This document provides an overview of stone columns, which are columns of compacted aggregate installed in soft soils to improve their load-bearing capacity and reduce settlement. Stone columns function by transferring load to the stiffer column material, allowing drainage of pore water pressures. They are installed using ramming or vibro-replacement techniques. Failure typically occurs through bulging of the column into the surrounding soil. A case study demonstrates that a highway embankment treated with stone columns at 2m spacing experienced 25% less settlement than untreated ground.

1. STONE COLUMNS: AN OVERVIEW
S.V. Abhishek, PG Student
V. Tarachand, PG Student
Department of Civil Engineering
College of Engineering (A), Andhra University
Visakhapatnam, Andhra Pradesh, India
svabhi.92@gmail.com
Department of Civil Engineering
College of Engineering (A), Andhra University
Visakhapatnam, Andhra Pradesh, India
vtarachandg@gmail.com
Abstract—The prime requirement for the development of any
country is sufficient infrastructure of buildings, roads, tunnels,
bridges and other civil engineering works. India has a vast area
of soft soils (especially along the coast) which possess low bearing
capacity and exhibit high compressibility. For the construction of
high rise buildings and other important structures, the soft soil is
bypassed by providing piles socketed into hard strata. However
for low rise buildings and flexible structures such as liquid
storage tanks and rail/road embankments that can tolerate some
settlement, adoption of ground improvement techniques such as
stone columns are preferable and more economical. In the
present paper, a broad overview on stone columns with regard to
their behavior, functions, failure mechanism and construction
techniques, is presented. The overview is complemented by a case
history of a highway embankment raised on stone column treated
ground.
Index
Terms—Stone
columns,
vibro-replacement, settlement
unit
cell,
bulging,
INTRODUCTION
Amongst various techniques for improving in-situ ground
conditions, stone columns/granular piles are probably the most
versatile, due to their ability to serve a variety of important
geotechnical functions. The use of stone columns for ground
improvement originated in Germany in the late 1950’s while in
India, the use of stone columns began in the early 1970’s.
Stone columns are basically load bearing columns of well
compacted coarse aggregate installed in soft, non-compactable
cohesive soils such as silts, clays; organic soils such as peat
and in granular soils having high fines content (in excess of
15%). Soils whose undrained shear strength range from 7 to 50
kPa and loose sandy soils also necessitate installation of stone
columns (IS:15284-Part 1).
Stone columns are often used to stabilize large areas of soil
mass to support flexible structures such as embankments, tank
farms and fills. However they are sometimes provided beneath
footing and rafts to carry structural loads. Due to the high
stiffness and angle of internal friction of the column material
when compared to that of in-situ weak soil, majority of the
applied load is transferred to the stone column. As a result, less
load is transferred to the surrounding weak soil which leads to
significant reduction in settlement. The present paper discusses
the various functions performed by stone columns, their
behavior under loads and failure mechanism. The techniques
used for installation of stone columns are described. A case
history of a highway embankment constructed on stone column
treated ground elucidates the effectiveness of this ground
improvement technique.
FUNCTIONS OF STONE COLUMNS
Stone columns perform the following important
geotechnical functions (cited by Madhav and Sivakumar,
2012): (a) Enhance the bearing capacity of foundations,
(b) Carry high shear stress by acting as stiff elements and
hence increase the stability of embankments founded on soft
ground, (c) Facilitate radial drainage (by acting as vertical
drains) and dissipate rapidly the excess pore water pressure
leading to acceleration of the consolidation process and
reduced post-construction settlements, (d) Mitigate the
potential for liquefaction and damage by preventing build up of
high pore pressure, providing a drainage path and increasing
the strength and stiffness of the ground.
INSTALLATION PATTERNS
Stone columns of diameter, D, are often installed in large
arrays at uniform spacing, S, in either square or triangular
patterns (Fig. 1).
Figure 1 Installation Patterns (IS:15284 – Part 1)

2. For such cases, a unit cell consisting of stone columns
surrounded by in-situ soil in the zone of influence can be
considered as representative of the treated area. In the unit cell
approach, the tributary area of soil surrounding each stone
column (in the form of hexagon and square for triangular and
square arrangement respectively) is closely approximated by an
equivalent circle of diameter, De. For an equilateral triangular
and square arrangement of stone columns, the equivalent circle
has an effective diameter of 1.05S and 1.13S respectively,
where ‘S’ is the centre to centre spacing of stone columns
(generally ranges from 2 to 3 m). The equilateral triangular
pattern gives the densest packing of stone columns in a given
area and is thus preferable. The resulting cylinder of composite
ground with diameter, De, enclosing the tributary soil and one
stone column is known as the unit cell.
(Hughes and Withers, 1974; Hughes et al. 1976). However,
when stone columns are installed in extremely soft soils having
undrained shear strength less than 7 kPa, the radial
confinement/restraint offered by the surrounding soil is
inadequate, resulting in excessive lateral displacement of stone
into the surrounding soil. In such circumstances, the load
carrying capacity of the stone column can be improved by
encasing the column in a suitable geosynthetic. Fig. 3 depicts
the load carrying mechanism of a single, isolated stone column
in compression. The length of stone column over which
bulging takes place is known as critical length (about 4 times
the diameter of the column).
FAILURE MECHANISMS
Stone columns are often constructed through soft soils fully
penetrating to an end bearing stratum. However, they may be
constructed as floating piles; the tips ending within the soft
layer but at depths where the strength of soil is adequate. Stone
columns may fail individually or as a group. The failure
mechanisms for a single, isolated stone column in compression
are illustrated in Fig. 2 indicating respectively, the possible
failures as a) bulging, b) shear failure and c) sliding/punching.
Figure 3 Pre-bulging failure mode of a single stone column
Apart from bulging, stone columns derive their load carrying
capacity through surface resistance or frictional resistance
developed between the column material and surrounding weak
soil acting upwards within the critical length, and also from the
passive resistance mobilized by the column material. The
portion of the stone column below the critical length does not
participate in load transfer but functions akin to a vertical drain
and accelerates the consolidation of the surrounding soft soil.
Figure 2 Failure mechanisms of a single stone column in a
homogeneous soft layer (IS:15284 – Part 1)
For single, isolated stone columns, the most probable failure
mechanisms are bulging or punching. Punching failure
mechanism controls the ultimate load for short columns resting
on soft to medium stiff bearing layer (the tip of the column is
floating in the soft soil) while bulging failure is most likely for
a long stone column irrespective of end bearing or floating
(Madhav et al. 1994). A long stone column is one whose length
is greater than its critical length (about 4 times the diameter of
the column). Practically since most stone columns are installed
upto depths of 10-15 m preferably into stiff end bearing
stratum, lateral bulging of the column into the surrounding
weak soil is the pre-dominant load transfer mechanism, i.e., the
stone columns derive their load carrying capacity from the
lateral earth pressure against bulging from the surrounding soil
INSTALLATION TECHNIQUES
The common techniques employed for installation of stone
columns are 1) Rammed Stone Column Technique and
2) Vibro-Replacement.
1) Rammed Stone Column Technique: In this technique, a
borehole is created by using a bailer. Stone chips are tipped
into the borehole and are compacted by using a rammer
(Fig. 4). Alternatively, a closed end pipe mandrel can be driven
to the desired depth. The tip valve is opened to discharge the
stone chips delivered through the pipe. The mandrel is
withdrawn until the valve can be closed and the same is used to
ram against the stone chips to expand and densify. A boring of
400-500 mm diameter generates a column of 700-900 mm in
diameter. The fill material should vary in particle size from
about 5-100 mm with not more than 15% material finer than
5 mm.

3. around the vibrator from the ground surface. The stone is
compacted and pressed into the surrounding soil by alternating
steps of retraction and re-penetration of the vibrator. Gradually,
in stages, by pouring the stone from the ground surface with
the help of a wheel loader and compaction of the stone by
vibrator, a stone column is constructed upto the platform level.
A schematic of the stone column installation by the wet
top-feed method is shown in Fig. 5.
Figure 4 Rammed stone column by bailer and chisel technique
2) Vibro-Replacement: The technique of vibro- replacement
employs a deep vibro poker (depth vibrator) which ensures
construction of properly compacted stone columns of required
diameter and depth as well as densification of the surrounding
soil between the columns. Vibro-Replacement is further
classified into two types depending on the mode of penetration
of the vibrator and feeding of stone into the borehole as,
(a) Wet Top-Feed Method and (b) Dry Bottom-Feed Method
(b) Dry Bottom-Feed Method
In this method, the vibrator supported by a purpose built
base machine called “Vibrocat”, displaces the surrounding soil
and penetrates to the required treatment depth aided by the
action of vibrations, compressed air and pull down facility
from the winch. Initially, the vibrocat positions the vibrator
over the required location of the compaction point and
stabilizes itself using hydraulic supports. A wheel loader then
fills the bucket with stones of size typically ranging from
20 mm to 40 mm.
The bucket is lifted and its contents are emptied into the air
chamber provided with an air lock. Once the air lock is closed
the aggregate is supplied to the tip of the vibrator through a
special stone tube with the assistance of pressurized air. The
vibrator penetrates into the ground and upon reaching the
design depth, it is retracted by about 0.5 to 1.0 m, causing the
aggregate in the pressurized stone tube to exit and fill the
cavity so created. The vibrator then re-penetrates into the
in-filled space resulting in effective compaction of the
aggregates into surrounding soil.
(a) Wet Top-Feed Method
In this method, the depth vibrator hung from a crawler
crane, penetrates to the required treatment depth under its own
self weight (about 17.8 kN) aided by high pressure water jets
which are an integral part of the vibrator. The high pressure
water jets create a momentary quick condition ahead of the
vibrator tip by washing out the fine soil particles and thus
permit penetration of the vibrator into the ground.
Figure 6 Schematic of Dry Bottom-Feed Method
(Courtesy of Keller Group)
Figure 5 Schematic of Wet Top-Feed Method
(Courtesy of Keller Group)
Upon reaching the required depth, stones of size typically
ranging from 40 mm to 75 mm are tipped into the annular gap
The stone column is constructed in alternating steps of
retraction and re-penetration until the aggregates in the stone
tube are exhausted. Thereafter, another charge of aggregate is
loaded into the stone tube. In this way the stone column
construction process continuous upto the platform level. After
completion of installation of all the stone columns, the surface
is leveled and if necessary compacted with a vibratory roller. A
schematic of the installation procedure is depicted in Fig .6.

4. CASE HISTORY
Oh et al. (2007) presented a case history of a highway
embankment constructed over soft estuarine clay with high
sensitivity and low undrained shear strength. The embankment
was divided into three sections, section 1 with no stone
columns, section 2 with stone columns at 2 m spacing and
section 3 with stone column at 3 m spacing. The embankment
was constructed in two stages. Each stage consisted of a fill
height of 2 m and thus the final height of the embankment was
4 m. The side slopes of the embankment were 1(V):2(H) and
the base width was 20 m. The diameter and length of the stone
columns are 1 m and 14 m respectively. The columns were
installed in square pattern using the vibro-replacement
technique. Fig. 7 depicts the geometry of the embankment over
stone column treated ground while Fig. 8 shows the variation
of the liquid limit (wL), plastic limit (wP), natural moisture
content (wn) and undrained shear strength (su) of the soft clay
with depth.
(from Oh et al. 2007)
The soil profile at the site consisted of 14 m thick deposit
of very soft to soft estuarine clay overlying moderately dense
to dense sandy sediment stratum. The natural moisture content
of the soft clay varied from 60-100% and was greater than the
liquid limit. The undrained shear strength of the soft clay was
low and varied from 5-20 kPa. The compressibility of the soft
clay ranged from 0.5-3.5 m2/MN while the coefficient of
consolidation varied from 0.2 to 0.3 m2/year. Settlement gauges
were installed at various locations in the embankment to
monitor the deformations of the underlying soft clay.
Figs. 9 (a), (b) and (c) show the measured settlement profiles
corresponding to the embankment on untreated soft clay,
embankment with stone columns at 3 m and 2 m spacing
respectively, at different monitoring periods. It can be observed
that the settlement of the embankment increased with time for
both untreated as well as stone column treated case. The
maximum measured post-construction settlements were about
520 mm, 495 mm and 390 mm for the untreated, 3 m spaced
and 2 m spaced stone columns respectively. The reduction in
settlement was about 5% and 25% of the settlement for the
untreated case, corresponding to the 3 m and 2 m spaced stone
columns, respectively.
Figure 7 Geometry of embankment over stone column treated
ground, all dimensions in m (after Oh et al. 2007)
(a) Untreated soft clay
Figure 8 Variation of wL, wp, wn and su with depth

5. (b) Stone columns at 3 m spacing
well as seismic conditions. The case history presented,
compares the response between an untreated embankment and
embankment treated with stone columns installed at 2 m and
3 m spacing over soft estuarine clay. The embankment treated
with stone columns spaced at 2 m centre to centre experienced
the least settlement when compared to the other cases.
REFERENCES
(c) Stone columns at 2 m spacing
Figure 9 Measured Settlement Profiles (from Oh et al. 2007)
CONCLUSIONS
Stone columns are one of the most versatile techniques for
engineering the ground. They can be installed to improve a
variety of ground conditions through several variants of the
technique such as rammed stone columns and
vibro-replacement (wet top-feed and dry bottom-feed
methods). The in-situ ground is improved by reinforcement,
densification and drainage functions performed by the stone
columns. Further, they are equally effective under normal as
[1] Hughes, J.M.O. and Withers, N.J. (1974) “Reinforcing of Soft
Cohesive Soils with Stones Columns”, Ground Engineering,
Vol. 7, No.3, pp. 42-49.
[2] Hughes, J.M.O., Withers, N.J. and Greenwood, D.A. (1976) “A
Field Trail of Reinforcing Effect of Stone Column in Soil”,
Proceedings of Ground Treatment by Deep Compaction,
Institution of Civil Engineers, pp. 32-44.
[3] IS:15284 – Part 1 (2003) “Design and Construction for Ground
Improvement-Guidelines for Stone Columns”, Bureau of Indian
Standards, New Delhi, India.
[4] Madhav, M.R., Alamgir, M. and Miura, M. (1994) “Improving
Granular Column Capacity By Geogrid Reinforcement”,
Proceedings of 5th International Conference on Geotextiles,
Geomembranes and Related Products, Singapore, pp. 351-356.
[5] Madhav, M.R. and Sivakumar, V. (2012) “Perspectives in
Granular/Stone Columns Engineered Ground”, Proceedings of
the International Conference on Ground Improvement and
Ground Control”, Australia, pp. 621-628.
[6] Oh, E.Y., Balasubhramaniam, A.S., Sorarak, C., Bolton, N.,
Chai, G.W.K., Huang, M. and Braund, M. (2007) “Behaviour of
a Highway Embankment on Stone Column Provided Estuarine
Clay”, Proceedings of 16th South East Asian Geotechnical
Conference, Kaula Lumpur, Vol. 1, pp. 567-572.

6. (b) Stone columns at 3 m spacing
well as seismic conditions. The case history presented,
compares the response between an untreated embankment and
embankment treated with stone columns installed at 2 m and
3 m spacing over soft estuarine clay. The embankment treated
with stone columns spaced at 2 m centre to centre experienced
the least settlement when compared to the other cases.
REFERENCES
(c) Stone columns at 2 m spacing
Figure 9 Measured Settlement Profiles (from Oh et al. 2007)
CONCLUSIONS
Stone columns are one of the most versatile techniques for
engineering the ground. They can be installed to improve a
variety of ground conditions through several variants of the
technique such as rammed stone columns and
vibro-replacement (wet top-feed and dry bottom-feed
methods). The in-situ ground is improved by reinforcement,
densification and drainage functions performed by the stone
columns. Further, they are equally effective under normal as
[1] Hughes, J.M.O. and Withers, N.J. (1974) “Reinforcing of Soft
Cohesive Soils with Stones Columns”, Ground Engineering,
Vol. 7, No.3, pp. 42-49.
[2] Hughes, J.M.O., Withers, N.J. and Greenwood, D.A. (1976) “A
Field Trail of Reinforcing Effect of Stone Column in Soil”,
Proceedings of Ground Treatment by Deep Compaction,
Institution of Civil Engineers, pp. 32-44.
[3] IS:15284 – Part 1 (2003) “Design and Construction for Ground
Improvement-Guidelines for Stone Columns”, Bureau of Indian
Standards, New Delhi, India.
[4] Madhav, M.R., Alamgir, M. and Miura, M. (1994) “Improving
Granular Column Capacity By Geogrid Reinforcement”,
Proceedings of 5th International Conference on Geotextiles,
Geomembranes and Related Products, Singapore, pp. 351-356.
[5] Madhav, M.R. and Sivakumar, V. (2012) “Perspectives in
Granular/Stone Columns Engineered Ground”, Proceedings of
the International Conference on Ground Improvement and
Ground Control”, Australia, pp. 621-628.
[6] Oh, E.Y., Balasubhramaniam, A.S., Sorarak, C., Bolton, N.,
Chai, G.W.K., Huang, M. and Braund, M. (2007) “Behaviour of
a Highway Embankment on Stone Column Provided Estuarine
Clay”, Proceedings of 16th South East Asian Geotechnical
Conference, Kaula Lumpur, Vol. 1, pp. 567-572.