Tunnelling & underground design (Topic5-hard & weak rock tunnelling)

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1 of 59 Tunnelling Grad Class (2015) Dr. Erik Eberhardt

EOSC 547:

Tunnelling & Underground Design

Topic 5: Hard & Weak Rock

Tunnelling


Tunnel Excavation in RockIt 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, it is necessary to induce additional fracturing and fragmentation of the rock.

The peak strength of the rock must be exceeded.

This introduces three critical aspects of excavation:

The in situ block size distribution must be changed to the required fragment size distribution.

By what means should the required energy be introduced into the rock?

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Tunnel Excavation in Rock

StrengthThe tensile strength of rock is about 1/10th the compressive strength and the energy beneath the stress-strain curve is roughly its square. Therefore, breaking the rock in tension requires only 1/100th of the energy as that in compression.

Block Size

Hudson & Harrison (1997)

The fracturing of rock during excavation changes the natural block size distribution to the fragment size distribution. The goal therefore is to consider how best to move from one curve to the other in the excavation process.


Energy and Excavation ProcessOne 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 durations (cyclical – drill then blast, drill then blast, etc.).

Machine Excavation: Energy is input in smaller quantities continuously.

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Drill & BlastThe technique of rock breakage using explosives involves drilling blastholes by percussion or rotary-percussive 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. Hudson & Harrison (1997)


Conventional Drill & Blast Cycle

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Conventional Drill & Blast Cycle

Drill Load

Blast

Ventilate

ScoopScale

Bolt

Survey


Drill & Blast – Drilling Rates

0

1

2

3

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0 20 40 60 80 100 120 140

Uniaxial Compressive Strength (MPa)

Drilling

Rat

e (m

/min)

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1

2

3

4

5

0 100 200 300 400 500

Specific Energy (kJ/m3)

Drilling

Rat

e (m

/min)

Thur

o &

Plin

ning

er(2

003)

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Drill & Blast – Drilling Rates

UN

IT-N

TH (1

995)


Blasting Rounds – Burn CutThe 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 into which rock broken by the blast can expand. This is generally achieved by a wedge or burn cut which is designed to create a clean void and to eject the rock originally contained in this void clear of the tunnel face.

Burn cut designs using millisecond delays.

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Blasting Rounds – Blast Pattern Design


Specialized Blasting TechniquesDuring blasting, the explosive damage may not only occur according to the blasting round design, but there may also be extra rock damage behind the excavation boundary. To minimize damage to the rock, a smooth-wall blast may be used to create the final excavation surface.

Hud

son

& H

arri

son

(199

7)

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 lightly charged parallel holes, designed to create a fracture plane connecting the holes through by means of coalescing fractures.

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Blasting Rounds – 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 waste has to be moved to surface for milling or disposal. Some waste 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 right, may be removed and used for road ballast or concrete aggregate (which can sometimes then be re-used in the tunnel itself).


Blasting – SummaryN

TNU

(199

5)

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Machine Excavation in Rock

Tunnel Boring Machine (TBM)


Machine Excavation in RockThere are two basic types of machine for underground rock excavation:

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

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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Machine Excavation in RockPartial-face machines are cheaper, smaller and much more flexible in operation.

cut

scoop

muck out


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

muck out

cut

scoop

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Machine Excavation in Rock


Machine ExcavationThe 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, the need to frequently replace cutters, blocked scoops, broken conveyors, etc.

The cutters may damage if the TBM is pushed forwards with too much force, or large blocks fall and strike them.

Broken conveyor

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TBM OperationFactors that may control TBM performance include:

• TBM Penetration Rate (meters/machine hour)

• TBM Downtime (minutes) • TBM Utilization (machine

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

shift)


Mechanics of Rock CuttingIn tunnelling terms, a TBM applies both thrust (Fn) and torque (Ft) during the cutting process. 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 cutters, how to avoid damaging the cutter mounts, and how to minimize vibration.

Hud

son

& H

arri

son

(199

7)

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Mechanics of Rock Cutting

NTN

U-A

nleg

gsdr

ift

(199

8)

Cutting involves a complex mixture of tensile, shear and compressive modes of failure. With thrust, the cutting disc penetrates the rock and generates extensive crack propagation 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.


Mechanics of Rock Cutting – Cutter Wear

newunevenwear

normalwear

heavywear

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. In general, 1.5 hours are required for a single cutter change, and if several cutters are changed at one time, each may require 30-40 minutes. Even higher downtimes can be expected with large water inflows, which make cutter change activities more difficult and time-consuming.

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


Mechanical Excavation – Cutter Heads

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TBM Excavation & DesignThe 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 rock deformability and strength, 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 the 12.9km long Pinglin tunnel in Taiwan. Ba

rla

& Pe

lizza

(200

0)

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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.

“Single” shield TBM

“Double” shield TBM

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U.S. Army Corps of Engineers (1997)



U.S. Army Corps of Engineers (1997)

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TBM Excavation & DesignTBM insertion through vertical shaft.

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

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


TBM Operation

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Tunnelling Breakthroughs


TBM Selection & Weak RockThe Yacambú-Quibor Tunnel is a prime example of 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.

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

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.

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.

Hoe

k (2

001)

Mining out the remains of the trapped TBM.

1979: During a holiday shutdown, squeezing rock conditions were left unchecked, resulting in the converging ground effectively “swallowing” one of the TBMs.

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

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Sequential Excavation & Design - BenchesBenched excavations are used for large diameter tunnels in weak rock. The benefits are that the weak rock will be easier to control for a small opening and reinforcementcan be progressively installed along the heading before benching downward. Variations may involve sequences in which the inverts, top heading and bench are excavated in different order.


Ground Reaction - ConvergenceA key principle in underground construction involving weak rock is the recognition that the main component of tunnel support is the strength of the rock mass and that it can be mobilized by minimizing deformations and preventing rock mass “loosening”.

Whi

ttak

er &

Fri

th (1

990)

During construction of a tunnel, some relaxation of the rock mass will occur above and along the sides of the tunnel.

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Terzaghi’s Rock LoadTerzaghi (1946) formulated the first rational method of evaluating rock loads appropriate to the design of steel sets.

The movement of the loosened area of rock (acdb) will be resisted by friction forces along its lateral boundaries and these friction forces help to transfer the major portion of the overburden weight onto the material on either side of the tunnel.

As such, the roof and sides of the tunnel are required only to support the balance which is equivalent to a height Hp. Terzaghi related this parameter to the tunnel dimensions and characteristics of the rock mass to define a series of steel arch support guidelines.


Terzaghi’s Rock Load

Terz

aghi

(194

6)

Deere et al. (1970)

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Tunnelling in Weak RockTerzaghi’s ”Rock Load” implicitly relates the benefits gained through the grounds natural tendency to arch. The essence of tunnelling in many respects is to disturb the natural arch as little as possible while excavating the material.

In weak rock, ground loosening breaches the integrity of this natural arch. The consequence is that without supporting the excavation soon after it is completed –the walls may squeeze together and the roof collapse.

Besides the strength of the rock mass, a second key factor controlling the extent of loosening is the size of the excavation. Several difficulties relating to the size of the face include:

• increased volume of ground disturbed• decreased accessibility to all parts of the face• increasing difficulty in supporting and controlling face stability


Building on Past Experiences – Ground Control

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Early Tunnel Experiences in Weak RockThrough much trial and error, the lesson commonly learned was that with a small tunnel face, the volume of ground moving and relaxing is also smaller and can often be tolerated or kept within acceptable limits by relatively simple timbering or other temporary support.

Belgium method


Early Tunnel Experiences in Weak RockBelgium method The method was first employed in building the

Chaleroy tunnel (in Belgium) in 1828. The great advantage claimed for the system by Belgian and French engineers was the speed whereby the roof of the tunnel could be secured, a desirable advantage in poor rock.

The method fell out of favour as a result of catastrophic experiences encountered during the construction of the Gotthard Tunnel (1872-1882). The key problem was that the sequencing following Stage 3 required the arch to be underpinned. However, this proved difficult in the yielding ground conditions encountered, leading to the timbers giving way, followed by the cracking or total collapse of the masonry arch. Be

aver

(197

2)

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Early Tunnel Experiences in Weak RockGerman system

The “German System” introduced the principle of leaving a central bench of ground to be excavated last and to use it to support roof and wall timbering.

This allowed the arching to be built in one operation, unlike the Belgium method which had the disadvantage of building the arch and walls separately.

The German system proved disastrous when applied to the Cžernitz tunnel in Austria (1866), where the timbers supporting the heading either pushed into the core, whereupon they became loose, or were crushed by swelling pressures that developed in the core.


Early Tunnel Experiences in Weak RockAustrian methodThe “Old” Austrian Tunnelling Method was first

used for the Oberau tunnel in 1837, which was constructed through marls, gneiss and granite. The method differed from others in that it required the full section to be excavated before the masonry was added, with the excavation being carried out in small sections.

A centre-bottom heading was first driven for a distance of about 5 m. This ‘pilot tunnel’ served to ventilate the workings, drain the surrounding area, and establish the tunnel alignment.

Sand

strö

m(1

963)

A centre-top heading then followed (driven for the same distance). Section 3 was then removed by men working from the top heading, enabling the top structures to rest on the undisturbed timbers below.

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Early Tunnel Experiences in Weak RockAustrian method

Sand

strö

m(1

963)

Once the excavation was fully opened, the masonry lining was built up from the foundations to the crown of the arch in consecutive 5 m long sections.

Breaking out of the tunnel to full width then began at the shoulders, working down.


Sequential Excavation Methods (SEM)Although the use of these early systems eventually died out due to the huge quantity and high cost of timber required, and the replacement of masonry linings with concrete, their underlying principles still live on. That is the benefits of driving one or more small headings that are later enlarged, enabling for ground deformations to be controlled better.

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The Observational Method in Design

“In geotechnical engineering, vast goes towards securing approximate values for required parameter inputs. Many additional variables are not considered or remain unknown. Thus, the results of computations are no more than working hypotheses, subject to confirmation or modification during construction.”

In the 1940’s, Karl Terzaghi introduced a systematic means to manage geological uncertainty in geotechnical design:

“These uncertainties require either the adoption of an excessive factor of safety, or else assumptions based on general experience. The first of these is wasteful; the second is dangerous as most failures occur due to unanticipated ground conditions.”

“As an alternative, the observational method provides a ‘learn as you go’ approach. First, base the design on whatever information can be secured, making note of all possible differences between reality and the assumptions (i.e. worst case scenarios), and computing for the assumed conditions, various quantities that can be measured in the field. Then, based on these measurements, gradually close the gaps in knowledge and, if necessary, modify the design during construction.” Te

rzag

hi&

Peck

(194

8)


The Observation Method in Design

a) Sufficient exploration to establish the general nature, pattern and properties of the soil deposits or rock mass;

b) Assessment of the most probable conditions and the most unfavourable conceivable deviations from these conditions;

c) Establishment of the design based on a working hypothesis of behaviour anticipated under the most probable conditions;

d) Selection of quantities to be observed during construction and calculation of their anticipated values on the basis of the working hypothesis;

In brief, the method embodies the following components:

e) Calculation of values of the same quantities under the most unfavourable conditions compatible with the available subsurface data;

f) Selection in advance of a course of action or modification of design for every foreseeable significant deviation of the observational findings from those predicted on the basis of the working hypothesis;

g) Measurement of quantities to be observed and evaluation of actual conditions;

h) Modification of design to suit actual conditions.

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Observation Method Example – Jubilee ExtensionThe Jubilee Line Extension to the London Underground, started in 1994 and called for twin tunnels 11 km long, crossing the river in four places, with eleven new stations to be built, eight of which were to be underground. One of the more problematic of these was a station placed right opposite Big Ben.


Observation Method Example – Jubilee Extension

The technical implications were immense. Built in 1858, Big Ben is known to be on a shallow foundation. It started to lean towards the North shortly after completion. Any ground movement in the vicinity would exaggerate this lean, and threaten the stability of the structure.

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Observation Method Example – Jubilee ExtensionTo deal with excavation-induced settlements that may irreversibly damage historic buildings in the area, the design called for the use of compensation grouting during tunnelling. In this process, a network of horizontal tubes between the tunnels and the ground surface is introduced, from which a series of grout holes are drilled. From these, liquid cement can be injected into the ground from multiple points to control/prevent movement during excavation of the main tunnels.


Observation Method Example – Jubilee ExtensionInstrumentation was attached to Big Ben and to the buildings in the vicinity to measure movement (with some 7000 monitoring points), and computers were used to analyze the data to calculate where and when the grout has to be injected.

For Big Ben, a movement of 15 mm at a height of 55m (approximately the height of the clock face above ground level) was taken to be the point at which movement had to be controlled. Throughout the 28 month construction period, experience had to be gained as to which tube to use for grouting, the volume of grout to be injected and at what rate.

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Observation Method Example – Jubilee Extension

It was calculated that without the grouting, the movement of Big Ben would have gone well over 100 mm, which would have caused unacceptable damage.

Following construction, the grouting pipes were left in place and monitoring continued. Thus, compensation grouting can be restarted if required. However, instrumentation is showing that no further grouting is necessary.


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

Beaver, P. (1972). “A History of Tunnels”. Peter Davies: London. 155 pp.

Burland JB, Standing JR & Jardine FM (2001). Building Response to Tunnelling - Case Studiesfrom Construction of the Jubilee Line Extension, London. Thomas Telford: London.

Deere, DU, Peck, RB, Parker, H, Monsees, JE & Schmidt, B (1970). Design of tunnel supportsystems. Highway Research Record, 339: 26-33.

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

Hoek, E & Guevara, R (1999). Overcoming squeezing in the Yacambu´-Quibor Tunnel, Venezuela.Rock Mechanics Rock Engineering, 42: 389–418.

Hudson, JA & Harrison, JP (1997). “Engineering Rock Mechanics – An Introduction to thePrinciples ”. Elsevier Science: Oxford, 444pp.

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.

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Lecture ReferencesNTNU-Anleggsdrift (1998). “Hard Rock Tunnel Boring: The Boring Process”. Norwegian Universityof Science and Technology (NTNU), Trondheim, Project Report 1F-98.Sandström, G.E. (1963). “The History of Tunnelling”. Barrie and Rockliff: London. 427pp.

Terzagi, K (1946). Rock defects and loads on tunnel support. In Proctor & White (eds.), RockTunneling with Steel Supports, pp. 15-99.

Terzaghi, K & Peck, RB (1948). “Soil mechanics in engineering practice”. Wiley: New York. 566pp.

Thuro, K & Plinninger, RJ (2003). Hard rock tunnel boring, cutting, drilling and blasting: rockparameters for excavatability. In: Proc., 10th ISRM Congress, Johannesburg. SAIMM:Johannesburg, pp. 1227-1234.

UNIT-NTH (1995). “Tunnel: Prognosis for Drill and Blast”. Department of Building andConstruction Engineering, 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.