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ICD Brookfield PlaceStrategy for Basement Excavation

Andrew Smith

Coffey Geotechnics Ltd.

CONTENTS

1. Project description and site history

2. Ground Conditions

3. Assessment of constraints to construction

4. Development of strategy for design and construction of perimeter wall

5. Some details of design and construction

ICD Brookfield PlaceProject details

ICD Brookfield Place will be constructed in the Dubai International Financial Centre (DIFC).

• 290m tall office tower, approximately 55 storeys.

• 3 podium levels above ground containing shops and leisure facilities.

• 7 basement levels below ground for car parking.

ICD Brookfield PlaceSite plan and nearby development

Plot GB03(previously developed

in 2007-09)

Plot GB04

Al Fattan Currency House

N

Existing Diaphragm WallsWest

South

North

East

ICD Brookfield PlaceSite plan and nearby development

Construction began in 2007 on Plot GB03:

• Excavation to -16.55 mDMD (GL ~ 2 mDMD)for five levels of basement parking

• 24 m deep diaphragm walls (600 mm to 640 mm thick) on three sides

• Temporary anchors to support the shoring walls.

• Excavation to -18.65 mDMDfor two linked towers

• Construction of 890 piles in excavation base

– 750 mm 16 m long tension piles in podium

– 900 mm 31 m long compression piles and

– 1500 mm 47 m long piles for towers

• Internal and external dewatering. NB water table then was at -12 mDMDdue to extensive construction in the area)

In 2009 construction was stoppedand the excavation was backfilled with sand,leaving the shoring walls and piles in-situ.

North

South

East

West

Previous development of Plot GB03Lighthouse Tower

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008

2007 Lighthouse Tower starts

Basement backfilled

Options StudyTenders

Construction starts

Tower foundation cast

Previous development of Plot GB03 (2009)

Requirements for ICD Brookfield Place

• Seven level basement – founding level -26.75 mDMD(10 m deeper than previously)(second deepest basement in Dubai)

• Single tower in different location from previous proposed Lighthouse TowersFounding level -28.75 mDMD

• Extension into Plot GB04

• Basement space for car parking had to be maximised

Requirements for ICD Brookfield Place

• Key Constraints

• Anchors in existing temporary wall are life-expired

• New excavation level will be below toe of existing wall

• Water table has recovered since 2008 – long term design level 0 mDMD

• A new temporary perimeter wall will be needed for stability and water control

• New wall and foundations will be constrained by existing wall and piles

Ground Conditions

-80

-70

-60

-50

-40

-30

-20

-10

0

0 2 4 6 8 10

Weak Sandstone

Made Ground(Sand Fill)

Sand

Weak Conglomorate

Level (m

DM

D)

New Excavation Levels

GB04GB03

Very weakSandstone / Calcaranite

Extremely weak Sandstone Design UCS profile

UCS (MPa)

• Options Report

• Presentations to:

– Project managers;

– Structural engineers;

– Architects;

– ECI Contractors

• Concept drawings for shoring wall tender documents;

• Tender review

Simplified Contractual Setup

Client - ICD Brookfield

Robert Bird GroupLead Consultant

Permanent Works Structural Designers

Foster & PartnersArchitects

Brewer Smith Brewer GroupLocal Architects for Design

Submissions

AecomProject Management

Brookfield MultiplexEarly Contractor Involvement

2017

2016

2015

2014

ICD Brookfield Place Options Study

Tenders

Construction starts


CoffeyBasement Temporary Support -

Concept DesignGeotechnical Interpretative Report

Piled Foundation Settlement Assessment



Brookfield Multiplex Ssangyong JVMain Contractor



Swissboring Overseas PilingBasement Temporary Support

D & B Subcontractor



Submissions



CoffeyBasement Temporary Support

Category III Check

WJ DewateringDewatering D & B

EirengStructural Check

2017

2016

2015

2014


Tenders

Construction starts





Existing Conditions

• Ground level at 1.5 to 2.5mDMD• Groundwater at 0mDMD• Existing wall

o 600-660mm thicko Varies in height

• Existing piles 600-1200mm from wall• P1 750 mm dia 16 m long

(tension piles for podium)• P2 900 mm dia 31 m long

(compression piles for towers)• P3 1500 mm dia 47 m long

(compression piles for towers)• Anchors past 2 year design life

East Wall (Al Sa’ada Street)

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008


Basement backfilled


Construction starts


Previous development of Plot GB03Close up of East Wall (2009)

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008


Basement backfilled


Construction starts


Previous development of Plot GB03Piling (2009)

Features to be Defined

Design Decisions Required

• Working level• Dewatering level

• Wall construction• Basement / pile construction

• New retaining wallo Type (secant or D-wall)o Thicknesso Toe level

Controlled by both stability and groundwater criteria

o Cut-down levelMay be controlled by water control criteria

• Construction clearance• Existing piles

o Feasibility of extractiono Extraction level

• New anchor levelso Protrusion of anchors

+ waling beams

Option Tree

Dewatering

External Internal

X Pile Extraction

Possible Not Possible

New Wall LocationDetermined by Existing Wall

Determined by Existing Piles and Wall

Working level determined by:

– Extent of reuse of existing wall

– Groundwater controlFor original construction ground water was lowered over a large area – not now possible

Southern Boundary – dealt with separately

Effects on neighbouring properties

Interactions Between Constraints

Construction Level Dewatering Level

Control over wall verticality

Better Control

Reuse of Existing D-wall

High (+2.5mDMD)

Low(-16.55mDMD)

Ease of construction

Increasing Difficulty with water pressures and flows

Risk of hitting buried piles

Reducing Risk

Wall watertightnessfor secant piles

Increasing Risk of Leaks

Risk of Unintended External Dewatering

Increasing Risk

Location of New Wall

Determined by:

– Location of existing wall

– Existing walings and anchor heads – vary according to location

– New walings and anchor heads – have to be related to existing

– Construction tolerances – e.g wall verticality and machine clearance

– Existing piles if we can’t extract them

D Wall vs Secant Wall

D Wall Secant Wall

Advantages

Disadvantages

Risks

WaterproofingPracticable Installation depthWaling construction easierHigher moment capacity

Flexibility in plan shapeCan break or go round obstructions

Can’t break obstructionsInflexibility in plan shape

Verticality issuesRestricted depthWaterproofingLower moment capacity

DeviationsWater ingress

Damage to hydrofraise on pile reinforcementNo Plan B

Constraints Imposed by Existing Piles

Existing D-Wall

Piles Envelope of Pile Toe Uncertainty

Pile Toe Uncertainty

= Constraint toNew Wall

Envelope ofPile Edges

5

4

3

2

1

00 20 40 60 80 100 120

Distance along Wall from North (m)

Off

set

fro

m E

xis

tin

g D

Wall (

m)

Edge of Pile TopsPotential Edge of Pile ToesNew 800 mm D-Wall Top at EGLPotential D-Wall at Pile Toe Level

Effect of Verticality Tolerances – East Side

• Nominal Pile gap 2400 mm

• Very Complex at South End

• Piles 1:75 verticality

• 800 mm D Wall

• Installed from EGL

• D Wall 1:100

• These graphs indicated that a D wall should be feasiblefor substantial lengths

And that there was little to be gainedfrom extracting piles

Dewatering for Installation of Wall

Risks

Unintended external

dewatering

High flows though faults

Advantages

Enables construction from a

lower level, hence:

– Easier to locate existing piles

– Easier to remove existing piles

– Shorter new wall

– - And less to remove later

Disadvantages

Need to dispose of water flow

Harder to install piles and D

wall against upward water

flow

Need to deal with

groundwater pressure and

flow from extracted pile voids

Need to anchor / prop existing

D wall

East Wall

Geometric Constraints

– Existing road adjacent

– Top of existing D Wall +2.5 mDMD

– Excavation level –16.55 mDMD (short length at -18.65 mDMD)

– Three or four rows of anchors

– Walings believed to be built in based on site photos

– Protruding anchor heads and strands

– Tension piles ~1200 mm gap to D Wall

– (7 No 900 mm 31 m long pilesand 3 No 1500 mm 47 m long piles in same row)

2.5 mDMD

-18.6 mDMD

-26.75 mDMD

-16.55 mDMD

-36.0 mDMD

East Wall

Design Decisions

– Construct new wall from former excavation level (-16.55 mDMD)

– Cut down wall to -18.6 mDMD

– No pile extraction

– Internal dewatering in two stages

– New anchors in old walls

East Wall

Design Decisions

– Construct new wall from former excavation level (-16.55 mDMD)

– Cut down wall to -18.6 mDMD

– No pile extraction

– Internal dewatering

– New anchors in old walls

– Diaphragm wall except secant wall in one location

– All former tension piles abandoned

– A few 900 mm and 1500mm piles could be reused

– North end used for access and North wall constructed last

– Non-structural cut-off wall installed across middle of site


2017

2016

2015

2014

2013

2012

2011

2010

2009

2008


Basement backfilled


Construction starts


New development of Plot GB03East Wall (2017)

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008


Basement backfilled


Construction starts



2017

2016

2015

2014

2013

2012

2011

2010

2009

2008


Basement backfilled


Construction starts


Previous development of Plot GB03Site access at North end (2017)

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008


Basement backfilled


Construction starts


Strategies for constrained lengths of East side

Northern end – cut the corner

5

4

3

2

1

00 20 40 60 80 100 120


Off

set

fro

m E

xis

tin

g D

Wall (

m)


Strategies for constrained lengths of East side

Towards south - original design was for secant wall to avoid existing piles

- after existing piles exposed, design was changed to have a thinner wall (600 mm) with an extra row of anchors

5

4

3

2

1

00 20 40 60 80 100 120


Off

set

fro

m E

xis

tin

g D

Wall (

m)


West Wall


– Truck Tunnel immediately adjacent

– Top of existing D Wall -8.5 mDMD – nature of connection to Truck Tunnel not known

– Excavation level –16.55 mDMD

– Two rows of anchors throughout – threaded between Truck Tunnel piles?

– Nature of waling not known – see note

– Tension piles ~600 mm gap to D Wall – then ~1500 mm to next row

West WallWest Side (Truck Tunnel)

– New anchors in old wall(Three rows instead of twoin same vertical plane)

– New wall top at -18.6mDMD

– Space between walls grouted with pressure relief wells

Truck Tunnel

-8.5 mDMD

-18.6 mDMD

-26.75 mDMD

-28.75 mDMD

-20.5 mDMD

Analysis for Cat III Check

• Complex interaction at overlap of walls

• Therefore use two software packages, FREW and PLAXIS .

• Full construction sequence modelled, taking into account 2007 construction.

• West side – Upper (Old) Wall

– External surcharge to model truck tunnel

– External water level at base of truck tunnel

• West side – Lower (New) Wall

– External surcharge to model depth of top of wall

• PLAXIS can model complex geometry (both walls)and entire construction sequence

-8.5 mDMD

-18.6 mDMD

-18.6 mDMD

-26.75 mDMD

ShearMomentDisplacement

-40

-35

-30

-25

-20

-15

-10

-5-1200 -800 -400 0 400 800 1200 1600

Existing NewSLS LimitsFrewPlaxis

Level (m

DM

D)

Moment (kNm/m)






Brookfield Multiplex Ssangyong JVMain Contractor



Swissboring Overseas PilingBasement Temporary Support

D & B Subcontractor



Submissions



CoffeyBasement Temporary Support

Category III Check

WJ DewateringDewatering D & B

EirengStructural Check

2017

2016

2015

2014


Tenders

Construction starts


~18m

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008


Basement backfilled


Construction starts


Existing diaphragm wall (GB03)• 600mm thick• 3 rows of new anchors, new walers• New wall to be installed in front of existing wall

(-18.6mDMD to -36mDMD)

~18m

December 2016New diaphragm wall (GB04)• 800mm thick• 5-7 rows of anchors• 38.5m deep (GL to -36mDMD)

2017

2016

2015

2014

2013

2012

2011

2010

2009

2008


Basement backfilled


Construction starts


New diaphragm wall (GB03)• 800mm thick• 2 rows of new anchors, new walers• Installed in front of existing wall

(-18.6 mDMD to -36 mDMD)

North Wall


– Al Fattan Tower immediately adjacent

– Top of existing D Wall -11.5 mDMD– nature of connection to Al Fattan Tower not known

– Variable previous excavation level –18.65 mDMD in centre and -16.55 mDMD elsewhere

– Cantilever at ends – two rows of anchors in centre– threaded between Al Fattan piles?

– Tension piles ~600 mm gap to D Wall

– (also 7 No 900 mm 31 m long piles)

– Anchors not permittedbetween Al Fattan piles

North Wall

• Original Design

– Anchors not permittedSupport old wall with site access ramp

– Install new wall from top of rampthen cut it down

– Then strut to part-completed permanent basement

-11.5 mDMD

-16.0 mDMD

-18.6 mDMD

-26.75 mDMD

North Wall

• Original Design

– Anchors not permittedSupport old wall with site access ramp

– Install new wall from top of rampthen cut it down

– Then strut to part-completed permanent basement

• Alternative Design

– Raking strut to base slabinstalled while bund still in place

– Raking anchors to go beneath Al Fattan piles

-11.5 mDMD

-16.0 mDMD

-18.6 mDMD

-26.75 mDMD

Some Conclusions

• The existing structure posed serious constraints to new construction

• Many of these constraints interacted with each other

• Initial assessment of the constraints and their interactionenabled a coherent construction strategy to be developed

• The problem was dominated by geometry

• Continuity of involvement was crucial to coherent design

Some Conclusions

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