Lecture+4+-+4+-+Stone+Column-1.ppt

University of Technology, Sydney
Faculty of Engineering
49119 – Problematic Soils and
Ground Improvement Techniques
Week 4
Stone Columns Design and Construction
Subject Coordinator: Yujie Qi
(PhD, MEng, BEng)
Yujie.qi@uts.edu.au

Spring 2008 Vibro Techniques 2
Content
 Introduction to vibro techniques
 Vibro process
 Vibro compaction
 Process
 Design
 Vibro replacement
 Process
 Design
 Improvement factor
 Examples

Introduction
 Vibro Techniques
 Very cost effective method for compaction of loose and soft grounds
(comparable to deep dynamic compaction)
 Vibro compaction
 Suitable for loose granular soils
 Involves penetration of vibrating probes to densify the soil
 Vibrator can be jetted into the ground to the required depth
 Vibrated during withdrawal while compacting the backfill
 Vibro replacement
 Suitable for fine grained soils with low plasticity
 Vibrator forms cylindrical cavities in the ground
 The cavities are filled with suitable material and compacted
 Compaction results
 Lower post-construction settlement
 Higher bearing resistance

Costs

Suitability of the application
Soil type Vibro-compaction Vibro-replacement
Sands Excellent Not applicable
Silty sands Good Excellent
Silts Poor Good
Clays Not applicable Good
Dumped fills Depends on nature of fill Good

Vibrator
 Vibrator
 Specially-designed, poker-type depth vibrators
 Length: 2-5 m
 Diameter: 0.3 – 0.5m
 Weight: 1.5 to 5 tonnes
 Cause of vibration
 Eccentric weight within the vibrator
 Rotates by electric or hydraulic motor
 Rotational speed of 1500-3000 rpm
 Vibration amplitude of 10 – 50 mm
 Acceleration of up to 5g
 Generates horizontal force of 150-700kN

Vibrator

Vibro Process
 Insertion to required depth
 Normally due to vibrator weight
 May be aided by water jet or air in sand
 Water and air flow are stopped when vibrator reaches the
required depth
 Compaction
 Vibrator compacts the surrounding soil
 Makes a temporary cavity in fine grained soils
 Reduces voids in granular soils, compacts the supplied soils
 ~ o.5-1.0 m lift after 30 – 60 sec vibration
 Cavity is filled and compacted by vibrator
 Fills are added during compaction in granular soil
 Fills are added after cylindrical cavity is created
https://www.youtube.com/watc
h?v=fI54TqFbbgM

Vibro Compaction Process

Vibro Replacement Process

Vibro Process
http://cee.engr.ucdavis.edu/faculty/boulanger/geo_photo_album/GeoPhoto.html

Vibro Techniques
 Treatment can be:
 Through the whole depth of soft / loose soil layer
 Under high loading area
 At specified locations

Spring 2008 Vibro Techniques
15
Vibro Techniques: applications

16
Vibro Techniques: applications

Advantages of Vibro Techniques
 Increased bearing capacity
 Increased shear resistance
 Reduced settlement
 Mitigation of liquefaction and lateral spreading
 Uniformity of site after treatment
 Cost and time savings over conventional systems
 Can be applied close to existing structures
 In situ treatment, thus avoiding excavation and
replacement

Advantage in Foundation Design
 Bearing Capacity
 A function of soil shear strength parameters, c and f.
 Vibro techniques increase foundation strength by
 Increasing values of c and f of in-situ soil
 Constructing columns of strong materials
 Settlement
 A function of stiffness of the soil, E or mv
 Indirectly related to moisture content and/or void ratio
 Vibro techniques reduce ground settlement by:
 Reducing the voids, increasing the stiffness of in-situ material
 Adding columns of stiff material into the ground
Density Very
loose
Loose Medium
dense
Dense Very
dense
Relative density, Dr <15 15-35 35-65 65-85 85-100
SPT N-value <4 4-10 10-30 30-50 >50
CPT, qc (MPa) <5 5-10 10-15 15-20 >20
Dry unit weight, gd (kN/m3) <14 14-16 16-18 18-20 >20
Modulus of deformation
(MPa)
15-30 30-50 50-80 80-100 >100
Friction angle, f (o) <30 30-32.5 32.5-35 35-37.5 >37.5

Design Steps
 Perform site investigation
 Normal procedures in Geotechnical Engineering
 SPT / CPT and soil gradation
 Check performance of native soil
 Bearing capacity, settlement, instability, liquefaction, …
 Establish treatment requirements
 Degree of densification required
 Develop and design appropriate compaction plan
 Method, extent, depth, spacing, energy, etc.
 Develop testing criteria
 Consistent with initial tests
 Able to evaluate degree of treatment

Vibro Compaction
 Suitable for clean granular soil
 Silt % < 10%
 No cohesion
 CPT: Friction ratio between 0 -1, Tip resistance < 3MPa
 Range of treatable soil types
 Grading curve
 Suitability number, SN < 30
2
10
2
20
2
50 )
D
(
1
)
D
(
1
)
D
(
3
7
.
1
SN 



Vibro Compaction 2
10
2
20
2
50 )
D
(
1
)
D
(
1
)
D
(
3
7
.
1
SN 


Particle size (mm)
Finer
%
100
90
80
70
60
50
40
30
20
10
0
Clay Silt Sand Gravel
0.001 0.01 0.1 1 10

Vibro Compaction- Densification
2
10
2
20
2
50 )
D
(
1
)
D
(
1
)
D
(
3
7
.
1
SN 



Autumn 2022 Vibro Techniques 23
Vibro Replacement
 Involves:
 Insertion of vibro probe to create a cylindrical cavity
 The cavity is filled with granular soils and compacted
 Compaction is achieved by the vibro probe
 Suitable for:
 Silt and low plastic clay
 Columns of gravel or crushed stone produced
 ~1m diameter
 ~1-3m spacing
 Spacing depends on soil conditions, equipments, and construction
procedure
 A drainage blanket of granular soil covers the finished
ground

Backfill Material
 Generally coarse grained material
 Including crushed stone, gravel, coarse sand
 Suitability number:
 SN: 0 – 10 => Excellent for backfill
 SN: 10 – 20 => Good
 SN: 20 – 30 => Fair
 SN: 30 – 40 => Poor
 SN: > 50 => Unsuitable
 Degree of improvement is partially based on quality
of backfill material
 And also of the lateral support provided by the native soil
2
10
2
20
2
50 )
D
(
1
)
D
(
1
)
D
(
3
7
.
1
SN 



Strength of Stone Column
 Mechanism of failure of stone columns
s
t
gz
gz+2cu
cu
gz + 2cu
gz
gz+2cu

Strength of Stone Column
 Mechanism of failure of stone columns
f s
t
Kpc (gz + 2cu)
gz + 2cu
gz
Kpc (gz + 2cu)
gz+2cu
 Failures occur at excessively large deformation

Stiffness of Stone Columns
 Serviceability criterion is more relevant in design of
stone columns
 Priebe’s method gives acceptable estimate of:
 Settlement
 Strength parameters
 Based on estimation of improvement factor, n
 Can be estimated with a good level of approximation
treatment
with
Settlement
treatment
without
Settlement
n 

Improvement Factor
 Simplified assumptions:
 Stone column is assumed to be incompressible
 Change in stiffness due only to lateral deformation of stone
column
 No effects of column compressibility
 No effects of density variation between stone column and
native soil
 Some of the effects will be included later
 Basic equation:













 1
)
A
,
A
,
(
f
.
K
)
A
,
A
,
(
f
5
.
0
A
A
1
n
c
s
ac
c
s
c
o A
/
A
2
1
)
A
/
A
1
).(
1
(
)
A
,
A
,
(
f
c
s
c
s
c
s








Kac=Coefficient of active earth pressure of stone column

Improvement Factor
 For Poisson’s ratio of 0.33:
  











 1
A
/
A
1
K
4
A
/
A
5
A
A
1
n
c
ac
c
c
o

Effects of Column Compressibility
 Due to compressibility of stone column the real
improvement factor must be less than no
 Effects of compressibility can be included as an
additional area ratio
Taking mo=CEc/CEs:
1
K
4
)
1
m
(
K
16
1
K
4
5
)
2
m
(
K
4
2
1
)
1
K
4
(
2
5
)
2
m
(
K
4
A
A
ac
o
ac
2
ac
o
ac
ac
o
ac
1
c


























1
)
A
/
A
(
1
)
A
/
A
(
1
c
c 


)
A
/
A
(
A
/
A
1
A
A
c
c
m
c









  

















 1
)
A
/
A
(
1
K
4
)
A
/
A
(
5
A
A
1
n
m
c
ac
m
c
m
c
1

Effects of Column Compressibility

Effects of Density Difference
 Initial pressure difference between soil and column
results in bulging.
 It was assumed the difference is only to to applied pressure
 Density of column and soil also contributes to pressure
difference
 Effect is included as:
n2 = n1 × fd
s
c
oc
c
oc
c
oc
d
W
W
K
K
K
f


s
s

)
z
.
(
)
1
K
(
K
K
oc
c
oc
c
oc

g



s
s


Effects of Density Difference

Limits
 Limits of fd:
s
c
s
E
c
E
d
/
C
/
C
f
1
s
s


)
A
,
A
,
(
f
.
K
)
A
,
A
,
(
f
5
.
0
c
s
ac
c
s
s
c




s
s
 Limits of n:










 1
C
C
A
A
1
n
s
E
c
E
c
max

Limits

Shear Strength Improvement
 Change in shear strength is proportional to the ratio
of loads on stone columns, m
m = (n-1)/n
 Average friction angle, fm, and cohesion, cm, are
tan fm = m×tan fc + (1-m)tan fs
cm = cs×(1 - m)

Shear Strength Improvement

Settlement Calculation
 Settlement of a footing on a large treated area will be
reduced by n.
 Settlement of large footing on large treated area will
be one dimensional on homogeneous soil:
 Settlement of a footing on stone columns under the
footing only is approximated
 Priebe’s charts
2
s
E n
.
C
d
.
s
s



 Settlement of a footing on a large treated area will be
reduced by n.
 Settlement of large footing on large treated area will
be one dimensional on homogeneous soil:
 Settlement of a footing on stone columns under the
footing only is approximated
 Priebe’s charts
 For layered soil use the following equation
2
s
E n
.
C
d
.
s
s


 
u
u
l
1
2
s
E
d
.
)
s
/
s
(
d
.
)
s
/
s
(
n
.
C
s 
 
s



Lecture+4+-+4+-+Stone+Column-1.ppt

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