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�
Selection of Tunnelling Method & Lining
Alun Thomas
�contents
• Introduction
• Risk-based design
• Tunnelling method
• Lining type
• Conclusions
�Introduction
These are the important questions:
• Innovation vs reliability
• Short-term vs whole life cost
• Risk vs the cost of contingencies
• Rational design criteria
• Global best practice vs local needs
We need a flexible, logical, modern approach
�Risk based design
• A logical process for making complex decisions
• An audit trail
• A clear focus on the risks to safety, cost and time
• Good examples:
– San Diego Mission Valley
– T5 Airside Road Tunnel
(see ITA WTC 2007)
�Risk Reduction Process
Identify risks
Examine mitigation
meaures
Evaluate risks in
risk assessment
Are all risks
acceptable?
YES
NO
Revise design
Issue design &
risk register
Initial design
Can the risks
be reduced in
the design?
NO
YES
�Tunnelling methods
Consider sprayed concrete lined tunnelling (SCL or NATM) & TBMs
�Which one is best?
The choice depends on many factors:
• Ground & groundwater
– Rock or soil? High or low stress? Wet or dry?
• Shape of tunnel
• Length
• Budget of the client
• Time
Several options are always possible !
�SCL vs TBM ?
• TBMs are faster for longer tunnels with a constant, circular shape
– Also better in wet ground
– High quality permanent segmental lining
• SCL is good for:
– Shorter tunnels
– Non-circular cross-sections
– Junctions
– Very variable ground (e.g. fault zones or squeezing rocks)
�TBM Utilisation rates
0
500
1000
1500
2000
2500
3000
0.1 0.2 0.3 0.4 0.5 0.6 0.7
3 m/hr
5 m/hr
7 m/hr
~35 % ~55 %
Rate of advance increases rapidlyas utilization increases
Utilization factor, α
Advance r
ate
in m
/ m
onth
Utilization = how much of the time the TBM is excavating
�Open vs Closed face TBM ?
Risks
• Face stability
• Groundwater
• Fault zones
• Karstic features
�Q-TBM (Barton 1999)
�Case studies
Airside Road Tunnel
• 1.3 km tunnel in clay
• 8 m internal diameter
• Closed face TBM
• Dry, stable ground
• Tunnelling under a live
airport – high risk !!!
Hindhead Road Tunnel
• 1.8 km tunnel in sandstone
• 10.6 m internal diameter
• Fibre reinforced sprayed
concrete lined tunnel
• Dry, stable ground
• Cheaper than TBM
�Lining types
The choice partly dictated by the tunnelling method
Consider steel fibres and permanent sprayed concrete
�Requirements
• Ultimate capacity / serviceability
• Watertightness
• Fire resistance
• Durability and design life
• Constructability
• Finish (surface roughness & colour)
• Cost
�
New technologies
Single shell linings/Single shell linings/
““CompositeComposite”” LiningsLiningsDouble shell liningsDouble shell linings
SCL – Current & Future?
• Wet-mix process
• Alkali-free accelerators
• Low w/c using superplasticisers
• Nano/micro silica technology
• Fibre reinforcement
• Spray applied membranes
• Improved equipment & robotic spraying
• Higher skilled operatives
Temporary sprayed concrete
Geotextile & sheet membrane
Permanent cast in-situ concrete
1
2
3
1
2
3
Permanent sprayed concrete
Spray applied membrane
1
2
1
2
1
�Single Shell vs Double Shell
• Balancing increased
material costs for
permanent sprayed
concrete vs savings in
quantities, time, equipment
(e.g. formwork)
Dimmock 1998
See Prague seminar in 2006
�Requirements for permanent SCL
• Compressive strength
– Early age strength – J2 class (OBV Guidelines 2006)
– Long term strengths C32/40
• Energy absorption at 28 days
> 700J
• Durability - Water Permeability
< 1 x 10-12 m/s
• Drying shrinkage at 28 days
< 0.03 %
�Tunnels with fibre reinforced sprayed permanent lining
• Heathrow Express Crossover (railway)
• Jubilee Line Extension Crossover (railway)
• Mission Valley East – San Diego (tram)
• Kárahnjúkar Hydroelectric Project (water)
• Heathrow Terminal 5 (railway & road)
… and no waterproof membrane
�Fibre reinforcement
• Steel
– often used instead of bar reinforcement
• Structural plastic
– New ! - used structural reasons
• Polypropylene
– used for fire protection
Fibres are used in both cast concrete (e.g. segments) and sprayed concrete
�Segmental linings
• Many types: bolted; wedge-block; tapered; trapezoidal; hexagonal …etc
• Traditionally bar reinforcement
• Steel fibres are now often used for structural reasons:
– Durable
– Good under impact loading
– Easy to add to concrete mix; saves time during production
• Polypropylene fibres are now often used for fire protection
�Synthetic mono/multi filament pp fibres for anti-spalling fire protection
�Pre-Cast Concrete Liningswith Steel Fibres
– Heathrow Baggage Handling Tunnel (1994) 1.4 km
– London Jubilee Line Extension (1996) 2.2 km
– Channel Tunnel Rail Link (2004) 20 km
– Heathrow PiccEx & HexEx Tunnels (2005) 3.5 km
�Heathrow HexEx Tunnel
• Geometry
– 5.675m diameter (expanded)
– Ring width 1.0m, thickness 220mm
– 9 segments & key
– Radial joints convex (radius = 1375mm)
• Steel Fibres
– Dramix
– 30kg/m3
• Note: bolted lining was bar reinforced
�Heathrow PiccEx Tunnel
• Geometry
– 4.5m diameter (expanded)
– Ring width 1.0m, thickness 150mm
– 7 segments & key
– Radial joints convex (radius = 1500mm)
• Steel Fibres
– Dramix
– 30kg/m3
�HexEx & PiccEx Tunnels
• Design
– Compressive strength – 60 N/mm2
– Polypropylene fibres
– BS8110 – tensile strength (0.5 x √fcu)
– Design of radial joints dependent on full
scale verification testing
�Typical usage in segments
• Steel fibres: ~30 kg/m3
• Polyproyplene fibres: 1 kg/m3
• Fewer segments damaged during
building; faster manufacture ?; thinner segments ?
�Value Engineering Case Study
1. SFRS Primary lining
2. PVC sheet membrane
3. Plain cast in-situ concrete lining
12
3
Benefits of Option B:
• Materials and Equipment Cost saving ~ £1.5M
• Programme saving 3 - 4 months, and increased flexibility
1. SFRS Primary lining
2. Spray applied membrane
3. Sprayed concrete lining
4. Plain cast in-situ concrete lining
12 3
4
�Conclusions
• A wide variety of options exist for tunnelling methods and linings
• A systematic evaluation of risks can help choose the most appropriate ones
• A flexible, logical, modern approach for the best value for money
�
www.tunnels.mottmac.com
Questions ?