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 Rock
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, 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
Strength
The 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 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
durations (cyclical – drill then
blast, drill then blast, etc.).
Machine Excavation: Energy is
input in smaller quantities
continuously.

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Drill & Blast
The 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
Scoop
Scale
Bolt
Survey
Drill & Blast – Drilling Rates
0
1
2
3
4
5
0 20 40 60 80 100 120 140
Uniaxial Compressive Strength (MPa)
DrillingRate(m/min)
0
1
2
3
4
5
0 100 200 300 400 500
Specific Energy (kJ/m3)
DrillingRate(m/min)
Thuro&Plinninger(2003)

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Drill & Blast – Drilling Rates
UNIT-NTH(1995)
Blasting Rounds – Burn 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 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 Techniques
During 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.
Hudson&Harrison(1997)
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 – Summary NTNU(1995)

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Machine Excavation in Rock
Tunnel Boring Machine (TBM)
There 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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Partial-face machines
are cheaper, smaller
and much more flexible
in operation.
cut
scoop
muck
out
Full-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
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, 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 Operation
Factors 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 Cutting
In 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.
Hudson&Harrison(1997)

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Mechanics of Rock Cutting
NTNU-Anleggsdrift(1998)
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
new
uneven
wear
normal
wear
heavy
wear
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 Heads
Delays: When the tunnel boring machine is inside the tunnel, the cutters
must be changed from the inside the cutting head.

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TBM Excavation & Design
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 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.
Barla&Pelizza(2000)

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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 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 insertion through
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.g. rock
bolts, meshing, shotcrete).
TBM Operation

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Tunnelling Breakthroughs
TBM Selection & Weak Rock
The 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.
Hoek(2001)
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 - Benches
Benched 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 reinforcement
can 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 - Convergence
A 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”.
Whittaker&Frith(1990)
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 Load
Terzaghi (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
Terzaghi(1946)
Deere et al. (1970)

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Tunnelling in Weak Rock
Terzaghi’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 Rock
Through 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
Belgium 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.
Beaver(1972)

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German 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.
Austrian 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.
Sandström(1963)
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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Austrian methodSandström(1963)
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.”
Terzaghi&Peck(1948)
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 Extension
The 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.
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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To 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.
Instrumentation 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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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 References
Barla, 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 Studies
from Construction of the Jubilee Line Extension, London. Thomas Telford: London.
Deere, DU, Peck, RB, Parker, H, Monsees, JE & Schmidt, B (1970). Design of tunnel support
systems. 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 the
Principles ”. 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 References
NTNU-Anleggsdrift (1998). “Hard Rock Tunnel Boring: The Boring Process”. Norwegian University
of 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.), Rock
Tunneling 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: rock
parameters 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 and
Construction Engineering, Norwegian University of Science and Technology (NTNU), Trondheim,
Project Report 2B-95.
Whittaker, BN & Frith, RC (1990). “Tunnelling: Design, Stability and Construction”. Institution of
Mining and Metallurgy: London, 460pp.

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