Rock Engineering Practice & Design - ISRM

Rock Engineering Practice & Design Lecture 11: Excavation Methods

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Erik Eberhardt – UBC Geological Engineering

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Author’s Note: The lecture slides provided here are taken from the course “Geotechnical Engineering Practice”, which is part of the 4th year Geological Engineering program at the University of British Columbia (V (Vancouver, Canada). C d ) The Th course covers rock k engineering i i and d geotechnical design methodologies, building on those already taken by the students covering Introductory Rock Mechanics and Advanced Rock Mechanics. Mechanics Although the slides have been modified in part to add context, they of course are missing the detailed narrative that accompanies any l lecture. It is also l recognized d that h these h lectures l summarize, reproduce and build on the work of others for which gratitude is extended. Where possible, efforts have been made to acknowledge th vvarious the ri us ssources, urc s with ith a list of f references r f r nc s being b in provided pr vid d att the th end of each lecture. Errors, omissions, comments, etc., can be forwarded to the author at: [email protected] 2 of 45


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The Excavation Process It is instructive to consider the fundamental objective of the excavation process – which is to remove rock material (either to create an opening or to obtain material for its inherent value). In order to remove part of a rock mass, mass it is necessary to induce additional fracturing and fragmentation of the rock. This introduces three critical aspects p of excavation: The peak strength of the rock must be exceeded. The in situ block size distribution must be changed to the required fragment size distribution. distribution By what means should the required energy be introduced into the rock? 3 of 45


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In Situ Block and Fragmentation Distribution Rock is naturally fractured and consists of rock blocks of certain sizes. The fracturing of rock during excavation changes this natural block size distribution to the fragment size distribution. The engineer can consider how best to move from one curve to the other in the excavation process.

Hudson & Harrison (1997) 4 of 45


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Energy and Excavation Process Although the objective during excavation is large-scale fragmentation, at the same time we wish to minimize any damage to the wall rock as this would work towards weakening the rock mass which may result in ground control problems. problems Fragmentation and rock mass damage are both related to the amount of energy used and whether its applied instantaneously or continuously.

Hudson & Harrison (1997)

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The tensile strength of rock is about 1/10th the compressive strength and the energy beneath the h stress-strain curve is roughly hl its square. Therefore, breaking the rock in tension requires only 1/100th of the energy as that in compression. Erik Eberhardt – UBC Geological Engineering

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Energy and Excavation Process One objective in the excavation process may be to optimize the use of energy, i.e. the amount of energy required to remove a unit volume of rock (specific energy = J/m3). There are two fundamental ways of inputting energy into the rock for excavation: Blasting: Energy is input in large quantities over very short h durations (cyclical – drill then blast, drill then blast, etc.). Machine Excavation: Energy is input in smaller quantities continuously.

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Conventional Drill & Blast The technique of rock breakage using explosives involves drilling blastholes by percussion or rotarypercussive means, loading the boreholes with explosives and then detonating the explosive in each hole in sequence according to the blast design. The explosion generates a stress wave and significant gas pressure. Following the local fracturing at the blasthole wall and the spalling of the free face, the subsequent gas pressure then provides the necessary energy to disaggregate the broken rock rock. 7 of 45

Hudson & Harrison (1997)


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Conventional Drill & Blast Load

Drill

Survey

Blast

V Ventilate il

Bolt

Scoop

Scale

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Drill & Blast - Drilling

water flush ((chip p removal) 9 of 45

rotation


thrust force

percussion hammer Thuro (1997) ISRM Edition

Drill & Blast - Drilling The bit' Th bit's ability bilit tto penetrate t t th the rock k efficiently ffi i tl d depends d on th the contact t t surface of the buttons, their shape and number, the bits' flushing characteristics and the brittleness, or drillability, of the rock. Button Shape

Spherical

SemiBallistic

Conical (Ballistic)

Characteristics

Application

• • • •

"non aggressive" shape minimum drilling rates low bit wear excavation mainly by impact

Rock with high UCS and high abrasivity (e.g. quartzite, granite, gneiss, amphibolite)

• • • •

"aggressive" gg shape p moderate drilling rates moderate bit wear excavation mainly by shearing/cutting

Rock with mid UCS and less abrasivity (e.g. slate, sandstone, limestone, weathered rock)

• • • •

"very aggressive" shape maximum drilling rates high bit wear excavation mainly by shearing/cutting

Rock with low UCS and low abrasivity (e.g. shale, weak sandstone, phyllite)

Thuro (1997) 10 of 45


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Plin nninger et al. l. (2002)

Drill & Blast – Drill Bit Buttons

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Drill & Blast – Blast Pattern Design One of the basic principles of designing the configuration and sequential detonation of blastholes in a one blast, is the presence of a free face parallel or sub-parallel to the blast holes, as detonation occurs. In some cases these free faces may already be present (benches in an open pit cases, mine), but in other cases may need to be created by the blast itself (a tunnel face).

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Practical application of the free-face free face concept using one form of the burn cut. Erik Eberhardt – UBC Geological Engineering

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Drill & Blast – Burn Cut The correctt d Th design i of f a blast bl t starts t t with ith the th first fi t h hole l to t b be detonated. d t t d In the case of a tunnel blast, the first requirement is to create a void into which rock broken by the blast can expand. This is generally achieved by a wedge g or burn cut which is designed g to create a clean void and to eject j the rock originally contained in this void clear of the tunnel face.

Burn cut designs using y millisecond delays.

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Drill & Blast – Blast Pattern Design

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Specialized Blasting Techniques

Hud dson & Harrison (1997)

During D i blasting, bl ti the th explosive l i d damage may nott only l occur according di to the blasting round design, but there may also be extra rock damage behind the excavation boundary. To minimize damage to the rock a pre-split rock, pre split blast (surface excavation) or smooth-wall smooth wall blast (underground) may be used to create the final excavation surface.

Pre-split blast: First a series of small-diameter, parallel boreholes are drilled along the plane of the required final excavation boundary (i.e. rock cut slope). slope) 15 of 45


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PrePre -Split & Smooth Smooth-Wall Blasting The principle is then to tailor the explosive parameters such that detonation of the explosives l i iin these h initial i i i l holes will primarily create a plane of intersecting holes through the coalescence of several induced fractures.

The smooth-wall blast follows a similar process to the pre-split blast, except in the reverse order. First a rough opening is formed using a large bulk blast, and then the smooth-wall blast follows along a series of closely spaced and lightly charged parallel holes. Hudson & Harrison (1997) 16 of 45


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PrePre -Split Blasting When, subsequently, the main body of rock is blasted to form the cutting, the pre-split reflects the stress waves back into the rock being excavated and dissipates excess gas pressure, such that the bulk blast has little effect of the rock behind the pre-split plane. plane

pre-split bl t d blasted 17 of 45

normal bulk bl blasted d


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Drill & Blast – Explosives Commercial explosives are mixtures of chemical compounds in solid or liquid form. Detonation transforms the compounds into other products, mostly gaseous. The following are the main criteria applied to select an explosive for a given type of blasting: 

available energy per unit weight of explosive (i.e. strength)



density of the explosive



detonation velocity



sensitivity (ease of ignition)



reaction ti rate t



temperature and pressure



stability y (chemical ( and storage) g )

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Drill & Blast – Explosives The following are the more common explosives used in hard rock excavation:  dynamites (nitroglycerin made stable by dissolving it in an inert

bulking agent – moderate bulk strength)

 ANFO (Ammonium Nitrate & diesel Fuel Oil – low bulk strength)  slurries (water gels – high bulk strength for wet conditions)  emulsions

ANFO is the most prevalent explosive used in mining because it is the least expensive and the safest to transport and handle. ANFO type explosives are susceptible to water and, and therefore, are not suitable for wet blastholes. 19 of 45


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Drill & Blast – Stemming “Stemming” materials (e.g. pea-sized gravel), are used to top-off the blastholes. The stemming material acts to provide confinement, confinement preventing the explosive gases and energy from travelling (venting) up through the drill hole, and instead are contained i d within i hi the h rock k mass.

Effects of poor stemming. 20 of 45


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Drill & Blast – Blasting Caps & Delays A blasting cap is a small explosive device generally used to detonate a larger, more powerful explosive. Most blasting caps contain what is c ll d a primary called p im explosive. xpl si A primary p im explosive xpl si is a high explosive compound that will explode from flame, heat or shock. Word to the wise: Do not crimp fuses to blasting caps using your teeth.

Few blasts are fired instantaneously. Instead, delays are used to sequence giving g better fragmentation, g the blast g more efficient use of the explosive, reduced vibration, and better control of the fly rock. Generally, delay detonators are produced in measures of milliseconds or seconds. 21 of 45


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Drill & Blast – Fly Rock Fly rock Fl k iis a constant t t concern to t blasters bl t and d their th i co-workers. It can be controlled if proper preparation, blasting techniques and safety procedures are followed. Four of the major p j causes of excessive fly rock are: • • • •

Geology conditions. Inaccurate drilling g and loading. g Poor hole design. Poor pattern timing.

Incidents have been recorded where flyrock has travelled in excess of 1 km and resulted in significant damage, injury and/or death.

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Drill & Blast – Fragmentation How efficiently muck from a working tunnel or surface excavation can be removed is a function of the blast fragmentation. Broken rock by volume is usually 50% greater than the in situ material. In mining, both the ore and nd waste t h has to t be b moved m d to t surface f for f milling millin or disposal. di p l Some S m waste t material can be used underground to backfill mined voids. In tunnelling, everything has to be removed and dumped in fills – or if the material is g , may y be removed and used for road ballast or concrete aggregate gg g right, (which can sometimes then be re-used in the tunnel itself).

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NT TNU (1995)

Drill & Blast – Summary

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Mechanical Excavation Tunnel Boring Machine (TBM)

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Mechanical Excavation There are two basic types of machine for underground excavation:

Partial-face Partial face machines: use a cutting head on the end of a movable boom (that itself may be track mounted).

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Full-face Full face machines: use a rotating head armed with cutters, which fills the tunnel cross-section completely, and thus almost always excavates circular tunnels.


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Mechanical Excavation Partial-face P ti l f m machines hin are cheaper, smaller and much more flexible in operation. p

cut

muck out

scoop

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Mechanical Excavation Full-face machines – when used for relatively straight and long tunnels (>2 km) – permit high rates of advance in a smooth, automated construction operation. operation

scoop muck out

cut

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Mechanical 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 the machine is cutting rock). Factors contributing to low utilization rates are difficulties with ground support and steering, steering the need to frequently replace cutters, blocked scoops, broken conveyors, etc. The cutters may jam if the TBM is i pushed h d forwards f d with too much force. Then they might scrape against the rock and become fl tt flattened d on one side. id

Broken conveyor 29 of 45


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Mechanical Excavation – Tool Wear Delays: When the tunnel boring machine is inside the tunnel tunnel, the cutters must be changed from the inside the cutting head.

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Mechanics of Rock Cutting – Tool Wear The primary Th i impact i t of f tool t l wear on costs t can be b so severe that th t cutter tt costs are often considered as a separate item in bid preparation. For TBM cutters, 1.5 hours are required for a single cutter change, and if several cutters are changed g at one time, m , each may m y require q 30-40 m minutes. Even higher downtimes can be expected with large water inflows, which make cutter change activities more difficult and time-consuming. uneven wear

new

normal wear

heavy wear

Thuro & Plinninger (2003) 31 of 45


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Mechanics of Rock Cutting

Hud dson & Harrisson (1997)

During D i the th cutting tti process, both b th thrust th t (Fn) and d torque t (Ft) are applied. li d I In selecting the proper cutting tool, the engineer wishes to know how the tools should be configured on a machine cutting head, how to minimize the need to replace p cutters,, how to avoid damaging m g g the cutter mounts, m , and how to minimize vibration.

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Mechanics of Rock Cutting

NTNU U-Anleggsdrrift (1998)

Cutting C tti iinvolves l a complex l mixture i t of f tensile, t il shear and compressive modes of failure. With thrust, the cutting disc penetrates the rock and generates extensive extens ve crack propagat propagation on to the free surface. Further strain relief occurs as the disc edge rolls out of its cut, inducing further tensile cracking and slabbing at the rock surface.

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The two main factors that will stop tunnel boring machines are either the rock is too hard to cut or that the rock is too soft to sustain the reactionary force necessary to push the machine forward. TBM’s will operate within certain ranges of f rock k deformability d f bili and d strength, h where the machine can be tailored to a specific range to achieve maximum efficiency (the risk being if rock conditions diverge from those the TBM is designed for) .

Instability problems at the tunnel face, encountered during excavation of g Pinglin g tunnel in the 12.9km long Taiwan. 34 of 45


Barla & Pelizza (2 2000)

TBM Excavation & Design

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Barla a & Pelizza (2000)


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TBM Excavation & Design Single & Double Shield TBM’s – Single-shield TBM’s are cheaper and are the preferred machine for hard rock tunnelling. Double shielded TBMs are normally used in unstable geology (as they offer more worker protection), or where a high rate of advancement is required.

“Double” shield TBM “Single” shield TBM

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TBM Excavation & Design Open- & Closed-Face O Cl d F Shields Shi ld – When Wh th the tunnel t l face f does d nott require i a continuous ti and pressure balanced support, the TBM is operated in ‘Open Mode’. The face is mechanically supported by the cuttinghead while the flood control doors regulate muck flow from the face to the cuttinghead chamber. The excavated muck is rapidly idl extracted t t d by b the th conveyor. With a closed-face, l d f an airlock i l k and d bulkhead b lkh d are used to allow the “excavation chamber” to be pressurized with compressed air to aid face support.

Open-face shields Closed-face shields

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TBM Excavation & Design Earth Pressure Balance (Closed-Face Shield) – This method provides continuous support to the tunnel face by balancing earth pressure against machine thrust. As the cutterhead rotates and the shield advances, the excavated earth is mixed with foams in the cutterhead chamber to control its viscosity. The pressure is then adjusted by means of the rate of its extraction (by screw conveyor) to balance the pressure exerted by the ground at the tunnel face. This enables near surface tunnelling in bad ground conditions with minimal surface settlement.

clay y foam injector j

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U S Army Corps of Engineers (1997) U.S.

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U.S. Army Corps of Engineers (1997)

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TBM Selection & Geological Risk The Yacambú-Quibor Th Y bú Q ib Tunnel T l iis a prime i example l of f tunnelling blind – the geology was largely unfamiliar and unpredictable. With little previous experience, it was unknown how the rock would react, especially under the high stresses of the Andes. Geology: Weak, tectonically sheared graphitic phyllites were encountered giving rise to serious squeezing problems, which without adequate support would result in complete closure of the tunnel. 1975: Excavation begins on the 24 km tunnel, for which the use of a full-face TBM is specified (for rapid excavation).

1979: During a holiday h ld shutdown, h d squeezing rock k conditions d were left unchecked, resulting in the converging ground effectively “swallowing” one of the TBMs. 1980’s: A decision is made to p permit the tunnel to be excavated by drill & blast. Recently completed, it took more than 33 years to tunnel the full 24 km. 41 of 45

Mining out the remains of the trapped TBM.


Hoe ek (2001)

1977: The weak phyllites fail to provide the TBM grippers with enough of a foundation to push off of. Supporting squeezing ground was another defeating problem.

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TBM Excavation & Design TBM insertion i ti through th h vertical shaft.

TBM gripper used to provide reactionary force for forward thrust by gripping onto sidewalls of tunnel.

TBM working platform for installing support (e (e.g. g rock bolts, meshing, shotcrete). 42 of 45


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TBM Excavation & Design - Pre Pre-Cast Linings

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Tunnelling Breakthroughs

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Lecture References Barla, G & Pelizza, S (2000). TBM Tunneling in difficult conditions. In GeoEng2000 - Proceedings of the International Conference on Geotechnical & Geological Engineering, Melbourne. Technomic Publishing Company: Lancaster, pp. 329-354. Hoek, E (2001). Big tunnels in bad rock (the Thirty-Sixth Karl Terzaghi Lecture). Journal of Geotechnical and Geoenvironmental Engineering 127(9): 726-740. Hudson, JA & Harrison, JP (1997). Engineering Rock Mechanics – An Introduction to the Principles . Elsevier Science: Oxford. NTNU (1995). (1995) Tunnel: Blast Design. Design Norwegian University of Science and Technology (NTNU): Trondheim, Project Report 2A-95. NTNU-Anleggsdrift (1998). Hard Rock Tunnel Boring: The Boring Process. Norwegian University of Science and Technology (NTNU): Trondheim, Project Report 1F-98. Plinninger, RJ, Spaun, G & Thuro, K (2003). Prediction and classification of tool wear in drill and blast tunnelling. In Proc., 9th Congress of the IAEG, Durban. CD-ROM, paper #395, pp. 2226-2236. Thuro, K (1997). Drillability prediction: Geological influences in hard rock drill and blast tunnelling. g Rundschau 86(2): ( ) 426-438. Geologische Thuro, K & Plinninger, RJ (2003). Hard rock tunnel boring, cutting, drilling and blasting: Rock parameters for excavatability. In Proc., 10th ISRM Congress, Johannesburg. SAIMM: Johannesburg, pp. 1227-1234. U.S. U S Army A C Corps of f Engineers E i (1997) Engineering (1997). E i i and d Design D i - Tunnels T l and d Shafts Sh ft in i Rock. R k Publication EM 1110-2-2901. 45 of 45


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Rock Engineering Practice & Design - ISRM

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