Geosynthetic Encased Stone Columns - · PDF fileGeosynthetic Encasement for Stonger and Stiffer Stone ColumnsStonger and Stiffer Stone Columns K. Rajagopal Professor, Department of

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: [email protected]

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


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


6Encased 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


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)


How does a stone column resist vertical loads?How does a stone column resist vertical loads?


Failure mechanism Failure mechanism –– Single SCSingle SCgg


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


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


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


(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


Construction of Encased Stone ColumnConstruction of Encased Stone Column

Alexiew et al. (2005)



Construction of Encased Stone Column

Alexiew et al (2005)Encased Stone Columns 23

Alexiew et al. (2005)

Geotextile encased


sand column

Hamburg GermanyHamburg, Germany

A380 factory site




Pre-fabricated ESC column being lowered into a drill borehole


Another type of encased stone column

Courtesy:Dipl.-Ing. Holger Pohlmannp g g

Naue Fasertechnik GmbH & Co. KG

P dPresent study


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.


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.


Construction of Encased Stone ColumnConstruction of Encased Stone Column


Encased Stone ColumnEncased Stone Column


11C i t t th tiCompression tests on geosynthetic

encased stone aggregatesencased stone aggregates


Geosynthetic Encased Stone aggregateGeosynthetic Encased Stone aggregate


Compression test on Encased Stones


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


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


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


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


Stone column arrangements

s

Square pattern Triangular pattern

Influence radius = 0.564s Influence radius = 0.525s


f

Schematic of setup – Unit CellSchematic of setup Unit CellStrain controlled loadingProving ring

To strain read out unit

Proving ring


Sof500 mm

StonesSoft

Clay Strain gaugesy Strain gauges

Unit cell tank

Sectional plan

soft clay

Encased Stone Columns 44210 mmSectional plan


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


11 In-situ void ratio 1.25

Strain gauges in encasementStrain gauges in encasement


Stone Column in Unit Cell


ESC in Unit cell tank


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


50

Bulging in stone columnsg g

OSC ESC


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


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


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


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


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


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


Diameter of stone column

33L d t t i l tLoad tests on single stone

column in large tankcolumn in large tank


Load tests on stone column installed in large test tankinstalled in large test tank

Vertical loading Dial gauges

Stone columns


600

Soft Clay

600 mm

y


1200 mm

Load test on single stone column


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


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


Configuration of group of Stone columns Pl- Plan

75 mm Ø @ 150 mm c/cSoft Clay

Loading plate 280 mm Ø

1200 mm


1200 mm

Group of Stone column – Triangular pattern


Loading plate fitted with pressure ll G t tcell – Group test

Loading

To read out unit

Loading

Pressure cells


Load test on group of stone columns


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


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


Numerical simulations


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


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


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)


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)


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


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)


(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


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)


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


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)


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


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


250

FE Analysis with c

embankment loadingMaximum embankmenrtEmbankment fill embankmenrt height = 5m

Stone

Foundation soil depth = 5 m

column


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 ( )


height of ebankment (m)

Design chart for encased stone columns


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.


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.


Thank youy

Geosynthetic Encased Stone Columns - · PDF fileGeosynthetic Encasement for Stonger and Stiffer Stone ColumnsStonger and Stiffer Stone Columns K. Rajagopal Professor, Department of

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