study of extent of soil improvement by employing vibro
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International Conference on Case Histories in Geotechnical Engineering
(2013) - Seventh International Conference on Case Histories in Geotechnical Engineering
02 May 2013, 4:00 pm - 6:00 pm
Study of Extent of Soil Improvement by Employing Vibro-Study of Extent of Soil Improvement by Employing Vibro-
Compaction Technique Compaction Technique
Tejas Belani TATA Consulting Engineers Ltd., India
Shailendra Nagrikar TATA Consulting Engineers Ltd., India
Deepak Raj Keller Ground Engineering (I) Pvt. Ltd., India
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STUDY OF EXTENT OF SOIL IMPROVEMENT BY EMPLOYING
VIBRO-COMPACTION TECHNIQUE Tejas Belani
TATA Consulting Engineers Ltd.
Mumbai – 400083, Maharashtra, India.
Email: [email protected]
Shailendra Nagrikar TATA Consulting Engineers Ltd.
Mumbai – 400083, Maharashtra, India.
Email: [email protected]
Deepak Raj
Keller Ground Engineering (I) Pvt. Ltd.
Mumbai - 400059, Maharashtra, India.
Email: [email protected]
ABSTRACT
India has seen a substantial growth in various Soil Improvement Techniques over last decade. A particular technique has been chosen
based on Project location, Sub soil profile, Technical requirements, Time available and allowed budget for foundation. An expansion
was planned for a leading fertilizer plant in North India. Project was located in high seismic zone having PGA of 0.24g. The site was
underlain by liquefiable loose fine sands up to depth of 10m to 12m. Existing plant structures were built on Stone Columns few years
back. This paper presents a case study where Vibro-compaction technique was successfully used to mitigate liquefaction and to
increase bearing capacity. It also outlines the detailed geotechnical investigation carried out using standard penetration test (SPTs) and
Electric Cone Penetration Test (ECPTs) to confirm the soil improvement in the treatment area. In addition, radial effect of Vibro-
compaction outside the treatment area is also studied and reported.
Paper No. 6.44a 1
INTRODUCTION
A leading Fertilizer plant located in North India is in service
for more than two decades. In order to ease out current
operations and incorporate automations, the expansion of
existing Wagon Loading Platform (WLP) needed to be
increased by 375.0m in length and 18.0m in width along with
associated MCC room (MCC) of size 21.0m x 12.5m.
(Fig.1).The plant is located in high seismic zone having
magnitude of 7 and PGA of 0.24g. Existing Plant structures
were built on Stone columns. Due to time constraint and limited allocation of budget for project, Engineers were asked
to choose a cost effective but technical sound foundation
system. Fig 1 indicates the location plan of expansion works at
site. Geotechnical investigation at site, revealed its
susceptibility to liquefaction under seismic event, due to
presence of loose sandy soil and high ground water table.
Thus, site required soil improvement to mitigate liquefaction
potential and enhance the soil safe bearing pressure. Vibro-
compaction technique was chosen due to sandy strata
available and to economize the cost of foundation.
GEOTECHNICAL MODEL
Site Location Plan
The locations of WLP and MCC are shown in Fig 1.
Fig.1. Location Plan for WLP and MCC area.
At site, to assess the subsoil condition, Standard Penetration
Tests (SPTs) were conducted by drilling two boreholes BH1
and BH-2 up to a maximum depth of 16.0m below existing
ground level (EGL). Three Electric Cone Penetration Tests
(CPTs) CPT-2, CPT-3 and CPT-4 were also carried out up to a
maximum depth of 11.0m below EGL at WLP and MCC room
area. Mechanical refusal was noted in CPTs beyond 11.0m
Paper No. 6.44a 2
from EGL. Several field tests have gained common usage for
estimation of in-situ subsoil condition; however SPTs and
CPTs have been preferred because of more extensive
databases and past experience with these tests parameters,
especially in evaluating liquefaction potential for the site.
Site Subsoil Profile
The subsoil layers encountered at BH-1 and BH-2 are
presented in Fig-2. The subsoil profile consists of top 2.0 -
3.0m sandy clayey silt to silty clayey sand (CL/ML-CL)
containing fines more than 50%. From 3.0m to 12.0m soil
consists of poorly graded saturated fine sand (SP-SM) in loose
to medium dense state with fines less than 10%. This layer is
underlain by stiff to hard silty clay to clayey silt / dense to
very dense sand up to termination depth.
Ground Water Table
The ground water table was observed at a depth below 3.0m to
3.2m from EGL from boreholes BH-1 and BH-2. However,
for design analysis, it was considered at a depth of 1.50m
below EGL considering the fluctuations in water levels.
Fig. 2. Subsurface profile pre soil improvement.
ASSESSMENT OF LIQUEFACTION POTENTIAL AND
SAFE BEARING CAPACITY
Liquefaction Potential Analysis
The liquefaction phenomenon is a state where the soil looses,
most of the shear strength under seismic forces due to sudden
rise in pore water pressure of soil. This is more prominently
observed in loose saturated cohesion less soils.
The soil is susceptible to liquefaction if the estimated cyclic
stress ratio (CSR) based on shear stress caused due to seismic
event exceeds the estimated cyclic resistant ratio (CRR) based
on resistance offered by the particular soil strata. At site,
presence of loose saturated sand falling under the soil
classification of ‘SP’ with low SPT N values up was observed up to 9.0m depth from EGL indicated its susceptibility to
liquefaction.
Liquefaction potential was analyzed for earthquake magnitude
of 7.0 and peak ground acceleration (PGA) of 0.24g (Seismic
zone IV - IS 1893) as per project specifications. Detail
analysis have been carried out using simplified procedure as
described in the 1996 NCEER and 1998 NCEER /NSF
workshops on evaluation of liquefaction resistance of soils
(Youd et.al., 2001) using SPTs and CPTs test results.
Computation of CSR and CRR
Two variables are required for evaluation of liquefaction
resistance of the soils (1) the seismic demand on a soil layer,
expressed in terms of cyclic stress ratio (CSR) and (2) the
capacity of soil to resist liquefaction, expressed in terms of
Cyclic Resistance Ratio (CRR).
CSR. It is evaluated from selected PGA, total and effective overburden stresses at various depths and correction factors
using equation formulated by Seed and Idris (1971) for
calculation of the cyclic stress ratio.
CRR Based on SPTs. The SPT N values measured in the field
are normalized to an effective overburden pressure σvo’ of approximately 100 kPa, other correction factors like hammer
energy ratio (ER), borehole diameter, Rod length, Sampling
method as indicated in Summary Report of NCEER Workshop
(2001) are applied to obtain SPT (N1)60 values. The SPT (N1)60
values have been further corrected for the fines content to
obtain SPT (N160)cs (Youd et. al., 2001). Fig. 3, represents evaluation of CSR and CRR based on the SPT tests results
obtained from site.
Paper No. 6.44a 3
Fig.3. CSR – CRR vs Depth Pre Soil Improvement from SPT.
CRR based on CPTs. The normalized cone penetration resistance (qc1N)cs is obtained from qc measured in the field,
after accounting overburden stress σv0’ at effective equivalent
overburden pressure of approximately 100 kPa, soil behavior
Index Ic, Friction ratio F (%) and correction factor for fines
content kc.
Thus, the CRR obtained from SPTs and CPTs were modified
for magnitude scaling factor. (Youd et al., 2001)
Fig.4. CSR – CRR vs Depth Pre Soil Improvement from CPT.
Figure 4, indicates evaluation of liquefaction computed in
terms of CSR and CRR based on the CPT test results
conducted in WLP and MCC area.
Allowable Net Safe Bearing Pressure (SBP) of soil
SBP estimated from the geotechnical investigation tests results
for WLP and MCC, are indicated in table 1 below.
Table 1. SBP for WLP and MCC Structure Foundations.
Area Footing Type
Footing Depth
below
EGL
Footing Size
SBP for
25mm
settle
ment
(kPa)
Required SBP @
25mm
Settlemen
t (kPa)
WLP Isolated 2.0m
2.0m x
2.0m 104
120 3.0 x
3.0m 78
MCC Isolated 2.5m
2.5m x
2.5m 89
150 3.5m x
3.5m 87
The above detailed assessment indicated need for mitigating liquefaction and enhancing allowable soil bearing pressure by
using suitable soil improvement technique in WLP and MCC.
SUITABILITY OF SOIL IMPROVEMENT METHOD
Site being underlain by loose clean sand below founding
depth, Vibro-compaction technique was chosen as suitable
ground improvement technique against stone column or driven
piling. The selection was done after taking into account of the
following factors.
Technical Compliance
A geotechnical investigation carried out by employing CPTs
indicated friction Ratio, F in the range of 0.5% to 2.2% with
average lower than 2.0% and relative density of soil in the
range of 15% to 35% from 2.5m to 11.0m depth below EGL
(Fig. 10), laboratory tests results indicated fines content < 10%
from 3.0m to 10.5m. Hence, above parametric study indicated
suitability of vibro-compaction technique.
0
2
4
6
8
10
12
14
16
0.0 0.2 0.4 0.6 0.8
Dep
th (
m)
CSR - CRR
CSR
Pre
CRR,
BH-2
0
2
4
6
8
10
12
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
DE
PT
H (m
)
CSR - CRR
CSR
CRR Pre
CPT-2
CRR
PRE
CPT-3
Paper No. 6.44a 4
The employed technique helped to mitigate the assessed
liquefaction potential by rearranging the particles and
increasing the relative density of soil mass to minimum of
60% RD.
Quality Assurance and Quality Control Methods by Using
Computerized Equipments.
By employing advanced construction equipment and
technology, the quality assurance and quality control of the
work executed was constantly monitored; using real time
monitoring system equipped with automated data loggers.
Material Availability and Convenience
The backfill material (Sand) used for vibro-compaction technique was easily available and in plenty. Thus, the
technique selected was well suited with the backfill material
available compared to next available solution of stone
columns where in the required stone aggregate would need to
be borrowed from a quarry located about 150 to 200 Kms
away from the site. Driven Piling would have been very costly
and more time consuming solution.
Time and Cost
Significant saving in time and cost could be achieved by
employing vibro-compaction technique against driven piles or
stone column construction.
OVERVIEW OF CONSTRUCTION PROCEDURE OF
VIBRO-COMPACTION USING VIBROFLOT.
By employing Vibro-compaction technique, deep in-situ loose
soil grains were rearranged into a denser array by insertion of
a vibratory probe with simultaneous vibration and saturation.
The basic principle behind the process is that particles of non cohesive soils can be arranged into denser state by means of
Vibration.
In order to design the effectiveness of spacing for vibro-
compaction and extent of soil improvement, the trial area of
30.0m x 30.0m was selected, which was set out from the
established benchmark at site using conventional land
surveying techniques on a square grid pattern. At the site,
spacing of compaction points in a square grid of 2.75m x
2.75m was adopted predetermined based on various trials and degree of compaction achieved.
The soil improvement was extended beyond foundation line
corresponding to an angle of 30 degree to vertical axis from
foundation periphery. During the trial area works,
contamination of top clayey silty soil with underlain sand
layer of about 2.0m vertical depth was observed based on
friction ratio obtained from post CPT test results. MCC room
founding level below 1m of EGL, a combination of Vibro
compaction with Vibro Stone columns were adopted to increase stiffness of top fine grained soil. A schematic
diagram is shown in Fig. 5.
Fig.5. Sketch showing the arrangement combination of vibro
techniques at MCC Room of the plant
Equipments for vibro-compaction work at site comprised of a
Vibroflot probe, power supply system, water pump, crane and
front end wheel loader. The vibrator was connected to a source
of electric power and a high pressure water pump. Extension
tubes were added as necessary, depending on the treatment
depth. The vibrator suspended from a crane, when, lowered down into the ground, vibro driver activated the probe to
vibrate in the lateral direction and imparted a lateral impulse.
Thus, the combination of vibration and high pressure water
jetting enabled Vibroflot to penetrate the soil. The Vibroflot
was penetrated to desired depth and was hold for designed
time or designed amperage, whichever occurred earlier before
extraction. Then the vibrator was pulled up in short steps of
0.5m vertical interval. The effectiveness of compaction was
visible at ground surface in the form of a cone shaped
depression (crater formation) (Fig.6). At surface, the crater of
diameter 1.45m to 1.55m was observed for the peripheral
(outer) columns while it decreased to about 1.25 to 1.35m diameter for internal points. This was due to the densification
achieved by the peripheral / adjacent columns. The depression
around the extension tubes were continually filled with clean
sand by front end wheel loader. The procedure was continued
step wise until the vibrator reached the surface. On completion
of each panel the area was leveled, prior to post compaction
tests. Consumption of backfill material (sand) was observed in
the range of 10% to 12% after treatment up to 12.0m depth.
The complete process of penetration and compaction was
monitored using real time automatic computerized data logger.
Paper No. 6.44a 5
Fig. 6. Crater Formation during vibro-Compaction.
PERFORMANCE ASSESSMENT POST SOIL
IMPROVEMENT
Post soil improvement, the field tests SPTs and ECPTs were
conducted within and adjacent to the treatment area.
Mechanical refusal beyond 5.0m depth below EGL was noted
during Electric cone penetration tests. Further PLTs were
conducted at footing locations to confirm the reduction in
settlement at desired loading intensity as explained later part
of the paper.
Performance Assessment based on Standard Penetration Tests
(SPTs)
Figure 7, indicates the comparison of corrected SPT N values
vs depth for pre soil improvement (Pre) and post soil
improvement (Post) for BH-1 and BH-2 conducted at site.
Fig.7. Measured SPT N, Pre and Post soil improvement for
boreholes BH-1 and 2.
Figure 8, indicates Post CRR values have significantly
improved compared to Pre CRR values for boreholes BH-1
and BH-2.
Fig.8. SPT based CSR – CRR Post Soil Improvement.
Performance Assessment based on Cone Penetration Tests
(CPTs)
Figure 9, indicates Post CRR values have improved
considerably compared to Pre CRR values below 2.5m depth.
Fig. 9. CSR – CRR Post Soil Improvement based on CPTs.
0
2
4
6
8
10
12
14
16
5 10 15 20 25 30 35
Dep
th (
m)
Measured SPT N
Pre
BH-1
Pre
BH-2
Post
BH-1
Post
BH-2
0
2
4
6
8
10
12
14
16
0.00 0.20 0.40 0.60 0.80 1.00
DE
PT
H (m
)
CSR - CRR
CSR
Post
CRR,
BH-1 Post
CRR,
BH-2
0
2
4
6
8
10
0.0 0.2 0.4 0.6 0.8
DE
PT
H (m
)
CSR - CRR
CSR
CRR Post
CPT-2
CRR POST
CPT-3
CRR POST
CPT-4
Paper No. 6.44a 6
In top 0.0m to 2.5m not much improvement is seen in values.
This is because of inadequate compaction due to presence of
fines in top depths. Below 2.5m depth from EGL, considerable
improvement in CRR value is observed.
Figure 10, indicates the comparison of Pre and Post cone
resistance qc (MPa), vs depths for CPTs conducted in WLP
and MCC and equivalent qc (MPa) for 60% and 35% relative
density of soil obtained using Schmertmann’s co-relation.
Fig.10. Measured cone resistance, qc (MPa) Pre and Post soil
improvement, average qc -pre soil improvement and
equivalent qc for Relative Density, RD-35% and 60%.
Performance Assessment based on Plate Load Test (PLTs)
Plate load tests were carried out with procedures stipulated in
IS 1888-1982, at two locations in WLP and single column
load test at as per IS 15284 (Part-1) at one location in MCC
room area, to assess, the settlement characteristics and
improvement in SBP.
Wagon Loading Platform Area: Two plate load tests were
conducted at a depth of 2.0m below EGL using a square plate
of size 0.6m x 0.6m up to design load intensity of 120 kPa.
The test was performed two times the design load intensity i.e. up to 240 kPa. Total net settlement at design load of 120 kPa
was observed to be 2.91mm and 2.42mm. For a footing size of
2.0m width and design load intensity of 120 kPa, the
settlement was estimated as 4.0mm from plate load test
results.
MCC Room Area: Since MCC room was improved by
combination of vibro-compaction with vibro Stone Columns due to elevated founding level of 2m below EGL, a single
column load test was done up to design load intensity of 150
kPa with test load up to 1.1 times the design load. Tributary
area of the test column was excavated up to 2.0m below EGL.
The square plate of size 1.8m x1.8m was used for the test. The
final net settlement for design load intensity of 150 kPa was
noted as 1.42mm.
RADIAL EXTENT OF IMPROVEMENT OUTSIDE THE
COMPACTION AREA
Generally, half the grid spacing is expected to achieve desired
degree of compaction from the last vibro-compaction point.
To study the radial extent of compaction achieved, cone
penetration tests CPT-2 at a distance of 0.25m, CPT-9 at a
distance of 1.95m, CPT-15 at a distance of 4.70m and CPT-7
at a distance of 7.25m from last vibro-compaction point were
conducted to compare the improvement. As observed in Fig.
11, post soil improvement the test results indicated that the qc
value at CPT-2, CPT-9, CPT-15 and CPT-7 had enhanced to
average 124 %, 97%, 45% and 7%, respectively.
Fig. 11. Pre and post soil improvement qc values at various
radial distances from last Vibro-Compaction (VC) point.
0
2
4
6
8
10
12
0.0 5.0 10.0 15.0 20.0 25.0
DE
PT
H (
m)
Measured qc (Mpa)
Pre CPT-2
Pre CPT-3
Pre CPT-4
Avg. Pre CPT
Post CPT-2
Post CPT-3
Post CPT-4
Equivalent to
35% RD
Equivalent to
60% RD
0
2
4
6
8
10
12
0 2 4 6 8 10 12 14 16 18 20
Dep
th (
m)
Measured qc (Mpa)
Post CPT-2
@ 0.25m
from VC pt
Post CPT-9
@ 1.95m
from VC pt.
Post CPT-
15 @
4.70m from
VC pt.
Post CPT-7
@ 7.25m
from VC pt.
Average-
Pre CPT-
2,3,4
Paper No. 6.44a 7
Figure 12, indicated the radial extent of improvement in terms
of relative density of soil pre and post vibro-compaction. It
was observed average 35% relative density pre soil
improvement has enhanced to about 40% to 70% at indicated
radial distance from last vibro-compaction point.
Fig. 12. Relative Density vs Radial Distance from last vibro-
compaction point.
CONCLUSION
The use of Vibro-compaction as suitable ground improvement
technique for loose sandy soil is demonstrated successfully to
meet project requirements i.e. to mitigate liquefaction and to
increase bearing capacity. Over 5,500 square meter area was
treated in 1 month time using 1 rig showing speed of the
method. Localized sand was used as back fill material,
minimizing the construction cost compared to other foundation techniques.
The results from SPTs and CPTs showed similar trend pre and
post soil improvement. A relative density of more than 60%
was achieved in improved sand layers.
Care, must be taken in the use and construction of Vibro-
compaction, as their design and construction is not a routine
process. It is vital that its design be conducted by experienced engineer based on subsoil properties and suitable design
methodology. Its installation shall be performed and
monitored closely by the contractor and engineer. This method
can be extended for higher bearing capacity requirement for
future expansion of the plant after giving careful consideration
in design.
ACKNOWLEDGMENT
The authors wish special thanks to owner of the development,
TATA Chemicals for their kind permission to publish this
information. In addition, a special thanks to TATA Consulting
Engineers Limited and M/s. Keller Ground Engineering India Ltd. for support and contribution to this manuscript.
REFERENCES
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Resistant Design of Structures”, Bureau of Indian
Standards.
3) IS 15284 (Part 1) [2004], “Design and construction of
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4) K. Rainer Massarsch and Bengt H. Fellenius, [2005].
“Deep vibratory compaction of granular soils, Chapter 19
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5) Purshothama Raj [1999], “Ground Improvement
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8) Schmertmann J. H, “Guidelines for CPT performance and
design”, US Department of Transportation, Federal Highways Administration Report – FHWA – 78-209.
9) TATA Chemicals Limited [2010] “Final Report on Soil
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0 1 2 3 4 5 6 7 8
Rel
ativ
e D
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Distance (m)