geoss event seminar 4 nov 2013

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


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


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


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


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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36

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.

38

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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39

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

40

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)

42

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

44

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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45

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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47

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.

48

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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49

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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geoss event seminar 4 nov 2013

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