Efficacy of Densification Techniques for Bearing Capacity Improvement
IGC 2009, Guntur, INDIA
EFFICACY OF DENSIFICATION TECHNIQUES FOR BEARING
CAPACITY IMPROVEMENT
N. Unnikrishnan & A.S. Johnson
Lecturer in Civil Engineering, College of Engineering Trivandrum–695 016, India.
E-mail: [email protected], [email protected]
ABSTRACT: Compaction sand piles are being employed for the ground improvement of a variety of subsoil conditions. The
efficacy of compaction sand piles in achieving the required strength is found to depend on a number of parameters. This
makes prediction of improved strength difficult and necessitates post-improvement verification through in-situ tests. The
failure of the technique to achieve the desired improvement results in the need to consider alternate proposals. This causes loss of
time and project time overruns. Case histories of success, moderate success and failure of the application of compaction sand
piles are discussed. The possible mechanisms behind the observed performances are indicated. Densification through the
installation of compaction sand piles is an effective ground improvement technique under suitable ground conditions.
1. INTRODUCTION
Sand is considered to be a good material to support
foundation loads if confined and with good relative density.
Densification techniques such as sand piling and dynamic
compaction have found to improve the relative density of
loose sand deposits resulting in improved bearing capacity.
Among the two techniques, compaction sand pile has gained
wider acceptance due to the simple and scalable nature of
equipment involved, environment friendliness and cost
savings compared with other techniques. In compaction sand
piling, densification is achieved by driving a casing pipe with
a detachable non-retrievable shoe at the end to the required
depth, filling the pipe with well graded sand and ramming
the same while withdrawing the pipe. Figure 1 shows the
heavy duty compaction sand pile installation rig. The lateral
push of the pipe, vibrations of driving and the injected
volume of additional sand results in improved relative
density. The technique has found to work effectively in the
case of pure sands. In reality, pure sand deposits are rare. The
subsoil available in many locations of the state of Kerala is
has sand as a major constituent. Often, the sand deposits do
contain silt and clay fractions in the range of 15-40%. The
efficacy of sand piling is affected by the properties of this
small but influential fine content. In addition to this, the
initial relative density also has found to influence the
efficacy of compaction sand piles.
Strength of the densified ground has been investigated by
several researchers. Standard Penetration Test (SPT) has
been employed for this purpose by a number of investigators
(e.g. Nakayama et al. 1973). Several case histories of
densification through compaction sand piles have also been
published (Moh et al. 1981). Methods for estimating the
effectiveness of compaction sand piles have also been
suggested (Farias et. al. 2005, Shamoto et. al. 1997).
However, certain site restrictions, skill of labourers, quality
of sand used and equipment limitations hinder the application
of any such predictive methods.
The compaction sand pile technique has been widely adopted
by the Geotechnical community as a ground improvement
technique (Mitchel, 1970). Owing to its capability to reduce
the liquefaction potential of loose saturated sand deposits,
research on the technique has gained prominence in the
countries like Japan (Tokimatsu et al. 1990, Tsukamoto et al.
2000, Okamura et al. 2003, Murali Krishna et al. 2006).
451
Drop
Weight
Casing
pipe
Engine
Winch
Detachable cone
Fig. 1: Schematic Diagram of Compaction Sand Pile Rig
Efficacy of Densification Techniques for Bearing Capacity Improvement
The significance of in-situ geotechnical conditions in
determining the efficacy of the technique is explained with
the help of few case histories. The situations that led to the
choice of sand piling as a foundation solution are explained.
Strength of improved ground was investigated through postimprovement in-situ testing. The analysis of such case
histories throw light into the underlying phenomenon and the
also the parameters involved embarking upon a densification
programme.
2. CASE HISTORY I
It was proposed to construct a five storied hotel building at a
site on the side of the NH by pass in the Trivandrum City. A
three storied building already existed within the site. The
geotechnical investigation details of the existing building
were not available. The building was reported to be
supported on isolated column footings, dimension of which
were again not available. The new building was to be
constructed in close proximity to the existing building. The
site is known to have been filled with imported lateritic soil
up to a depth of 2.5 to 3 m above the existing natural surface.
Preliminary investigation was carried out using hand auger
boring up to a depth of 15m below the existing ground level.
Standard Penetration Test (SPT) was conducted at regular
intervals. The samples were analysed for particle size, natural
moisture content and shear parameters as deemed necessary.
Three boreholes were taken at the site. The typical borehole
profile is shown in the Figure 2 (a). Since the site was being
used for parking heavy vehicles, the top lateritic soil fill soil
got compacted over a period of time. However, since the
building was proposed to have a basement floor (cellar)
below the existing ground, the compacted fill needs to be
removed during the construction.
Below the lateritic soil fill (2.5 to 3 m depth), a layer of silt
sized material with presence of decayed vegetation was
encountered. This extended for a thickness of 1.5 to 2 m.
Below this layer, silty fine sand was obtained. The SPT
penetration resistance (N value) varied in the range of 7–13.
The building had several constraints in the disposition of
column positions due to mandatory requirements as well as
architectural considerations. This resulted in large column
spans. A soil stratum capable of supporting deep foundations
was not encountered. Water table occurred at depths of 2.1 to
2.4 m below the ground level. Due the nearness of the
existing structure, the presence of water table and the
presence of silty sand with little fine content, it was not
possible to excavate below the proposed basement levels and
remove the organic material and replace the same with good
quality material. It was thought that the presence of organic
layer will render densification techniques ineffective. Hence
it was recommended to carry out mechanised boring to
explore deeper depths. The design of deep foundations could
then be based on the availability of hard stratum within
reasonable depths. In deep boring, the boreholes were
extended up to a depth of about 45 m. Beyond a depth of
15 m, layers of silty fine and medium sand as well as clayey
silt/silty clay was encountered up to a depth of 45 m. Bearing
stratum of consistent characteristics and good shear strength
was not encountered in either of the boreholes. Exploring
further depths was considered unnecessary by the client as
the cost of deep foundations at greater depths would be very
high.
Under these circumstances and left with no other options, the
possibility of improving the ground through densification
and supporting the building on raft foundation was
considered. Densification through installation of sand
compaction piles was decided upon. The rationale behind the
consideration was that the organic layer may get partially
replaced by the injected sand, resulting in reasonably good
shear strength. Compaction sand piles 100 mm diameter
spaced at 45 cm centre to centre were installed to a depth of
6m below the existing ground level. A zigzag pattern was
followed. The pile was installed by driving a casing, 10 cm
in diameter with a detachable (non-retrievable) concrete
conical shoe at the bottom. The hole was back filled with
well-graded sand while withdrawing the casing pipe. After
installing a few sand piles, it was decided to check the
efficacy of the technique through in-situ tests. Figure 2 (b)
shows the site profile of the improved ground.
It was found that the technique did not result in marked
improvement in shear strength of the organic layer. To
encourage the replacement of organic content with the
injected sand, attempt was made to reduce the spacing and
even install one sand pile over the other. Though there was
considerable increase in the penetration resistance above and
below the organic layer (Figure 2 (c)), the N value obtained
in the organic layer remained zero. Thus the densification
technique had to be abandoned, as it did not result in the
required improvement. Ultimately, a system of piled raft was
adopted, with the short piles resting in the sand deposit
below the organic layer. This layer had undergone
densification due to the installation of compaction sand piles.
The capacity of the piles was considerably low due low shear
strength of the bearing stratum and also taking the possibility
of negative drag.
The reason for the failure of the attempt to densify could be
that the organic layer with N < 0 could not be replaced
through the compaction sand pile installation method.
However, the improvement brought about below the organic
layer was adequately used to support the system of short
piles.
452
Efficacy of Densification Techniques for Bearing Capacity Improvement
BH2
1.5m
100
80
60
40
20
N Values
0
Profile
+0m
1.5m
2.6m
3m
4.5m
4.5m
6m
Profile
+0m
BH2
80
60
40
20
7.5m
7.5m
N Values
0
Profile
+0m
100
6.5m
6.6m
0.5m
1.5m
0.4m
9.3m
9m
9m
9m
1.5m
10.5m
10.5m
N Values
0
6m
100
BH2
5.1m
80
4.9m
60
3m
40
2.5m
20
100
80
60
40
20
sand piles with casing diameter of 100 mm and depth of
4.5 m, the piles being spaced at 350 mm centre to centre.
After carrying out the improvement, SPT was carried out at
the site. Figure 3 shows the N values post improvement. It
was found that the N values almost doubled after the
improvement. The building could be supported over isolated
column footings on the improved ground.
BH2
N Values
0
Profile
+0m
3m
10.5m
3m
11.6m
4m
4.5m
3.9m
12m
4.5m
13.1m
13.5m
5.3m
14.8m
6.8m
6m
14.1m
(a)
(b)
6m
7m
7.5m
7.5m
8.7m
9m
10m
10.5m
8.3m
BH2
5.7m
9m
10.3m
9.7m
10.5m
100
80
60
40
20
N Values
0
Profile
+0m
10.3m
Fig. 3: SPT Before and After Installation of Compaction
Sand Piles
1.5m
2.5m
3m
4. CASE HISTORY III
4.5m
5.3m
In another site that was situated on the side of a river in the
interior of the Trivandrum City, soil investigation for a three
storied building was carried out with the help of hand auger.
The samples obtained in SPT revealed the presence of silty
sand for most of the depths. The proportion of silt was in the
range of 35–40%. The sand comprised mainly of fine
fraction and medium fraction to a lesser extent. Exploration
was continued up to 15 m below the ground. Hard stratum
capable of supporting deep foundations was not encountered
within these depths. N values in the range of 3–6 were
obtained within depths up to 6 m.
6m
7m
7.5m
8.7m
10.5m
9m
10.5m
(c)
Fig. 2: SPT Before and After Installation of Compaction
Sand Piles
3. CASE HISTORY II
This a case history relating to the ground improvement
carried out at a site on the coastal reaches of the Trivandrum
City. An investigation was carried out for the proposed three
storied guest house of an Institution. The site directly faced
the sea and was on the side of a river estuary. Preliminary
investigation was carried out using hand auger boring and
SPT at regular intervals. The test revealed that the soil
stratum comprised of loose fine and medium sand deposits
with N values in the range of 6–8 up to a depth of 4.5 m.
Dense sand stratum capable of supporting deep foundations
(N > 100) was encountered towards the bottom of the
borehole (10.5 m). Exploring further depths was not
considered feasible. Water table was at a depth of 0.5–0.8m
below the ground level. It was decided to carry out a
densification programme as outlined earlier for improving
the relative density of sand. The densification was carried out
over the entire plan area of the building using compaction
Compaction sand piles were chosen as a method to improve
the relative density of the silty sand deposits within shallow
depths. Accordingly, sand piles were installed by driving
100 mm diameter casing pipe with detachable shoe up to a
depth of 5 m. SPT was carried out after the improvement to
assess the degree of improvement achieved. The postimprovement SPT revealed that the N values could only be
improved only marginally to a range of 5–9. The building
was eventually designed to be supported on raft foundation at
a depth of 2 m below the ground.
5. DISCUSSIONS
The compaction sand pile is a technique widely being used
for ground improvement, especially in the case of loose sand
deposits. The degree of improvement needs to be decided
based on the existing sub soil conditions and the requirement.
Higher the volume of sand injected, better will be the
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Efficacy of Densification Techniques for Bearing Capacity Improvement
improvement. In addition to the soil that is being introduced,
the shock vibration of the dynamic driving of the casing also
leads to densification. However, it has been noticed that if
the initial relative density of the sand deposit is very low, the
degree of improvement achieved is very low, irrespective of
closer spacing of the piles. This may be due to the fact that,
as the initial shear strength is very poor (N < 3), the driving
of the casing results in punching failure. Due to this, the soil
below the driven casing pipe does not move sideways. In the
case of sand deposits with relatively better shear strength, the
driving results in lateral movement of soil, rather than
punching. A soil profile with progressively increasing
relative density can be improved further as the chances of
punching shear are less.
It can be noticed from the above case histories that, the other
conditions remaining same, the efficacy of sand compaction
piles depends a lot on the composition of the in-situ soil (viz.
the proportion of fine content and the grain size distribution
of coarse content). A well-graded sand can be densified to a
greater degree than a poorly graded or uniform graded sand.
This is because the particles can move to a compact
arrangement with the driving vibrations and the sideways
movement. However, as the proportion of fines increases, the
efficacy of sand compaction pile diminishes. This
phenomenon is manifest in the Case History 3.
In the Case History 1 described above, it was noticed that the
sand compaction pile installation has resulted in improved
relative density above and below the organic layer. The
technique would have been the most suitable for the site
conditions, but for the organic layer. With the cellar floor,
the building foundation after improvement was poised to be
founded just above the organic layer. The presence of water
table and another framed building in the site prevented a
complete replacement of the organic layer. The idea of
completely replacing the organic layer even with compaction
sand piles at very close spacing and even one over the other
did not yield favourable results. This was revealed by SPT
after three stages of rigorous improvement.
6. CONCLUSIONS
Compaction sand piles are a cost effective and suitable
ground improvement technique. The technique assumes
importance in the light of the vast deposits of loose sand
especially near the coastal regions. The typical case histories
discuss the efficacy of the technique under a variety of
ground conditions. A perfect understanding of the sub soil
profile is essential before choosing the compaction sand pile
technique. The efficacy of the technique is affected by
presence of fine grained soil as well as the gradation of the
sand. The initial relative density also plays an important role
in deciding the effectiveness of the technique. The technique
has been successful in improving the ground to requisite
levels under favourable ground conditions. The experience
gained through the described case histories may help
Geotechnical Engineers to judiciously choose such
favourable site conditions.
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