ICD Brookfield Place, Dubai. Risk Management for a Deep Basement Excavation. Andrew Smith. Coffey Geotechnics Ltd.

Transcription

1 ICD Brookfield Place, Dubai Risk Management for a Deep Basement Excavation Andrew Smith Coffey Geotechnics Ltd.

2 CONTENTS 1. Project description and site history 2. Ground Conditions 3. Assessment of risks in basement construction 4. Development of strategy for design and construction of perimeter wall

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

4 ICD Brookfield Place Site plan and nearby development Al Fattan Currency House North Plot GB03 (previously developed in ) N West Existing Temporary Diaphragm Walls South Plot GB04 East

5 Previous development of Plot GB03 Lighthouse Tower Construction began in 2007 on Plot GB03: North Excavation to mdmd (GL ~ 2 mdmd) for five levels of basement parking m deep temporary diaphragm walls (600 mm to 660 mm thick) on three sides Temporary anchors to support the shoring walls. Excavation to mdmd for two linked towers West 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 East Internal and external dewatering. NB water table then was at -12 mdmd due to extensive construction in the area) In 2009 construction was stopped and the excavation was backfilled with sand, leaving the shoring walls and piles in-situ. South

6 Previous development of Plot GB03 (2009) 2007 Lighthouse Tower starts Basement backfilled Options Study Tenders Construction starts Tower foundation cast

7 Requirements for ICD Brookfield Place Single tower in different location from previous proposed Lighthouse Towers Founding level mdmd Basement space for car parking had to be maximised Seven level basement founding level mdmd (10 m deeper than previously) (second deepest basement in Dubai) Extension into Plot GB04

8 Key Constraints Requirements for ICD Brookfield Place New excavation level will be below toe of existing temporary 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 Anchors in existing temporary wall are life-expired New wall and foundations will be constrained by existing wall and piles

9 Level (mdmd) Ground Conditions 0-10 GB03 Made Ground (Sand Fill) UCS (MPa) GB04 Sand Extremely weak Sandstone Design UCS profile -20 New Excavation Levels Very weak Sandstone / Calcaranite Weak Conglomorate Weak Sandstone

10 Construction stopped and basement backfilled Simplified Contractual Setup Client - ICD Brookfield Foster & Partners 2013Architects 2014 Brewer Smith Brewer Group Local Architects for Design Submissions Aecom Project Management Robert Bird Group Lead Consultant Permanent Works Structural Designers Brookfield Multiplex Early Contractor Involvement ICD Brookfield Place Options Study Tenders Construction starts Tower foundation cast Continuity Coffey Basement Temporary Support - Concept Design Geotechnical Interpretative Report Piled Foundation Settlement Assessment Coffey Basement Temporary Support Category III Check Brookfield Multiplex Ssangyong JV Options Report Main identification Contractor of risks (constraints) Presentations to: Project managers; Swissboring Structural Overseas engineers; Piling Basement Temporary Architects; Support D & B Subcontractor ECI Contractors WJ Dewatering Dewatering D & B Eireng Structural Check Concept drawings for shoring wall tender documents; Tender review

11 Existing Conditions East Wall (Al Sa ada Street) Ground level at 1.5 to 2.5 mdmd Groundwater at 0 mdmd Existing wall o mm thick o Varies in height Existing piles mm from wall P1 750 mm dia 16 m long (tension piles for podium) P2 900 mm dia 31 m long (compression piles for towers) P mm dia 47 m long (compression piles for towers) Anchors past 2 year design life Each wall was different

12 Previous development of Plot GB03 Close up of East Wall (2009) 2007 Lighthouse Tower starts Basement backfilled Options Study Tenders Construction starts Tower foundation cast

13 Previous development of Plot GB03 Piling (2009) 2007 Lighthouse Tower starts Basement backfilled Options Study Tenders Construction starts Tower foundation cast

14 Features to be Defined Design Decisions Required Working level Dewatering level Wall construction Basement / pile construction New retaining wall o Type (secant or D-wall) o o Thickness Toe level Controlled by both stability and groundwater criteria o Cut-down level May be controlled by water control criteria Construction clearance Existing piles o Feasibility of extraction o Extraction level New anchor levels o Protrusion of anchors + waling beams

15 Option Tree Dewatering Effects on neighbouring properties External X Internal Pile Extraction Possible Not Possible New Wall Location Working level determined by: Extent of reuse of existing wall Determined by Existing Wall Determined by Existing Piles and Wall Groundwater control For original construction ground water was lowered over a large area not now possible

16 Interactions Between Constraints Reuse of Existing D-wall Construction Level Dewatering Level High (+2.5mDMD) Low (-16.55mDMD) Ease of construction Wall watertightness for secant piles Better Control Increasing Difficulty with water pressures and flows Reducing Risk Increasing Risk Increasing Risk of Leaks Control over wall verticality Risk of hitting buried piles Risk of Unintended External Dewatering

17 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

18 D Wall vs Secant Wall Advantages Disadvantages Risks D Wall Waterproofing Practicable Installation depth Waling construction easier Higher moment capacity Can t break obstructions Inflexibility in plan shape Damage to hydrofraise on pile reinforcement No Plan B Secant Wall Flexibility in plan shape Can break or go round obstructions Verticality issues Restricted depth Waterproofing Lower moment capacity Deviations Water ingress

19 Constraints Imposed by Existing Piles Existing D-Wall Piles Envelope of Pile Edges Pile Toe Uncertainty Envelope of Pile Toe Uncertainty = Constraint to New Wall

20 Offset from Existing D Wall (m) Effect of Verticality Tolerances East Side Nominal Pile gap 2400 mm Very Complex at South End Distance along Wall from North (m) These graphs indicated that a D wall should be feasible for substantial lengths And that there was little to be gained from extracting piles 1 2 Edge of Pile Tops Potential Edge of Pile Toes 3 4 5

21 Geometric Constraints Existing road adjacent Top of existing D Wall +2.5 mdmd East Wall Excavation level mdmd (short length at 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 piles and 3 No 1500 mm 47 m long piles in same row)

22 East Wall Design Decisions Construct new wall from former excavation level ( mdmd) Cut down wall to mdmd No pile extraction Internal dewatering in two stages New anchors in old walls Pressure relief wells between old and new walls 2.5 mdmd mdmd mdmd mdmd mdmd

23 Design Decisions East Wall Construct new wall from former excavation level ( mdmd) Cut down wall to mdmd No pile extraction Internal dewatering New anchors in old walls Pressure relief wells between old and new walls Diaphragm wall except secant wall in one location (later changed to thinner diaphragm wall with extra row of anchors) All former tension piles abandoned (now too short) 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

24 New development of Plot GB03 East Wall (2017) 2007 Lighthouse Tower starts Basement backfilled Options Study Tenders Construction starts Tower foundation cast

25 Previous development of Plot GB03 Close up of East Wall (2017) 2007 Lighthouse Tower starts Basement backfilled Options Study Tenders Construction starts Tower foundation cast

26 Some Conclusions The existing structure posed serious constraints (risks) to new construction Continuity of involvement was crucial to reduction of risk and achievement of coherent design Simple visual aids and diagrams enabled risks to be understood and explained to others The problem was dominated by geometry

27 Some Conclusions

28 Interactions Between Constraints Reuse of Existing D-wall Construction Level Dewatering Level High (+2.5mDMD) Low (-16.55mDMD) Ease of construction Wall watertightness Better Control Increasing Difficulty with water pressures and flows Reducing Risk Increasing Risk Increasing Risk of Leaks Increasing Knowledge Control over wall verticality Risk of hitting buried piles Risk of Unintended External Dewatering Opportunities to change design during construction