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23.1
FJELLSPRENGNINGSTEKNIKK
BERGMEKANIKK/GEOTEKNIKK 2017
STABILITY PROBLEMS IN DRILL-AND-BLAST TUNNELS
CAUSED BY FAULTED ROCK – COULD TBM TACKLE MOST
OF THESE CASES, OR NOT?
Stabilitetsproblemer i konvensjonelt drevne tunneler i
forkastningssoner - kan TBM-er håndtere disse, eller ikke?
Javier Macias, JMConsulting-Rock Engineering, Norway
SUMMARY
The drill and blast method (D&B) and tunnel boring method (TBM) are widely used with
success as tunnelling methods in a wide range of rock mass quality. A 'faulted rock mass' is
difficult ground conditions for tunnelling in general, and for TBMs in particular, and comprises
from extremely fractured rock mass to completely sheared weak rock. Some of the main
geotechnical problems and instability situations associated to faulted rock as well as the most
common stabilization methods, which apply for D&B and TBM tunnelling, are briefly
discussed in the paper. It is not intended to be a thorough description, but rather a general
overview and state of the art.
The D&B method can adjust more easily to situations caused by faulted rock, solving the
majority of instabilities caused by faulted rock. Technically speaking, the TBM method can
tackle most of the cases of stability problems caused by faulted rock. However, important
considerations need to be taken from initial study stages. Anyway, D&B and TBM methods are
complementary methods and the most appropriate method should be selected by considering
the complete ground conditions for both methods. They have the same purpose, and the use of
"hybrid" solutions must be considered according to the particular characteristics of the project.
SAMMENDRAG:
Konvensjonelt drevne tunneler (D&B) og fullprofilboring (TBM) brukes i stor grad med
suksess som drivemetode i et bredt spekter av bergmassekvalitet. En "forkastningssone" er
vanskelige grunnforhold for tunneldrift generelt, og spesielt for TBM-er, og varierer fra
ekstremt oppsprukket bergmasse til fullstendig skjærdeformert svak bergmasse. Noen av de
viktigste bergtekniske og stabilitetsproblemer som er knyttet til forkastningssoner og de mest
vanlige stabiliseringsmetodene som gjelder for konvensjonell drift og fullprofilboring er kort
beskrevet i artikkelen. Det er ikke ment å være en grundig beskrivelse, men snarere en generell
oversikt og "state of the art".
D&B-metoden kan lett justeres til situasjoner forårsaket av forkastningssoner og løser det store
flertallet av ustabiliteter forårsaket av disse. Teknisk sett kan TBM-metoden håndtere de fleste
tilfellene av stabilitetsproblemer forårsaket av forkastningssoner. Men viktige hensyn må tas i
betraktning fra første planfase. Uansett er D&B- og TBM-metodene komplementære metoder,
og den mest hensiktsmessige metoden bør velges ved å vurdere de samlede grunnforholdene
for begge metoder. De har samme formål, og bruken av "hybrid"-løsninger må vurderes i
henhold til prosjektets spesifikke egenskaper.
23.2
INTRODUCTION
Underground construction industry has been experiencing a strong development where drill and
blast excavation method (D&B) and tunnel boring machine method (TBM) are widely used
with success in a wide range of rock mass quality.
A 'faulted rock mass' is a difficult ground condition, from extremely fractured rock mass to
completely sheared weak rock. Faulted rock is a challenge for underground excavation in
general. The D&B method can adjust more easily to situations caused by faulted rock mainly
due to the high flexibility of the method solving the vast majority of instabilities caused by
faulted rock. However, 'faulted rock' has a significant impact of faulted rock on hard rock TBMs
mainly because the low flexibility and high geological risk of the TBM method. TBMs need to
be properly designed and prepared in advance from initial evaluations, otherwise long
downtime may occur and they should be expected.
Some of the main geotechnical problems and instability situations associated to faulted rock as
well as the most common stabilization methods are discussed in the paper. It is not intended to
be a thorough description but rather a general overview of the 'state of the art'.
The literature shows too many cases trying to "save the day" where, besides delay and extra
cost, health and safety are exposed. It seems that some of these cases may have been solved by
being prepared in advance. Still nowadays we are leading with "unexpected" situations and risk
situations that should not be acceptable.
Technically speaking, the TBM method can tackle most of the cases of stability problems
caused by faulted rock mass. Several considerations need to be considered in terms of sufficient
pre-investigations, appropriate TBM design from preliminary stages, use of continuous ground
investigation during excavation, consider the use of pre-treatment of faults (e.g. pre-grouting…)
and assume and plan time delays when faulted rock is expected.
The D&B and TBM methods are complementary methods and they should not be considered
as competitors. The most appropriate method should be applied by considering the complete
ground conditions for both methods They have the same purpose and the use of "hybrid"
solutions must be considered according to the particular characteristics of the project.
23.3
FAULTS AND FAULTED ROCK
A definition of 'fault' (in geology) is "a planar or gently curved fracture in the rocks of the
Earth’s crust, where compressional or tensional forces cause relative displacement of the rocks
on the opposite sides of the fracture".
Figure 1 shows diverse types of faulting, normal and reverse faulting where the rock masses
slip vertically past each other and in strike slip faulting, where the rocks slip past each other
horizontally.
Figure 1 Types of faulting: In normal and reverse faulting, rock masses slip vertically past each other. In strike-slip faulting,
the rocks slip past each other horizontally. (Encyclopædia Britannica, 2015).
A 'fault' is an unfavourable and undesirable geological structure. The length may be from
centimetres to many hundreds of kilometres with thicknesses from millimetres to metres
(Figure 2).
Rarely, however, does the shifting motion occur along a single, simple plane. As the rocks grind
past each other, they get crushed and broken, and movement can happen anywhere within that
volume of disturbed rocks. Due to this, it is often referred to the crushed volume of rock as a
fault zone.
Many faults (and fault zones) are not vertical. They make an angle with a horizontal plane (or
the ground surface). That angle is known as the dip of the fault. The line of intersection between
the fault and that horizontal plane is called the strike of the fault. The adjacent ground may be
disturbed and weakened by associated structures: drag folds and/or secondary faulting.
Figure 2 shows a conceptual diagram of a fault zone across a fault. The diagram shows the main
components of a fault zone (i.e. fault core, damage zone) and structural elements and features
in each component.
23.4
Figure 2 Conceptual diagram of a fault zone across a fault (modified from Choi et al., 2015). The diagram shows the main
components of a fault zone (i.e. fault core, damage zone) and structural elements and features in each component.
The 'fault core' is the structural, lithologic and morphologic part of the fault zone where most
of the deformation is accommodated. It consists of low permeability fault rocks, characterised
by a high amount of fine grained rock matrix.
The 'damage zone' comprises the bounds the fault core and is characterised by subsidiary
structures such as small faults, veins, fractures and fold, causing heterogeneity and anisotropy
in fault zone permeability. The damage zone will be greatly responsible for the permeability of
the fault zone.
Fault zones might not further develop, resulting in an apparent lack of a fault core. In rock
engineering, fault zones with these damage zone only or highly fractured characteristics have
however to be considered in the same way as fully developed fault zones.
A 'faulted rock mass' is a difficult ground condition, from extremely fractured rock mass to
completely sheared weak rock. Figure 3 shows a photography of fault on surface and during
tunnel construction.
Figure 3 Examples of faults. Left: Photograph of fault on surface (Lofoten Islands, Norway) and right: Jagdberg Tunnel in
Thuringia, Germany (www.tunnel.com, 2010).
23.5
GENERAL CONSIDERATIONS ON THE SELECTION OF THE
EXCAVATION METHOD: D&B VERSUS TBM
Underground construction industry has been experiencing a strong development with enormous
technological improvement. In rock tunnelling, D&B and TBM methods are widely used with
success in a variety of projects and rock mass conditions.
Following, the main general considerations in the selection of the excavation method in rock
tunnelling are listed (Macias and Bruland, 2014):
Hard rock tunnelling methods
Widely used with success
Complex choice
Rarely clear at early project stages
Extensive analysis of the parameters necessary
Every project is unique
Comprehensive and detailed study
Wrong choice may result in severe situations
Stability problems in D&B tunnelling caused by faulted rocks
The selection of the excavation method is not a simple issue. It may result in undesirable
situations as experience has shown. The choice is more complex than a simple economic issue
and rarely clear from the initial stages of the projects. It is necessary to have an entire overview
of the parameters involved in the excavation method choice.
The excavation methods are not mutually exclusive. Hybrid solutions should be considered
taking advantage of them whenever circumstances allow (Barton, 2013).
Many parameters are involved with different role in every project case; project characteristics
and purpose, environmental aspects or even social issues are involved. Every project is unique
and a comprehensive and detailed study should be carried out.
The main parameters involved on the excavation method choice may be grouped (Macias and
Bruland, 2014). Following, the grouped parameters are listed:
o Project design considerations
o Final Purpose considerations
o Start-up time
o Health, Safety and Working Environment
o Advance Rate
o Flexibility
o Risk
o Ground stability
o Operation and construction crew
o Costs
o Overbreak and tunnel profile quality
o Environmental disturbance
o Temporally access and implantation layout
o Contractual considerations in the choice of the excavation method
23.6
STABILITY PROBLEMS IN TUNNELLING CAUSED BY FAULTED
ROCK
Following, some of the main geotechnical problems associated to faulted rock in rock tunnelling
are listed:
Instability of the excavation face area
Excessive overbreak
Excessive deformation (squeezing ground)
Frequent changes of stresses and displacements
Large water inflows
The main instability situations associated to faulted rock may listed as following:
Slabbing
Collapses
- Fractured rock mass
- Highly fractured/crushed rock mass
- Cataclastic material
Swelling
Squeezing
Flow ground
Figure 4 illustrates the most frequent instability problems in tunnelling caused by faulted rock.
Instability
Slabbing
Fractured rock
mass/single joint
Highly
fractured/Crushed
rock mass
Cataclastic material
Swellin
g
Squeezing
Figure 4 Illustration of the most frequent instability problems caused by faulted rock (modified from Lunardi, 2008).
23.7
A 'cataclastic' rock is a type of metamorphic rock that has been wholly or partly formed by the
progressive fracturing and comminution of existing rock, a process known as 'cataclasis', and
is mainly found associated with fault zones.
Squeezing can be caused by overstress conditions in weak rock formations and highly jointed
rock masses.
Figure 5 includes several pictures illustrating some of the main geotechnical problems
associated to faulted rock in tunnelling.
Figure 5 Pictures illustrating some of the main geotechnical problems associated to faulted rock in tunnelling. a) Fractured
rock mass and overbreak in drill and blast tunnelling, b) Collapse area due to two parallel filled joints at Hanekleiv tunnel,
Norway (Nilsen, 2011), c) tunnel collapse due to a highly fracture zone and d) tunnel collapse with flowing material (Trinh,
2016).
COMMON STABILIZATION METHODS IN D&B AND TBM
TUNNELLING
The stabilization methods in rock tunnelling can be grouped into "ground improvement ahead
the tunnel face" and "rock support" according to their moment of application during tunnelling:
prior or post-excavation.
The most common stabilization methods applied in rock tunnelling, included in the two main
groups, are following listed.
23.8
Ground improvement ahead the tunnel face (pre-excavation):
o Spiling bolts
o Pipe umbrella support system
o Face bolting
o Pre-excavation rock mass grouting
o Jet grouting
o Ground freezing
Rock support (post-excavation):
o Bolting, rock "straps" and sprayed concrete
o Reinforced sprayed concrete (ribs and others)
o McNally TBM support system
o In situ concreting
o Segmental lining
The ground improvements ahead the tunnel face in TBMs are similar to the applied to D&B
tunnelling previously discussed but considering the difficulties of implementation due to the
lower flexibility of the TBM method. In addition, when excavating tunnels in faulted rock, it
must be considered short blast rounds and even may be necessary the division of the cross
section.
Shield machines have general advantages in faulted rock. Double shield TBMs have the
possibility to push off the last ring of precast elements, in conditions where gripper thrust is lost
in faulted, clay-bearing or over-breaking rock.
However, the risk to become 'trapped' is higher with blocky rock and squeezing conditions due
to the distance from the tunnel face where the rock support is applied. This may special
significant with Double Shield TBMs and faulted rock with short stand-up time.
Following, the stabilization methods are briefly described. Only main characteristics and
principles are included.
Spiling bolts
Spiling bolting is considered as temporary pre-support and it is usually applied with fractured
rock. It has the main purpose is to maintain the theoretical cross section until the installation of
the permanent rock support.
The material used for the bolts is usually of ordinary reinforced steel quality and they are
installed in grout with 25 – 32 mm diameters frequently used. The bolt lengths are usually 6
metres with a typical bolt spacing 0.2 – 0.6 m and recommended angle to the tunnel axis of 10-
15 (NFF, 2010). The distance between the bolt rows or overlap is around 0.5 m.
Figure 6 illustrates support of poor rock masses by spiling and it illustrates an example of the
use of spilling combined with sprayed concrete ribs and concrete invert.
23.9
Figure 6 Illustration of support of poor rock masses by spiling. It shows an example of the use of spilling combined with
sprayed concrete ribs and concrete invert (NGI, 2015).
Pipe umbrella support system
Pipe Umbrella System (also referred to as Umbrella Arch System) is considered as a temporary
pre-support providing rock support ahead of the tunnel face. The main purpose of the Pipe
Umbrella system is to support the overburden from collapsing into the tunnel before the
permanent rock support is applied.
The method consists on the installation of a "screen" of steel pipes ahead of the tunnel face
covering the entire or part of the roof of the tunnel profile. It is a stiffer reinforcement than
spiling bolting and it applies in extremely poor geological conditions (Strømsvik et al., 2016).
Steel pipes have normally a diameter of 75-200 mm with thickness 5-25 mm. The typical length
is 10-30 m, pipe angle 5-8, spacing 30-50 cm and minimum overlap between rows 3 m. (NFF,
2010, Oke et al., 2014). Figure 7 illustrates the design parameters.
Figure 7 Illustration of the design parameters of Pipe umbrella or Umbrella Arch support system (Oke et al, 2014).
23.10
The steel pipes are grouted by pumping grout through the pipe, which also fills the openings
between the pipe and the rock, as well as penetrating into the rock mass along the pipe (NFF,
2010).
In addition, it is possible to inject the pipes in sections at different distances from the tunnel
face, by using valve tubes or perforated pipes, providing a more controlled penetration of the
injection along each pipe.
Face bolting
The purpose is to strength the core-face using special fibre glass reinforcement. A series of
holes are drilled into the face, sub parallel to the axis of the tunnel, evenly distributed over the
face and normally longer than the diameter of the tunnel. Fibre glass and cement mortar are
normally used.
Pre-excavation rock mass grouting
Rock mass grouting by pre-grouting may have different applications: groundwater control
and/or improve stability contributing to increase stand-up time.
Pre-excavation grouting to reduce rock mass permeability has become important on
underground excavation and it is normally a standard procedure in Norwegian tunnelling (NFF,
2011).
Figure 8 shows an illustration of the principle of pre-excavation grouting. Typical drilling
injection holes for grout cover lengths between 18 and 24 m with and overlap between grout
rounds 6-10 m (NFF, 2011). The jointing characteristics of the rock are of critical importance
in the drilling plan. For pre-grouting, cement-based grouts are the most common.
Figure 8 Illustration of the principle of pre-excavation grouting (NFF, 2011).
Jet grouting
Jet-grouting is applied in loose material that can be fragmented and washed out (e.g. clay, silt
and sand fractions). The purpose is to improve the mechanical properties of the ground.
A special steel pipe, equipped with radial nozzles, is inserted by drilling into the actual rock
mass and to a certain length. Cement mortar at high pressure (approx. 400 bars) is flushed
through the nozzles while the drilling pipe is rotated simultaneously pulled back.
23.11
The loose material is exposed to stress due to the high-pressure mortar and the loose material
become fragmented. A mixture of fragmented loose material and cement mortar is obtained.
The columns are typically from 200 to 1200 mm diameter.
Figure 9 shows examples of jet grouting consolidation applied in tunnelling.
Figure 9 Photos of jet grouting consolidation applied in tunnelling (www.pietrolunardi.it).
Artificial ground freezing
Artificial ground freezing is to be applied in loose material and it is a temporally stabilization
until the permanent rock support is applied.
Several different approaches are applied regarding to the diverse tunnelling applications. A
commonly used approach involves horizontally drilling of the freeze pipes around the tunnel
perimeter, freeze from near tunnels or access specifically built and freeze the entire alignment
ground. It needs to be planned according to the tunnel drive schedule and the frozen properties
of the ground. The freeze of the ground typically takes several weeks.
In TBM tunnelling, ground freezing has been applied with success in particular and challenge
cases as reported by Barla and Pelizza (2000) or Leung et al. (2012).
Bolting, rock "straps" and sprayed concrete
Rock bolting is the most widely used means of rock support for underground excavations (Li,
2017). In addition, rock "straps", wire mesh and sprayed concrete are a common stabilization
method which applies for a wide range of rock masses quality from high rock mass quality to
highly fractured rock mass.
Figure 10 shows rock bolts and rock "straps" in a moderately fractured rock mass.
23.12
Figure 10 Rock bolts and rock "straps" in a moderately fractured rock mass (Andersson, 2016).
Various rock mass classification systems can be used as a guideline for rock support design (i.e.
Q-system, RMR system). The Q value and the equivalent dimension define the rock support to
be applied post-excavation (Figure 11). The Q values are plotted along the horizontal axis and
the equivalent dimension along the vertical axis.
Figure 11 Rock support chart and permanent rock support recommendations based on the Q-system. Shallow area indicates
the rock mass quality area covered by faulted rock (Barton et al., 1974; NGI, 2015).
23.13
It is considered likely that geological mapping in bored tunnels may result in better assessments
of rock quality properties than in drill-and-blasted tunnels (Macias, 2016b). Barton (2000)
discussed findings from the Svartisen tunnel (Løset, 1992), where Q-values mapped in the TBM
tunnel section was 1.5 to 3.0 times higher than those from the drill-and-blasted section for Q-
values in the middle range (from 4 to 30).
In TBMs must be considered during the machine design stages the mounting rock bolting drills
and spayed concrete robot close behind the cutterhead. In good rock mass conditions is expected
that normal rock support (i.e. bolting and sprayed concrete) may be installed while boring. In
smaller TBM diameters, due to limited space might be more difficult.
McNally TBM support system
The 'McNally support system' was introduced and patented by McNally and McNally (2002)
and it is a system for progressively placing the roof structure in place as the tunnel is being
bored with a TBM. This system is applied in open or main beam TBMs in situations where the
rock on the roof is somewhat unstable.
The McNally TBM support system has been reported by Brox (2013) as adequate to address
severe overstressing, such as rock bursting (Figure 12).
Figure 12 McNally TBM support system (Tunneltalk, 2012).
Reinforced sprayed concrete (ribs and others)
Steel ribs/arches combined with sprayed concrete normally comprises one or more layers of
sprayed concrete. The steel ribs/arches will be placed with specific spacing. It is considered
Sprayed concrete ribs with lattice girders is a rock support type that consists of rolled rebar
girders (most simple triangular) that are prefabricated according to the theoretical tunnel
contour. It applies for a wide range of rock quality conditions including poor and very poor
rock masses. Figure 13 shows the principle of construction of reinforced ribs of sprayed
concrete.
23.14
Figure 13 Principle of construction of reinforced ribs of sprayed concrete (NGI, 2015).
When this rock support is going to be expected during tunnel boring, a mechanical ring-arm
erector for the installation of steel ribs support must be considered in addition to the sprayed
concrete robot.
In situ concreting and segmental lining
Concrete lining is applied while excavating through weakness zones with heavy rockfall,
massive swelling clay zones, crushed zones with substantial water problems and in the portal
areas. Reinforcement, anchorage and possible concrete invert must be assessed case to case.
Diverse kinds of shield or formworks are used to apply in-situ concreting according to the
characteristics of the specific case.
Figure 14 Example of segmental lining (Hurt, 2016).
23.15
Segmental lining or circular gasketed segments (the elements are made of reinforced pre-cast
concrete), offer an economical and efficient method of constructing tunnel linings, especially
in soils and weak rocks (BTS and ICE, 2000). The method avoids placing personnel at
unsupported tunnel face. The use of steel fibres can give the base concrete greater ductility and
significantly improve post-cracking behaviour (DAUB, 2013).
Segmental lining is commonly and well suited for TBM tunnelling. Rings are positioned in
relative short time and with precision. Figure 14 shows an example of segmental lining. The
figure includes the different structural parts.
Built in TBM stabilization systems
In addition to the 'traditional' TBM stabilization methods, 'built' system for ground stabilization
during tunnel boring have been recently introduced and applied.
Multi-speed cutterhead drives
Shield design for continuous advance
Convergence measuring system
Cutterhead inspection camera
Improve ground detection (e.g. probe drilling, seismic…)
Improvement in ground treatment ahead
The design with multi-speed cutterhead drives enables the machine to have the ability to work
with high torque at low rpm while keeping normal conditions for hard rock boring. The machine
can keep boring in collapse face event or fault zones where a high potential for jamming exists.
The use of shortest possible shield length with steeped shields if necessary (especially with
double shield machines) improves the capability for TBM advance. Each successive shield
having a slightly smaller diameter than the last. In addition, radial ports in the shield can be
used for application of bentonite to provide lubrication between the shield and the ground
(Robbins, 2016).
Figure 15 Bentonite shield lubrication (Robbins, 2016; Home, 2016a).
23.16
In squeezing or blocky ground, the convergence measuring system uses a hydraulic cylinder
mounted on top of the shield and connected to the TBM control system. It measures the gap in
the tunnel crown detecting squeezing or collapsing ground.
The cutterhead inspection camera can be used to remotely inspect the boring cavity without
intervention and to check water levels ahead of the TBM.
Home (2016b) reported the improvement in grout treatment by an adapted canopy tube and
positioner at Kargi Kizilirmark Project, Turkey (Figure 16).
Figure 16 Adapted canopy tube and positioner for grout treatment ahead (Home, 2016b).
Other important improvement is the possibility to have more drill ports including 360-degree
radius in shield machines for probe and pre-excavation treatment.
Recently, a "new" type of TBMs are available named multi-mode, convertible or hybrid
machines which include the possibility to adapt the "excavation mode" inside of the tunnel
(Bäppler. 2016). Figure 17 shows the main types of multi-mode, convertible or hybrid
machines.
Figure 17 Types of multi-mode, convertible or hybrid machines according to Hourtovenko (2015).
23.17
TBMS IN FAULTED ROCKS
General
Nowadays, diverse types of TBMs are available in the tunnelling market. Different
classifications can be found in the relevant literature (ITA_ITAES, 2000; EFNARC, 2005,
Maidl et al., 2012; Rostami, 2016). Figure 18 shows a general classification of TBMs regarding
to the ground conditions according to Rostami (2016).
Figure 18 General classification of TBMs regarding to the ground conditions (Rostami, 2016).
Faulted rock has a significant impact of faulted rock on hard rock TBMs mainly because the
low flexibility of the TBM method, it is very sensitive to changes in geology and its high cost.
In addition to the previously discussed stability problems in tunnelling, it exists some additional
geotechnical problems for hard rock TBMs when boring in faulted rock:
Cutterhead jamming
Large water inflow/chimney formation
Removal of over-break
Cutter and cutterhead damages
Figure 19 shows a photography taken during tunnel boring through faulted rock. Large water
inflow, probably resulting on erosion of faulted rock, results in problems for removing over-
break and most probably chimney formation.
23.18
Figure 19 Example of water inflow/erosion problem for hard rock TBMs in faulted rock (Andersson, 2016).
Figure 20 is a hypothetical comparison of D&B and TBM tunnelling rates (Barton, 2000).
Average rates in m/week (solid curves), in m/month and m/year (dotted lines) are shown against
the rock mass quality based on the Q-system.
Figure 20 Hypothetical comparison of D&B and TBM tunnelling rates in m/week (solid curves). Stippled curves represent
possible monthly and yearly rates. Shallow area indicates the rock mass quality area covered by faulted rock (modified from
Barton, 2000).
The figure emphasises a more 'marked' deceleration of TBM performances for monthly and
yearly advance rates and the sensitivity of the TBM performances to rock mass conditions
resulting in a high geological risk
23.19
Project cases
Several researches have analysed geotechnical problems and major damages during boring in
faulted rock (Grandori et al., 1995; Barla and Pelizza, 2000; Barton, 2000, 2012; 2015;
Swannell et al., 2016; Clark, 2016; Home, 2016). Some project examples are:
- Pont Ventoux (Italy) - 20m in 7 months (Barla and Pelizza, 2000)
- Evinos-Mornos (Greece) – 50 days standstill (Grandori et al., 1995)
- San Pedro Tunnel (Spain) – Double track 9.5 km – 2 open TBM: after approx. 400
m one trapped – D&B to finish the excavation of the tunnels– The 2nd TBM with
difficulties and injection from surface (www.diariodeleon.es , 2006)
- Chile mine tunnel – Due to over-break, 2.5-5 m/day for 1 month (Barton, 2015)
A recent example is the Kargi hydroelectric tunnel in central Turkey (Clark, 2015; Home,
2016a, b). A 9.84 m. diameter double shield TBM was selected to excavate the total length of
11.8 km including 2.5 km with segmental lining. The remainder of the tunnel was estimated to
be supported by shotcrete, rock bolts and wire mesh. The expected geology along the tunnel
alignment consisted of Kırazbası complex Kargı ophiolites (including sandstone, siltstone and
marl) for the initial 2,300 meters, followed by 1000 meters of Kundaz metamorphites (including
marble, metalava and metapelite), and the remaining 8,500 meters consisted of Beynamaz
Volcanites (including basalt, agglomerates and andesite). The anticipated strength of the rock
was up to 140 MPa. Multiple fault zones and transition zones added to the complexity of the
geological conditions (Clark, 2015).
Almost immediately after start, the machine encountered substantially more problematic than
described in the preliminary geological reports. It was blocky rock, sand, clays and water
bearing zones. After 80 metres boring the TBM became stuck/trapped in a section of collapsed
ground with more than 10 m above the crown. In addition, continuous problems with crushed
rock in the telescopic shield were encountered. The solution to free the cutterhead and stabilize
the disturbed ground was to design a bypass tunnel and work procedures. The designed bypass
tunnel was crossing the top of the TBM cutterhead. The "hand-made" excavation by hand held
hydraulic hammer with limited section took a minimum of 14 days for each bypass (Home,
2016). A total of 7 bypasses were needed within the first 2 km (Home, 2016).
Figure 21 shows the monthly advance rates along the TBM sections and the total average by
D&B. The approximate average of the first two kilometres were 144 m/month (4.8 m/day) and
the average of the second part after the TBM modifications and other geology: approx. 407
m/month (13.5 m/day). The total average along the tunnel was 271 m/month (9 m/day), (Clark,
2015; Home, 2016a, b). However, Drill & blast was completing the tunnel with an average
advance rate of 173.8 m/month (5.8 m/day).
An important consideration from these performances, is the lower advance rate of the TBM for
the first two kilometres where "unexpected" difficult ground conditions were encountered
compared to the D&B section from the other adit. An interesting question would be whether,
with a proper pre-investigation and considering the advance rate performed by D&B, the use
of this method for the first two kilometres instead of TBM would have been beneficial for the
project.
23.20
Figure 21 Advance rate (m/month) along the tunnel sections excavated by TBM. Average of monthly advance rate with D&B
is included (Macias, 2016b).
In general, the literature shows too many cases trying to "save the day" where, besides delay
and extra cost, health and safety are exposed. Some of these cases may have been solved by
being prepared in advance on the basis of proper pre-investigation, machine design and
continuous ground investigation during excavation. Still nowadays we are leading with
"unexpected" situations and risk situations that should not be accepted.
CONCLUSIVE REMARKS
General
'Faulted rock mass' is a challenge for tunnelling in general. The D&B method can adjust more
easily to situations caused by faulted rock mainly due to the high flexibility of the method. In
general, the D&B method solves difficult conditions caused by faulted rock.
However, TBMs need to be prepared in advance from the design stage otherwise long downtime
may occur and they should be expected.
Could TBM tackle most of these cases, or not?
Technically speaking, the TBM method can tackle most of the cases of stability problems
caused by faulted rock, but it is of great importance to consider:
- Sufficient pre-investigations are demanded
- An appropriate TBM design from preliminary stages is needed
- Continuous ground investigation during excavation should be considered (e.g.
probe-drilling, geophysics…)
- Pre-treatment of faults should be considered (e.g. pre-grouting…)
- Assume and plan time delays when faulted rock is expected
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In addition, several considerations should be assumed:
- Higher investments due to machine type, additional equipment and more pre-
investigation needed
- Do not rely on assumptions regarding to geology. Enough pre-investigation is
demanded in order to reduce geological risk
- Higher operational cost
- Delays and pre-treatment resulting in extra cost
Final considerations
The most appropriate method should be applied by considering the complete ground conditions
for both methods. It is the result of the complete tunnel that should count.
The D&B and TBM methods are complementary methods and they should not be considered
as competitors. They have the same purpose and the use of "hybrid" solutions should be taken
from initial study stages.
ACKNOWLEDGMENTS
I would like to thank to my PhD evaluation committee Dr. Nick Barton, Dr. Heiko Käsling and
Dr. Rolf André Bohne for the suggestion of the subject "Stability problems in drill-and-blast
tunnels caused by faulted rock – Could TBM tackle most of these cases, or not?" for the public
trial lecture of my PhD defence (08.12.2016) that finally resulted on this paper.
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