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!