Innovative development in TBM tunneling – Case Histories
Desarrollo innovador en la construcción de túneles mediante TBM – Casos de historia
Büchi Ernst, Dr. phil. Consulting Geologist – GEO 96, Switzerland
ABSTRACT: This paper presents a brief summary for recent development in different types of tunnel boring machines
(TBM) for hard rock and soft ground or mixed face conditions. Benefits and risks are presented based on actual experience
from different projects. Finally some recent innovative developments – combination of already known concepts – hybrid
TBMs are presented.
RESUMEN: Este artículo presenta un resumen condensado sobre el reciente desarrollo en diferentes tipos de máquinas
perforadoras de túneles (TBM) para roca dura y para suelo blando o para condiciones de frente mixto. Se presentan
beneficios y riesgos basados en la experiencia real de diferentes proyectos. Finalmente algunos desarrollos innovadores
recientes – combinación de conceptos ya conocidos: TBMs híbridas, son presentados.
1. TECHNICAL DEVELOPMENT AND SOME
EXPERIENCE FOR HARD ROCK GRIPPER
TBMs
Larger – stronger – faster! This was definitely the
tendency for the last 30 years. For example: The
world’s largest gripper TBM 14.44 m diameter was
built some 7 years ago. Tunnelling at Niagara Falls
Hydropower project started in 2006 with temporary
rock support by steel ribs, rock bolts, wire mesh
and shotcrete. Major over-breaks at tunnel crown
required more intensive support works than initially
assessed and delayed the progress of the works. The
tunnel was then lined with double layer membranes
and a cast in-situ concrete.
To Conclude: For large diameter tunnels systematic
temporary support is a must primary by safety
aspects, this in addition to the requirements for rock
mass stabilisation. Automation for basic temporary
support work is the consequence for the design of
the TBM and its back up system. But there are
other aspects of gripper TBMs with significant
development in recent time, see the following
Chapters.
1.1 Cutters
A constant development happened with the hard
rock roller cutters. Generally speaking the size of
the cutters increased from 12” some 40 years ago to
15½ ” - 17” and 19” and 20” cutters. The bigger the
diameter of the cutter rings the longer the life time
before a change due to wear. The 19” cutter rings
provide more material for wear. Therefore less
cutter changes will be required for the excavation of
a certain volume of rock. Les cutter chnages results
in less down time on a project. But increasing the
size of the cutter ring allows as well increasing the
size of the hub and so the size of the cutter
bearings. With larger cutter bearings it was feasible
as well to increase the cutter load. The higher the
specific load of a cutter the higher the achievable
penetration rate and therefore faster advance speed
of the TBM. In other words the increased cutters
enable to improve the penetration (mm/revolution)
as well the life time of the cutters. Both finally
result in an improved daily production rate.
When Robbins started with 19” cutters in 1989 the
maximum loading was designed for 312 kN per
cutter ring [1]. This corresponded to a significant
increase of applicable cutter load when compared
with 17” cutters and maximum 220 kN cutter load.
One year later Atlas Copco Jarva introduced the
20” cutter with 350 kN/ cutter ring [2]. There
followed a long period of development of the new
cutter size by different TBM manufacturers until
reaching today’s situation with reliable 19” cutters
and cutter rings with loading capacity of 320 kN. At
the same time the loading capacity for 17” cutters
could be improved to today’s 267 kN/cutter. The
shape and quality (metallurgy and heat treatment)
of cutter rings were developed in parallel with the
increase for the cutter load. More specific designs
were now feasible in optimisation for a specific
project and its relevant rock types.
Latest development intends to monitor each single
cutter positioned on the cutterhead: Rolling yes/no,
rolling speed and cutter temperature should be
checked in a systematic and constant way. In
Austria there already exist project owners requiring
in the Tender Documents the monitoring of the
cutters – being part of technical TBM
specifications. Today different manufacturers
provide different technical solutions. But to my
knowledge no TBM has ever been equipped with
monitoring of all cutter positions. The wireless
transmission of all data from rotating cutterhead to
the operator’s cabin still seems to be the main
bottleneck. Different brands are tested in practise
by various manufacturers but so far without evident
success on a long term base.
1.2 Cutterhead rotation speed
For many years there was the magic figure of 2.5
m/s representing the maximum periphery cutting
speed of the outmost gauge cutter. In other words
the rule of thumb said: A cutterhead should have
the capacity to rotate at max. of revolution per
minute that the outmost gauge cutter would cut with
a maximum speed of 2.5 m/s. For a 10 m diameter
machine this would mean maximum 4.8 rpm: (10 x
3.14 x 4.8 = 150.8 m/min = 2.5 m/s). This was the
guideline for the design of the hard rock TBMs and
was part of many predictor formulas to assess the
TBM performance. In reality the cutterhead rotation
speed has to be adjusted to the actually encountered
rock mass qualities. Typically in block ground the
rpm of the cutterhead has to be reduced so to
minimise the potential peak-loads for the cutters
avoiding an increase in failed cutter bearings.
Today this maximum cutting speed has changed to
more than 3 m/s – a significant improvement by
+20% for the advance rate. Latest experience from
actual projects: Railroad Tunnel Wienerwald (boreØ = 10.7m): 2.7 – 3.2 m/s [3]; AlpTransit Lot
Erstfeld (bore-Ø = 9.56m): 3.2 m/s [4]; Tunnel de
Bure (bore-Ø = 12.53m): 3.28 m/s [5]; Karanjukar
(bore-Ø = 7.63m): 2.7 – 2.84 m/s [6]. Today it
represents the state of the art to have variable speed
drive for the cutterhead. This will allow for better
optimisation the advance rates and daily production
rates related to the actual geological conditions
encountered in the tunnel and its local variations.
1.3 Cutter spacing
This is a very special aspect where the development
is rather minimal through the last years. Already in
the 1980ies hard rock TBMs were built with a large
spacing of 4” = 101 mm. For example the Jarva
TBM MK22, this machine was applied in the T
ARP Project in Chicago boring with very good
success through hard dolomite with UCS up to 160
MPa. 15½“ cutters were in use. In 1990ies Atlas
Copco built a special cutterhead (Ø 5.0m) for a
R&D program to operate in massive granite (Äspö Research for Nuclear Fuel and Waste – Sweden).
This special design allowed for arranging the face
cutters at a spacing of 90 mm, 135 mm or 180mm.
The tests were very successful [7]. Despite these
promising single cases the spacing for face cutters
remained for many years in the order of 80 – 85 m.
Some TBM manufacturers were as low as 65 – 75
mm. Today however the state of the art presents a
cutter spacing of 90 mm for 17” cutters in hard to
extremely hard rock, and 95 – 100 mm for 19”
cutters accordingly. I am convinced that cutter
spacing will significantly increase once the
monitoring of each single cutter comes true in
practise.
1.4 Lifetime of gripper TBMs
Gripper TBMs are relatively easy to rebuild in its
excavation diameter (of course within a certain
limit). Therefore the reuse of a gripper TBM with a
changed diameter may offer a favourable
alternative to a new one. The very sturdy and robust
main body provides the option for an almost
endless rebuild and re-use of a hard rock gripper
TBM. The so called project specific machine design
is then restricted to the cutterhead and mainly to the
additional equipment installed for temporary rock
support measures like drill rigs immediately behind
the cutterhead, steel arch erectors etc. There are
machines which have excavated more than 50 km in many different projects and most different
geological conditions. Recently the news was
spread that a 37 year old gripper machine achieved
a record daily performance of 125 m/day. To
conclude: Gripper machines may live forever and
so may represent a good investment for the
Contractor.
Today’s development for typical hard rock gripper
machines mainly includes further automation for
installing temporary support measures, systematic
probing and installation of high capacity shotcrete
robotics on the back-up system. All these will be
combined with a tunnel conveyor system to allow
for new record performance.
1.5Specific benefit of gripper TBMs
In very good to good rock mass conditions the
progress rates of gripper TBMs can always result in
new world records. In addition - when compared
with the Drill & Blast method - the smooth
excavation process of tunnel boring versus the
blasting allows for a significant reduction of the
amount of temporary rock support measures to be
installed. A rule of thumb says that TBM
excavation improves the required rock support
measures by one class when compared with drill
and blast method.
In very poor ground with squeezing rock it is only
the gripper TBM which allows for very early and
flexible installation of heavy rock support close to
the face – when compared to shield TBMs. The
flexible steel arch support (Th profile) and
systematic bolting as required installed right after
the cutterhead permits the control of plastic
deformations in the tunnel (extended convergence).
Later stiff shotcrete support can be installed when
the deformations have stabilized. Of course there
still exist the risk to get stuck with the cutterhead
and its shield of a gripper TBM. In this worst case
the access however to free the machine is much
better and faster when compared with a SS-TBM or
DS-TBM.
Over-boring – the increase of the boring diameter
– is basically limited to a maximum of radial 4 to 5
cm. Spacers are installed in the saddles of the gauge
cutters to move its cutting position and so to extend
the TBM excavation radius. At the same time the
scrapers have to be replaced – extended
accordingly. It reflects my experience from many
projects that the use of flexible extendable cutters –
whatever the type of technical systems is –
represent a nice dream for design engineers, a must
but a nightmare for TBM manufacturers in case of
any guarantees required and definitely an always
“forget-about-it” for the Contractors.
Rock bursting situation: Here as well the gripper
TBM probably offers today the only option for
machine excavation as an alternative to Drill &
Blast method. The use of the McNally System in
combination with steel arches allows for a
“reasonable” excavation concept even in very
“poppy” ground. This concept has been applied in
an extended version for tunnelling the delivery
tunnel of the Olmos HPP and irrigation Project in
Peru. Very extensive rock bursts - up to 20 heavy
bursts a day have affected tunnelling works with an
open gripper TBM. Safety for labour and machine
were a big issue due to the unpredictability of time
and intensity of the bursts. The delay of the TBM
drive caused a basic analysis about feasible advance
rates before starting with the conventional
excavation method from the opposite side of the
tunnel [15]. The following Table presents the
comparison of achieved advance rates for the TBM
versus advance rates of D&B method.
Table 1: Average daily advance rates achieved related to the
different rock classes with the gripper TBM versus Drill & Blast
method, Olmos Trasandino Tunnel [10]:
Rock Support
Class
Class I
Class II
Class III
Class IV
Class esp. heavy
bursts
TBM advance
(m per day)
20
20
12
5.7
2.4
D&B advance
(m per day)
6.6
5.6
4.6
3.0
1.8
To conclude: The above figures indicate the
significant advantage for the gripper TBM versus
the D&B method. Despite the TBM got stuck
several times and suffered dramatically by heavy
rock bursts the very experienced Contractor
managed to come through and finish the tunnel in a
most impressive way [10].
2. TECHNICAL DEVELOPMENT AND SOME
EXPERIENCE FOR HARD ROCK SHIELD
TBMs
We can divide the shield TBMs in three basic
types:
- Open hard rock shield – single shield or double
shield system: Open tunnel face without active
support for stabilization
- Soft or mixed ground earth pressure balanced
shield TBM (EPB): Closed cutterhead area with
face stabilization by excavated material – may
require additional conditioning of soil / rock
material. Muck handling by screw conveyor,
practical limits: 4 – 6 bar pressure. Option for
pressurised of open mode operation (atmospheric
pressure).
- Soft or mixed ground slurry shield TBM: Closed
cutterhead area with tunnel face stabilization by a
slurry (including bentonite to create a membrane);
muck transfer by the slurry, pumping above ground
to a separation plant and recycling the slurry.
Practical limits 8 – 10 bars.
For many years these basic concepts are well
known. The technical development however went
on one hand to an increase of maximum pressure at
the face: For example: The TBM was designed to
maximum pressure of 15 bar for the Hallandsas
railway tunnel project in Sweden. On the other
hand development went towards larger diameters to
be successfully handled. The world’s largest EPBTBM with 17.48 m just has started in Seattle US.
This machine has a design max, pressure in the
chamber of 10 bar. Excellent performance was
recently achieved in the SPARVO Tunnel in Rome
– Italy with the so far largest EPB TBM with
excavation diameter of 15.62 m. In 19 month 5 km
of highway tunnel (two parallel tubes) have been
finished.
Figure 1: Monthly production rates and net daily advancement per month for SPARVO project in Italy [21].
2.1 Options for double shield TBM tunnelling in
good rock mass conditions
The use of a double shield TBM implies the basic
concept to erect precast concrete segments in the
tail shield and continuously line the tunnel. In good
to very good rock mass conditions such lining with
segments however is a overdoing the requirement
for rock support, which could be but spot bolting or
local shotcrete. In this case the DS-TBM can move
forward without erection of the segmental ring. The
gripper shield stabilizes the TBM during boring
process. Then only the invert segment is installed
for continuation of the rails at the same level.
The following experience of the hydropower
project San Francisco in Ecuador describes in a
typical way the benefit for this concept of flexible
segmental lining. The drive of headrace tunnel of
approx. 7 km was expected in granite, gneiss and
crystalline schist. The rock mass quality mainly was
predicted to be good to very good (approx. 80%
rock class I and II).
The tunnel alignment included the crossing of three
major valleys with large rivers. The local rock
cover corresponded to 25 – 30 m only. For all tree
crossings of these valleys it was decided to apply
full ring segment lining from early beginning. This
resulted in a total length of 600 m. For the
remaining tunnel section only invert segment
should be installed. In case of poor ground a local
change to segmental lining was foreseen.
Figure 2: Headrace Tunnel HPP San Francisco – Ecuador: Selection of the support with a double shield TBM Ø 7.03m:
areas with full ring segmental lining:
, other tunnel section with invert segment only [9]
Figure 3: Daily advance rates for double shield excavation: Days with invert segment lining – black colour, full ring segment
lining – grey colour: Headrace Tunnel HPP San Francisco Ecuador. [9]
Table 2: Average daily advance rates achieved related to the
different rock classes [10]:
Rock Class
Class I
Class I – II
Advance
21 m/day
26 m/day
Class III
13 m/day
Class IV
16 m/day
Rock support installed
invert segment only
local support in tunnel
crown
systematic bolting +
shotcrete
full segment lining
To conclude: Production rates were very successful
in support class I and II. In class III the systematic
installation of rock support measures reduced the
daily advance rates in a significant way. The
average production was below the production rates
achieved with full ring segment lining. After some
time for a learning curve the change from invert
segment lining only to full ring lining could be
realised within less than one shift. In several fault
zones full ring lining was installed, in addition to
the river crossings.
The major delay of the TBM drive by the
geological accident will be briefly discussed in a
following Chapter.
2.2 Options for double shield TBMs in poor rock
mass conditions
The double shield TBM has the possibility to
operate in the so called “single shield mode”. This
would be required in case the rock mass is too poor
for successful gripping. Consequently the thrust
pressure is reacting against the already installed
segmental lining. This corresponds to the same
thrust concept applied with a single shield TBM.
The tunnel Guadarrama – the high speed train
project from Madrid to Segovia in Spain – consists
of two 31 km tunnels in parallel tubes with an
excavation diameter of 9.5 m. The geological
prognosis of the portal zones and an intermediate
depression zone indicated for occurrence of an
extended length with limited rock cover in
weathered rock formations. The 4 TBMs applied –
two from both portals – basically operated as
typical DS-TBMs with reacting grippers to the
tunnel walls. In poor ground areas however these
machines had to thrust against the installed
segments. The decision to work in double or single
shield mode was taken by the production manager
of each machine and was related to actual
experience from the site. The outcome was quite
different for the four TBM drives: Tunnel West:
28%, Tunnel East: 22 % operating in single mode.
The following Table indicates the actually achieved
cycle time for the different operating modes in
different Tunnels (1 and 2 South, 3 and 4 Nord in
parallel) [11].
Table 3: Cycle time for different operating modes: double
shield (DS), single shield (SS) [11]
Cycle time DS
(min)
Cycle time SS
(min)
Time extension
for SS
Tunnel
1
61
Tunnel
2
52
Tunnel
3
57
Tunnel
4
51
70
76
67
56
+ 13%
+ 18%
+ 15%
+ 9%
To conclude: This project indicates that the
production loss when operating in SS-mode instead
of DS–mode results in an increase of cycle time by
approx. +14% or approx. +8 min. It is worth
mentioning that the rock mass conditions for DSmode are much better and consequently net boring
time is longer per cycle when compared with SSmode in poor ground conditions with soft, strongly
weathered or sheared rock. The significant
difference between the two tunnels requiring single
shield mode operation indicates the risks involved
when assessing in advance the expected portions
for SS-mode based on the geological prognosis.
2.3 Risks to get stuck with a DS- and SS-TBM
Today there is a general trend for automation of
tunnelling works. The use of a DS-TBM in
combination with a precast concrete segment lining
provides a safe approach for systematic tunnelling
progress at a fixed cost level per meter of tunnel.
This is a conservative design approach since the
design of the segments has to be adequate even for
the worst case. Consequently the segmental lining
is overdoing most of the time. For the Client the
costs may be higher. But there exists several
benefits like reduced risk for time extension and
additional costs of the project related to unforeseen
geological conditions. Daily discussions about rock
mass conditions and adequate temporary support
and final lining can be avoided. The Contractor can
achieve maximum production rates due to
systematic routine work. In addition safety for
labour and equipment is at a higher level when
compared with an open gripper TBM drive or with
drill and blast excavation.
But the main risks remaining with hard rock shield
tunnelling are:
High ground water inflows (high volumes high
pressure) requiring intensive pre-grouting to reduce
inflow rates. Intensive cleaning work of material
washed into the shield TBM and back-up system.
Sediment deposits along the tunnel invert.
Local poor ground conditions resulting in large
over-breaks at tunnel face and tunnel crown;
collapsing material may block the cutterhead and
fill up the cutterhead chamber with loose blocky
rock material. Or the cutterhead continues to rotate
and loading material while the collapse is ongoing,
producing a “chimney” and finally a sink hole at
surface.
Coincidence of shear zones jamming
cutterhead and or front-shield of the TBM.
the
Squeezing ground: The in-situ stress situation
leads to significant and quick plastic deformations
in the tunnel (convergence) jamming the TBM
shield. Available thrust is too low to move the
machine forward.
Water: Case history for project experience with
heavy water inflows: In an ongoing project for a
delivery tunnel of fresh water to a major city three
shield TBMs are applied to excavate the tunnel.
One TBM started from each portal and the 3rd one
from an intermediate shaft. Two machines are
mining at a downwards and only one at the
preferred upwards grade. TBM one has finished
very successfully its excavation of 9.5 km arriving
at the intermediate shaft 8 month ahead of schedule.
Number 2 TBM - driving downwards from the
intermediate shaft - hit a zone with increasing water
inflows. Tunnelling works had to stop for
installation of additional pump and piping
capacities and for intensive grouting works. More
than 6 months are lost due to these works. TBM 3
excavating upwards hit a fault zone with high
inflows of ground water and a lot of fine rock
material. The by-pass tunnels 1 and 2 had to be
excavated for pre-grouting and consolidation
works. After 7 month the excavation could restart at
slow rates. The poor rock mass within the fault
zone still requires intensive pre-grouting to control
the ground water inflows and stabilize the tunnel
face.
Figure 4:
Initial flooding of the machine with
high water inflows combined with loose
rock material from the fault zone.
water level
Collapsing ground: Graphitic schist is well known
for its poor rock mass stability and its tendency for
ravelling and ongoing collapses. Stabilization of the
tunnel face may result in a major concern in case of
encountering graphite schist in a tectonic
disturbance zone with intensively fractured and
sheared rock material. In most cases the over-break
at tunnel face stabilises itself in some distance
ahead or above the tunnel crown. Otherwise a major
chimney is developing. Therefore the Contractor
usually continues to rotate the cutterhead emptying
the cutter chamber from collapsing material as long
as possible. With this approach he may avoid the
risk to block the cutterhead due to the large amount
of collapsing material. Quite often this concept ends
Figure 5:
Clean water from tunnel face is flooding
the tunnel invert well above the rails,
estimated flow rate 250 l/s. This picture
was taken approx. 3 month after initial
flooding. [12]
up with a success. Then the empty space in front
and above the tunnel can be consolidated by
shotcrete and/or filled up by a silica foam before the
excavation can be continued. Typically positive
results are known for example from high speed
railway tunnel in Austria – Wienerwald Tunnel:
Hard rock shield TBM in collapsing soft sandstone
and marls; or by-pass highway tunnel Flüelen
Switzerland: Hard rock shield TBM in collapsing
ground of typical tectonic disturbance zones in
hard brittle limestone. But there exists negative
results as well where the collapse in the chimney
does not stabilize by its self but continues all the
way up to the surface, see the following Figure.
Figure 6:
Sinkhole at surface in
graphite
schist,
approx. 30 – 40 m
wide crater on top of a
120 m deep vertical
chimney above the
tunnel level. [13]
Coincidence of shear zones: This has been the
case for the geological accident encountered in the
headrace tunnel of the HPP San Francisco Ecuador, see Figure 2 and 3 above. The DS-TBM
got trapped in the area of a coincidence of two fault
zones. Initially the front shield got trapped while
the rear shield as well the cutterhead could be
moved. At the beginning there were no water
inflows, later on water started and reached inflow
rates of approx. 40l/s at a pressure of > 15 bar. All
that happened within a zone of sound mica gneiss at
a rock cover of approx. 700 m. A pilot tunnel above
the machine and crossing the risky zone for a total
length of approx. 40 m was required to free the
TBM and allow for continuation of TBM boring. In
addition – for the worst case scenario – a by-pass
tunnel by drill and blast was started as well in case
of an even longer delay of the main tunnel
excavation [14].
Squeezing ground: Squeezing ground is the
consequence of high in-situ stress – caused by
lithostatic pressure (rock cover x specific density)
and / or by tectonic stresses – in an area with too
low strength of the rock mass. This results in fast
and significant plastic deformation of the rock mass
surrounding the tunnel excavation. Squeezing starts
quite fast and close to the excavation face. This is a
basic difference to swelling rock mass conditions.
Swelling causing plastic deformations is a very
slow process when related to squeezing ground.
There is an increased risk to get trapped in case of
tunnelling with a shield TBM in squeezing ground.
In practise there exist several technical approaches
to reduce this risk to get trapped with a shield
TBM: Increase the overcut, conical shape of the
shields, additional thrust cylinders to push the
shield forward, additional lubrication of the shield
etc. The following examples demonstrate the
situation for two ongoing tunnel drives in squeezing
ground:
Example A: DS-TBM driving a headrace tunnel for
a HPP project; excavation diameter 4.0 m. The
maximum rock cover corresponds to approx. 950
m. After a good start the TBM encountered black
clayey shale inter-bedding thin bedded limestone
instead of the anticipated hard and massive
limestone formation. The rock mass tends to
immediately squeeze within few hours trapping the
shield as soon the inter-bedded shale and limestone
is additionally disturbed by a tectonic shear or
fracture zone. Several times the DS-TBM got
trapped and was consequently freed by bypass
tunnels starting behind the tail-shield. Each
jamming of the shield requires a stop of 2 – 6
weeks to free the machine (including repair work of
the machine). These works usually require
temporary support by wood and shotcrete.
Example B: DS-TBM driving a delivery tunnel for
a HPP project; excavation diameter 9.9 m. The rock
cover corresponds to 350 m with a steady increase
to future maximum of 650 m. The machined got
trapped several times already. Usually this starts by
face instability causing a stoppage of the advance
and then consequently jamming the shield by the
time required to free the cutterhead and for preconsolidation. By-pass tunnels usually start in the
area of the telescopic shield reaching the face area
for further pre-consolidation. In this case stoppages
to free the TBM and do the consolidation works
may last one to several months.
S
S
Figure 7: By-pass tunnel to free the DS-TBMs: left side Ø 4.0 m excavation diameter, right side Ø 9.9 m; S = shield outside
2.4 Benefit of DS-TBM drive versus Drill and Blast
Recent experience was gained in the hydropower
project Palomino in Dominican Republic when
excavating the headrace Tunnel by DS-TBM and
the tailrace tunnel by Drill & Blast method. For
some sections both tunnel excavations were
performed in the same rock formation (Flysch). The
following Table illustrates the average daily
production rates achieved for both excavation
methods:
Table 4: Comparison of daily advance rates achieved by DSTBM and by Drill & Blast method, related to different rock
classes:
DS-TBM
(m/day)
D&B
(m/day)
Rock
class 2
24
Rock
class 3
27
Rock
class 4
23
Rock
class 5
26
5.3
5.4
4.3
1.5
Above values indicate a clear benefit for DS-TBM
versus the D&B method.
3. OPTIONS AND DEVEOLPMENT FOR EPB TBMs
mode operation in the stable bedrock formations.
Problems with stabilisation of the TBM (guiding)
and problems with the installed segmental lining
(ovalisation of segment rings) led to the
consequence to change the basic tunnelling concept
in the bedrock. In this rather soft rock formation the
TBM had to be operated in closed mode as well
with the goal that additional trust forces required in
the closed mode will enable the correct steering of
the machine. In addition the increased thrust forces
provide sufficient stabilisation of the installed
segment ring up to the time of complete backfilling
of the annular gap behind the segments by peagravel.
To conclude: Tunnelling works within the stable
but rather soft bedrock could only be completed
with good success due to the additional option to
operate the TBM in closed mode. Only this option
allowed for proper steering the machine and correct
positioning of the segmental lining as well within
the stable bedrock tunnel sections. The bedrock has
a compressive strength in the order of 1 – 20 MPa.
There was the beneficial fact that no changes were
foreseen from screw conveyor (EPB-drive sections)
to TBM conveyor (stable tunnel sections in
bedrock).
3.1 EPB - TBM operating in closed mode in soft
bedrock formations
3.2 EPB TBM operating in open or closed mode in
hard rock but collapsing rock mass
A typical example for such a situation is the
Highway Tunnel in Biel – Switzerland. On one
hand the two parallel tunnels pass through soft
ground of alluvial, lake and glacial deposits at
portal area and between two consecutive tunnel
sections. On the other hand the tunnels cross
through bedrock of soft sandstone, siltstone and
marl formation. The basic tunnelling concept
corresponded to: Closed mode operation in soft
ground and weathered rock (portal area) and open
New Kaiser Wilhelm railway tunnel in Germany.
Here as well the tunnel could be divided in two
different types of ground: 3.5 km hard quartzitic
schist and quartzitic sandstone, rock cover 20 – 250
m. approx. 500 m the non-disruptive crossing of the
City of Cochem with minimum rock cover in loose
material and/or completely weathered rock and
mixed faces. The Client required for the entire
length of tunnelling works to apply a shield TBM
with screw conveyor. No change was foreseen from
open hard rock concept with TBM conveyor to
screw conveyor in closed mode EPB drive was
foreseen. Within the hard rock section the
geological longitudinal profile indicated a 70 m
section of a “fault zone” to be excavated in closed
mode. All the other hard rock tunnel section was
foreseen to excavate in open mode. At the end
followed the section with loose material and
minimum rock cover under the city Cochem. For
this part closed mode operation was specified [17].
During excavation the hard rock formations turned
out to be less stable than expected. Major rock
Figure 8:
collapsed blocked the cutterhead causing significant
delay time. The Client and the Contractor jointly
agreed – in co-operation with the TBM
manufacturer – to define a critical thrust level for
the machine when the change from open mode to
closed has to be made and vice-versa. The goal was
to anticipate further blockage of the cutterhead due
to instabilities at tunnel face in closely fractured
rock mass conditions. This was critical thrust value
was systematically applied with good success; see
the following figure.
New Kaiser Wilhelm Tunnel: „
“ represents tunnel areas excavated in closed mode operation of the EPB
machine.
: expected fault zone: was not encountered and so excavated in open mode.
A total of 18 tunnel sections within the bedrock
section of the tunnel were excavated in closed mode
operation representing a total length of 164 m. The
70 m of expected fault zone was not encountered
and therefore could be excavated in open mode. A
significant optimisation was feasible as well for the
non-disruptive crossing of the City of Cochem.
Instead of operating one long section of 530 m in
closed mode there were 6 different sections in
closed mode operation, summing up to 301 m total
length. In-between the machine operated in open
mode allowing for regular inspection (at
atmospheric pressure) of the cutterhead operating in
very abrasive ground conditions.
To conclude: The basic decision of the Client to
require 100% of tunnel excavation with an EPB
TBM and screw conveyor (as well in 3.5 km of
hard abrasive bedrock) was very favourable for the
project. With this approach the Contractor was
always “ready” for a change from open to closed
mode and vice versa. The tunnelling mode could
easily be adapted to the actual geological conditions
encountered. The definition and jointly agreement
for the critical thrust level of the cutterhead to
decide about a required change of the operating
mode allowed avoiding many time consuming
meetings and discussions. This concept enabled the
successful excavation of the tunnel including many
unforeseen changes of operating modes. Additional
positions jointly agreed in the bill of quantity
provided a reasonable arrangement for Contractor’s
payment: The successful result of detailed
geotechnical understanding and TBM knowledge of
all participant parties.
4. COMBINATION OF DIFFERENT CONCEPTS
– HYBRID TYPE SHIELD TBMs
4.1. EPB in combination with slurry mucking
Project Port of Miami Tunnel consisted of two
parallel highway tunnels with non-disruptive
crossing of the channel to the cruise port in Miami.
The ground for the tunnel consists of the change
from loose sand – close to surface – to solid
bedrock of different limestone formations,. Typical
mixed face conditions were foreseen for the start
and end section of the 1.2 km tunnels. The bedrock
partly consist of limestone formations with locally
extremely high porosity (up to 51 %), and partly
with inter-bedding of hard and abrasive sandstone.
In addition the limestone contains as well major
cavities of 0.25 – 1.0 m size, locally filled with
loose sand. Sand lenses and sandstone tended for
increased wear rate to the TBM [18].
Figure 9: Highly
porous limestone
S
Figure 10: Geological profile for the project – East Tunnel; rock cover under the channel approx. 10m, max. water pressure
2.2 bar, top level: loose sand. TBM - excavation diameter 12.85m [19]
The general requirement defined a TBM capable to
excavate hard rock and/or loose sand below
groundwater level with pressure of approx. 3 bar,
partly in highly porous and karstified limestone
sequences. The latter required a special Water
Control Process (WCP) resulting in a specific
layout of an EPB TBM.: The muck – a mix of loose
sand and rock chips – was extracted from the
cutterhead chamber by a screw conveyor. Then – in
case of need – a rock crusher, a “slurrifier” and a
slurry-system followed.
About 2/3 of the excavation length were done in
standard EPB closed mode. The passage of the
channel however was performed with the WCP
concept. A construction period of 19 months was
required for both tunnels, including the resetting /
turn of the TBM.
To conclude: The combination of EPB screw
conveyor with potential coupling with rock crusher
and pressurised slurry system enabled the
optimisation of the excavation process and relevant
muck handling with a minimised risk in case of
major water inflows at the face.
4.2 EPB TBM with extended/double screw conveyor
Project Emisor Oriente – TEO in Mexico: This
project includes the application of EPB machines
with increased length of the screw conveyor for
muck extraction form the cutterhead chamber.
There are two screw conveyors with a gate inbetween. Both screws can be operated
independently. One of the basic ideas is that the two
screw conveyors can provide two separate
chambers being separated by a gate. Closing and
opening separately can be done for both chambers.
This would be needed in case of the non-sufficient
pressure reduction through the entire length of both
screw conveyers, for example in the situation of
high ground water inflows at the face or inadequate
conditioning of the encountered rock material.
Figure 11: EPB shield TBM with extended / double screw conveyor and additional gate in-between.
In case of standard EPB drive the pressure release
can be achieved within the first screw conveyor.
Starting the project it was realised that the
synchronisation of both screw conveyors provides
some operational problems and extended learning
curve for the operator. In some areas the second
screw would not be required. Consequently the
option should exist to load the muck to a regular
tunnel conveyor already at this point after the first
screw conveyor. Less operating problems and less
wear on the second crew would allow for further
optimisation of the daily production rates.
To conclude: The concept of a double screw
conveyor – extended length – with an additional
gate in-between reduces the risk for non-sufficient
pressure release within the screw conveyor section.
In case of need the screws can be operated in an
alternating mode to control the required pressure
release. The extended length of the screw conveyor
provides as well the option of pressure release when
starting at a higher pressure at the cutterhead
chamber and / or can reduce the risk in case of
problematic / most demanding conditioning of the
muck. At present stage there seems to be several
technical aspects requiring further optimisation to
allow to take full profit of this concept [20].
4. SUMMARY
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