geoss event seminar 4 nov 2013
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8/13/2019 GeoSS Event Seminar 4 Nov 2013
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CSGE/GEOSS SEMINAR
Prof Harry TanDate: 4 November2013
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Nature of ground displacements
Available theories
Cantabria/MIT and Swedish Study
Case of Mixed Type PilingCase of Jack-in piles in Deep Soft Clays
Some Conclusions
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Two Relevant Papers• Paper 1 from Cantabria and MIT
• Paper 2 from Skanska Sweden
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Nature of ground
displacements
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Swedish empirical method
SSPM – Shallow Strain Path Method
Direct FEM – UnDrainedVolumetric
Strains or Prescribed Cavity Expansion
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• Approximate answers to be used for
simplified cases only
• Basic assumption is UnDrained
Volume displacements takes on a
simple form of lateral and vertical
movements
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Closed form solutions are
remarkably so simple
Prof Harry Tan
Date: 15 OCT 2013
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Assume single pile R=0.5m
L=5m
SSPM single pile surface displacements
R (m) L (m) r (m) dr (m) dv (m)
0.50 5.00 1.00 0.123 0.100
0.50 5.00 2.00 0.058 0.039
0.50 5.00 3.00 0.036 0.0200.50 5.00 4.00 0.024 0.012
0.50 5.00 5.00 0.018 0.007
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E=2MPa,
nu’=0.3
E=50MPa,
nu’=0.3E=50MPa,
nu’=0.3
E=10MPa,
nu’=0.3
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E=2MPa,
nu’=0.3
E=50MPa,
nu’=0.3
E=50MPa,
nu’=0.3
E=10MPa,
nu’=0.3
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Ground Heave Radial Lateral Displacement
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SSPM
SSPM single pile surface displacements
R (m) L (m) r (m) dr (m) dv (m)
0.50 5.00 1.00 0.123 0.100
0.50 5.00 2.00 0.058 0.039
0.50 5.00 3.00 0.036 0.020
0.50 5.00 4.00 0.024 0.012
0.50 5.00 5.00 0.018 0.007
For r>3D, FEM heave agrees very well with SSPM
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SSPM
SSPM single pile surface displacements
R (m) L (m) r (m) dr (m) dv (m)
0.50 5.00 1.00 0.123 0.100
0.50 5.00 2.00 0.058 0.039
0.50 5.00 3.00 0.036 0.020
0.50 5.00 4.00 0.024 0.012
0.50 5.00 5.00 0.018 0.007
For r>3D, FEM heave agrees well with SSPM
BUT E values have some small influence on results
E=20 and 50 MPa
E=2,5,10 MPa
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For r>3D, FEM surface lateral displacement agrees well with SSPM
SSPM single pile surface displacements
R (m) L (m) r (m) dr (m) dv (m)
0.50 5.00 1.00 0.123 0.100
0.50 5.00 2.00 0.058 0.039
0.50 5.00 3.00 0.036 0.020
0.50 5.00 4.00 0.024 0.012
0.50 5.00 5.00 0.018 0.007
SSPM
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For r>3D, FEM surface lateral displacement agrees very well with SSPM
BUT E values have some small influence on results
SSPM single pile surface displacements
R (m) L (m) r (m) dr (m) dv (m)
0.50 5.00 1.00 0.123 0.100
0.50 5.00 2.00 0.058 0.039
0.50 5.00 3.00 0.036 0.020
0.50 5.00 4.00 0.024 0.0120.50 5.00 5.00 0.018 0.007
SSPM
E=20 and 50 MPa
E=2,5,10 MPa
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100% Vol Strain:
Esp-xx=50% and Esp-zz=50%
Prescribed displacements:
Uxx=0.19m from radius of pile
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Use HS-small model to characterize
highly non-linear response of soils
Soft Clay
(UnDrained)
Loose Sand
(Drained)
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• Lateral displacements are almost the same by Volumetric Strain or Prescribed
Displacement method (Linear Elastic Model)
• Linear Elastic and Non-linear models also nearly same results5/11/2013
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Lateral Displacements for Linear model using
Vol Strain vs Prescribed Displacements; and
HSsmall models for Sand vs Soft Clay (using
Prescribed Displacements)
From circumference of
pile
• Heave are different by Volumetric Strain or Prescribed Displacement method
• Linear Elastic and Non‐linear models also gave different results
• Influence of small strain stiffness is important for accurate modeling of these kind of
problems 5/11/2013
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Vol Strain: E=2 MPa
Pres Disp:
E=2 MPa
Soft Clay (UnDrained)
Loose Sand
(Drained)
From circumference of
pile
Vertical Displacements
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Compare 3D Plaxis with SSPM
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SSPM vs FEM
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Fig.2 Soil Profile at Piling Site in Main Workshop Area
FILL Sandy Silts
N=10 to 20
Soft Marine Clay
Su=10 to 20 KPa
Stiff Jurong
Residual Soils
Sandy Silts
N=30 to 100
0m
9m6.5m7m
0m
14m18m
14m17m
BH175BH184
BH176
Max tension force expected in
Bored Piles near base of Soft
Clay
Upheave
bored pile
gripped by
Firm Soils
Bored pile held
down by
embedment in Stiff
Soils
0m
25m25m25m
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1st Ring2nd Ring
3rd Ring
Fig.7 Simplified Ring Model of Driven Piles
surrounding Single Bored Pile
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Fig.8 Axis-symmetric model of single bored pile
with equivalent rings of driven piles around it
1st ring Vol‐strain=13%
2nd ring Vol‐strain=17%
3rd ring Vol‐strain=9%
Upper 12m of bored
pile has nominal steel
cage
Sandy FILL, E=20
MPa
Stiff Clay, E=50
MPa
Soft Clay, E=2 MPa
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Fig.9 Progressive Ground Upheave from Equivalent Rings of Driven
Piles installed using Axis-symmetric 2D-FEM
1st Ring
2nd Ring
3rd Ring
Single bored pile of
radius=300mm
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St=3000
kPa
Tension
failure
Sandy
FILL
Soft CLAY
Stiff CLAY
Fig.10 Tension Failure in Bored Pile below steel cage due to upheave by soil
displacements from close proximity driven RC piles
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Tension Failure
of Bored Pile
(see tension
cutoff points)
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Fig.11 3D-FEM model of single bored pile upheave due to closely
spaced driven piles close to it
Single bored
pile upheave
> 25 mm
Ground upheave
between 140 and
180mm
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600mm diameter
single bored pile
Fig.12 Progressive Ground Upheave from Rings of Driven Piles installed
using 3D-FEM, with single softer bored pile in the center
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Pile cap heave about
50 mm
Axial forces
(1624
kN)
in
bored piles exceeded
maximum tension
capacity of 500 kN
Ground heave by
300 to 400 mm
Fig.13 Realistic 3D-FEM model of bored pile group followed by
closely spaced 400 square RC driven piles installation
Pile cap
with
4 600mm
diameter bored piles
(cyan color)
Arrays
of
closely
spaced
driven 400mm square RC
piles (green color)
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Site piling plan with about half of the pile groups (in green color) that
were installed without any mitigation measures taken
Installed pile
groups (in green
color)
Piles not installed
yet
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Due to the existence
of very deep softmarine clay (about
27~30m thick) at site,
see BH-2 as example,
with SPT N=0~2, the
RC pile installation
induced excessive
ground lateral
movement and heave
and caused
undesirable cracks(nonstructural) to
adjacent properties.
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It is impossible to simulate the whole site without 3D FEM numerical simulation
difficulty. As such, the 1st 3D FEM impact analysis will focus on the zone as
indicated below with many installed pile groups to back-analyze its impact to
adjacent properties, and evaluate future superposing effect of the trailing outstanding
piles with & without mitigating measures
Installed pile
groups (in green
color)Piles not installed
yet
Referred to as
“Zone-1”
hereinafter
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To make the 3D FEM feasible, the 2nd 3D FEM impact analysis
will focus on the zone as indicated below with most of the pile
groups NOT installed yet, and evaluate its impact to adjacent
properties with & without mitigating measures
Piles not installed
yet
Referred to as
“Zone-2”
hereinafter
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Top view of the 3D FEM mesh with the Installed Piles with imposed volumetric strain
expansion over the full length of each of the installed piles
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Visualization of induced lateral ground movement pattern (Largest lateral
ground movement around the pile groups and dissipate with
distance away from piles)
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Visualization of induced ground heave pattern (Largest ground heave around the
pile groups and dissipate with distance away from piles)
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The calculated lateral ground movement at the frontline of adjacent properties is
about 94mm (inclinometers only recorded about 30~50mm as it is at corner of
piles location,
thus
the
analysis
is
on
the
conservative
but
realistic
side),
and
this
help explains the observed cracks damages in the buildings
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The calculated ground heave at the frontline of adjacent prosperities is about
58mm (Settlement markers only recorded about 30~40mm, thus the analysis may
be on the conservative side), and this explains the observed cracks around the
buildings
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Going forward, the scenario was simulated that the outstanding RC piles are to be
installed WITHOUT any mitigating measures…
Outstanding RC
piles to be
installed without
any mitigating
measures
46
There will be an ADDITIONAL lateral ground movement at the frontline of adjacent
properties of about 31mm with accompanying ground heave of about 18mm, which
are deemed to be too much additional movements for the adjacent properties.
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As
such,
it
is
proposed
to
take
the
following
mitigating
measures
before resumption of the site works:
(1) Installation of perimeter sheetpile wall with length of 18m to
help cut off the propagation of induced ground movements;
(2) 1m x 1m trench will be formed behind the above sheetpile
wall to further cut off the propagation of ground movement
at the ground level;
(3) Last but most importantly, the top 24m of each pile location
will be pre‐bored with sufficiently large diameter (say about
400mm) to eliminate the volumetric strain expansion of the
soft ground, which is most crucial in view that the buildings
are founded on footings near the ground surface.
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In the 3D FEM simulation, the top 24m was NOT imposed with volumetric strain as it
has been pre‐bored. On the other hand, the lower part will be imposed with full
volumetric strain accordingly.
For the Outstanding RC
piles, top 24m will NOT
be imposed with
volumetric strain as it
has been pre‐bored.
For the Outstanding RC
piles, the lower part will
be imposed with 100%
lateral volumetric strain
accordingly.
18m sheetpile
1mx1m trench
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The figure below clearly shows that with the mitigation measures, the
induced lateral ground movement mainly occurs in the deep ground
elevations, while
the
induced
ground
surface
lateral
movement
is
quite
minimal.
SSPM theory on
Pre-boring
Effects
50
The induced ADDITIONAL lateral ground movement at the location of the adjacent
properties is about 11mm which mainly occurs at much deeper elevations, while the
lateral ground movement at the ground surface where the buildings are seated is
less than 1mm. However, there is an accompanying ground heave of about 3mm
which is deemed to be acceptable.
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Ground displacements by UnDrained Volume expansion of
installed solid piles had been extensively studied
SSPM and FEM approach produce close agreements in predicted
ground movements some distance away from the installed piles
These approaches were applied to a well monitored Swedish
case, and showed good reliable predictions
The same methods were applied to two local cases with success
to give insights into the field problems
The methods
have
good
potential
for
applications
to
predict
and
mitigate excessive ground movements that may cause potential
damages to close‐by buried and surface structures in crowded
urban built‐up environment like Singapore
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