�

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 ?