat comprises a specially-
constructed track mounted supporting
unit and a vibrator, which incorporates a
stone tube with compression chamber and
stone-feed hopper to ensure properly
formed compacted stone columns to the
required diameter and depth. A special
feature of the dry method is that it does not
require water jetting for penetration and
therefore eliminates the need to handle the
collected water. Furthermore, this method
can be used where limited working space is
available, especially in developed or urban
areas, or where no water source can be
found nearby. This technique provides
effective drainage paths to ensure rapid
consolidation. It also has a built-in real-
time computer monitoring system to
provide quality control on compaction
effort throughout the construction process.
Residential Complex Case HistoryUrban Tree Infrastructures (Urban Tree)
recently completed a residential project in
Chennai, India. The project consisted of 198
units with ground-level parking and four
floors above, on a 2.5 acre (10,100 sq m)
development site.
The site subsurface consisted of
desiccated clay and medium dense sand up
to about 3.5 m (11.5 ft) underlain by
relatively weak clay and sandy clay up to a
depth of 6 m (19.7 ft). This top 6 m (19.7 ft)
of soil with highly varying consistency is
underlain by about 8 m (26.2 ft) of medium
dense sand and stiff clay deposits below
which there is a 6 m (19.7 ft) thick layer of
medium stiff consistency. Denser sand layers
and hard clay layers form the remaining
subsurface profile. The groundwater table
was encountered at about 3 m (9.8 ft) below
the existing ground level.
• Design Approach
The performance of shallow foundations
for the residential complex would not be
e ffec t ive due to weak so i l l ayers
Footprint of the raft foundation Typical soil profile showing ground improvement arrangement Vibro cat in action at project site
Results of single column routine plate load test
The complex had a load intensity at the
foundation level of approximately 75 to 85
kPa (1,566 to 1,775 psf). A total footprint
area of about 5,500 sq m (59,200 sq ft)
under the raft foundation was treated. The
foundation raft was divided into six pours
for ease of construction. Individual column
loads from the super structure varied from
25 to 185 tonnes (27.5 to 204 tons). Though
the raft foundation transmits uniform
pressure to the bearing soil, a denser grid
was adopted due to the large column loads.
Designs, in terms of strength and
deformation characteristics for the proposed
loading conditions, were made using Priebe
(1995) design methodology and appropriate
geometry (stone column diameter, spacing,
pattern and depth). The final scheme was
reviewed and vetted by Geotechnical
Solutions, Chennai, a third-party specialty
geotechnical consultant.
• Quality Control and Monitoring
In order to measure and assure the quality
of stone columns being constructed, it was
necessary to adopt stringent quality control
and quality assurance procedures to meet
the specifications, and to satisfy the client’s
requirements at various stages of execution
of the project.
• During Construction
An automated computerized recording
device fitted to the Vibrocat recorded the
installation of each stone column. This
instrument yielded a computer record (M4
Graph) of the installation process in a
continuous graphical mode, plotting depth
versus time and power consumption
(compaction effort) versus time. The infor-
mation provided included:
• Stone column reference number
• Date of installation
• Start and finish times for installation
• Period required for installation
• Maximum depth
• Compaction effort during penetration
and compaction process
The above parameters allowed the qual-
ity of the stone columns being installed to be
monitored. Further, the diameter of the stone
column and consumption of backfill were
continuously monitored by the site person-
nel to estimate the in-situ achieved diameter.
78 • DEEP FOUNDATIONS • SEPT/OCT 2016 DEEP FOUNDATIONS • SEPT/OCT 2016 • 79
immediately below the existing ground
level. The high structural loads planned for
the complex would induce large settle-
ments of about 220 mm (8.7 in), with the
top 6 to 8 m (19.7 to 26.2 ft) of soil
inducing about 115 mm (4.5 in). As a
result, driven cast-in-situ piles bearing in
the hard clay layers 25 m (82 ft) below were
selected. However, a residential neighbor-
hood nearby strongly resisted pile driving
activities, mainly due to environmental
issues including noise pollution, ground
vibrations and the carbon footprint. This
called for a rethinking of the foundation
sys tem; the feas ib i l i ty o f ground
improvement was therefore assessed.
The developer contacted Keller India to
design and construct the ground improve-
ment works. Considering the project
boundary conditions, vibro replacement
(stone columns with dry bottom feed
method) was selected as a viable method for
soil improvement and a full raft foundation
supported by the treated ground as the
alternative foundation system. The selected
method of ground improvement addressed
the environmental issues raised at the pro-
ject site. In this method, the stone columns
are installed by displacement technique
(without removing any soil) so the site
would be comparatively clean. In addition,
benefits including economizing the foun-
dation cost and optimizing construction
time proved invaluable for the project.
Vibro TechniquesVibro replacement is an accepted method
for soil improvement in which columns of
coarse grained material are installed in the
soil by means of vibrators. Performance of
this composite system consisting of stone
columns and weak soil mass can be
established theoretically and also be
verified by full-size field plate load tests.
Contrary to vibro compaction, which
densifies noncohesive soils through
vibration, vibro replacement improves
cohesive and noncohesive soils by
reinforcing the weak soil with load bearing
columns of well-compacted, coarse-
grained material. When the weak soil is
fully replaced with a well-compacted
coarser material, there is no complexity in
understanding its improved load carrying
capacity and corresponding deformations.
But, when the weak soil is partially replaced
and displaced by the introduction of these
stiffer reinforcing elements at regular grid
patterns, response of this modified ground
becomes complex. There are ways for
arriving at an equivalent stiffness matrix of
a system that replaces some part with a
material of larger stiffness. Similarly, there
are ways and means to establish the
modified density and stiffness when the
entire soil mass is densified. But when the
improvement is attributed to both
displacement and replacement, the
quantification of improvement is difficult
to determine. Considerable efforts such as
large-scale load tests can only prove the
effectiveness of the installed stone
columns. An improvement factor is
established to compare how much stone
columns increase the stiffness of the soil
they are installed in. It can be calculated
based on the area replacement ratio and the
reinforcing material used for the stone
columns.
The deformation modulus of the
composite system is another one of the
basic inputs for finalizing the design of
stone columns. However,
the reality is that in many
p r a c t i c a l c a s e s t h e
reinforcing effect of stone
columns installed by
vibro replacement is
c o m b i n e d w i t h t h e
densifying effect of vibro
compaction, i.e., the
insta l la t ion of s tone
columns densifies the soil
between grids increasing
its K0 (coefficient of earth
pressure at rest) and Kp
(coefficient of passive
earth pressure). In such
cases, the densification of
the soil has to be evaluated on the basis of
original soil data and correspondingly the
design of vibro replacement can be
modified as needed to achieve particular
improved site conditions.
Dry Vibro Stone Column MethodologyA custom-built machine called the Vibrocat
was developed by Keller for installation of
vibro stone columns without using water.
The Vibrocat comprises a specially-
constructed track mounted supporting
unit and a vibrator, which incorporates a
stone tube with compression chamber and
stone-feed hopper to ensure properly
formed compacted stone columns to the
required diameter and depth. A special
feature of the dry method is that it does not
require water jetting for penetration and
therefore eliminates the need to handle the
collected water. Furthermore, this method
can be used where limited working space is
available, especially in developed or urban
areas, or where no water source can be
found nearby. This technique provides
effective drainage paths to ensure rapid
consolidation. It also has a built-in real-
time computer monitoring system to
provide quality control on compaction
effort throughout the construction process.
Residential Complex Case HistoryUrban Tree Infrastructures (Urban Tree)
recently completed a residential project in
Chennai, India. The project consisted of 198
units with ground-level parking and four
floors above, on a 2.5 acre (10,100 sq m)
development site.
The site subsurface consisted of
desiccated clay and medium dense sand up
to about 3.5 m (11.5 ft) underlain by
relatively weak clay and sandy clay up to a
depth of 6 m (19.7 ft). This top 6 m (19.7 ft)
of soil with highly varying consistency is
underlain by about 8 m (26.2 ft) of medium
dense sand and stiff clay deposits below
which there is a 6 m (19.7 ft) thick layer of
medium stiff consistency. Denser sand layers
and hard clay layers form the remaining
subsurface profile. The groundwater table
was encountered at about 3 m (9.8 ft) below
the existing ground level.
• Design Approach
The performance of shallow foundations
for the residential complex would not be
e ffec t ive due to weak so i l l ayers
Footprint of the raft foundation Typical soil profile showing ground improvement arrangement Vibro cat in action at project site
Results of single column routine plate load test
The complex had a load intensity at the
foundation level of approximately 75 to 85
kPa (1,566 to 1,775 psf). A total footprint
area of about 5,500 sq m (59,200 sq ft)
under the raft foundation was treated. The
foundation raft was divided into six pours
for ease of construction. Individual column
loads from the super structure varied from
25 to 185 tonnes (27.5 to 204 tons). Though
the raft foundation transmits uniform
pressure to the bearing soil, a denser grid
was adopted due to the large column loads.
Designs, in terms of strength and
deformation characteristics for the proposed
loading conditions, were made using Priebe
(1995) design methodology and appropriate
geometry (stone column diameter, spacing,
pattern and depth). The final scheme was
reviewed and vetted by Geotechnical
Solutions, Chennai, a third-party specialty
geotechnical consultant.
• Quality Control and Monitoring
In order to measure and assure the quality
of stone columns being constructed, it was
necessary to adopt stringent quality control
and quality assurance procedures to meet
the specifications, and to satisfy the client’s
requirements at various stages of execution
of the project.
• During Construction
An automated computerized recording
device fitted to the Vibrocat recorded the
installation of each stone column. This
instrument yielded a computer record (M4
Graph) of the installation process in a
continuous graphical mode, plotting depth
versus time and power consumption
(compaction effort) versus time. The infor-
mation provided included:
• Stone column reference number
• Date of installation
• Start and finish times for installation
• Period required for installation
• Maximum depth
• Compaction effort during penetration
and compaction process
The above parameters allowed the qual-
ity of the stone columns being installed to be
monitored. Further, the diameter of the stone
column and consumption of backfill were
continuously monitored by the site person-
nel to estimate the in-situ achieved diameter.
80 • DEEP FOUNDATIONS • SEPT/OCT 2016
• Post Construction Stage
A full-size field plate load test is one of the
accepted ways to assess the performance of
the improved soil treated with stone
columns. The size of the test pad and the
magnitude of the test load can vary
according to the stone column layout,
treatment depth, load and type of
structure. Routine stone column load tests
were performed to ascertain the effec-
tiveness of design and performance of the
ground improvement works. The observed
settlements are within the acceptable limits
of 75 to 100 mm (2.9 to 3.9 in) for raft
foundations according to the stipulations
specified in Indian Standard Code of
Practice (IS 1904-1986) for the applied
design load intensity of 100 kPa (2088 psf).
AcknowledgementsThe authors wish to thank the
developer of the project for their
permission to publish the information
contained in this paper and Keller
management, for its continuous
support and encouragement as well as
valuable comments and suggestions
that helped the authors to refine and
improve the quality of the paper.
Thanks are also due to the Keller
India site team for providing photo-
graphs and other data.
This is an abridged version of a paper
from the Singapore Soft Ground
Conference last year. This project was
presented at the DFI-India 2016
conference in Kolkata.
• Real-time Settlement Monitoring
Success of the foundation system was
proved by full-scale monitoring of
foundation settlement during and after
completion of the project. Post con-
struction real-time monitoring offered
confidence on the engineering judgment
taken at various stages of the project
completion. The predicted design
settlements that are calculated using
conventional methods were compared with
the actual settlement occurring at the site by
adopting proper monitoring systems.
To assess the post construction
performance of the structure, 14 locations
were selected on the raft foundation to
monitor settlements during and post
construction. After the ground improve-
ment was installed, the raft foundation was
placed on the treated ground. The entire
building foundation area was divided into
six zones which were delineated based on
concrete pour.
The measured settlements were
substantially lower than the predicted
settlement, which proved the efficiency of
Settlement monitoring points
Results of observed settlements
Completed residential complex
the raft foundation resting on improved
ground. The load in the superstructure
increased with increasing number of floors
and the corresponding settlements
increased accordingly.
The superstructure load was increased
from 0 to 80 kPa (0 to 1670 psf) in 20
weeks and correspondingly predicted
settlements (analytical method) increased
from the 0 to 64 mm (0 to 2.5 in). However,
t h e o b s e r v e d s e t t l e m e n t s w e r e
considerably less than the predicted
settlements as well as the allowable
settlements of 75 to 100 mm (2.9 to 3.9 in)
for raft foundations bearing on clayey soils.
The total settlement observed at the start of
maximum loading (after 18 weeks) was
about 30 mm (1.18 in) that gradually
increased to about 50 mm (2 in) during the
next 17 weeks and remained mostly
uniform thereafter. This suggests that long-
term settlements will also be a smaller
range than predicted.
ConclusionsVibro stone columns proved to be an
effective ground improvement solution to
support the residential building on variable
weak soil deposits. It was also shown by the
results of extensive monitoring that the
required performance was achieved. In
addition to improving shear strength and
compressibility parameters, the ground
improvement solution provided a shorter
overall construction schedule and enabled
the project to be completed within the
stipulated duration.
The ground improvement works were
completed within six weeks (compared to
six months for that of pile foundations) that
was made possible through effective
project management. The project was
delivered to the end users ahead of time as a
result of the construction speed of the
alternative foundation solution. The
savings in time benefited all those involved
in the project including the end users,
suppliers, financiers and the developer.
DEEP FOUNDATIONS • SEPT/OCT 2016 • 81
80 • DEEP FOUNDATIONS • SEPT/OCT 2016
• Post Construction Stage
A full-size field plate load test is one of the
accepted ways to assess the performance of
the improved soil treated with stone
columns. The size of the test pad and the
magnitude of the test load can vary
according to the stone column layout,
treatment depth, load and type of
structure. Routine stone column load tests
were performed to ascertain the effec-
tiveness of design and performance of the
ground improvement works. The observed
settlements are within the acceptable limits
of 75 to 100 mm (2.9 to 3.9 in) for raft
foundations according to the stipulations
specified in Indian Standard Code of
Practice (IS 1904-1986) for the applied
design load intensity of 100 kPa (2088 psf).
AcknowledgementsThe authors wish to thank the
developer of the project for their
permission to publish the information
contained in this paper and Keller
management, for its continuous
support and encouragement as well as
valuable comments and suggestions
that helped the authors to refine and
improve the quality of the paper.
Thanks are also due to the Keller
India site team for providing photo-
graphs and other data.
This is an abridged version of a paper
from the Singapore Soft Ground
Conference last year. This project was
presented at the DFI-India 2016
conference in Kolkata.
• Real-time Settlement Monitoring
Success of the foundation system was
proved by full-scale monitoring of
foundation settlement during and after
completion of the project. Post con-
struction real-time monitoring offered
confidence on the engineering judgment
taken at various stages of the project
completion. The predicted design
settlements that are calculated using
conventional methods were compared with
the actual settlement occurring at the site by
adopting proper monitoring systems.
To assess the post construction
performance of the structure, 14 locations
were selected on the raft foundation to
monitor settlements during and post
construction. After the ground improve-
ment was installed, the raft foundation was
placed on the treated ground. The entire
building foundation area was divided into
six zones which were delineated based on
concrete pour.
The measured settlements were
substantially lower than the predicted
settlement, which proved the efficiency of
Settlement monitoring points
Results of observed settlements
Completed residential complex
the raft foundation resting on improved
ground. The load in the superstructure
increased with increasing number of floors
and the corresponding settlements
increased accordingly.
The superstructure load was increased
from 0 to 80 kPa (0 to 1670 psf) in 20
weeks and correspondingly predicted
settlements (analytical method) increased
from the 0 to 64 mm (0 to 2.5 in). However,
t h e o b s e r v e d s e t t l e m e n t s w e r e
considerably less than the predicted
settlements as well as the allowable
settlements of 75 to 100 mm (2.9 to 3.9 in)
for raft foundations bearing on clayey soils.
The total settlement observed at the start of
maximum loading (after 18 weeks) was
about 30 mm (1.18 in) that gradually
increased to about 50 mm (2 in) during the
next 17 weeks and remained mostly
uniform thereafter. This suggests that long-
term settlements will also be a smaller
range than predicted.
ConclusionsVibro stone columns proved to be an
effective ground improvement solution to
support the residential building on variable
weak soil deposits. It was also shown by the
results of extensive monitoring that the
required performance was achieved. In
addition to improving shear strength and
compressibility parameters, the ground
improvement solution provided a shorter
overall construction schedule and enabled
the project to be completed within the
stipulated duration.
The ground improvement works were
completed within six weeks (compared to
six months for that of pile foundations) that
was made possible through effective
project management. The project was
delivered to the end users ahead of time as a
result of the construction speed of the
alternative foundation solution. The
savings in time benefited all those involved
in the project including the end users,
suppliers, financiers and the developer.
DEEP FOUNDATIONS • SEPT/OCT 2016 • 81
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