design analysis deep excavations session01

November 2009 Excavation Overview

Wong Kai Sin 1

A Short Course on

DEEP EXCAVATIONS

By

WONG Kai Sin

[email protected]

1

November/December 2009

New Zealand

Excavation Overview

Time Session Topic

Session 1 Overview

p09:00 – 10:30 1 Overview10:30 – 11:00 Coffee Break

11:00 – 12:30 2 Design (Part 1)12:30 - 01:30 Lunch

01:30 – 03:00 3 Mohr-Coulomb Soil Model &

2

Design (Part 2)03:00 – 03:30 Coffee Break

03:30 – 05:00 4 How to reduce wall deflection

Excavation Overview


Wong Kai Sin 2

Overview of Braced Excavation

• Types of walls

•Methods of excavation

•Modes of failure

3Excavation Overview

Anatomy of a Braced Excavation

struts

wall

walers

walers

struts

kingposts

4

wall

Excavation Overview


Wong Kai Sin 3

Types of Retaining Walls for Excavation

5

Cantilever Wall Anchored or Propped Wall

Braced Wall

Excavation Overview

Wall Types of Deep Excavations

Diaphragm WallDiaphragm Wall

Sheetpile Wall

Bored Pile Wall

Soldier Pile Wall

6

DCM or Grout Mixed Pile Wall

Excavation Overview


Wong Kai Sin 4




Wong Kai Sin 5

Concrete Diaphragm Wall

9

Stop‐ends & water‐proofs

Excavation Overview



Wong Kai Sin 6

Braced Excavation with Sheetpile Wall

struts

wall

walers

walers

struts

kingposts

11

wall

Excavation Overview

Sheetpile Installation



Wong Kai Sin 7

Bored Pile Walls

13

Contiguous Bored Pile Wall Secant Bored Pile Wall

Excavation Overview

18.5m Cantilever CBP wall at NUS

Contiguous Bored Pile Wall



Wong Kai Sin 8

Construction of Secant Bored Pile Wall


Braced Excavation with Soldier Pile Wall



Wong Kai Sin 9

Construction of DCM or Grout Mixed Pile Wall


Grout Mixed Pile Wall



Wong Kai Sin 10

struts

wall

walers

walers

walers

struts

wall

kingposts

Conventional Bottom‐Up

19

wallBottom‐Up Excavation Method

Excavation Overview

Top‐Down Excavation



Wong Kai Sin 11

Top‐Down Excavation at Boon Keng Station


Up‐Down Construction Method



Wong Kai Sin 12

Up‐Down Construction Method


Island Method of Excavation



Wong Kai Sin 13

Under Water Excavation

atat

Marina South



November 2009 Deformation & Failure Modes

Wong Kai Sin 1

Wall and Ground Movements in Excavation

FILL

MARINE CLAY

200 to 300 m

Damage ?? Damage ??Damage ??

1

OLD ALLUVIUM

SAND

Deformation & Failure Modes

Main Cause of Wall DeflectionUndrained Shear Distortion in Clay

Vaterland 1, Oslo (NGI, 1962)

Ad

As

2

No volume change!

Ad ~ As

Ad

As



Wong Kai Sin 2

Wall Deflection and Ground settlementVaterland 1, Oslo (NGI, 1962)

3Deformation & Failure Modes

Wall Deflection and Ground settlementVaterland 1, Oslo (NGI, 1962)

A

4

B C

Area “A” = Area “B” + Area “C”



Wong Kai Sin 3

Wall Deflections at Different Stages of

Excavation

Deformation & Failure Modes 5

Example of questionable wall deflection profiles

Variations of Piezometric Levels

with Time

-2

0

2

4

12-S

ep-

9811

-Nov

-98

10-J

an-

9911

-Mar

-99

10-M

ay-

9909

-Jul

-99

07-S

ep-

9906

-Nov

-99

05-J

an-

0005

-Mar

-00

04-M

ay-

0003

-Jul

-00

01-S

ep-

0031

-Oct

-00

30-D

ec-

0028

-Feb

-01

29-A

pr-

0128

-Jun

-01

27-A

ug-

01

wn

(m)

Casting of base slab


4

6

8

10

Dra

wdo

w

GWP3071 GWP3081 GWP3051-1 GWP3051-2 GWP3051-3


Wong Kai Sin 4

Development of Strut Forces during Excavation


Development of Strut Forces during Excavation



Wong Kai Sin 5


Development of Bending Moment During Excavation



Wong Kai Sin 6

Major Factors Affecting Soil and Wall Movements in Deep Excavations

• Depth of penetration, D• Undrained shear strength, cu

11

p p ,

•Wall stiffness, EI

• Vertical strut spacing

• Construction workmanship

g , u

• Depth of excavation, H

•Width of excavation, B

• Depth to hard stratum, T


Effect of Shear Strength on Wall

Deflection



Wong Kai Sin 7

Effect of Shear Strength on Wall

Deflection


Effect of Excavation Width

on Wall Deflection



Wong Kai Sin 8

Effect of Clay Thickness on Wall

Deflection


Sheetpile wall

Diaphragm wall

Effect of Wall Stiffness on Wall

Deflection

wall wall



Wong Kai Sin 9

Modes of Failure

Collapse Excessive Deformation


Basal Heave Stability

qo

qult

18

When qo > qult, failure in imminent.



Wong Kai Sin 10

Basal Heave Failure in Taipei


Basal Heave Failure due to Stockpiling

Excessive Surcharge

q = 20 kPa



Wong Kai Sin 11

Basal Heave Failure due to Stockpiling

Stockpile

21Courtesy of Dr Lim PC



Wall Rotation



Wong Kai Sin 12


Wall Raking



Lifting of King Posts

Jet Grout Slab



Wong Kai Sin 13

Lifting of Kingpost due to Bottom Heave



Not all basal heave instability lead to catastrophic failures

26

Formation Level



Wong Kai Sin 14

Piping &

Loss of Fines


Strut Failure



Wong Kai Sin 15

Strut Failures


Effect on Adjacent StructuresStructures



Wong Kai Sin 16

Effect of Excessive Movements

Damaged Pavement

31

After Choong (2003)


Buildings on Shallow Foundations

32

Race Course Road



Wong Kai Sin 17

Buildings on Shallow Foundations


Building on Mixed Foundations(Shirlaw et al., 2003)

NewSundaySchool

New kindergarten section Old

Church, wrapped in extension



Wong Kai Sin 18

Building on Mixed Foundations


Lowering of Ground Water Table

Causes:

Pumping

Well pointWell point

Pumping

Wall leakage Sand

Marine Clay

36

Old Alluvium

Lowering of ground water table will increase the effective stress in soft clay and result in consolidation settlement.



Wong Kai Sin 19

Effect of Lowering of Groundwater Table



November 2009 Observational Method

Wong Kai Sin 1

Observational Approach in Deep Excavation

Design

1

ConstructionMonitoring

Obervational Method

In a perfect world …….

All we need is one analysis & one design!

2Obervational Method


Wong Kai Sin 2

In the imperfect world …. with imperfect design!

Too many uncertainties:

• Soil conditionsSoil conditions

• Ground water

• Loading

• Construction sequence

• Workmanship

• Time delayNot easy to account for all

3

y

• 3‐D effect

• Proficiency of designer

• Limitations of software

these uncertainties in the design.

Obervational Method

In the imperfect world …….

•Watch every steps carefully.

Design

•Make adjustments when necessary.

• Improvise as excavation proceeds.

4


Obervational Method


Wong Kai Sin 3

Role & Responsibility

Design


Team Role and Responsibility

Design • Produce a good design• Monitor performance of system• Modify design

C t ti B ild di t d i

g

5

Construction • Build according to design• Monitor performance of system• Feedback problems & unusual findings

Monitoring • Install and monitor instruments• Feedback problems & unusual findings

Obervational Method

What is Observational Approach?

It is a check‐and‐balance process that enable us to:

• Check adequacy of the design• Anticipate potential problems • Modify the design where necessary• Optimise the design

6

• Make contingency plan• Avoid unnecessary delay

Obervational Method


Wong Kai Sin 4

An Example on Application of Observational Method

(9th Rankine Lecture by R. B. Peck, 1969)

Peck’s APD

7

Design

Obervational Method

Predict Modify Design

Implementation of Observational Approach

0

5

0 20 40 60 80 100

Wall Deflection (mm)

Excavate

Monitor

Compare

Design

Re‐analyse

8

5

10

15

20

25

30

Dep

th (m

)

Compare

Back‐analyseFavourable?Yes No

Obervational Method


Wong Kai Sin 5

Designer, Contractor and Instrumentation Specialist have to work together!


Excavate

Monitor

Compare

Design

Re‐analyse0

5

0 20 40 60 80 100


Computed

9

Compare


10

15

20

25

30

Dep

th (m

)

Measured

Alert LevelObervational Method

Back‐Analysis: To calibrate input parameters to produce reasonable agreement with field measurements.


Excavate

Monitor

Compare

Re‐analyse

00 20 40 60 80 100


10


5

10

15

20

25

30

Dep

th (m

) Computed

Measured

Design

Back‐Analyzed

Obervational Method


Wong Kai Sin 6

Predict

Excavate

Modify Design

Re‐Analysis: Use calibrated parameters to re‐analyse

0

5

10

0 20 40 60 80 100


(m)

Excavate

Monitor

Compare

Re‐analyse

15

20

25

30

Dep

th (

0

5

0 20 40 60 80 100 120


11

Back‐analyseFavourable?Yes No10

15

20

25

30

Dep

th (m

)

Obervational Method

Designing Temporary Work is a Continuous Process

Design &Analysis

ConstructionControl

InstrumentationMonitoring

Initial Design Final Design

12

(Work Drawings) (As‐Built)

Start Finish

Excavation

Obervational Method

November 2009 Site Investigation

Wong Kai Sin 1

Site Investigation

Site Investigation 1

How many borings should we drill?

In general, the more the merrier!

It depends on ho m ch e kno abo t the site and theIt depends on how much we know about the site and the nature of project. It is a very subjective decision.

Be careful of “Penny Wise but Dollar Foolish”!



Wong Kai Sin 2

Problem of having too few borings


Boring 1 Boring 2

Problem of having too few borings

Bedrock

Unforeseen Ground Conditions?



Wong Kai Sin 3

Surprise!


$$$

Problem of having too many borings

S.I. will take a long time

Tight schedule affects quality

Multiple drillers and multiple laboratories

Design completed ahead of S.I.

BCA’s Advisory Note: 1 boring every 300 m2.



Wong Kai Sin 4

Site Investigation

Which one is better practice?

[B] 45 borings [A] 15 borings


Site Investigation

150 m

How many borings? Where?

90 m

BCA’s Advisory Note: 1 boring every 300 m2.

Therefore, 45 borings over 13,500 m2.Site Investigation 8


Wong Kai Sin 5

Site Investigation

150 m

It is better to conduct it in phases!

90 m

Phase 1 Phase 2 Phase 3


FillEE

Fill

Control Section for Construction Control

Control Section E

UMC

F2 upper

LMC

F2 lowerF2

LMC

LMC

OA N = 20

F2 upper

UMC

E

JGP1

JGP2

Section

F2 lower

OA N = 20OA N = 30

OA N = 70

OA N = 100OA N = 70OA N = 30OA N = 20

JGP2

Additional borings at Control Section!



Wong Kai Sin 6

Site Investigation

Type of soil sampler

SPT split‐barrel

Shelby tube

Piston

M i

Type of in‐situ tests

SPT

FVT

CPT

P tMazier

Thick‐walled tube

Pressuremeter

Permeability

The Designer must instruct the driller what to do!Site Investigation 11

Undrained Shear Strength of Clay cu

Laboratory Tests(Undisturbed Samples)

In‐Situ Tests

FVTUCUUCUCKoUTCCKoUTECK UDSS

SPTCPTPMTDMT

Each test gives a CKoUDSSMiniature VaneConeTorvanePocket Penetrometer

gdifferent cu!

Which test should we do?



Wong Kai Sin 7

What should we do with the Factual Reports?


Scrutinize the test results

Test A Test B

cu= 40 kPaφu = 0

cu= 28 kPaφu = 0

Which one is more reliable?Site Investigation 14


Wong Kai Sin 8

Proper way of trimming soil sample


An expedient way to “trim” a sample

How to prepare a “disturbed” sample?!



Wong Kai Sin 9

You get what you paid for!


Trimmed Sample Untrimmed Sample

UU CU

Mazier Samples



Wong Kai Sin 10

0

10

0 20 40 60 80 100 120 140 160

Cu from UU Tests (kPa)

UU test results

Example 1 ‐‐ Undrained Strength of Marine Clay

Sand Fill

10

20

30

40

Dep

th (m

)

Upper Marine Clay

Lower Marine Clay

40

50

60

Old Alluvium


Example 1 ‐‐ Are field vane strength more reliable?

0

10

0 20 40 60 80 100 120

Cu (kPa)

10

20

30

40

Dep

th (m

)

Vane

UU

50

60



Wong Kai Sin 11

How do we differentiate between good and bad data?

i h dibl ?

0

0 20 40 60 80 100 120

Cu (kPa)

M t I dibl ?Highest Incredible?

10

20

30

Dep

th (m

)

Vane UU

Most Incredible?

Fully consolidated

Before reclamation

Worst Incredible?

40

50

60


Example 1 ‐ Results of field vane tests from two companies

0

5

0 20 40 60 80 100 120

Cu (kPa)

S dfill

0

5

0 20 40 60 80 100 120

Cu (kPa)

5

10

15

20

25

30

35

Dep

th (m

)

Sandfill

Marine Clay

5

10

15

20

25

30

35

Dep

th (m

)

Sandfill

Marine Clay

35

40

45

50Old Alluvium

Company B

35

40

45

50Old Alluvium

Company A



Wong Kai Sin 12

Data from Company A

Data from Company B

Should we engage another company for 3rd opinion?

Should we use the average values from A and B?


Example 2 – cu from CPTU



Wong Kai Sin 13

0

10

0 20 40 60 80 100 120

Undrained Shear Strength (kPa)

CPT-1Contractor

(qt – σv)cu = ‐‐‐‐‐‐‐‐‐‐‐‐‐

NktContractor

20

30

40

Dep

th (m

)

AC - Nk=12

Client - Nk=20

Consultant A Nkt=12

C lt t C N 20

Vane

Must calibrate cone factor “Nkt” against other data

50

60

AC - Nk=14Client - Nk=20

Consultant B Nkt=14Consultant C Nkt=20


p’o & p’c (kPa)

0

5

0 50 100 150 200 250 300 350 400

P'c from Consol

P'o

0

5

0 10 20 30 40 50 60 70 80

Cu (kPa)

BH3-UUBH3-vaneBH3-consolBH4-consol0.22*P'oBH4-UU

cu (kPa)

Example 3 ‐‐ parameters from a uniform soil deposit

10

15

20

25

30

Dep

th (m

)

10

15

20

25

30

Dep

th (m

)

BH4-vaneBH1-vaneBH2-vaneBH1-UUBH2-UU

35

40

If all data fall within a narrow band, we can use the best fitting curve for the entire site.

30

35

40

Data from entire siteData from entire site



Wong Kai Sin 14

Example 4

Importance of location specific soil condition

0 10 20 30 40 50 60 70 80

Cu (kPa)cu (kPa)

0 10 20 30 40 50 60 70 80

Cu (kPa)

A

A

cu (kPa) 0

5

10

15

20

25

Dep

th (m

)

BH3-UUBH3-vaneBH3-consolBH4-consol0.22*P'oBH4-UUBH4-vaneBH1-vaneBH2-vaneBH1-UUBH2-UU

0

5

10

15

20

25

0 10 20 30 40 50 60 70 80

Dep

th (m

)


30

35

40

Entire Site

25

30

35

40

Section A‐A


A

A

0 10 20 30 40 50 60 70 80

Cu (kPa)cu (kPa)

0 10 20 30 40 50 60 70 80

Cu (kPa)cu (kPa)

Example 4

Importance of location specific soil condition

0

5

10

15

20

25

Dep

th (m

)

A3-UUA3-vaneA3-consolA4-consol0.22*P'oA4-UUA4-vaneM2081-vaneM2081A-vaneM2081-UUM2081A-UUCPT-CR4-Nkt=30

0

5

10

15

20

25

Dep

th (m

)


Section A‐A

30

35

40

30

35

40

Entire Site



Wong Kai Sin 15

10

11

12

13

14

15

16

17

18

0 20 40 60 80 100 120 140 160 180 200 220 240

Effective Stress (kPa)

Dep

th (m

)

Example 5

Importance of location specific soil condition at

a reclaimed site

10

11

12

13

14

0 40 80 120 160 200 240 280

Effective Stress (kPa)

)

19

20

21

22

po'

CPT1

UU-BH1

consol - BH1Fill

Soft Clay

Silty Sand / Sandy Silt15

16

17

18

19

20

21

22

Dep

th (m

)

po'

CPT2

UU - BH2

Stress and strength of soft clay at

reclaimed site can be very variable.


Example 6 ‐‐ Reliability of SPT blowcount in sand

Fill

Marine Clay

F1 N = 2 EquivalentOld

Alluvium

F1 N = 2 Equivalent N~10


design analysis deep excavations session01

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