Hard Rock Tunnelling Methods

8/16/2019 Hard Rock Tunnelling Methods

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EOSC 547EOSC 547::

Tunnelling &Tunnelling &Underground DesignUnderground Design

Topic 4:

1 of 40 Tunnelling Grad Class (2012) Dr. Erik Eberhardt

Hard Rock TunnellingHard Rock TunnellingMethodsMethods

Tunnel Excavation in RockTunnel Excavation in Rock

It is instructive to consider the fundamental objective of the excavationprocess – which is to remove rock material (either to create an opening orto obtain material for its inherent value). In order to remove part of arock mass, it is necessary to induce additional fracturing andra mentation o the rock.

The peak strength of the rockmust be exceeded.

This introduces three critical aspects of excavation:

The in situ block sizedistribution must be changed


to the required fragment sizedistribution.

By what means should therequired energy be introducedinto the rock?


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Tunnel Excavation in RockTunnel Excavation in Rock

Strength The tensile strength of rock is about

1/10th the compressive strength andthe energy beneath the stress-straincurve is roughly its square.Therefore breakin the rock intension requires only 1/100th of theenergy as that in compression.

Block size

The fracturing of rock duringexcavation changes the natural


Hudson & Harrison (1997)

fragment size distribution. The goaltherefore is to consider how best

to move from one curve to theother in the excavation process.

Energy and Excavation ProcessEnergy and Excavation Process

One objective in the excavation process may be to optimize the use ofenergy, i.e. the amount of energy required to remove a unit volume ofrock (specific energy = J/m3). There are two fundamental ways ofinputting energy into the rock for excavation:

Blasting: Energy is input in largequantities over very shortdurations (cyclical – drill thenblast, drill then blast, etc.).

Machine Excavation: Energy isinput in smaller quantities


continuously.


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Drill & BlastDrill & Blast

The technique of rock breakage using explosives

involves drilling blastholes by percussion or rotary-percussive means, loading the boreholes withexplosives and then detonating the explosive in each

.

The explosion generates astress wave and significantgas pressure. Following thelocal fracturing at theblasthole wall and thes allin of the free face,


the subsequent gaspressure then provides the

necessary energy todisaggregate the brokenrock.

Hudson & Harrison (1997)

Conventional Drill &Conventional Drill & Blast CycleBlast Cycle

Drill Load

Blast

VentilateBolt

Survey


ScoopScale


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Drill & BlastDrill & Blast –– Drilling RatesDrilling Rates

2

3

4

5

R a t e

( m / m i n )

0

1

0 20 40 60 80 100 120 140

Uniaxial Compressive Strength (MPa)

D r i l l i n g

4

5

/ m i n )

( 2 0 0 3 )


0

1

2

3

0 100 200 300 400 500

Specific Energy (kJ/m3)

D r i l l i n

g

R a t e

(

T h u r o & P l i n n i n g e r

Drill & BlastDrill & Blast –– Drilling RatesDrilling Rates

( 1 9 9 5 )


U N I T - N T


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Blasting RoundsBlasting Rounds –– Burn CutBurn Cut

The correct design of a blast starts with the first hole to be detonated.

In the case of a tunnel blast, the first requirement is to create a void intowhich rock broken by the blast can expand. This is generally achieved by awedge or burn cut which is designed to create a clean void and to eject therock originally contained in this void clear of the tunnel face.


Burn cut designs usingmillisecond delays.

Blasting RoundsBlasting Rounds –– Blast Pattern DesignBlast Pattern Design



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Specialized Blasting TechniquesSpecialized Blasting Techniques

During blasting, the explosive damage may not only occur according

to the blasting round design, but there may also be extra rockdamage behind the excavation boundary. To minimize damage to therock, a smooth-wall blast may be used to create the final

.

d s o n & H a r r i s o n ( 1 9 9 7 )


H u

The smooth-wall blast begins by creating a rough opening using a large bulk blast.This is followed by a smooth-wall blast along a series of closely spaced and lightlycharged parallel holes, designed to create a fracture plane connecting the holesthrough by means of coalescing fractures.

Blasting AccessoriesBlasting Accessories

ExplosivesDelays: used to

orchestraterotational firing.


Primacord: ignition velocity isapprox. 6,400 m/s.

Safety fuse: Gives miner time to lightall fuses and still have time to seeksafety before the blast occurs.


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Blasting RoundsBlasting Rounds –– FragmentationFragmentation

How efficiently muck from a working tunnel or surface excavation can be

removed is a function of the blast fragmentation. Broken rock by volumeis usually 50% greater than the in situ material. In mining, both the oreand waste has to be moved to surface for milling or disposal. Some waste

. ,everything has to be removed and dumped in fills – or if the material isright, may be removed and used for road ballast or concrete aggregate(which can sometimes then be re-used in the tunnel itself).


BlastingBlasting –– SummarySummary


N T N U

( 1 9 9 5 )


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Mechanical Excavation in RockMechanical Excavation in Rock

Tunnel Boring Machine (TBM)



There are two basic types of machine for underground rock excavation:


Partial-face machines: use acutting head on the end of amovable boom (that itself may betrack mounted).

Full-face machines: use a rotating headarmed with cutters, which fills the tunnelcross-section completely, and thus almostalways excavates circular tunnels.


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Partial-face machines

are cheaper, smallerand much more flexiblein operation.

cut

scoop

muckout



Full-face machines – when used for relativelystraight and long tunnels (>2 km) – permit highrates of advance in a smooth, automatedconstruction operation.

muckout

scoop


cut


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Mechanical ExcavationMechanical Excavation

The advance rate at which the excavation proceeds is a function of the

cutting rate and utilization factor (which is the amount of time that themachine is cutting rock). Factors contributing to low utilization rates aredifficulties with ground support and steering, the need to frequentlyre lace cutters blocked scoo s broken conve ors etc.

The cutters may jam if theTBM is pushed forwardswith too much force. Thenthey might scrape againstthe rock and becomeflattened on one side.


Broken conveyor

TBMTBM OperationOperation

Factors that may controlTBM performance include:

• TBM Penetration Rate(meters/machine hour)

• • TBM Utilization (machine

hours/shift hours)• Tool Wear (tool changes per

shift)



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Mechanics of Rock CuttingMechanics of Rock Cutting

In tunnelling terms, a TBM applies both thrust (Fn) and torque (Ft) during

the cutting process. In selecting the proper cutting tool, the engineerwishes to know how the tools should be configured on a machine cuttinghead, how to minimize the need to replace cutters, how to avoiddamaging the cutter mounts, and how to minimize vibration.

( 1 9 9 7 )


H u d s o n & H

a r r i s o n

Mechanics of Rock CuttingMechanics of Rock Cutting

Cutting involves a complex mixture of tensile,shear and compressive modes of failure. Withthrust, the cutting disc penetrates the rock andgenerates extensive crack propagation to thefree surface. Further strain relief occurs as the

( 1 9 9 8 )

disc edge rolls out of its cut, inducing furthertensile cracking and slabbing at the rock surface.


N T N U - A n l e g g s d r i f t


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Mechanics of Rock CuttingMechanics of Rock Cutting –– CutterCutter WearWear

The primary impact of disc wear on costs can be so severe that cutter

costs are often considered as a separate item in bid preparation. Ingeneral, 1.5 hours are required for a single cutter change, and if severalcutters are changed at one time, each may require 30-40 minutes. Evenhigher downtimes can be expected with large water inflows, which make

newunevenwear

cutter change activities more difficult and time-consuming.


normalwear

heavywear

Mechanical ExcavationMechanical Excavation –– Cutter HeadsCutter HeadsDelays: When the tunnel boring machine is inside the tunnel, the cuttersmust be changed from the inside the cutting head.



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Mechanical ExcavationMechanical Excavation –– Cutter HeadsCutter Heads





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TBM Excavation & DesignTBM Excavation & Design

The two main factors that will stop

tunnel boring machines are eitherthe rock is too hard to cut or thatthe rock is too soft to sustain thereactionar force necessar topush the machine forward. TBM’swill operate within certain rangesof rock deformability and strength,where the machine can be tailoredto a specific range to achievemaximum efficiency (the risk beingif rock conditions diverge fromthose the TBM is designed for) .

0 )


Instability problems at the tunnelface, encountered during excavation ofthe 12.9km long Pinglin tunnel inTaiwan.

B a r l a & P e l i z z a ( 2 0 0


0 )


B a r l a & P e l i z z a ( 2 0 0


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Single & Double Shield TBM’s – Single-shield TBM’s are cheaper and are the

preferred machine for hard rock tunnelling. Double shielded TBMs are normallyused in unstable geology (as they offer more worker protection), or where a highrate of advancement is required.

“Double” shieldTBM


“Single” shieldTBM



U.S. Army Corps of Engineers (1997)


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U.S. Army Corps of Engineers (1997)


TBM insertion throughvertical shaft.

TBM gripper used to providereactionary force for forward thrustby gripping onto sidewalls of tunnel.


TBM working platform forinstalling support (e.g. rockbolts, meshing, shotcrete).


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TBMTBM OperationOperation


TBM Excavation & DesignTBM Excavation & Design -- PrePre--Cast LiningsCast Linings



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Tunnelling BreakthroughsTunnelling Breakthroughs


TBM Selection & Geological RiskTBM Selection & Geological Risk

The Yacambú-Quibor Tunnel is a prime example oftunnelling blind – the geology was largely unfamiliarand unpredictable. With little previous experience,it was unknown how the rock would react, especiallyunder the high stresses of the Andes.

1975: Excavation begins on the 24 km tunnel, for which theuse of a full-face TBM is specified (for rapid excavation).

1977: The weak phyllites fail to provide the TBM gripperswith enough of a foundation to push off of. Supporting

Geology: Weak, tectonically sheared graphitic phyllites wereencountered giving rise to serious squeezing problems, which withoutadequate support would result in complete closure of the tunnel.


.

H o e k ( 2 0 0 1 )

Mining out the remains ofthe trapped TBM.

1979: During a holiday shutdown, squeezing rock conditionswere left unchecked, resulting in the converging groundeffectively “swallowing” one of the TBMs.

1980’s: A decision is made to permit the tunnel to beexcavated by drill & blast. Recently completed, it tookmore than 33 years to tunnel the full 24 km.


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Sequential Excavation & DesignSequential Excavation & Design -- BenchesBenches

Benched excavations are used for largediameter tunnels in weak rock. The benefitsare that the weak rock will be easier tocontrol for a small opening and reinforcementcan be progressively installed along theea ng e ore enc ng ownwar . ar at ons

may involve sequences in which the inverts, topheading and bench are excavated in differentorder.


Lecture ReferencesLecture ReferencesBarla, G & Pelizza, S (2000). TBM Tunneling in difficult conditions. In GeoEng2000, Melbourne .Technomic Publishing Company: Lancaster, pp. 329-354.

Harding, H (1981). “Tunnelling History and My Own Involvement”. Golder Associates: Toronto,258pp.

Hudson, JA & Harrison, JP (1997). “Engineering Rock Mechanics – An Introduction to the Principles”. , .

Maidl, B, Herrenknecht, M & Anheuser, L (1996). “Mechanised Shield Tunnelling”. Ernst & Sohn:Berlin, 428pp.

NTNU (1995). “Tunnel: Blast Design”. Department of Building and Construction Engineering,Norwegian University of Science and Technology (NTNU), Trondheim, Project Report 2A-95.

NTNU-Anleggsdrift (1998). “Hard Rock Tunnel Boring: The Boring Process”. Norwegian University ofScience and Technology (NTNU), Trondheim, Project Report 1F-98.

Thuro, K & Plinninger, RJ (2003). Hard rock tunnel boring, cutting, drilling and blasting: rockarameters for excavatabilit In: Proc 10th ISRM Con ress Johannesbur SAIMM: Johannesbur


. ., , . ,

pp. 1227-1234.

UNIT-NTH (1995). “Tunnel: Prognosis for Drill and Blast”. Department of Building and ConstructionEngineering, Norwegian University of Science and Technology (NTNU), Trondheim, Project Report 2B-95.

Whittaker, BN & Frith, RC (1990). “Tunnelling: Design, Stability and Construction”. Institution ofMining and Metallurgy: London, 460pp.

Documents

Hard Rock Tunnelling Methods