Deep Excavation in Hong Kong – Design and Construction Controlhkieged.org/HKIE/download/S2-3.pdf · Deep Excavation in Hong Kong – Design and Construction Control ... • makes

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Deep Excavation in Hong Kong –Design and Construction Control

Jack Pappin, John Endicott & John Clark

(James Sze)

Content of the Presentation

• Part I – Design Considerations

• Part II – Numerical Modelling

• Part III – Observational Method

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Part I – DESIGN CONSIDERATIONS

Design Codes and Guides

• CIRIA 104

• BS 8002

• DD ENV

• BD 42/00

• Piling handbook

• CIRIA R185

• CIRIA C517

• CIRIA SP95

• GEOGUIDE 1 2nd Ed.

• GCO Publication No. 1/90

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Conventional Retaining System Design Approach

Figure 29, GCO 1/90 (Navfac, 1982b)

CIRIA Report C580 Design

• Aim - holistic, consistently reliable & economic ELS design

• Limit states design• Maintain simplicity over different

uncertainties – adopt partial factor of safety on soil strength only

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• Review Group set up by GeotechnicalEngineering Office in 2004

• Recommendation concluded in “Notes on Design of Excavation and Lateral Support Works Using the Limited State Partial Factor Method in the CIRIA Report No. C580”

• Promulgated by BD/GEO in early 2005

• alternative to global factor of safety approach in GCO Publication No. 1/90

• review in 2 years before long-term implementation

Use of C580 in Hong Kong

• Most probable parameters• Unfactored• Unfactored surcharge load• Most unfavorable groundwater level under normal circumstances

• BM, SF & prop loads (SLS) × LF• Lateral wall deflection

SLS Analysis

BM, SF & prop loads envelope (structural design)

Simplified Flowchart of C580 Design Approach (SSI)

• Moderately conservative parameters• Apply partial factors• Unplanned excavation(0.5m or 0.1H)• Unfactored surcharge load• Worst credible groundwater level

ULS Analysis

• BM, SF & prop loads (ULS)• Toe-in requirement

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Conventional vs C580 Design Approach - Example

FSP-VL steel sheetpile wall

Existing Diaphragm wall

-6mPD

-16mPD

Sze & Lo (2005)

Sensitivity analyses – Wall Bending Moment

0

1

2

3

4

5

6

7

8

9

10

-2000 -1000 0 1000 2000

Max. bending moment (kNm/m)

Dep

th o

f em

bedm

ent (

m)

ULS (+ve) ULS (-ve)

SLS x 1.4 (+ve) SLS x 1.4 (-ve)

Mu = 788kNm/m

-20

-15

-10

-5

0

5-600 -400 -200 0 200 400 600

Ultimate Bending moment (kNm/m)

Leve

l (m

PD)

Conventional (Max.) Conventional (Min.)

C580 (Max.) C580 (Min.)

C580 toe level

Conventional toe level

-6.0mPD Dig Level

Soil side Excavation sideSoil side Excavation side

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-20

-15

-10

-5

0

5-600 -400 -200 0 200 400

Ultimate Shear force (kN/m)

Leve

l (m

PD)

Conventional (Max.) Conventional (Min.)

C580 (Max.) C580 (Min.)

C580 toe level


-6.0mPD Dig Level

Sensitivity analyses – Wall Shear Force

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000

Max. shear force (kN/m)

Dep

th o

f em

bedm

ent (

m)

ULS SLS x 1.4

Vu = 669kN/m

Soil side Excavation side

Sensitivity analyses – Strut Force & Wall Deflection

0

1

2

3

4

5

6

7

8

9

10

0 500 1000 1500 2000

Max. strut force (kN/m)

Dep

th o

f em

bedm

ent (

m)

ULS - S1 ULS - S2

SLS - S1 x 1.4 SLS - S2 x 1.4

-20

-15

-10

-5

0

50 20 40 60 80 100

Horizontal Wall deflection (mm)

Leve

l (m

PD)

Conventional C580 - SLS

C580 toe level


-6.0mPD Dig Level

S1

S2

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Fill

Alluvium

CDV

Rock

Control of Groundwater at Passive Zone

E.g. KCRC Spurline Kwu Tung Station

Pump well

Buildability

• Difficult in constructing

• Unrealistic movement criteria

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E.g. KCRC Tsuen Wan West Station

Buildable Design

• Easy and fast construction

• Less risks

• Safer working environment

• Could be more cost effective

Over-excavation

Unworkable sequence vs Opportunistic contractor

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Optimization of ELS Design at Kowloon Bay

Original Design

Fill

MD

ALL

CDG/MDG

Optimized

vert. 2m c/c

Development at Sheung Shing Street, KLN

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Bottom-up with Raking Strut Option

Unrealistic Movement Criteria - Diaphragm Wall

Settlement measured at Chater Station in mid 70s

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How about Sheetpile Wall Installation ?

0

5

10

15

20

25

30

35

40

45

40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82

Chainage (m)S

ettle

men

t (m

m)

2005-03-08 2005-03-14

6mm

Installation may attribute a substantial part of induced movements

Sheet piling – Giken (Silent) piling

E.g. East Rail TSTE Mody Road Subway, CLP Cable Tunnel TWS access shaft

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Conclusions of Part I

• C580 design approach would result in consistently reliable design.

• Shortest Toe-in might not be most economic.

• Buildability - one of key factors for successful execution of ELS works.

• Get Contractor’s involvement at earlier stage if possible.

Part II – NUMERICAL MODELLING

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Numerical Modelling

• Earth pressure Coulomb 1776Sub-title

Numerical Modelling

• Earth pressure Coulomb 1776

• Applied Pressure Diagram Terzaghi & Peck 1948

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Numerical Modelling



• Beam on Springs 1976

Numerical Modelling




• Finite Elements/Differences 1986

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Numerical Modelling





• Bricks on Strings Simpson 1992

Numerical Modelling





• Bricks on Strings Simpson 1992

• 3D Applications in practice 2002 - 2005

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Layout Plan of Extension to the Tsim Sha TsuiStation Concourse

Typical Cross Section showing Ground Conditions and the Proposed Works

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View of the 3D FLAC Model Developed showing the Lateral Wall Deflections along the length of the Excavation

Plan view from FLAC 3D Model Showing the Displacement of the Tunnel Linings along the length of the Excavation

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Wall Deflections and Tunnel Deformations from FLAC 3D at a Section about 40 m from the southern end

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-20-16-12-8-4048

121620

10 Dec 02 09 J an 03 08 Feb 03 10 Mar 03 09 Apr 03 09 May 03 08 J un 03 08 J ul 03 07 Aug 03 06 Sep 03 06 Oct 03 05 No v 03 05 Dec 03 04 J an 04

1 - 4 1 - 5 1 - 6 1 - 7 7 - 2 7 - 3 7 - 4

123

45

67

1- 4 1- 5 1- 6 1- 7 2- 7 3- 7 4- 7

Monitoring of Tunnel Convergence – Downtrack Tunnel

Convergence in mm

Measured Deformation of Excavation Lateral Support Wall

0.00

5.00

10.00

15.00

20.00

25.00

30.00

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30

Displacement (mm)

Dep

th (

m)

INC01

INC02

INC03

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• 3-D modelling can be useful for complex geometries


• Often variations in ground conditions or construction procedure render sophisticated constitutive models redundant

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• KIS = Keep It Simple



• KIS = Keep It Simple• Less likely to get a plausible result based on wrong reasons• Easy to check for approval• Easy to compare with monitoring results• Easy to back analyse

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Part III - USE OF OBSERVATIONS MADE DURING EXCAVATION

What Constitutes a Well Performing Excavation?

• Common Objectives of Client and Contractor

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• Common Objectives of Client and Contractor• totally safe against collapse


• Common Objectives of Client and Contractor• totally safe against collapse• adequate protection to nearby structures and roads etc.

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• Common Objectives of Client and Contractor• totally safe against collapse• adequate protection to nearby structures and roads etc.• makes provisions for a safe working environment


• Common Objectives of Client and ContractorProtective Factors• totally safe against collapse• adequate protection to nearby structures and roads etc.• makes provisions for a safe working environment

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• permits reasonably rapid progress of work



• permits reasonably rapid progress of work• cost as low as possible after considering the above

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• Common Objectives of Client and ContractorProtective Factors• totally safe against collapse• adequate protection to nearby structures and roads etc.• makes provisions for a safe working environmentCommercial Factors• permits reasonably rapid progress of work• cost as low as possible after considering the above

Dominating Factor... Ground Conditions Uncertainty

• The state-of-the-art in modern geotechnics gives us...

• increasingly powerful design tools• easy access to information on the outcome of many past

projects

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• But still views diverge on the correct balance between protective and commercial aspects.

• variance between analytical tools• significance of disturbance to field samples• use of average values or more conservative• whether soil properties may change during the excavation

process

The Only Sure Way is the Observational Way

The Observational Method in ground engineering is the continuous review and refinement of a design based on observations of field behaviour

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Examples

• Tseung Kwan O Station• observational method was used to justify omission of a layer of

props and a buried prop at later stages of the project

• otherwise it seems that use of observational design is the exception in Hong Kong

Example of Observational Method

Typical Section of Excavation

Strut S5 deleted by Observation Approach

Strutting

Formation Level

Jet Grout Raft

Ex. Ground Level Ex. Ground Level Diaphragm Wall

S1S2

S3

S4S5

Sandy Fill

Stiff Silty Sand

Sandy Fill

Marine Clay

MD Silty Sand

Hard Boundary Clay

Hard Boundary Clay

Marine Clay

Stiff Silty Sand

Clayed Sand

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Why Use Observational Design?

Direct Alteration of Soil Properties

Heating / Dessication due to Jet Grout Hydration

Why isn't OM used for every excavation?

• time lag between measurement and reporting and interpretation

• reliability of reported measurements

• authority approval for change in sequence of work

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Web Based Real Time Reporting

dataloggerswith solar panel

Instruments (eg.strain

gauges on struts)

Users connected

over the Internet

Server

Instrument Reliability

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Authority Approvals Strategy

• Submit two designs for approval• Design A – Adopts "moderately conservative" parameters

agreeable to the authority (i.e. normal practice)• Design B – Targets more favourable parameters

• Proponent (probably the contractor) decides how much better performance is expected to be... could be "most probable" or even more optimistic

• Identify a Test Stage in the Excavation• Up to the test stage, A and B follow the same path, but diverge

thereafter• Identify test conditions to verify that more favourable parameters

are safe • At design approval stage, the authority approves only the test

conditions, not the target parameters

Test Stage

Ø1, C1

Moderately Conservative Parameters

S1

S2

S3

S4S5

Design A Design B

Ø2, C2

MostProbable

Parameters

S1

S2

S3

S4

S1

S2

S3

a

b

c

def

a

b

c

d

a

b

c

d

f

Test Condition –

Strut load, deflection

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Issues to Address in adopting the Observational Method

• Most critical is to avoid a delay of say one month waiting for consent after submission of a report on reaching the test stage, therefore...

• make consistently up-to-date archive of observations available to the authority and the supervising engineer

• build a simple control framework into the web based real time reporting system so that compliance with test stage conditions can be instantly verified

• adopt a "self-regulation" process whereby the Supervising Engineer (RSE) certifies the compliance with the test stage conditions

Summary

• Limit State Design• for consistency and economy

• Buildability• most problems are the result of buildability not being properly

considered in the design, the leanest solution may not be the best

• Sophisticated 3D modeling• invaluable for unusual, complex analysis, esp. for effects on

existing sensitive structures, but requires specialist expertise in its application

• Observational Method• viable tool for reducing cost and improving safety that can and

should be integrated into existing procedures for authority control

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Thank you

Documents

Deep Excavation in Hong Kong – Design and Construction Controlhkieged.org/HKIE/download/S2-3.pdf · Deep Excavation in Hong Kong – Design and Construction Control ... • makes