Post on 25-Mar-2018
Geosynthetic Encasement for Stonger and Stiffer Stone ColumnsStonger and Stiffer Stone Columns
K. RajagopalProfessor, Department of Civil Engineering
IIT M d Ch i 600036IIT Madras, Chennai 600036e-mail: gopalkr@iitm.ac.in
Problems due to soft clay soils• Low bearing capacity• Excessive settlementsExcessive settlements• Deep seated foundation failure
embankment
soft Clay li i l
failure wedge
soft Clay
Firm Soil
slip circle
Encased Stone Columns 2
Solutions to the Problem• removal of soft material and replacement with
Solutions to the Problemremoval of soft material and replacement with quality fill
• displacement of soft materialili• piling
• Pre-consolidation: sand drains, PVDs, vacuum consolidationsand drains, PVDs, vacuum consolidation
• lightweight fill• Stone columns• Geosynthetics – many possibilities
Encased Stone Columns 3
IntroductionIntroductionIntroductionIntroductionStone columns/Granular piles
P l d i t t h i
Stone columns/Granular piles
Popular ground improvement technique
Flexible structures like Embankment, Storage tank etc.
Structures with large loaded areas, Parking garages etc.
Liquefaction mitigation
Encased Stone ColumnsEncased Stone Columns 44
Stone Column Construction Stone Column Construction th dth dmethodsmethods
Rammed Stone columnsRammed Stone columns Vibro – Replacement
Vib Di l t Vibro – Displacement
Encased Stone ColumnsEncased Stone Columns 55
6Encased Stone Columns
7Encased Stone Columns
Encased Stone Columns 8
9Encased Stone Columns
Encased Stone Columns 10
11Encased Stone Columns
42.5 m dia x 14.6 m high external floating roof tanks for storing high/low sulphur diesel for Chochin Refineries Limited
Encased Stone Columns 12
Historical DevelopmentHistorical Developmentpp
First applied in France in 1830 to improve a First applied in France in 1830 to improve a native soil
Used extensively in Europe since the lateUsed extensively in Europe since the late 1950's in marginal soils
Stone columns are commonly employed in Sto e co u s a e co o y e p oyedIndia.
Barksdale and Bachus (1983)
Encased Stone ColumnsEncased Stone Columns 1313
How does a stone column resist vertical loads?How does a stone column resist vertical loads?
Encased Stone ColumnsEncased Stone Columns 1414
Failure mechanism Failure mechanism –– Single SCSingle SCgg
Encased Stone ColumnsEncased Stone Columns 1515
Plan arrangements of stone columnsPlan arrangements of stone columnsPlan arrangements of stone columnsPlan arrangements of stone columnsSquare pattern Triangular pattern
ds
d
D Ds
Dia. of unit cell = 1.128 s Dia. of unit cell = 1.05 s i ll 0 866* *
s
Encased Stone ColumnsEncased Stone Columns 1616
Unit cell area = s*s Unit cell area = 0.866*s*s
Thorburn (1975)Thorburn (1975)
Li iti b iLimiting bearing pressure on stone
lcolumns= 25cu
Encased Stone ColumnsEncased Stone Columns 1717
Improving the load capacity of Improving the load capacity of th t lth t lthe stone columnthe stone column
• Reinforcing with iron rodsReinforcing with iron rods
• Skirting • Rao and Ranjan (1985)
• Layered reinforcement (Geogrids)• Layered reinforcement (Geogrids)• Sharma (1998) and Sharma et al. (2004)
• Encasing the Stone Column• (ESC)
Present study
Encased Stone ColumnsEncased Stone Columns 1818
(ESC)
Encasing the Stone ColumnEncasing the Stone Column Bearing capacity enhanced by
Passive pressure
Geosynthetic encasement
Additional fi t
pressure+
encasementconfinement
Stone columnSectional plan
Encased Stone ColumnsEncased Stone Columns1919
From........Clay........to........PileFrom........Clay........to........PileClay OSC ESC Pile
eneo
us
posi
te
rigi
d
gid
Hom
oge
Com
p
Sem
i-
Rig
Encased Stone Columns 20
Construction of Encased Stone ColumnConstruction of Encased Stone Column
Alexiew et al. (2005)
Encased Stone Columns 21
Encased Stone Columns 22
Construction of Encased Stone Column
Alexiew et al (2005)Encased Stone Columns 23
Alexiew et al. (2005)
Geotextile encased
Encased Stone Columns 24
sand column
Hamburg GermanyHamburg, Germany
A380 factory site
Encased Stone Columns 25
Encased Stone Columns 26
Encased Stone Columns 27
Pre-fabricated ESC column being lowered into a drill borehole
Encased Stone Columns 28
Another type of encased stone column
Courtesy:Dipl.-Ing. Holger Pohlmannp g g
Naue Fasertechnik GmbH & Co. KG
P dPresent study
Encased Stone Columns 29
Advantages of EncasementAdvantages of EncasementAdvantages of EncasementAdvantages of Encasement
1 Imparts lateral confinement1. Imparts lateral confinement
2. Increases the load capacity & stiffness by many fold
3 Stresses are transferred to deeper3. Stresses are transferred to deeper strata.
4. Higher lengths of stone column are possible.
Encased Stone ColumnsEncased Stone Columns 3030
p
Advantages of EncasementAdvantages of EncasementAdvantages of EncasementAdvantages of Encasement
5 Lateral squeezing of stones is prevented5. Lateral squeezing of stones is prevented
6. Higher degree of compaction can be g g pachieved
7. Prevents the clogging of stone columns
8 Strength properties of the aggregate are8. Strength properties of the aggregate are preserved.
Encased Stone ColumnsEncased Stone Columns 3131
Construction of Encased Stone ColumnConstruction of Encased Stone Column
Encased Stone Columns 32
Encased Stone ColumnEncased Stone Column
Encased Stone Columns 33
11C i t t th tiCompression tests on geosynthetic
encased stone aggregatesencased stone aggregates
Encased Stone Columns 34
Geosynthetic Encased Stone aggregateGeosynthetic Encased Stone aggregate
Encased Stone Columns 35
Compression test on Encased Stones
Encased Stone Columns 36
Tensile load-strain behaviour of geosynthetic samples with seamgeosynthetic samples with seam
5Woven geotextileN t til
3
4
N/m
)
Non-woven geotextileSoft grid - 1Soft grid - 2
2
3
Load
(kN
1
L
00 10 20 30 40 50
St i (%)Encased Stone Columns 37
Strain (%)
Geosynthetic ConfinementGeosynthetic Confinement
C fi i d t bConfining pressure due to membrane
2H k l d Gilb t (1952)cM
3 Henkel and Gilbert (1952)
1c
ad
1 3Axial Capacity, pK 1 3p y, p
Encased Stone Columns 38
Experiment Vs. Henkel & Gilbert(Non woven geotextile)(Non-woven geotextile)
150050 mm
E i
1000kPa)
75 mm100 mm150 mm
ExperimentHenkel & Gibert
500ress
ure
(k
500Pr
00 10 20 30 40
A i l t i (%)Encased Stone Columns 39
Axial strain (%)
Experiment Vs. Henkel & Gilbert(woven geotextile)(woven geotextile)
50 mm Experiment1500
kPa)
50 mm75 mm100 mm
ExperimentHenkel & Gibert
1000
ress
ure
(k 150 mm
500Pr
00 5 10 15 20 25
Encased Stone Columns 40
Linear strain (%)
Influence of the diameter on the ultimate load carrying capacityultimate load carrying capacity
1500Woven geotextile
1000ss (k
Pa
gNon-woven geotextileSoft grid - 1
500mat
e st
res
500
Ulti
m
00 50 100 150
Encased Stone Columns 41
Diameter of the sample (mm)
22L d t t i l tLoad tests on single stone
column in a Unit cellcolumn in a Unit cell
Encased Stone Columns 42
Stone column arrangements
s
Square pattern Triangular pattern
Influence radius = 0.564s Influence radius = 0.525s
Encased Stone Columns 43
f
Schematic of setup – Unit CellSchematic of setup Unit CellStrain controlled loadingProving ring
To strain read out unit
Proving ring
Geosynthetic encasement
Sof500 mm
StonesSoft
Clay Strain gaugesy Strain gauges
Unit cell tank
Sectional plan
soft clay
Encased Stone Columns 44210 mmSectional plan
Encased Stone Columns 45
Clay bed undergoing consolidation
Properties of ClaySl.No Properties Value
1 Liquid limit 49 %2 Plastic limit 17 %3 Specific Gravity 2.594 M i t t t ft lid ti 47±1%4 Moisture content after consolidation 47±1%5 In-situ vane shear strength 2.5 kPa6 Consistency Index 0.066 Consistency Index 0.067 Dry unit weight 11.56 kN/m3
8 CBR value 0.11 %9 IS Classification System CI (Silty clay of medium plasticity)
10 Degree of Saturation 96 %11 I i id i 1 25
Encased Stone Columns 46
11 In-situ void ratio 1.25
Strain gauges in encasementStrain gauges in encasement
Encased Stone Columns 47
Stone Column in Unit Cell
Encased Stone Columns 48
ESC in Unit cell tank
Encased Stone Columns 49
Load settlement curve for stone columns encased in non woven geotextileencased in non-woven geotextile
0 100 200 300 400Pressure (kPa)
00 100 200 300 400
ESC - 50 mmESC - 75 mm
10
20t (m
m)
SC 75ESC - 100 mmOSC - 50 mmOSC - 75 mmOSC - 100 mm0
30
ettle
men
t OSC 100 mmClay
40Se
Encased Stone Columns 50
50
Bulging in stone columnsg g
OSC ESC
Encased Stone Columns 51
Hoop strain variation in the Geosynthetic encasementGeosynthetic encasement
0 0 1 0 2 0 3 0 4 0 5Hoop strain (%)
00 0.1 0.2 0.3 0.4 0.5
100
200mm
)
300
Dep
th (m
50 mm Ø50 II T i l
400
500
50 mm II Trial75 mm Ø100 mm Ø
Encased Stone Columns 52
500
Encasement with different geosyntheticsEncasement with different geosynthetics
0 200 400 600Pressure (kPa)
00 200 400 600
ESC (woven geotextile)ESC (nonwoven geotextile)
10
20(mm
)
ESC (nonwoven geotextile)ESC (soft grid - 1)ESC (soft grid - 2)OSCCl20
30
ettle
men
t Clay
40
Se
Encased Stone Columns 53
50
Load settlement curve for stone columns d i t tilencased in woven geotextile
Pressure (kPa)
00 200 400 600 800
ESC- 50 mm
10
20(mm
)
ESC - 75 mmESC - 100 mmOSC - 50 mmOSC - 75 mm
20
30tlem
ent ( OSC - 100 mm
Clay
40
Sett
Encased Stone Columns 5450
Analytical predictionsAnalytical predictions
• Limiting stress on OSC (ordinary stone column)Limiting stress on OSC (ordinary stone column)
- IS 15284: Part-1 (2003) 4v ro u colpc K
Where,
v ro u colp
ro - Initial effective radial stress
U d i d h i f di lcu - Undrained cohesion of surrounding clay
Kpcol - Coeff. of passive earth pressure - stone
Encased Stone Columns 55
Analytical predictionsAnalytical predictions
• Lateral stress due to encasement in ESCLateral stress due to encasement in ESC
1 1 a
Circumferential strain in the encasement1c
a
Circumferential strain in the encasement,
2JAdditional lateral confining stress , 2c
Jpd
WhWhere,
J - Tensile load in the encasement corresponding to strain c
Encased Stone Columns 56
d - Diameter of the stone column.
Experimental Vs. AnalyticalExperimental Vs. Analytical1000
tESC(woven)-ExperimentESC(woven)-AnalyticalESC( ) E i t
800
settl
emen
t ESC(non-woven)-ExperimentESC(non-woven)-AnalyticalOSC-ExperimentalOSC-Analytical
400
600
t 50
mm
s
200
400
ress
ure
at
025 50 75 100 125
Pr
Encased Stone Columns 57
Diameter of stone column
33L d t t i l tLoad tests on single stone
column in large tankcolumn in large tank
Encased Stone Columns 58
Load tests on stone column installed in large test tankinstalled in large test tank
Vertical loading Dial gauges
Stone columns
Geosynthetic encasement
600
Soft Clay
600 mm
y
Encased Stone Columns 59
1200 mm
Load test on single stone column
Encased Stone Columns 60
Encased stone column (non-woven) Si l t l– Single stone columns
Pressure (kPa)
00 20 40 60 80 100
Pressure (kPa)
ESC - 50 mm
10
mm
)
ESC 50 mmESC - 75 mmESC - 100 mmOSC - 50 mmOSC - 75 mmOSC 10020
30lem
ent (
m OSC - 100 mmClay
30
40
Settl
Encased Stone Columns 6150
Experiment Vs. AnalyticalExperiment Vs. Analytical125
ESC - ExperimentESC A l ti l
75
100
ss (k
Pa) ESC - Analytical
OSC - ExperimentOSC - Analytical
50
75
iting
stre
s
25Lim
i
025 50 75 100 125
Di t ( )Encased Stone Columns 62
Diameter (mm)
44L d t t G fLoad tests on Group of
stone columns in large tankstone columns in large tank
Encased Stone Columns 63
Configuration of group of Stone columns Pl- Plan
75 mm Ø @ 150 mm c/cSoft Clay
Loading plate 280 mm Ø
1200 mm
Encased Stone Columns 64
1200 mm
Group of Stone column – Triangular pattern
Encased Stone Columns 65
Loading plate fitted with pressure ll G t tcell – Group test
Loading
To read out unit
Loading
Pressure cells
Encased Stone Columns 66
Load test on group of stone columns
Encased Stone Columns 67
Stress concentration on the stone l G t tcolumns – Group test
12
8ratio
n
ESC (woven)ESC (non-woven)OSCClay (ESC - woven)Cl (ESC )
s con
cent
r Clay (ESC-nonwoven)Clay (OSC)
4
Stre
ss
00 10 20 30 40 50
Encased Stone Columns 68
0 10 20 30 40 50Settlement (mm)
Embankment loading exerting a lateral thrust th t l i th t ition the stone columns in the extremities
Embankment Potential slip circle causing deep seated failure
Stone columns
Embankment Potential slip circle causing deep seated failure
Soft clayy
Encased Stone Columns 69
Numerical simulations
Encased Stone Columns 70
Numerical simulation experiments –OSC i U it llOSC in Unit cell
0 20 40 60 80 100Pressure (kPa)
00 20 40 60 80 100
50 mm Exp10
20(mm
)
50 mm -Exp50 mm FEM75 mm - Exp75 mm - FEM20
30ttlem
ent ( 100 mm - Exp
100 mm - FEMClay - ExpCl
40
Set Clay - FEM
Encased Stone Columns 7150
Numerical simulation experiments –ESC i U it ll
Pressure (kPa)
ESC in Unit cell
00 100 200 300 400 500
( )
10
mm
)
50 mm -Exp50 mm - FEM75 mm - Exp75 FEM20
30lem
ent (
m 75 mm - FEM100 mm - Exp100 mm - FEM
30
40
Settl
Encased Stone Columns 7250
Finite Element Parametric studies
Modified Hyperbolic Uniform Pressure
cFinite Element Parametric studies
model with plasticity
AxisymmetricStone
8 Node, Continuum Elements Soft clay
Stone
column
Elastic model forgeosynthetic
5mGeosynthetic encasement
Surcharge modeled as 200 kPa pressure
Encased Stone Columns 73Influence
radius
1 O di St C lCases considered for analysis
1. Ordinary Stone Column
2. Geogrid Encased Stone Column Pressure on Stone Column only
Parameters VariedColumn only & all over the area
1. Diameter of the stone Column
2. Pressure on the stone Column
3. Spacing of the stone columns (i.e. Influence radius)
4. Height of encasementg
5. Stiffness of the geosynthetic
6 Shear strength of the surrounding clayEncased Stone Columns 74
6. Shear strength of the surrounding clay
Validation of GEOFEM program ith H d G b (2002) M i ttl t
180Unreinforced - Present study
with Han and Gabr (2002) – Maximum settlement
160
ent (
mm
) Unreinforced Present studyUnreinforced - Han&Gabr Reinforced - Present StudyReinforced - Han&Gabr
120
140
m se
ttlem
e
100
Max
imum
800 1 2 3 4 5
Height of embankment (m)
Encased Stone Columns 75
Height of embankment (m)
Results and DiscussionsEffect of Geogrid Encasement for Stone Column
20(%)
16tlem
ent (
OSCESC
12
alis
ed S
et ESC
80 1 2 3 4 5 6
Nor
ma
Due to encasement the stone column settlements have reduced up
0 1 2 3 4 5 6Influence radius (m)
Encased Stone Columns 76
Due to encasement the stone column settlements have reduced up to 20% for all spacing.
Results and Discussions …
28%)
Variation of settlement in stone column with diameter
20
24
ttlem
ent (
OSCESC
12
16
alis
ed se
t ESC
8
12
0 5 10 15 20 25 30
Nor
m
0 5 10 15 20 25 30Area ratio (%)
Beyond certain diameter the encasement effect is very minimal
Encased Stone Columns 77
Beyond certain diameter the encasement effect is very minimal
Confining pressure in stone column
0 25 50 75 100
Confining pressure (kPa)
0
10.6 m Ø - ESC
0 6 Ø 1 12 aM
2
pth
(m)
0.6 m Ø -OSC1 m Ø - ESC
30
21
a
a
Md
3
4
Dep
5Bathurst and Rajagopal(1993) Rajagopal et al (1999)
Encased Stone Columns 78
(1993), Rajagopal et al. (1999) and Latha et al. (2006)]
Contours of mobilised shear strength
Encased Stone Columns 79(a) OSC (b) ESC with J = 5000 kN/m
Influence of Stiffness of the encasement (1m Ø Stone column)(1m Ø Stone column)
0 50 100 150 200 250 300pressure (kPa)
00 50 100 150 200 250 300
50
t (m
m)
100ettle
men
t 10000 kN/m2500 kN/m500 kN/m250 kN/m
150
s 50 kN/mOSC OSC
Encased Stone Columns 80
150
Lateral bulging of stone column (1 m Ø)
Normalised lateral bulging (z/ro) (%)
Lateral bulging of stone column (1 m Ø)
00 1 2 3 4
1
2m)
OSC50 kN/m2
3Dep
th (m 250 kN/m
500 kN/m1000 kN/m2500 kN/m
45000 kN/m10000 kN/m
(vertical pressure =200 kPa)
Encased Stone Columns 81
5
Influence of stiffness of encasement on the lateral confining stress (1 m Ø)the lateral confining stress (1 m Ø)
0 25 50 75 100Confining pressure (kPa)
0
1
0 5 50 75 00
OSC1
2(m)
OSC
ESC, J= 10000 kN/m
3Dep
th ( OSC
250 kN/m1000 kN/m
42500 kN/m5000 kN/m10000 kN/m
Encased Stone Columns 82
5
Influence of stiffness of encasement on settlement reductionsettlement reduction
100
60
80
ion
(%)
40
60
ent r
educ
ti
0.6m Ø Stone column
20
Settl
em
0.6m Ø Stone column1m Ø Stone column
00 1000 2000 3000 4000 5000
Stiffness of encasement (kN/m)
Encased Stone Columns 83
Stiffness of encasement (kN/m)
Hoop tension in the encasement(1m Ø stone column)
0 5 10 15 20 25Hoop tension (kN/m)
(1m Ø stone column)
00 5 10 15 20 25
1
2m)
50 kN/m250 kN/m2
3Dep
th (m
250 kN/m500 kN/m1000 kN/m
42500 kN/m5000 kN/m10000 kN/m
Encased Stone Columns 845
10000 kN/m
Influence of shear strength of di il (1 Ø)surrounding soil (1 m Ø)
Pressure (kPa)
00 100 200 300 400
50
100n (m
m)
100
150
ettle
men
t in
OSCESC 250 kN/mESC 5000 kN/mESC 10000 kN/m
200
Se
Cohesion = 20 kPaCohesion = 10 kPa
Encased Stone Columns 85
250
FE Analysis with c
embankment loadingMaximum embankmenrtEmbankment fill embankmenrt height = 5m
Stone
Foundation soil depth = 5 m
column
Geosynthetic encasement
Encased Stone Columns 86Influence radius 3 m
Variation of stress intensity factor with h i ht f b k theight of embankment
2.510000 kN/m
2tor,
SIF
10000 kN/m5000 kN/m2500 kN/m1000 kN/m250 kN/m2
ensi
ty fa
c OSC
1.5
stre
ss in
te
10 1 2 3 4 5 6
h i ht f b k t ( )
Encased Stone Columns 87
height of ebankment (m)
Design chart for encased stone columns
Encased Stone Columns 88
ConclusionsConclusionsConclusionsConclusions Encasement is more effective in lesser diameter stone
columns because of mobilisation of larger confining stresses.
Significant improvement can be achieved by encasingSignificant improvement can be achieved by encasing the top portions to depths of 2 to 3 times the diameter.
The load capacity of encased columns is not as sensitive t th h t th f th di ilto the shear strength of the surrounding soils as compared to OSCs.
The magnitude of loads transferred into the encased e g ude o o ds s e ed o e e c sedstone columns from the embankments can be increased by using stiffer encasement.
Encased Stone ColumnsEncased Stone Columns 8989
AcknowledgementsAcknowledgementsAcknowledgementsAcknowledgements
• MHRD for sponsoring a research project titledMHRD for sponsoring a research project titled“Investigations on Modern Technologies forconstruction of Road/rail embankments on soft claysoils” at IIT Madras
• Dr. Murugesan, formerly Ph.D. scholar at IITMadras for doing all this work.
Encased Stone ColumnsEncased Stone Columns 9090
Thank youy