Recent Advances in Column
Technologies to Improve Soft
Foundations
Jie Han, Ph.D., PE
Professor
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
Material
Granular
Chemically-stabilized
Concrete
Composite
Rigidity
Flexible
Sand compaction piles, stone columns, rammed
aggregate piers
DM columns and grouted columns
Concrete columns, cement-flyash-gravel (CFG)
columns
Geosynthetic-encased soil columns, stiffened DM
columns, and composite spun piles
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
Rotation
direction
1
Grouting
2
Grouting
3
Mixing
Mixing
Mixing
4
Grouting
5
Mixing
Grouting
6
Mixing
Mixing
7
Mixing
8
Courtesy of S.Y. Liu
T-shape Deep Mixing
Courtesy of S.Y. Liu
Hollow Concrete Columns
Referred to as Large Diameter Pipe Pile Using
Cast-in-place Concrete (PCC) by Prof. Liu
Courtesy of H.L. 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
-
Jet pressure =220 bar
-
Diameter =0.60 m
- L=7.00 m
Courtesy of Bergado
SDCM pile construction
Composite Columns - Grouted Spun Pile
Cement mix
Spun pile
Welding
Bhandari et al. (2009)
Pile-Column Combined Method
Pile
Column
Huang and Li (2009) and Zheng et al. (2009)
DM-PVD Combined Method
Embankment
DJM column
PVD
Liu et al (2008)
Inclinometer
Settlement plate
Earth pressure cell
Piezometer
Not to scale
DM
column
Ye et al (2008)
PVD
The Most Commonly Used Application
– Column-supported Embankments
Ds0
Geosynthetic-reinforced
Geosynthetics
fill platform
Embankment
Ds0
Columns
Firm soil or bedrock
Load Transfer Mechanisms
Equal Strain vs. Equal Stress
c
s
Ec
Ss
Es
Sc
Ec
Ss
Ec
s
c
Ss
Es
Sc
Ec
Ss
(a) Equal strain = rigid loading (b) Equal stress = flexible loading
DS
Columns
How about a column-supported embankment?
Stress Concentration under Equal V. Strain
Stress Concentration Ratio, n =
c
Ds
Sc = Ss
1-D unit cell
z =
c
Dc
n=
=
Dc
Ds
s
Ds
s
c
s
Dc
c
Ec
s
Es
h
Sc = Ss
Unit cell with lateral deformation
z’ - (x’ - y’)
z - (x - y)
z =
=
Es
Ec
Ec
>
n
Es
Stress Concentration Ratio vs. Strain
Equal vertical strain condition
Stress
c2
c3
c4
c1
Yielding
Stress concentration ratio, n
c
n
s
Column
Yielding
Soil
s4
s3
s2
s1
Strain
(a) Stress-strain relationship
0
Strain
(b) Stress concentration ratio
E.g., stone column: qcult = 15 to 25 cu, qsult = 5 to 6 cu
n = qcult / qsult = 2 to 5
Stress concentration ratio, n
Influence of Column Lateral
Deformation and Yielding
Castro and Sagaseta (2011)
Influence of Modulus Ratio
and Column Yielding
Stress concentration ratios
70
Rigid
column
L/d e=4
as =0.1
kc/kv=1
60
50
40
Ec/E
10
30
50
20
100
Semi-rigid
Flexible
10
0
0.1
1
10
1000
100
Time (days)
10000
100000
Jiang et al. (2010)
Stress Concentration vs. Consolidation
kPa
2040kPa
Yin and Fang (2008)
n vs. Ec/Es
10
Stress Concentration Ratio, n
9
8
7
6
Cutoff ratio
for stone columns
n = 1 + 0.217 (Ec /Es - 1)
5
4
Barksdale and Bachus (1983)
3
2
1
0
0
10
20
Modulus Ratio, Ec /Es
30
40
Stress Transfer
under Unequal Vertical Strain
Settlement, S(z)
Shear stress, t(z)
Average vertical
stress, (z)
Equal settlement
(upper plane)
Fill
hc
Soft
soil
re
Sc
Column
t<0
Ss
0 s f
rc
t>0
Equal settlement
(lower plane)
Bearing layer
z
Sc at r < rc
Ss at r = r e
c
z
t at r = rc
z
c at r < rc
s at r = r e
Modified from Schlosser and Simon (2008)
Stress Transfer in Geosynthetic-reinforced
Column-supported Embankment
t
Hcr
W
t
ps
T

c
s
H
Es
Ec
Effects: (1) modulus ratio effect, (2) soil arching,
(3) tensioned membrane/slab stiffening
Modified from Han (1998)
Stress Concentration Ratio, n
Field Stress Concentration Ratio
70
All plate loading test data from Han and Ye (1991)
60
50
40
30
20
10
0
0
100
Flexible column
PLT/lime columns
200
300
400
Applied pressure, p (kPa)
Semi-rigid column
PLT/DM columns
PLT/stone columns
GCSE/DM columns
PLT = Plate loading test
CSE = Column-supported embankment
GCSE = Geosynthetic-reinforced column-supported embankment
Findings:
500
600
Rigid column
PLT/VCC
PLT/concrete columns
GCSE/VCC
GCSE/concrete columns
CSE/concrete columns
(1) n increases with stress level
(2) n increases with rigidity of loading
Han and Wayne (2000)
09
DEM Modeling of Dynamic Behavior
Loading
19
20
21
22
23
9
Unreinforced
Reinforced
14
15
16
17
18
Embankment
1.3m
9
Optional
geogrid
4
10
5
1
Pile cap
11
6
2
12
7
13
8
0.3 m
3
Stress concentration ratio
8
7
6
5
4
3
2
1
0
0
5
10
15
20
25
Cycle
0.3 m
Findings:
0.9 m
0.3 m
(1) geosynthetic increases rigidity of loading
(2) n decreases with soil arching
30
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
ss  mv,s Dz H
Settlement of treated ground
ssc  m'v,s D'z H  m'v,ss Dz H
Settlement ratio
m 'v ,s
s sc

s
s s m v ,s
If assume mv,s = mv,s’
ssc
1
 s 
ss
1  a s (n  1)
Stress reduction factor
Aboshi et al. (1978)
Stress Reduction Factor Method
vs. Numerical Method
Consolidation settlement (mm)
300
H/d e = 4
as = 0.1
k c/k v = 1
250
200
Numerical
Simplified
150
100
50
0
0
20
40
60
80
100
/Es
EEcc/E
Jiang et al. (2013)
Improvement Factor Method
Basic Method
Assume incompressible columns with bulging over
column length
Improvement
factor


5  as
If  1  a s 
 1
2
o
 41  a s  tan 45  c / 2 
Settlement of stone column
foundation


ss
s sc 
If
Modified Method
In addition to column bulging, column compressibility
and overburden stress are considered
Priebe (1995)
Basic Improvement Factor Method
8
Friction angle
of column (deg.)
7
Improvement Factor
35
37.5
6
40
5
42.5
45
4
3
2
1
0
0.1
0.2
0.3
0.4
0.5
Area Replacement Ratio
Priebe (1995)
Elastic-Plastic Solution for
Stone Columns
• 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
Pulko and Majes (2005)
Castro and Sagaseta (2009)
Column Penetration Method
Equivalent unimproved
zone thickness due to
column penetration
Area
replacement Improvement
depth ratio
ratio
Hc = HL f() g() h()
Pressure
strength ratio
Chai et al. (2010) and Chai (2012)
Pier-raft Approach for Settlement of Soilcement or Concrete Columns
Eeq  Es  E p  Es 
Raft
Es
deq
Atp
Ag
Horikoshi and Randolph (1999)
Eeq
Ag
K pr 
Pp  Pr
Spr

K p  K r 1  2cp 
2
1  K r / K p cp
Randolph (1984)
Han et al. (2009)
Calculated Settlements by Pier-raft Aproach
10m
0.8m
10m
(a) Plan view
Settlement (cm)
Method
Group
7.4m
Analytical
15MN
Raft
0.5m
Numerical
Equivalent pier
15.9 (16.9*)
15.6
16.9
Lp =10m
DM columns
(Ep=100MPa)
h = 30m
* Without considering finite depth effect
(b) Cross section
Es=5MPa
Han et al. (2009)
Consolidation of Stone Columns
(Han and Ye, 2001; 2002)
Rate of consolidation
due to radial flow:
de
p
Ur  1 e
Drainage surface
rs
Stone column
Ec
rc
z
Es
kv
kc ks
8
Tr'
'
Fm ( N )
Modified time factor
in radial flow
H
2H
c'r t
T  2
de
'
r
kh
Drainage surface

r
re
1 

c  c r 1  n s 2 
N 1

'
r
Degree of Consolidation
0
0.2
Barron (1947)
0.4
U
n=10
0.6
n=1
Free-draining
stone column
Balaam and Booker (1981)
Han and Ye (2001)
0.8
1
0.0001
0.001
0.01
0.1
Tr
Han & Ye (2001)
Khine (2004)
Dissipation of Excess Pore Pressure
Dissipation of Average Excess Pore
Water Pressure, D u/p
1.0
N=3, ns=5
0.9
0.8
Due to stress reduction
0.7
0.6
0.5
0.4
0.3
Due to drainage
0.2
0.1
0.0
0
0.02
0.04
0.06
0.08
0.1
0.12
Time Factor, Tr
Han and Ye (2001)
Well Resistance Effect
Time (day)
0
20
40
60
80
100
120
0
10
Settlement (mm)
20
30
40
Field data (Tan et al., 2008)
No well resistance (Han and Ye, 2002)
Well resistance (Han and Ye, 2002)
50
60
70
80
90
Han (2010)
Consolidation of Column-improved
Soft Foundation over Soft Soil
Zhu and Yin’s (1999) closed-form solution for consolidation
of two-layered soils can be used for calculation of
consolidation rate
Chai and Pongsivasathit (2009)
Consolidation of Soil-cement
Column-improved Foundations
0
Average degree of consolidation (%) .
10
20
30
40
50
kc = ks
60
70
80
90
100
0.0001
Ec/Es
5
10
50
100
0.001
0.01
2
Time factor Tv =cv t/H
0.1
1
Jiang et al. (2013)
Stability
Column Failure Modes
under Embankment Loading
Soft soil
Columns
Embankment
Embankment
Embankment
Columns
Soft soil
Columns
Soft soil
Stiff layer
Stiff layer
Stiff layer
Sliding direction
(a) Sliding
(b) Collapse (rotational)
(c) bending
o
Embankment
Embankment
Berm
Embankment
S
Columns
Soft soil
Columns
Stiff layer
(d) Circular shear
Soft soil
Stiff layer
(e) Horizontal shear
Columns
Tensile
failure
Bending
failure
(f) Combined
Modified from Kitazume (2008) and Broms (1999)
Factor of Safety under Undrained
Condition for Stone Columns
b
a
Backfill
water level
Backfill
water level
Stone columns
Equivalent area
Clay
Clay
Sand
Sand
FS (individual) = 0.9 FS (equivalent)
Abusharar and Han (2010)
Numerical Modeling with DM Columns
6
Numerical
Bishop
Factor of Safety
5
Han et al. (2005; 2010)
4
3
Shear
Bending
Rotation
2
1
0
0
100
200
300
400
Cohesion of DM Walls (kPa)
500
600
Centrifuge Tests with Rigid Columns
Single column
Column group
Zheng et al. (2011)
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 transfer
 Columns under embankment loading can fail under
shear, tension, bending, rotation, or a combination.
Bending and rotation failure are dominant for semirigid and rigid columns
Thank You!