Lecture Tunnelling

Tunnelling

1. Tunnelling methods

2. Excavation techniques

3. Rock mass characterisation

4. Examples

Brick lined, hand dug tunnel in to London Clay under the River

Thames

• Victorian London Underground: 1st deep-level tube line;

1890 between Monument and Stockwell. Diameter: 3.10 m

Cut and cover tunnel cut in Gault Clay at Castle Hill, near Folkestone

• Tunnel portal on the UK side of Channel Tunnel

Shallow tunnel in soils: Cut and Cover Technique

1. Excavate trench by removing overburden

2. Install infrastructure (railway system)

3. Install roofing structure

NATM Singapore – opened 2006 New Austrian Tunnelling Method: uses Drill and Blast method

• ‘Design on the go’, rock behaviour monitored while tunnelling and

tunnel support constantly assessed and altered accordingly

• Economical tunnelling method

Ramsgate Harbour – 760m bored tunnel using a TBM,

opened 2000.

Submersible tube tunnel

Prefabricated in dry dock

Commonly used for short

road and rail crossings

across rivers and estuaries

1. Excavate a channel by

dredging

‘Underwater cut and fill’

2. Float section out to sea

3. Remove bulkhead and

sink

4. Position and dock

5. Add backfill and rock

armour

6. Dewater: pump out water

Conwy Bypass: UK’s first immersed tunnel,

1080 m

Crosses the River Conwy, N. Wales

Geology & Tunnels

Geology determines: tunnel route, design and

construction

Ground investigation allows identification of most

suitable unit to tunnel through:

1. Rock weathering causes rock strength to reduce

Mudrocks: fissure & soften; Basalts: micro-fracture

2. Discontinuities affect tunnel roof stability:

Orientation relative to tunnel axis:

(Bell, 2007)

Main tunnelling methods in soils &

weak rocks: <20 Mpa Major issue is tunnel ‘stand-up

time’, excavations collapse

Material with short stand-up time:

Shielding method uses:

Closed-face tunnel boring machine

Cutting face is pump pressurised

with bentonite slurry or grout

Permanent pre-cast concrete lining

Soils influenced by water table:

Sands flow as viscous liquid

Madehow.com

Conveyor belt

Shield

Main tunnelling methods in Rock

1. 2.

3.

Drill and blast – in any

hard rock and for large

caverns

1. Drill cavity (50 mm)

2. Insert explosive & blast

3. Muck out and repeat

Blasting damages rock

mass; fracturing

Weaken rock mass

Not suitable for shales of

schists; cleavages

Main tunnelling methods in Rock

Three ways:

Full-face

driving

Top heading

& bench

Top heading

Main tunnelling methods in Rock

Expresslink.hk

Full face blasting of a tunnel in Hong Kong

Blasting Sequence:

1. Blast out a cut

2. Free-face blasting

3. Smooth blasting

Smooth blasting

of perimeter holes

Blast holes

Main tunnelling methods in Rock

Roadheader – mobile milling head for use in rocks

(60 - ~150 Mpa UCS)

Rotating cutting head

Mounted drag picks

Some double headed

Remote controlled

Stratified formations:

High degree of control

Cut a range of tunnel shapes

Do not excavate a full face profile like a TBM

Roadheader Machine

Main tunnelling methods in Rock

Payline

Shored up by netting

Shotcrete?

Tunnel boring machine

(TBM)

Rotating heads cutting up to

30 m/day in soft rock

Excavation by cutter head

equipped with cutters

Smooth cylindrical tunnel

Diameter: 1 m to 19.25 m

Constant speed rotation

Hydraulic pushing system:

Gripper system pushes

against tunnel lining

Main tunnelling methods in Rock

• Minimal damage to adjacent rock

• Overbreak minimised: 5 % compared to 25 % in drill

and blast; less tunnel support needed

Tunnelling by TBM most

frequently used method due to:

• Increased rates of advance

• Versatile as can bore weak to

strong rocks

• Bore rock up to 150 Mpa UCS

Tunnel boring machine (TBM)

However:

1. Hard rock: cutter wear &

larger thrusts

2. Low fracture density: slow

progress

3. Geological structure can

deviate the TBM

4. Constant surveying

necessary

5. ~400 m turning circle

6. Only economical in tunnels

longer than 1 km

7. High upfront costs

8. Transport logistics

Geophysical Survey in Ground Investigation

for Tunnel

• Electrical resistivity tomography used to identify

difficult ground before tunnelling began

• Highly fractured limestone and clay filled zones

Tunnelling Considerations

Faults must be treated with caution; Falling wedges of fault gouge

Prone to swelling which can damage support

Lots of parasitic faults create zone of shattered rock

High groundwater flows present serious difficulties Unexpected water-bearing zones

Avoided by estimating water inflow by identifying hydrogeological boundaries

Overbreak Rock falls from above crown

Thinly bedded and jointed

Tunnelling Considerations: Rockfall hazard within unlined section of tunnel

• Due to failure to correctly characterise the rock mass

Tunnelling Considerations

Rock burst (occurs > 600 m with UCS > 140 MPa) Rock breaks from side of tunnel with explosive force

Popping: less violent form at lower depths; rocks foliate

Squeezing and swelling ground (where UCS < 2 MPa) Squeezing: slow subsidence of tunnel sides in soft clays

Swelling: expansion due to water infiltration in clays rich in montmorilonite (swelling clay)

Variable rockhead – major hazard Break through to water bearing rocks; tunnel acts as drain

Temperature increases (2-4°C/100 m) Ventilation to keep temperature below 25°C

The partial collapse of a tunnel being constructed over the Chiltern line at

Gerrards Cross closed the line to all services. The line is being roofed over

to provide space for a Tesco supermarket above the tracks. [2005-07-01]

Tunnel liner failed due to applied stress

Rock Mass Characterisation

1. Suitability of ground for tunnelling by determining ‘stand up time’ of a tunnel

2. Identify necessary stability measures

Unsupported tunnel stand up time

Tu

nn

el W

idth

(m

)

Bieniawski 1989:

Rock Mass Rating System

Divides rock masses in to 5

groups depending on

suitability for tunnelling

Collapse

immediately

Stand up

considerable time

1st: Geomechanics system of Rock Mass Rating

Parameter Assessment of values and rating

Intact rock UCS

(MPa)

Rating

>250

15

100 - 250

12

50 – 100

7

25 – 50

4

1 – 25

1

RQD %

Rating

>90

20

75 – 90

17

50 – 75

13

25 – 50

8

< 25

3

Mean fracture

spacing

Rating

>2 m

20

0.6 -2 m

15

200-600mm

10

60-200 mm

8

<60mm

5

Fracture

conditions

Rating

Rough

tight

30

Open

<1mm

25

Weathered

20

Gouge

<5mm

10

Gouge

>5mm

0

Groundwater

Rating

Dry

15

Damp

10

Wet

7

Dripping

4

Flowing

0

Fracture

orientation

Rating*

Very

favourable

0

Favourable

-2

Fair

-7

Unfavourable

-15

Very

Unfavourable

-25

*note negative values Rock Mass Rating (RMR) is the sum of the six ratings

Tunnel support derived from: rock strength, groundwater

and nature of discontinuities. Range: 0-100 (higher better)

2nd: Norwegian Q System

• Was developed as RMR

doesn’t account for support

derived from:

1. Joint roughness

2. Frictional strength of joint

infilling material

3. Stress reduction due to

tunnelling; loosening of tunnel

wall rocks

Waltham, 2009

Norweigan Q System successfully

multiplies rating values to determine the

rock mass quality (Q) as:

Q= (RQD/Jn) x (Jr/Ja) x (Jw/SRF)

RQD – rock quality designation (100-10)

Jn – Joint set number (1-20)

Jr – Joint roughness factor (4-1)

Ja – Joint alteration and clay infill (1-20)

Jw – Joint water inflow or pressure (1-0.1)

SRF – Stress reduction factor due to tunnelling

(1-20)

Q values range between <0.001 to > 1000

Guidelines properties for Rock Mass Classes

Rock Mass

Class

I II III IV V

Description Very good

rock

Good

rock

Fair rock Poor rock Very poor

rock

RMR 80-100 60-80 40-60 20-40 <20

Q Value >40 10-40 4-10 1-4 <1

Friction angle (°) >45 35-45 25-35 15-25 <15

Cohesion (kPa) >400 300-400 200-300 100-200 <100

SBP (Mpa) 10 4-6 1-2 0.5 <0.2

Safe cut slope (°) >70 65 55 45 <40

Tunnel support none Spot

bolts

Pattern

bolts

Bolts &

shotcrete

Steel ribs

Stand up time

for span

20 years

for 15m

1 years

for 10m

1 week

for 5m

12 hours

for 2m

30 mins

for 1m

Results of RMR or Q system converted in to:

• Rock mass classes (I-V)

• Tunnel stand up time and recommended support

Rock quality & stability improvement

methods

Rock Bolts

Rock quality & stability improvement

methods

Shotcrete

• For poor quality rock stability measures are erected

immediately after excavation:

• Clamp discontinuities closed

• Used along side drill and blast

• Sprayed on excavated surface

• 15 cm can eradicate rockfalls

Improving ground ahead of face

Spiling An arch of rock bolts or grout inserted at 10°

to tunnel axis ahead of tunnel advance

(Waltham, 2009)

Channel Tunnel, 1992 Longest undersea tunnel in the World

Links Folkestone, UK to Calais, France

4 years to construct

50 km rail tunnel – two bores of 7.6 m and smaller

service tunnel 4.8 m under the English Channel

Average 45 m depth below seabed

Cost: £4.65 Billion GBP

80% overspend

11 lives lost

2012: 18M passengers

Tunnel Transport System • Three tunnels:

• 2 for Eurostar trains

• 1 vehicle service tunnel

Lined with precast concrete

segments 0.3-0.6 m thick

5-8 segments

Planned Channel Tunnel Route

• 85 % tunnelled in Chalk Marl: UCS 5-9MPa, 30-40% clay

• Low fracture density, moderately strong, little support

required and impermeable

• 2 main tunnels cut by 11 TBMs of 8.7 m diameter

• Central tunnel probed 1 km ahead; pioneer hole

• Maintain 20 m of sound rock between crown and seabed

Cretaceous Wealdon – Boulonnaise Dome

Tates’ Cairn Tunnel, HK, 1991 Longest tunnel in HK

10.7 m wide, 8 m high, 4 km long

through strong granite

Drill and blast – two 10hr

shifts/day; advanced at 60m/week

Drilling 3 hrs; round of 90 holes,

50mm diameter, 4.5 m deep each

taking 3 mins

Charging and firing 2.5 hrs

Mucking out 4 hrs – front loader

fills 20t dumptruck in 2 mins, 1000t

per round, rock bulks by 50%

Summary

Types of tunnel: soft ground; cut and cover;

submerged tubes; bored

Identifying tunnelling conditions

Tunnel support

Tunnelling problems

Hand dug tunnels Hand drill and blast

Main tunnelling methods in soils &

weak rocks: <20 Mpa

Lotschberg tunnel 1908 – Swiss tunnel heading drove through rockhead

into saturated gravels 185 m below valley floor after a false assumption of

the sediment depth. There was no geomorphological input so there was no

allowance for any reverse gradient on the rock floor of the glaciated valley that

was buried beneath alluvial infill. The technology at the time excluded the use

of deep boreholes but probes could have been drilled ahead of the tunnel drive.

The water in rush killed 25 miners and the tunnel had to be re-routed (see

Geology Today, Vol. 23, No. 3, May/June 2007)