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Chapter 8.5 Tunnels and underground excavations (caverns)

Dr. Robert Hack

ITC, University Twente

For credits and references is referred to ‘2018 PAO-Chapter 8.5 - Tunnels and

underground excavations (caverns) – by Robert Hack’.

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Tunnels and underground excavations (caverns)

Excavation & support

Why is support needed

Excavation

Support

Tunnel Boring Machine (TBM)

Groundmass properties and parameters important for underground excavations

Tunneling risks

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Excavation & support

Excavation & support are linked items

excavation depends on support and vice versa

and

determined by many parameters

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Excavation & support

Support only required (if required at all) to keep ground material in the excavation wall at its place; the ground caries itself !

(see Chapter 4, Ground Mechanics)

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Excavation methods

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Mechanical vs blasting – Non-cyclic vs cyclic

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Weaker vs stronger rock mass

Ease of excavation & more support vs difficult excavation & less support

Everybody can excavate everything; even with bare hands and nails

but

may take forever and not be very economical

Economics determine excavation (and support) methods

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Cutting & grinding

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Hammering

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Blasting

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Old-fashioned vs modern blasting

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Specials – chemical expansion

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Very specialplasma, microwave

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Support

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Grouting - cement milk in pores and discontinuities

Freezing – groundwater becomes ice in pores and discontinuities

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Shield tunneling – tunnel boring machine

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Shotcrete

Shotcrete most popular means of support

Keeps ground at position even the smallest particles

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Bolts, dowels & anchors

Keep the ground together and increase shear strength

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Anchors

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Combined support

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Soil nailing

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Yielding steel arches

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Concrete blocks

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Timber

Timber is bad – compresses too much – allows ground to move –reduction shear strength and integrity

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Tunnel Boring Machine (TBM)

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Tunnel Boring Machine (TBM) – soft ground)

‘Soft’ ground types of TBM may also be used in ‘mixed’ (hard & soft) ground or ‘hard’ ground with water bearing and discontinuities filled with loose material

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Cutter heads

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Jacked tunnel

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Groundmass properties

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Tunneling risks

Everything on the scheme is a risk

and many more….

Even a manager who forgets his cell phone may be a major hazard for the project (nobody can contact him if something has gone wrong)

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Tunneling risks

Best approach is to study a tunnel project nearby in similar ground,

and do this also for progress and production !

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Tunnel progress and production

Wonderful articles are written on tunnel progress, production, and risks, but what the value is ?

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Tunnelingrisks Subsidence Rijswijkse Golfclub (2005) due to

jacked sewage tunnel from Ypenburg to The Hague. Tunnel diameter 1.9 m, depth ~ 25 m in Pleistocene sand

surface settlement down to 1 m

over area 100 x 50 m2

reason ????

Should have been a simple, more-or-less risk free project………