ground improvement for foundations of structures using
TRANSCRIPT
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Ground improvement for foundations of structures using stone column - case study on road connectivity to ICTT, Vallarpadam port in Cochin, Kerala, INDIA Avik Kumar Mandal, S. Sailesh & Pradyot Biswas LEA Associates South Asia Pvt. Ltd., New Delhi, India, ABSTRACT Vallarpadam ICTT Port in the state of Kerala was connected with NH-47 by 17.20 km long Four Lane Highway. Starting from the junction with NH 47, the first 8.40 Km length of the project road was on land area. The rest of the project corridor was routed across reclaimed land and last partof road was in island of port area. Deep Soft Clay deposits were found along the project road. In the landarea of project stretch, Stone Column had been adopted for the foundations of RCC Retaining Wall and Box Underpass structure as an alternative to pile foundations. The details of foundation adopted, design involving evaluation of load bearing capacity of stone columns treated soft ground for the retaining wall and underpass structures, results of load tests on stone columns are provided here in this paper. Details of machinery used, procedures adopted in the installation of vibratory stone columns and different field controls adopted to ensure the proper formation of stone columns are presented here. 1 INTRODUCTION Vallarpadam area of Cochin in the State of Kerala is a port hub and is located in marine backwater area. The ICTT Port at Vallarpadamarea was connected with NH-47 by 17.20 km long Four Lane Highway. The initial 8.40 Kmlength of the Four Lane Highway Connecting ICTT Port was in the land portion and it had limited right of way to accommodate both the main carriageway and service road. The maximum height of main carriageway embankment was about 8.0 m in the land area and subsoil is having soft soil (SPT, N < 5) deposits up to 12.0 m. So the high embankment of main carriageway had to be confined by RCC Retaining Wall. However, problems in inadequate bearing capacity and rotational stability had to be overcome for the retaining walls separating the high embankment of main carriageway and service road. In addition to that, due to the presence of deep soft clay deposit in subsoil of land area, the required safe load bearing capacity was not available for the shallow foundations of RCC Box type underpass structure in original in-situ ground condition.So the ground improvement measures by “Vibrofloat Stone Column” were adopted for the foundations of RCC retaining wall as well as for the RCC Box Underpass structure in the land portion of the project road. Brief details of design and construction methodology of stone column adopted including the machinery used for installation and different field quality control tests for ensuring the formation of compacted stone column are discussed here in this paper. 2 DETAILS OF TYPICAL SECTIONS The details showing typical cross-sections for the RCC retaining wall supporting the main carriageway embankment and longitudinalsection for the RCC Box
type Underpass structure along the land portion of ICTT port connecting road are givenbelow. 2.1 RCC retaining wall in road section The typical cross section of RCC retaining wall in between the main carriageway and service road is shown in Figure 1 below. The maximum height of embankment was up to 8.00m and the design height of RCC retaining wall was varying from 4.00 m to 8.00 m from the founding level. Generally, the side slope adopted for the retaining wall backfill soil embankments was 1V: 2H. Locally available Moorum was used as backfill embankmentmaterial which had cohesion (c)
3.0 t/m2, angle of internal friction ()27deg. and dry
density (d) 1.90 t/m3.
Figure 1. Typical details of road cross-section with retaining wall 2.2 RCC box underpass structure The underpass of size 1 x 6.00 m (span) x 2.50 m (height) was provided to give access to the cross road traffic in the land area of the project road. The typical longitudinal section of RCC Box type Underpass structure is shown in Figure 2 below.
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Figure 2. Typical details of longitudinal section of rcc box type underpass. 3 GEOTECHNICAL INVESTIGATIONS AND
TYPICAL PROPERTIES OF FOUNDATION SOIL For the engineering assessment of foundation of structures, a detailed “Geotechnical Investigation Work” comprising of both field and laboratory tests was undertaken at retaining wall and underpass locations along the project road. Total 37 numbers of boreholes of adequate depth were explored all along the alignment of project road in the land area.
The field investigation program consisted of explorationof minimum 150 mm diameter boreholes up to the desired depth and the recording of subsurface profile along with all related field tests and also the collection of different types of foundation soil samples following the related BIS guidelines.
The detailed laboratory investigations namely soil classification tests (Grain Size Analysis, Atterberg Limits i.e. LL, PL, PI and SL), natural moisture contents (NMC), in-situ density, specific gravity, shear strength tests (cohesion and angle of shearing resistance), settlement parameters (compression index, recompression index, void ratio and co-efficient of consolidation) of the collected soil samples and chemical tests of subsoil and ground water were conducted as per the relevant BIS guidelines.
The water table was found almost at the original ground level as observed through the exploratory boring.
Following section gives the broad range of the foundation soil stratifications below the existing ground level as found through investigations. 3.1 For retaining wall supported road section Total 35 numbers of boreholes of depth varying from 10.0m to 30.0m were drilledalong the retaining wall locations in the land area of the project road. Typical geotechnical engineering properties of the foundation soil layers in the retaining wall locations of land area are mentioned here in Table 1, from which it is clear that the foundation soil was of soft nature with low shear strength and the compressibility of the foundation soil was also high.
Table 1. Typical geotechnical engineering properties of
foundation soil layers in retaining wall locations
Layer
No.
Th
ickness o
f Layer(
m)
SP
T V
alu
es
Unit W
eig
ht i.e. b
ulk(t
/m3)
Shear Strength
Parameters
Compressibility Parameters
Cohesio
n i.e
. c(t
/m2)
Angle
of In
tern
al F
rictio
n i.e
. (
deg.)
Com
pre
ssio
n I
ndex (
Cc)
Recom
pre
ssio
n Index (
Cr)
Initia
l V
oid
Ratio (
e0)
Coeff
icie
nt
of
Consolid
atio
n i.e
. C
v (
m2/d
ay)
Pre
consolid
atio
n P
ressure
i.e
. p
c (
t/m
2)
Layer
1
4.2
-
5.3
2 1.7
0 1.50 0 0.75
0.07
5
2.0
0
0.00
4 5
Layer
2
2.5 4 1.8
0 2.50 0 0.45
0.04
5
0.7
1
0.00
4 10
Layer
3
2.9 5 1.8
5 1.30 26 0.25
0.02
5
0.7
1
0.00
8 15
Layer
4
9.9 8 -
14
1.9
5 4.50 15 0.20 0.02
0.6
8
0.00
8 25
Layer
5
11.
2
18
-
50
2.0
0 0.00 30 - - - - -
Layer 1: Soft Clayey Silt / SiltyClay. Layer 2: Soft to Medium Stiff Sandy Silty Clay. Layer 3: Loose Clayey Silty Sand. Layer 4: Stiff Lateritic Silty Sandy Clay. Layer 5: Medium Dense to Dense Clayey Silty Sand. 3.2 For RCC underpass structure Two numbers of exploratory boreholes of 18.00 m to 20.00 m depth were conducted in one RCC Box type Underpass location along land area of the project road. The typical range of geotechnical engineering properties of the foundation soil layers as found from the boreholes in the underpass locations are given in Table 2. From this table, the clear presence of very soft / soft to medium stiff and compressible clayey subsoil layers is observed in the underpass location. Table 2. Typical geotechnical engineering properties of foundation soil layers in underpass location
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Layer
No.
Th
ickness o
f Layer
(m)
SP
T V
alu
es
Unit W
eig
ht i.e. b
ulk (
t/m
3)
Shear Strength
Parameters
Compressibility Parameters
Cohesio
n i.e
. c (
t/m
2)
Ang
le o
f In
tern
al F
rictio
n i.e
. (
deg)
Com
pre
ssio
n I
ndex (
Cc)
Recom
pre
ssio
n Index (
Cr)
Initia
l V
oid
Ratio (
e0)
Coeff
icie
nt
of
Consolid
atio
n i.e
. C
v(m
2/d
ay)
Pre
consolid
atio
n P
ressure
i.e
. p
c(t
/m2)
Layer
1
7.8
5 1
1.2
5 1.20 0 1.21
0.1
9
3.0
0
0.00
05 5
Layer
2
3.2
8 5
1.8
0 3.00 5
0.251
0.0
25
0.8
5
0.00
40 15
Layer
3
8.1
3 30
1.9
0 - 30 - - - - -
Layer 1: Very Soft Silty Clay/ Clayey Silt with organic matter / decomposed wood. Layer 2: Soft to Medium Stiff Sandy Silty Clay. Layer 3: Medium Dense to Dense Clayey Silty Sand. 4 GEOTECHNICAL DESIGN OF FOUNDATION OF
STRUCTURE AND ITS GROUND TREATMENT Geotechnical design for the shallow i.e. open foundations of structures comprised of the estimation of safe load bearing capacity of foundations against the shear failure of foundation soil without exceeding the permissible total settlement. This was done as per the relevant guidelines namely IS: 6403, IS: 8009 (Part 1) and IRC: 78-2014.
The load bearing capacity analysis for the foundations of structures was performed in original ground conditions i.e. without any kind of ground treatment as well as with ground treatment condition.
The results of load bearing capacities for retaining wall and underpass structures are given separately in the following paragraphs. 4.1 For RCC retaining wall The shallow foundation for retaining wall was strip footing with minimum foundation embedment depth as 2.00 m satisfying the requirement given in IRC: 78-2014.
The results of “Load Bearing Capacity Analysis” for the shallow foundations of RCC retaining wall in original ground i.e. without any kind of ground treatment condition are presented below in the Table 3.
Table 3. Summary of Load Bearing Capacity Analysis for Shallow Foundations of Retaining Wall in “Original Ground Condition”
Desig
n
Heig
ht
(m)
Fo
undatio
n W
idth
(m
)
Depth
of F
oundatio
n fro
m O
GL
(m)
Safe
Load B
earin
g C
apacity o
f
open f
oundatio
n (t
/m2)
Required L
oad B
earin
g
Capacity (t
/m2)
Estim
ate
d T
ota
l S
ettle
me
nt
corr
espondin
g t
o t
he R
equired
Load B
earin
g C
apacity
(mm
)
Rem
ark
s
4.00 3.65 2.00 2.00 -
3.65 7.50 36 - 171
NOT
OK
5.00 4.45 2.00 2.00 -
6.00 9.80 94 - 279
NOT
OK
6.00 5.85 2.00 2.85 -
6.75 11.20 96 - 210
NOT
OK
7.00 6.70 2.00 4.10 -
5.75 11.20
135 -
169
NOT
OK
8.00 7.40 2.00 4.00 11.80 185 NOT
OK
From the above Table 3it is clear that the load bearing capacities of the foundations of RCC Retaining Wall in original ground condition i.e. without any ground treatment condition were not adequate and the estimated total settlement corresponding to the required load intensity was also high. So to satisfy the requirement of safe load bearing capacity of shallow foundation, it was required to adopt the ground treatment under the foundation base of retaining wall.
To achieve the required safe load carrying capacity for the foundation of retaining wall, the ground treatment measures in the form of “Removal and Replacement of existing top weak compressible soil” was examined first considering the ease of construction, time and economics. However, because of very high ground water table and close proximity of soft soil, removal of existing foundation soil was found economical and practical upto a maximum depth of 2.50m below OGL. In view of that a maximum of 2.50m depth of removal and replacement had been considered and analyzed.
The results of “Load Bearing Capacity Analysis” for the shallow foundations of RCC retaining wall with ground treatment i.e. after replacement of top soft compressible soil with compacted granular soil are presented below in the Table 4. Table 4. Summary of Load Bearing Capacity Analysis for Shallow Foundations of Retaining Wall after doing “Ground Treatment by Removal and Replacement”
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Desig
n
Heig
ht
(m)
Fo
undatio
n W
idth
(m
)
Depth
of F
oundatio
n fro
m
OG
L (
m)
Th
ickness o
f R
em
oval &
Repla
cem
ent w
ith G
ranula
r
Ma
teria
l (m
)
Safe
Load B
earin
g C
apacity
of
open foundatio
n (t
/m2)
Required L
oad B
earin
g
Capacity (t
/m2)
Rem
ark
s
4.00 3.65 2.00 1.50 -
2.50
3.40 -
7.00 7.50
NOT
OK
5.00 4.45 2.00 1.00 -
2.50
3.00 -
8.50 9.80
NOT
OK
6.00 5.85 2.00 1.00 -
2.50
3.50 -
7.75 11.20
NOT
OK
7.00 6.70 2.00 2.50 5.00 -
6.50 11.20
NOT
OK
8.00 7.40 2.00 2.50 5.60 11.80 NOT
OK
Form the above table it is found that even after replacing 2.50 m thick top soft soil below foundation base by compacted granular material, the available safe load bearing capacity of the foundation was not satisfying the requirement. Hence, for achieving the required load carrying capacity with less settlement it was recommended to adopt the ground treatment by special method like STONE COLUMN. 4.2 For RCC box underpass The shallow foundation for RCC Box Underpass was raft foundation with minimum foundation embedment depth as 0.500 m from the OGL.
The results of “Load Bearing Capacity Analysis” for the shallow foundations of RCC Box underpasswithout any kind of ground treatment condition i.e. in original ground condition are presented below in the Table 5. Table 5. Summary of Load Bearing Capacity Analysis for Shallow Foundations of Underpass in “Original in-situ Ground Condition”
Fo
undatio
n W
idth
(m
)
Depth
of F
oundatio
n
from
OG
L (
m)
Safe
Load B
earin
g
Capacity o
f open
foundatio
n (t
/m2)
Required L
oad
Bearin
g C
apacity
(t/m
2)
Estim
ate
d T
ota
l
Settle
me
nt
corr
espondin
g t
o t
he
Required L
oad
Bearin
g C
apacity
(mm
)
Rem
ark
s
7.80 0.500 1.75 8.00 350 NOT
OK
From the above Table 5it is clearly noted that the load bearing capacities of the foundations of RCC Box Underpass in original ground condition i.e. without any
ground treatment condition were not adequate and the estimated total settlement corresponding to the required load intensity was also quite high.
So to achieve the required safe load carrying capacity for the foundation of underpass, the ground treatment measures in the form of “Removal and Replacement of existing top weak compressible soil” wasattempted first considering the ease of construction, time and economics.
However, because of very high ground water table, removal of existing top foundation soil was found economical and practical upto a maximum depth of 2.50 m below OGL. In view of that a maximum of 2.50 m depth of removal and replacement had been considered and analyzed.
The results of “Load Bearing Capacity Analysis” for the shallow foundations of underpass structureafter ground treatment by replacement of top soft compressible soil with compacted granular soil are presented below in the Table 6. Table 6. Summary of Load Bearing Capacity Analysis for Shallow Foundations of Underpass after doing “Ground Treatment by Removal and Replacement
Fo
undatio
n W
idth
(m
)
Depth
of F
oundatio
n fro
m
OG
L (
m)
Th
ickness o
f R
em
oval &
Repla
cem
ent w
ith G
ranula
r
Ma
teria
l (m
)
Safe
Load B
earin
g C
apacity
of
open foundatio
n (t
/m2)
Required L
oad B
earin
g
Capacity (t
/m2)
Rem
ark
s
7.80 0.500 2.50 2.50 8.00 NOT
OK
For the underpass structure also, it is noted from the above table that even after replacing 2.50 m thick top soft soil below foundation base by compacted granular material, the available safe load bearing capacity of the foundation was not satisfactory. Hence, for achieving the required safe load carrying capacity with permissible settlement it was recommended to adopt the ground treatment by STONE COLUMN which was preferred over the conventional deep foundation like pile foundation because of overall cost economy, time and available of specialized agency in the close vicinity. 5 GROUND IMPROVEMENT FOR FOUNDATION
OF STRUCTURE BY STONE COLUMN In order to construct the structures namely RCC retaining wall and Box type underpass in land area, safe against collapsibility and serviceability, the ground improvement by Stone Column was recommended, viewed from the foundation soil conditions, site and road requirements, construction ease and schedule. Around 1.50 Km length of retaining wall and one Box type Underpass along the land area of the project road was constructed over the ground improved with Stone Column.
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5.1 Design & detailing of foundations treated with stone column
Detailed design of ground treatment measures for structure foundations with Stone Column was carried out in accordance with “IS 15284 (Part 1): 2003, Design and Construction forGround Improvement – Guidelines, Part 1 Stone Column”. The other guidelines namely IRC - HRB - Special Reports 13 and 14 dealing with design and construction of embankments on soft groundwere also followed for finalization of detailing of the ground improvement.
The load carrying capacity of the stone column treated ground was determined by summing up contributions of each of the following three components as per IS: 15284 (Part 1): a) Capacity of the stone column resulting from the
resistance offered by the surrounding soil against its lateral deformation (bulging / cavity expansion) under axial load,
b) Capacity of the stone column resulting from increase in resistance offered by the surrounding soil due to surcharge over it and
c) Bearing support provided by the intervening soil between the columns.
The formulae for evaluation of above said individual load carrying component was considered from IS guidelines. The settlement of the stone column treated ground was estimated using “Reduced Stress Method” based on the “Stress Concentration Factor” and “Area Replacement Ratio” as defined and the procedure given under Annex B of IS:15284 (Part 1).
The results of load bearing capacities for foundations of retaining wall and underpass structures rested over of the stone column treated ground are given separately in the following sections. 5.1.1 For RCC retaining wall The outputs of “Load Bearing Capacity Analysis” including the “Settlement Analysis” and “Rotational Stability Analysis” for the foundations of retaining wall with ground treatment by Stone Columnare presented below in the Table 7. Table 7. Summary of analysis for foundations of retaining wall after doing “ground treatment by stone column”
Desig
n
Heig
ht
(m)
Fo
undatio
n W
idth
(m
)
Depth
of F
oundatio
n fro
m
OG
L (
m)
Safe
Load B
earin
g C
apacity
of
open foundatio
n (t
/m2)
Required L
oad B
earin
g
Capacity (t
/m2)
Estim
ate
d T
ota
l S
ettle
me
nt
of
Sto
ne C
olu
mn
tre
ate
d
Fo
undatio
n S
oil
(mm
)
Fa
cto
r of S
afe
ty in
Glo
bal
Sta
bili
ty
Rem
ark
s
4.00 3.65 2.00 7.50 7.50 24 - 89 1.35 OK
5.00 4.45 2.00 9.80 9.80 44 - 130 1.60 OK
6.00 5.85 2.00 11.20 11.20 76 1.35 OK
Desig
n
Heig
ht
(m)
Fo
undatio
n W
idth
(m
)
Depth
of F
oundatio
n fro
m
OG
L (
m)
Safe
Load B
earin
g C
apacity
of
open foundatio
n (t
/m2)
Required L
oad B
earin
g
Capacity (t
/m2)
Estim
ate
d T
ota
l S
ettle
me
nt
of
Sto
ne C
olu
mn
tre
ate
d
Fo
undatio
n S
oil
(mm
)
Fa
cto
r of S
afe
ty in
Glo
bal
Sta
bili
ty
Rem
ark
s
7.00 6.70 2.00 11.20 11.20 76 - 129 1.55 OK
8.00 7.40 2.00 11.80 11.80 86 1.50 OK
For dispersion of applied load from retaining wall foundation base to the top of stone column and to aid drainage of the pore water, stone columns were topped with a compacted granular blanket of thickness varying from 500 mm to 1150 mm. The depth of stone column was generally so decided that very soft / soft and medium stiff clay layers, which were most significant weak strata were treated completely. Minimum two numbers of stone columns had been kept below foundation throughout the length in square pattern to utilize maximum resistance against bulging within the limited base width. The spacing of stone column was varying between 1.65 m and 2.50 m to achieve overall economy. One additional row of stone column was also given on either side, beyond the base width of retaining wall, to ensure the lateral spreading of load through the projected portion of granular blanket over the column.
The details of “Ground Treatment with Stone Column and compacted granular blanket” under the base of RCC Retaining walls are given below in the Table 8. Table 8. Summary of details of “ground treatment with stone column and compacted granular blanket” for retaining wall foundations
Design Height (m)
4.00 5.00 6.00 7.00 8.00
Foundation
Width (m) 3.65 4.45 5.85 6.70 7.40
Diameter of Stone Column
(mm)
1000 1000 1000 1000 1000
Spacing of Stone Column
(mm)
1850 -
2500
1650 -
2250
1750 -
2000
1650 -
2000 1650
Type of Arrangement
Square Square Square Square Square
Length of Stone Column from Average OGL
(m)
6 - 11 6.5 -
12.0
7.5 -
14.0
8.0 -
14.0 8.0
Safe Load Carrying
Capacity of Single Stone
Column and its Tributary Soil
(ton)
24.6 -
44.0
22.4 -
49.0
29.0 -
43.0
29.0 -
43.0 29.5
6
Design Height (m)
4.00 5.00 6.00 7.00 8.00
Nos. of Stone Column under RCC Retaining
Wall along Foundation
Width (nos.)
2 nos.
equally
apart
from
C/L
2 - 3
nos.
equally
apart
from
C/L
3 nos.
with
center
column
along
C/L
3 - 4
nos.
with
center
column
along
C/L
4 nos.
equally
apart
from
C/L
Nos. of Stone Column under either side of the Projected Blanket along Foundation
Width (nos.)
1 no. 1 no. 1 no. 1 no. 1 no.
Thickness of Compacted
Granular Blanket (mm)
500 500 -
1000
700 -
1150
500 -
1150 800
Width of Granular
Blanket beyond Edge of Last Row of Stone
Column (mm)
500 500 500 500 500
The typical sectional elevation and plan showing the arrangement of Stone Columns and compacted granular blanket below the RCC Retaining is given in following Figure 3.
Figure 3. Typical elevation and plan showing arrangement of stone columns and granular blanket below retaining wall
5.1.2 For RCC box underpass The results of “Load Bearing Capacity Analysis” including the “Settlement Analysis” for the underpass foundations with ground treatment by Stone Columnare presented below inTable 9. Table 9. Summary of analysis for foundations of underpass structure after doing “ground treatment by stone column”
Fo
undatio
n W
idth
(m
)
Depth
of F
oundatio
n fro
m
OG
L (
m)
Safe
Load B
earin
g C
apacity
of
open foundatio
n (t
/m2)
Required L
oad B
earin
g
Capacity (t
/m2)
Estim
ate
d T
ota
l S
ettle
me
nt
of
Sto
ne C
olu
mn
tre
ate
d
Fo
undatio
n S
oil
(mm
)
Rem
ark
s
7.80 0.500 8.00 8.00 150 OK
A compacted granular blanket of thickness 1000 mm was provided over top of the compacted stone column for dispersion of applied load from underpass foundation base to the top of stone column and to aid drainage of the pore water. This blanket was extended 500mm beyond the edge of last stone column all round. The depth of Stone Column is generally so decided that very soft/ soft and medium stiff clay layers, which were most significant weak strata were treated completely.
The details of “Ground Treatment with Stone Column and compacted granular blanket” under the foundation of RCC Box type Underpass structure are given below in Table 10. Table 10. Summary of details of “ground treatment with stone column and compacted granular blanket” for underpass foundations
Foundation Width (m) 7.80
Depth of Foundation from OGL (m) 0.500
Diameter of Stone Column (mm) 1000
Spacing of Stone Column (mm) 1650
Type of Arrangement Triangular
Length of Stone Column from Average OGL (m) 12.00
Safe Load Carrying Capacity of Single Stone Column and its Tributary Soil (ton)
17
Projected Row beyond width 2 nos.
Projected Row beyond length 1 no.
Thickness of Compacted Granular Blanket (mm) 1000
Width of Granular Blanket beyond Edge of Last Row of Stone Column (mm)
500
The typical sectional elevation and plan showing the arrangement of Stone Columns and compacted
7
granular blanket below the RCC Box Underpass is given in following Figure 4.
Figure 4. Typical elevation and plan showing arrangement of stone columns and granular blanket below rcc box underpass 5.2 Construction materials and installation
methodology of stone column 5.2.1 Construction materials The crushed stone namely gravel was used forstone column as well as for granular blanket at top of the stone column. Those crushed stone i.e. gravel material consisted of clean, hard, angular, chemically inert, resistant to breakage and free from organic material, trash, or other deleterious materials and conforming to the gradation as given in following Table 11. The uniformity co-efficient of crushed gravel was greaterthan 3 and grain size distribution curve showed the material was well graded. The aggregate crushing
value of the stone was not more than 30% and the impact value was not more than 25%. Table 11. Gradation of aggregates for stone columns
Size of Crushed Aggregates % Passing
75 mm (3”) 90-100
50 mm (2”) 40-90
20 mm (3/4”) 0-10
12 mm (1/2”) 0-5
The granular blanket was compacted in layers to a
relative density of 75 to 80 percent. This blanket was exposed to atmosphere at its periphery for pore water pressure dissipation. 5.2.2 Construction procedure and stone column
installation methodology The construction of stone column involved creation of a hole in the ground which was later filled with granular material in compacted manner. The stone columns were installed by Vibrofloat method. The wet method of Vibro-replacement was adopted as it was suitable for soft to medium stiff soil with high water table condition where borehole stability was questionable. Crushed stones of desired gradation was fed by mechanical means i.e. use of loader/ hopper/ chute etc. The slush, muck and other loose materials at work site was removedsuitably by the contractor as instructed by the Engineer. Adequate measures were taken to ensure stability of bore holes made for installation of stone column.
The detailed installation procedure / method statement was submitted by the project contractor including the following:
• Mechanical arrangement for placing stones (s)
around the probe point
• Quality control, Quality Assurance Procedure
covering details on automatic recording devices to
monitor and record stone consumption
• Deployment of various equipments and machineries
• Manpower deployment
• The proposed sequence and timing for constructing
stone columns having regard to the avoidance of
damage to adjacent stone columns
• Bar chart for the entire foundation work
The construction technique and probe was capable of
producing and/or complying with the following:
• The hole was close to circular.
• The probe and follower tubes were of sufficient
length to reach the elevations shown on the plans.
• The probe, used in combination with the flow rate
and available pressure to the tip jet, was capable of
penetrating to the required tip elevation. Pre-boring
of stiff lenses, layers or strata was permitted
• The probe had visible external markings at one (1)
foot/suitable increments to enable measurement of
penetration and re-penetration depths
• The equipment used was instrumented with sensors
and the data processed by a micro-processing unit
to enable continuous monitoring and capturing of
8
data related to depth of vibrator and vibrator
movements (i.e. depth of penetration) and power
consumption (i.e. compaction effort) during
construction of each stone column
Data captured was continuously displayed on a LCD
unit and graphical output (plots of depth versus time
and power consumption) generated by automated
computerized recording device throughout the process
of stone column installation for each point was
submitted to the Engineer.
Sufficient quantity of wash water was provided to the
tip of the probe to widen the probe hole to a diameter to
allow adequate space for stone backfill placement
around the probe. The flow of water from the bottom jet
was maintained at all times during backfilling to prevent
caving or collapse of the hole and to form a clean stone
column. The flow rate was generally greater as the hole
was jetted in and decreased as the stone column came
up to the top.
After forming the hole, the vibrator was lifted up a
minimum 3 m, dropped at least twice to flush the hole
out. The probe was, however, not completely removed
from the hole. The column was formed by adding stone
in lifts having each lift height between 600 mm and
1000 mm.
The stone aggregate in each lift was compacted by
re-penetrating it at least twice with the horizontally
vibrating probe so as to densify and force the stone
radially into the surrounding in-situ soil. The stone in
each increment was re-penetrated a sufficient number
of times to develop a minimum ammeter reading on the
motor of at least 40 amps more than the free-standing
(unloaded) ampere draw on the motor, but no less than
80 amps total.
Stone column wasinstalled so that each completed
column was continuous throughout its length.
The typical photographs of installation of stone
column, laying granular blanketand construction of
Retaining Wall are given below in Figure5, 6, 7 and 8.
Figure 5. Typical photo of installation of Stone Columns
with Vibroflotation Method (Wet)
Figure 6. Typical Photo ofVibroflotation Probe
Figure 7. Typical Photo ofLaying of Granular Blanket
Figure8.Typical photo of retaining wall construction over stone column treated ground 5.2.3 Monitoring of Stone Column Installation During construction of each stone column, all details & parameters were recorded & maintained and was made available for inspection by the Engineer. The details were included but not were limited to the following:
• Location, reference number, diameter and depth
(top & bottom elevation) of stone column.
• Stack measurement of stone (s) used in the works
• Monitoring stone consumption vis-à-vis theoretical
requirements, information to include pressure gauge
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readings, depth versus time (time to penetrate and
time to form each stone column) etc.
• Computation of average quantity of back fill
materials consumed per linear meter of the column
• Vibrator power consumption during penetration of
vibrator and during compaction of stone column
• Details of obstructions, delays and any unusual
ground conditions
• Any other information as required by the Engineer
The following Figure9 shows vibrator power
consumption with respect to depth during making and
washing of bore of stone column and finally compaction
of stone column which helps monitoring its degree of
compaction
Figure9.Typical photo of vibrator power consumption during installation of stone column
The Section 1 in above Fig.9 shows advancement of borehole and section 4 shows corresponding power consumed. Section 2 shows the washing of hole by moving the vibrator up and down the hole. There was no variation of power consumed for this process as clearly shown in section 5. The section 3 shows the formation of column and corresponding power consumed is shown in section 6. Section 6 shows more power consumption than section 5 and section 4, this is because more energy is required for doing compaction ofthe stone column. This energy difference indicates that a well compacted stone column had been formed. 5.3 Load Tests of Stone Column Two different types of vertical load tests namely “Initial Load Test” over the “Test Column” and “Routine Load Test” over the “Working Column” were conducted for the stone column treated foundations.
The Initial load tests were performed at a trial test site to evaluate the load-settlement behavior of the soil-stone column system. The tests was conducted on a single and also on a group of minimum three columns in accordance with IS: 15284 (Part-1). The load settlement observations were taken for the test load of 1.5 times of the safe design load in Initial Load Test. The number of initial load tests was as follows:
• Single column tests - 1 test per 500 or part thereof stone columns
• Three column group tests – 1 test per 1000 or part thereof stone columns
The following Figure 10and 11show the typical photograph of conducting load tests over stone column.
Figure 10. Typical photo of showing “Reaction Kentledge” for load test over stone column
Figure11. Typical photo of showing arrangement for load test over group of stone columns
The results of initial load test over stone column was accepted when settlement recorded was 10 to 12 mm at design load for a single column test and similarly settlement observed was 25 to 30mm under design load for three column group tests.
The following Figure 12 shows typical load settlement graph of “Initial Load Test” over single stone column.
Figure12. Typical load settlement graph of Initial Load Test over single stone column
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The Routine load tests over working column was carried out on a single column as well as over group of stone columns in accordance with IS:15284 (Part-1). The job columnwas loaded for a test load of 1.1 times the design load intensity with reaction kentledge minimum 1.3 times the design load pattern.
The following Figure13 and 14 show the typical load settlement graphs of routine load test over single stone column and group of stone columns under retaining wall.
Figure13. Typical load settlement graph of “Routine Load Test” over single stone column
Figure14. Typical load settlement graph of “Routine Load Test” over group of stone column
The typical load settlement graph of routine load test over single column under RCC Box underpass is also shown below Figure 15.
Figure15. Typical load settlement graph “Routine Load Test” over single stone column under Box Underpass
5.4 Advantages of using Stone Columnin place ofPile Foundation
Stone columnwas better alternative to pile foundation where soft compressible deposits of foundation soil
exist below foundation of structure, because: • Installation of stone
column wasquite faster and around 50 numbers of stone columnsof 10 m depth were installed in one day.
• Cost of construction of stone column was much less as compared to construction of pile foundation.
• As a result there was reduction in overall cost of ground improvement for foundations.
• Small and insignificant settlement under the safe design load was observed in post stone column construction/installationstage.
• Generally, no waiting period wasrequired to be adopted and loads were placed shortly after installation of stone columns in foundation soils.
6 CONCLUSION
There were 4.00 to 12.00 m deep soft compressible clayey deposits in the foundation soil of RCC retaining walls and box type underpass structures in land area of the Cochin ICTT Port road connectivity project. The ground improvement with stone column was effectively provided for the foundations of structures as an economical and safe alternative solution to the pile foundation. The details of design philosophy and construction methodology as adopted for the ground improvement works by “Vibrofloat Stone Column” under foundations of structures are described here. The field quality monitoring of stone column installation and load testing of stone column treated ground is also provided in this paper.
The following typical photo shows the completed project road with retaining wall supported over stone column treated ground.
7 REFERENCES
IS 6403 : 1981, Code of Practice for Determination of Bearing Capacity of Shallow foundations
IS : 8009 (Part I) - 1976, Code of Practice for Calculations of Settlement of Foundations, Part I Shallow Foundations
IRC 78(2000), Standard Specifications and Code of Practice for Road Bridges, Section VII Foundations and Substructure.
IS 15284 Part1:(2003), Design and Construction for Ground Improvement - Guidelines Part 1 Stone Columns.
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IRC-HRB (1994), State of the Art: High Embankment on Soft Ground - Part A - Stage Construction &Part B - Ground Improvement