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