Stability Analyses and Face Support Design for Cross Passage of

Waterview Project, Auckland (NZ)

M. Wansbone, R. Bhargava

Parsons Brinckerhoff, Australia

Well-Connected Alliance – Waterview Connection

Content Cross Passage: Size, Function, Geometry Cross Passage XP17 Geotechnical Conditions Constraints: Surface and Subsurface Construction Method & Sequence

Stability Analyses Analytical Method/Calculations 3D Numerical Analyses

Results Summary

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Cross Passage Overview

Cross Passages 16 mined (total 18) ~ 150m spaced Internal Space: Electrical & Comm. Room (2m x ~) Emergency Egress (1.2 x 2.1m) Mech. Pump and Deluge(1.5m x ~) Cables & Conduit Trench

XP17

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XP17: Size & Geometry

• Size Internal: 5.2m x 5.6m

Excavation:6.4m x 6.8m

• Support / Lining Temporary: SFRS 100-300mm

Permanent: RC 300mm

Tanked Structure

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XP17 Geology South Portal

North Portal

XP17

XP17: Geology Cross Section

ER & EW critical to: - Face Stability - Face Deformations

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Constraints & Impacts

• XP17 beneath GTNR Road, HP Gas & Sewer

Ground Treatment: Road Closure & Utility Damages

Intentional Dewatering: Settlement & Damages

• Proposed Solution

Reinforcing ground from within the tunnel

Pipe Canopy for Roof, and GRP Dowels for Face

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Sequence & Method Sequence - Top Heading w/Footing

- Bench

Main Considerations - Enhance Face Stability

- Address Settlement Impacts on:

- GTNR, HP Gas and Sewer mains

Analyses Required - Validate Stability of Sequence

- Face Bolt Requirement

- Temp Support Design

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Stability Analyses First Principle Calculations - Wedge silo limit equilibrium

- Drained condition (EW sandy, 1 round/12hrs)

- Full face unstable, TH (stable but FOS<1.4)

- Top heading with face bolts (FOS>1.4)

Limitations - Excavation sequence unaccounted

- Stress & deformation unknown (extent of yield)

- Stratigraphy & tunnel influence (indirectly)

3D FE Analyses to address the limitations

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3D Analyses FLAC-3D model - Tunnel and XP opening support (props) & sequence included

- Cross passage sequence included

- GRP face bolts included

Face Bolts

Props

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3D Analyses Analyses Type - Both Undrained & drained ground behaviour modelled

- Drained case adopted for design

Stability in 3D Analyses - Strength Reduction Factor

- Shear Failure Mechanism (soil and highly weathered rock)

- Failure Condition defined by increase in face displacements for

applied strength reduction factor analyses run

- Factor of safety = inverse of strength reduction factor (ratio of

available to mobilized strength)

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Results

Outputs

- For each cross passage advance SRF runs carried out

- Ground contours plots obtained

- Face displacement at selected points on the face vs SRF of

heading and bench plotted

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Implementation of Design Permit to Tunnel Process

- 3 mandatory probe holes

- Ground Class Assessment

- Install Canopy Tube

- Install Face Bolt

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Summary • Stability of proposed sequence of excavation validated

• Face reinforcement pattern validated

• Installing face bolts much cheaper and less disruptive

than ground treatment

• Ground deformations, particularly longitudinal settlement

profile

• Impacts on overlying services and pipe assessed

separately using ground settlement profile from 3D

analyses

• It is useful to include factor of safety check as a part of

numerical analyses for temporary support design

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Acknowledgments • Well Connected Alliance

• Matt Wansbone (Co Author)

Thanks!

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Thank You

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Project Overview

• Location - Auckland (NZ)

• Project Cost - NZ$1.4b

• Project Objective - Complete western ring route

• Project Model - Alliance (NZTA client)

• Delivery by - Well-Connected Alliance

• Tunnel Construction - Commenced 2013

• Commissioning - April, 2017

Tunnel (key elements)

• TBM - 10th largest EPBM machine

• Diameter - 14m (largest tunnel in Australia-Pacific)

• Reinforcement - World’s largest SFRC lining

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3D Analyses