Excavation by tunnel boring
machine (TBM)
A TBM is a
complex set of equipment assembled to excavate a tunnel. The TBM
includes the cutterhead, with cutting tools and muck buckets;
systems to supply power, cutterhead rotation, and thrust; a bracing
system for the TBM during mining; equipment for ground support
installation; shielding to protect workers; and a steering system.
Back-up equipment systems provide muck transport, personnel and
material conveyance, ventilation, and utilities.
(1) The
advantages of using a TBM include the following:
-
Higher
advance rates
-
Continuous operations
-
Less
rock damage
-
Less
support requirements
-
Uniform
muck characteristics
-
Greater
worker safety
-
Potential for remote, automated operation.
(2)
Disadvantages of a TMB are the fixed circular geometry, limited
flexibility in response to extremes of geologic conditions, longer
mobilization time, and higher capital costs.
TBM system design and
operation
A TBM is a
system that provides thrust, torque, rotational stability, muck
transport, steering, ventilation, and ground support. In most cases,
these functions can be accomplished continuously during each mining
cycle. The TBM cutterhead is rotated and thrust into the rock
surface, causing the cutting disc tools to penetrate and break the
rock at the tunnel face. Reaction to applied thrust and torque
forces may be developed by anchoring with braces (grippers) extended
to the tunnel wall, friction between the cutterhead/shield and the
tunnel walls, or bracing against support installed behind the TBM.
Basic TBM performance
parameters.
TBM system
performance is evaluated using several parameters that must be
defined clearly and used consistently for comparative applications.
(a)
Shift time. Some contractors will use 24-hr shifting and
maintain equipment as needed �on the fly.� As used here, the shift
time on a project is all working hours, including time set aside
solely for maintenance purposes. All shift time on a project is
therefore either mining time when the TBM operates or downtime when
repairs and maintenance occur. Therefore,
Shift time
= TBM mining time + Downtime
(b)
Penetration rate. When the TBM is operating, a clock on the TBM
will record all operating time. The TBM clock is activated by some
minimum level of propel pressure and/or by a minimum torque and the
start of cutterhead rotation. This operating time is used to
calculate the penetration rate (PR), as a measure of the
cutterhead advance per unit mining time.
Therefore,
PR
=
(distance mined) / (TBM mining time)
PR
is
often calculated as an average hourly value over a specified basis
of time (i.e., instantaneous, hour, shift, day, month, year, or the
entire project), and the basis for calculation should be clearly
defined. When averaged over an hour or a shift, PR values can
be on the order of 2 to 10 m per hour. The PR can also be
calculated on the basis of distance mined per cutterhead revolution
and expressed as an instantaneous penetration or as averaged over
each thrust cylinder cycle or other time period listed above. The
particular case of penetration per cutterhead revolution is useful
for the study of the mechanics of rock cutting and is given the
notation PRev (penetration per revolution). Typical values of PRev
can be 2 to 15 mm per revolution.
(c)
Utilization. The percentage of shift time during which mining
occurs is the Utilization, U.
U
(%) =
[(TBM mining time) / (Shift Time)] � 100,
and is
usually evaluated as an average over a specified time period. It is
particularly important that U is reported together with the
basis for calculation - whole project (including start-up), after
start-up �production� average, or U over some other subset of
the job. On a shift basis, U varies from nearly 100 percent
to zero. When evaluated on a whole project basis, values of 35 to 50
percent are typical. There is no clear evidence that projects using
a reconditioned machine have a lower U than projects
completed with a new machine. Utilization depends more on rock
quality, equipment condition, commitment to maintenance, contractor
capabilities, project conditions (entry/access, alignment curves,
surface space constraints on operations), and human factors
(remoteness, underground temperature, and environment).
(d)
Advance rate (AR). AR is defined on the basis of shift
time as:
AR
=
(Distance mined) / (Shift time)
If U and PR
are expressed on a common time basis, then the AR can be equated
to:
AR
=
(PR (U (%)) / 100
Advance
rate can be varied by changes in either PR (such as
encountering very hard rock or reduced torque capacity when TBM
drive motors fail) or in U changes (such as encountering very poor
rock, unstable invert causing train derailments, or highly abrasive
rock that results in fast cutter wear).
(e)
Cutting rate (CR). CR is defined as the volume of intact rock
excavated per unit TBM mining time. The averaging time unit must be
defined clearly, and typical values of CR range from 20 to 200 m3
per TBM
mining hour.
General considerations for TBM application .
Important project features that
indicate use of TBM include low grades (<3 percent preferable for
tunnel mucking and groundwater management) and driving up hill. A
minimum grade of 0.2 percent is required for gravity drainage water
inflow. Horizontal curves in an alignment can be negotiated by an
open TBM with precision and little delay if curve radii are on the
order of 40 to 80 m. Most shielded TBMs and back-up systems are less
flexible, however, so that a minimum radius of 150 to 400 m should
generally be used for design purposes. Tighter curves should be
avoided or planned in conjunction with a shaft to facilitate
equipment positioning.
(1) Experience indicates that tunnel
depth has little impact on advance rates in civil projects,
providing that contractor has installed adequate mucking capacity
for no-delay operation. Therefore, tunnel depth should be chosen
primarily by location of good rock. Portal access, as opposed to
shafts, will facilitate mucking and material supply, but more
important is that the staging area for either shaft or portal be
adequate for contractor staging. Confined surface space can have a
severe impact on project schedule and costs. For long tunnels,
intermediate access points can be considered for ventilation and
mucking exits. However, assuming the contractor has made appropriate
plans for the project, a lack of intermediate access may not have a
significant impact on project schedule.
(2) In planning a project schedule,
the lead time needed to get a TBM onsite varies from perhaps 9 to 12
months for a new machine from the time of order, to perhaps 3 to 6
months for a refurbished machine, and to nearly no time required for
a direct re-use without significant repairs or maintenance. With
proper maintenance, used TBMs can be applied reliably, and there is
little need to consider specifying new equipment for a particular
project. TBM cutterheads can be redesigned to cut excavated
diameters different by 1 to 2 m, but the thrust and torque systems
should also be modified accordingly.
(3) With delivery of a TBM onsite,
about 3 to 6 weeks will be required for assembly, during which time
a starter tunnel should be completed. The start of mining rarely
occurs with the full back-up system in place. Decreased advance
rates on the order of 50 percent less than for production mining
should be expected for the first 4 to 8 weeks of mining, as the
back-up system is installed and the crew learns the ropes of system
operation.
Sample large-scale modern TBM machines:
Height
of an average person
Mitsubishi Heavy Industries:
15m diameter huge TBM for M30 Highway Road Tunnel in Madid, Spain
(2005). This TBM started to excavate a 3.65 km tunnel in April 2006
and finished in October 2006. Excavating speed reached to average
534 m/month and maximum of 802 m/month. The maximum ring number was
23 rings (46 m)/day. This TBM is equipped with 277,000 kN thrust,
85,700 kNm rotary torque, and Maximum 2.4 rpm rotary speed
electrically controlled by inverter system. Double erectors with
vacuum grip can assemble 10 pieces segments of 15 tons weight/piece
within 30 minutes. Seven backup gantries are provided for cabin,
hydaulic power units, VFD, transformers, foam, mortar, bentonite,
conveyers, segment feeder, segment crane, quick unloader,
ventilation system, fire fighting system, industrial/discharge water
system, reels, and others. Total weight of the TBM and BU comes to
4,200 tons. TBM was manufactures by MHI-duro felguera S.A. (joint
company of MHI, Japan and DF, Spain).
________________________
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