Recent Advances in Column Technologies to Improve Soft

Foundations

Jie Han, Ph.D., PEProfessor

The University of Kansas, USA

Outline of Presentation

Introduction

Innovations in Installation and Applications

Load Transfer Mechanisms

Settlement and Consolidation

Stability

Concluding Remarks

Introduction

Definition of Columns

A vertical sub-structural element, installed in-situ by ground improvement techniques (replacement, displacement, and/or mixture with chemical agents), that carries the load of the super-structure or earth structure with surrounding soil and transmits it to geo-media around and/or below, through compression, shear, or rotation

Classification of Columns Method Type Technology Examples

 Installation Replacement Stone columns

Displacement Sand compaction piles, stone columns

Mixture DM columns, grouted columns

Combination Rammed aggregate piers 

MaterialGranular

Sand compaction piles, stone columns, rammed

aggregate piers

Chemically-stabilized DM columns and grouted columns

ConcreteConcrete columns, cement-flyash-gravel (CFG)

columns

CompositeGeosynthetic-encased soil columns, stiffened DM

columns, and composite spun piles 

RigidityFlexible

Sand compaction piles, stone columns, rammed

aggregate piers

Semi-rigid DM columns, grouted columns, composite columns

Rigid Concrete columns

Functions

Densification• Increase density, modulus, strength, and liquefaction resistance of surrounding soil• Increase pre-consolidation stress of surrounding soil

Pile effect• Transfer loads to a deeper and competent geo-material • Stress concentration

Drainage• Accelerate consolidation • Increase liquefaction resistance

• Reinforcement • Increase shear, tensile, and/or bending resistance

Design Considerations

• Load transfer

• Bearing capacity (e.g., Bouassida et al., 1995)

• Settlement and consolidation

• Slope stability

• Liquefaction mitigation (e.g., Rollins et al.)

• Earth retaining (e.g., Shao et al.)

Innovations in Column Installation and Applications

T-shape Deep Mixed Columns

Rotationdirection

1 2 3 5 6 7 8Grouting

4Grouting Grouting GroutingMixing Mixing Mixing

Mixing MixingMixingMixing

Courtesy of S.Y. Liu

T-shape Deep Mixing

Courtesy of S.Y. Liu

Hollow Concrete Columns

Courtesy of H.L. LiuReferred to as Large Diameter Pipe Pile UsingCast-in-place Concrete (PCC) by Prof. Liu

X-shape Concrete Columns

Courtesy of H.L. Liu

Geosynthetic-encased Columns

Alexiew et al. (2005)

Composite Columns

Courtesy of G. Zheng

Composite Columns - Stiffened Deep Mixed Piles

SDCM pile construction

- Jet pressure =220 bar

- Diameter =0.60 m

- L=7.00 m

Courtesy of Bergado

Composite Columns - Grouted Spun Pile

Cement mix Spun pile

Welding

Bhandari et al. (2009)

Pile-Column Combined Method

Huang and Li (2009) and Zheng et al. (2009)

Pile Column

DM-PVD Combined Method

Liu et al (2008)

Embankment

Settlement plate Earth pressure cell Piezometer

DJM column

Not to scale

PVD

Inclinometer

PVD

DMcolumn

Ye et al (2008)

The Most Commonly Used Application – Column-supported Embankments

Embankment

Columns

GeosyntheticsGeosynthetic-reinforcedfill platform

Firm soil or bedrock

s0

s0

Load Transfer Mechanisms

Equal Strain vs. Equal Stress

(a) Equal strain = rigid loading (b) Equal stress = flexible loading

cs

Ss SsSc

Ec EcEs

c

s

Ss SsSc

Ec EcEs

Columns

S

How about a column-supported embankment?

Stress Concentration Ratio, n = c

s

n = Dc

Ds

c s

Dc DsSc = Ss

n Ec

Es

Stress Concentration under Equal V. Strain

Ec EsSc = Ss

h

c s

z = c

Dc

=s

Ds

z = z - (x - y)

Ec

z’ - (x’ - y’)Es

=

1-D unit cell Unit cell with lateral deformation

>

Stress Concentration Ratio vs. Strain

Stress

Strain

c1

s1

s2

c2

s3

c3c4

s4

s

cn

Yielding Stress concentration ratio, n

Strain0

(a) Stress-strain relationship (b) Stress concentration ratio

Yielding

Column

Soil

Equal vertical strain condition

E.g., stone column: qcult = 15 to 25 cu, qsult = 5 to 6 cu

n = qcult / qsult = 2 to 5

Influence of Column Lateral Deformation and Yielding

Castro and Sagaseta (2011)

Str

ess

conc

entr

atio

n ra

tio,

n

Influence of Modulus Ratio and Column Yielding

Jiang et al. (2010)

0

10

20

30

40

50

60

70

0.1 1 10 100 1000 10000 100000

Stre

ss c

once

ntra

tion

ratio

s

Time (days)

10

50

100

Ec/E

L/de=4as=0.1kc/kv=1

Rigidcolumn

Semi-rigidFlexible

Stress Concentration vs. Consolidation

Yin and Fang (2008)

20 kPa40 kPa

n vs. Ec/Es

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40

Str

ess

Co

nce

ntr

atio

n R

atio

, n

Modulus Ratio, Ec/Es

Barksdale and Bachus (1983)

n = 1 + 0.217 (Ec/Es - 1) Cutoff ratio for stone columns

Stress Transfer under Unequal Vertical Strain

Settlement, S(z)

SsSc

Shear stress, t(z)Equal settlement

(upper plane)

Equal settlement (lower plane)

t < 0

0 cfs

Fill

Average vertical stress, (z)

rcre t > 0

z z zSc at r < rcSs at r = re

t at r = rc c at r < rcs at r = re

hc

Softsoil

Bearing layer

Column

Modified from Schlosser and Simon (2008)

Modified from Han (1998)

W tt

ps

H

sc

THcr

Stress Transfer in Geosynthetic-reinforced Column-supported Embankment

Effects: (1) modulus ratio effect, (2) soil arching, (3) tensioned membrane/slab stiffening

EcEs

Field Stress Concentration Ratio

0

10

20

30

40

50

60

70

0 100 200 300 400 500 600

Str

ess

Con

cent

ratio

n R

atio

, n

Applied pressure, p (kPa)

All plate loading test data from Han and Ye (1991)

Flexible column

PLT/lime columns

PLT/stone columns

Semi-rigid column

PLT/DM columnsGCSE/DM columns

Rigid column

PLT/VCCPLT/concrete columnsGCSE/VCCGCSE/concrete columnsCSE/concretecolumns

PLT = Plate loading test CSE = Column-supported embankmentGCSE = Geosynthetic-reinforced column-supported embankment

Findings: (1) n increases with stress level(2) n increases with rigidity of loading

Han and Wayne (2000)

DEM Modeling of Dynamic Behavior

PFC2D 3.10Step 69970 22:11:09 T ue Sep 29 2009

View S ize: X: -6.307e-001 < = > 2.360e+ 000 Y : -8.952e-001 < = > 2.554e+ 000

W a ll

W a ll

B a ll

M e a s u r e m e n t C ir c le s

1 2 3

4 5 6 7 8

9 10 11 12 13

14 15 16 17 18

19 20 21 22 23

1.3m

0.3 m

0.9 m0.3 m 0.3 m

Embankment

Pile cap

Optional geogrid

Loading

Findings: (1) geosynthetic increases rigidity of loading(2) n decreases with soil arching

Settlement and Consolidation

Methods of Settlement Calculation

1. Stress reduction factor (e.g., Aboshi et al, 1978)

2. Improvement factor method (e.g., Priebe, 1995)

3. Elastic-plastic solution (e.g., Pulko and Majes, 2005; Castro and Sagaseta, 2009)

4. Column penetration method (e.g., Chai et al., 2010)

5. Pier-raft method (e.g., Han et al., 2009)

5. Numerical method

Stress Reduction Factor Method Settlement of untreated ground

Settlement of treated ground

If assume mv,s = mv,s’

Stress reduction factor

Aboshi et al. (1978)

Settlement ratio

Hms zs,vs

HmHms zs'

s,v'z

's,vsc

ss,v

's,v

s

sc

m

m

s

s

)1n(a1

1

s

s

ss

s

sc

Stress Reduction Factor Methodvs. Numerical Method

Jiang et al. (2013)

0

50

100

150

200

250

300

0 20 40 60 80 100

Con

solid

atio

n se

ttle

men

t (m

m)

Ec/E

Numerical

Simplified

H/de = 4as = 0.1kc/kv = 1

Ec/Es

Improvement Factor Method

Priebe (1995)

Assume incompressible columns with bulging over column length

Basic Method

12/45tana14

a5a1I

co2

s

ssf

Improvement factor

f

ssc I

ss Settlement of stone column

foundation

Modified Method

In addition to column bulging, column compressibility and overburden stress are considered

Basic Improvement Factor Method

Priebe (1995)

Elastic-Plastic Solution for Stone Columns

Pulko and Majes (2005)Castro and Sagaseta (2009)

• Assume soft soil is linearly elastic

• Assume stone columns are linearly elastic-perfectly plastic with Mohr-Coulomb failure criterion with a constant dilantancy angle

• Plasticity starts with the upper portion of the column and can extend deeper to the whole length of column with applied load

Column Penetration Method

Chai et al. (2010) and Chai (2012)

Hc = HL f() g() h()

Equivalent unimproved zone thickness due tocolumn penetration

Area replacement

ratio

Improvement depth ratio

Pressure strength ratio

Pier-raft Approach for Settlement of Soil-cement or Concrete Columns

Han et al. (2009)

g

tpspseq A

AEEEE

2

cppr

cprp

pr

rppr K/K1

21KK

S

PPK

Horikoshi and Randolph (1999)

Randolph (1984)

Raft

Esdeq

Eeq

Ag

Calculated Settlements by Pier-raft Aproach

MethodGroup Equivalent pier

Analytical

Numerical

Settlement (cm)

15.9 (16.9*)

15.6 16.9

* Without considering finite depth effect

Han et al. (2009)

10m

10m

0.8m

7.4m

(a) Plan view

0.5mLp =10m

DM columns(Ep=100MPa)

Raft

(b) Cross section

h = 30m

15MN

Es=5MPa

Consolidation of Stone Columns(Han and Ye, 2001; 2002)

de

2H

z Hrc

rre

kv

kh

Drainage surface

Drainage surface

Stone column

p

rs

kskc

Ec Es

Rate of consolidation due to radial flow:

'r'

m

T)N(F

8

r e1U

2e

'r'

r d

tcT

Modified time factor in radial flow

1N

1n1cc

2sr'r

Degree of Consolidation 0

0.2

0.4

0.6

0.8

1

0.0001 0.001 0.01 0.1

Tr

U

Balaam and Booker (1981)

Han and Ye (2001)

Barron (1947)

n=10 n=1

Han & Ye (2001)

Khine (2004)

Free-draining stone column

Dissipation of Excess Pore Pressure

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 0.02 0.04 0.06 0.08 0.1 0.12

Time Factor, Tr

Dis

sip

atio

n o

f A

vera

ge

Exc

ess

Po

re

Wat

er P

ress

ure

,

u/p

Due to drainage

Due to stress reduction

N=3, ns=5

Han and Ye (2001)

Well Resistance Effect

Han (2010)

Consolidation of Column-improved Soft Foundation over Soft Soil

Chai and Pongsivasathit (2009)

Zhu and Yin’s (1999) closed-form solution for consolidation of two-layered soils can be used for calculation of consolidation rate

Consolidation of Soil-cement Column-improved Foundations

Jiang et al. (2013)

0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1Time factor Tv=cv t/H2

Ave

rage

deg

ree

of c

onso

lidat

ion

(%)

.

51050100

Ec/Es

kc = ks

Stability

Column Failure Modes under Embankment Loading

Modified from Kitazume (2008) and Broms (1999)

Embankment

Soft soil

Stiff layer

Columns

Sliding direction

Embankment

Soft soilColumns

Stiff layer

Embankment

Soft soilColumns

Stiff layer

Embankment

Soft soil

Stiff layer

Columns

Embankment

Soft soil

Stiff layer

Columns

(a) Sliding (b) Collapse (rotational) (c) bending

(d) Circular shear (e) Horizontal shear

Columns

EmbankmentBerm

Tensile failure

Bendingfailure

S

o

(f) Combined

Factor of Safety under Undrained Condition for Stone Columns

Abusharar and Han (2010)

Backfill

Equivalent area

Sand

water level

Clay

b

Sand

Backfill

Stone columns

water level

Clay

a

FS (individual) = 0.9 FS (equivalent)

Numerical Modeling with DM Columns

Han et al. (2005; 2010) 0

1

2

3

4

5

6

0 100 200 300 400 500 600

Cohesion of DM Walls (kPa)

Fa

cto

r o

f Sa

fety

Numerical Bishop

Shear Bending Rotation

Centrifuge Tests with Rigid Columns

Zheng et al. (2011)

Single column

Column group

Concluding Remarks

A variety of column technologies have been developed and successfully adopted for different applications Composite columns or combined technologies with

columns have been increasingly used to combine their

advantages Stress concentration ratio depends on rigidity of

loading, modulus ratio, lateral deformation, yielding of

columns, stress level, and dynamic loading Columns can accelerate the rate of consolidation through drainage and/or stress transferColumns under embankment loading can fail under shear, tension, bending, rotation, or a combination. Bending and rotation failure are dominant for semi- rigid and rigid columns

Thank You!