Gotthard base tunnel rock burst phenomena in a fault zone ...

World Tunnel Congress 2008 - Underground Facilities for Better Environment and Safety - India
Gotthard base tunnel rock burst phenomena in a fault zone, measuring and
modelling results
Heinz Hagedorn & Rolf Stadelmann
Amberg Engineering AG, Switzerland
Stephan Husen
Swiss Seismological Service (SED) ETH Zurich, Switzerland
SYNOPSIS: During construction of the multifunction section Faido consisting of up to 5 parallel tunnels an
unknown fault zone has been encountered. Large deformations occurred during excavation especially in a
tunnel crossing the fault kernel. The excavation of the tunnel system has been given rise to a large area of
stress redistributions, causing stress concentrations in the hard rock adjacent to the fault. During 2005 rock
bursts started to occur at the excavation faces. The intensity of the rock burst increased with ongoing
construction. The tunnel’s owner ATG (AlpTransit Gotthard Base Tunnel AG) decided to monitor the
seismic activity by a dense network of seismic stations. The monitoring campaign including result
interpretation has been realized by the SED (Swiss Seismological Service, Swiss Federal Institute of
Technology). The seismic measuring results enabled to determine the micro tremor’s sources and to derive
the dynamic load to be applied on numerical models. The numerical investigations served to assess the
additional loading of the tunnel’s final concrete lining and the possible risk arising due to a micro seismic
event during operation of the tunnel.
Amberg Engineering AG is member of the Engineering JV Gotthard Base Tunnel South, GBTS, consisting
of: PÖYRY, Lombardi AG and Amberg Engineering AG.
1. INTRODUCTION
1.1 Overview
The longest tunnel of the NEAT (New Alpine
Transverse) through the Swiss Alps is the Gotthard
Base Tunnel (GBT) with a length of 57 km. The
tunnel consists of two parallel single track tubes
each with a diameter of 9.2 m. The maximum
overburden is 2350 meters. Three intermediate
points of attack divide the tunnel into five sections
of approximately equal length. These additional
attacks are necessary to attain a reasonable
construction time and for ventilation purposes.
Detailed information of the tunnel can be found
under http://www.alptransit.ch/pages/e/.
The intermediate points of attack at Sedrun and
Faido shall serve as Multi Function Sections (MFS)
during operation of the tunnel. These MFS enables
the trains to change the tunnel tube in case of
maintenance works and the specially ventilated
419
emergency sections serve the rescue of people in
emergency cases (Figure 1). The MFS Faido had to
be accessed by a 2.7 km long access tunnel
declining at 12.7% from the portal. The overburden
in the MFS Faido is approximately 1500 meters.
The excavation was done by Drill and Blast
1.2 Geological Prediction for the MFS FAIDO
The outcrops from quarries in the area of the MFS
Faido, the experiences made during construction of
the investigation system for the Triassic Piora basin
as well as various vertical exploration drillings
confirmed a favorable geological section for the
designed location the MFS consisting of Leventina
Gneisses with very good quality (Figure 2).

Cross cavern
East Tube EON
West Tube EWN
Figure 1 Schematic original layout of the multifunction section at Faido.
0 km
57 km
420
MFS Faido
Figure 2. Geology along the Gotthard Base Tunnel consisting mainly of gneiss and granite.
MFS Faido is located in the Leventina Gneiss of very good quality
1.3 Actual Geology in the area of the MFS Faido
During construction of the cross cavern (Figure 1) a
break down of fine grained quartz occurred in the
cavern’s roof forming a cavity of 8 meters in height.
The results of an extensive drill holes campaign
together with seismic reflection measurements
effectuated during construction revealed an
unknown large fault system in the area of the MFS
Faido. The main kernel of this fault strikes at an
average angle of about 15° to the tunnel axis and
dips at about 80°. The fault system encountered
during construction is shown in Figure 3. In the
fault’s kernel, layers of partially completely
decomposed rock (kakitrite) are embedded.
Adjacent to the east of the fault hard and brittle
Leventina gneiss is located. To the west of the fault
the rock mass consists of hard but less brittle
Lucomagno gneiss.
2. DIFFICULTIES DURING CONSTRUCTION
During the first rebuilding of the EWN tube to the
north of the cross cavern (Figure 1) a support
consisting of steel arches HEM 200 backfilled with
40 cm concrete was installed immediately after each
excavation step of 1 m. This rigid support was
intended to cater for the heavy pressure and
especially to protect workforce from break outs at
the face. In the rebuilt section, on a length of 250 m
(Figure 3) the loading of the support however gave
rise to radial displacements of up to 1 m. Strain

North
EON tube
EWN tube
Area of increased
rock bursts
800 m
Cross section for
2D modeling
421
Fault’s kernel
Cross Cavern
Access tunnel
East
West
Rescue Station
South
Figure 3. Actual geology recognized by January 2003. Fault’s kernel with intensively jointed rock to both sides of the
kernel. As a consequence the crossover has been shifted to the south (Figure 1).
measurements revealed yielding of the steel arches
already Figure 4 days after backfilling[1]. Severe
damage of the support developed (Figure 4) and the
critical section had again to be rebuilt with an
enlarged excavation radius of 1.5 m to allow for
additional displacements. A flexible support has
been installed using TH profiles with sliding
connections. Shotcrete slots were left open for
unhindered sliding of the arches and for avoiding
damage of the shorcrete lining due to further
movements (Figure 5).
Figure 4. Sheared off steel arch in the invert
3. NUMERICAL SIMULATION OF THE
STATIC LOAD CASE
In order to better understand the failure mechanisms
in the area around the tunnels of the MFS numerical
modeling was carried out. The model section is
Figure 5. Rebuilding of the critical section with enlarged
profile and slots in the concrete lining
indicated in Figure 3. From the known geology it
was obvious that a discontinuum model with
deformable blocks had to be used in order to
recognize the influence of the discontinuities and
the kakirite zones on the stability of the tunnel
section. For this purpose the program UDEC,
developed by ITASCA was used. The investigation
was based on a parametric study comprising the
variation of rock and joint properties. The model is
shown in Figure 6. The different fault regions in the
model have been selected according to the
geologist’s findings based on interpretation of
numerous borehole results. From overburden, rock
mass density and gravity a primary principal stress
level of σyy=42 MPa and σxx = 21 MPa was
defined at the top of the model. In Figure 6 the
vertical stresses yy are represented. The results

clearly show a considerable extension of the stress
redistribution due to the excavation of the tunnels.
In the area of the middle tunnel EWN both the σxx
and σyy stresses decreased to approximately 10
MPa. A general stress reduction occurred around all
of the three tunnels. To the east of the fault in the
hard brittle gneiss a stress concentration is
extending beneath the tunnel along the fault’s
eastern boundary The stress concentration at tunnel
level amounts to yy = 60 to 80 MPa (Figure 6). The
area of this stress is in accordance with the
hypocenters of the micro tremors (Figure 8).
Kernel Intensively
jointed
Hard rock, brittle
Stress
concentration
West East
Y Medium hard
rock
X
Figure 6. Static load case: Distribution of the
vertical stresses σyy
4. MICRO TREMORS
4.1 Development of the seismic activity
In this paper, we define micro tremors as a seismic
event generally occurring at a larger distance from
the tunnel (few hundred meters) whereas a rock
burst occurs in the direct vicinity of the tunnel.
During 2004 rock bursts took place mainly at the
face of the EON (Figure 3) to the north of the cross
cavern (Figure 1). Most of these bursts (ca. 75%)
occurred approximately Figure 3 hours after
blasting. During 2005 intensive rock bursts
happened independently of the excavation in the
back area of the eastern tube giving rise to partially
severe damage of the support. Constructional
adaptations of the support and excluding critical
tunnel sections for access were required to provide
for the safety of workforce and equipment [2].
Between March 2004 and June 2005 the Swiss
422
Seismological Service (SED) recorded an
accumulation of seismic activity in the area of the
MFS Faido, a region which normally shows a very
low seismicity. During the above mentioned period
the permanently installed Swiss Digital Seismic
Network (SDSNet) registered 10 seismic events
with local magnitudes ML between 0.9 and 1.9.
With the SDSNet located at the surface, the
epicenters could be associated with the area of the
MFS Faido within an accuracy of one kilometer.
Together with the M1.9 tremor of 1.6.2005 a strong
rock burst could be associated. The same holds for
two additional tremors of similar magnitude. On the
other hand no relations to rock bursts could be
identified for tremors during the period March to
April 2004. Since it could not be excluded that the
accumulation of the tremors correlated with the
construction of the MFS Faido the owner ATG
decided in July 2005 to form a Work Group ’Micro
tremors’ consisting of representatives from the
Swiss Seismological Service (SED), Geology,
Engineering joint venture GBTS, Supervision and
ATG, the owner of the tunnel. On March 25, 2006
the strongest micro tremor of ML 2.4 was registered.
This tremor was felt by the inhabitants of the village
Faido close to the jobsite. This tremor triggered a
stress drop nearby the EON causing heaving in the
invert.
4.2 Additional seismic measuring stations
With first priority, additional seismic stations were
installed at the surface and in the MFS Faido. For
precise monitoring and location of the seismic
activity a special local seismic network consisting
of nine stations at the surface, including one station
from the SDSNet, were installed in a circular
arrangement 10 to 15 km around the MFS Faido. In
addition, two additional stations were installed at
different locations in the tunnels of the MFS Faido.
The circular position of the seismic measuring
equipment allows a precise determination of the
epicenters whereas the measuring stations directly
above and inside the MFS Faido serve the
evaluation of the depths of the micro tremors’
sources. The readings of the measuring stations
were integrated in the SED’s data acquisition
system. The real time transmission of the measuring
data guaranteed a continuous survey of the seismic
activity allowing for an immediate alert of the
responsible organizations such as ATG, supervision
and authorities in case of a strong tremor. This was

of particular importance in case of the M2.4 tremor
occurring on 25.03.2006. Accurate seismic wave
velocities required for the hypocenter’s
determinations were derived from two calibration
shots carried out in the MFS Faido. An average Pwave
velocity of 5.33 km/s was calculated.
4.3 Chronology of the seismic events
Figure 7 shows the development of the recorded
micro tremors’ number and magnitudes as functions
of time. The highest seismic activity took place
during December 2005, March 2006 and Mai 2006.
The highest magnitude of 2.4 occurred on 25 th
March 2006. During the period from October 2005
to February 2008 a total of 112 micro tremors were
recorded. The magnitudes of most of the tremors
were below 1.0. With termination of excavation in
the MFS Faido the micro tremors’ number and
magnitudes decreased continuously. Since
September 2007 no more micro tremors have been
recorded above the measuring threshold of M = -1.0
in the area of the MFS Faido.
Figure 7. Chronological development of number and magnitudes of the micro tremors (with courtesy form SED)
423
4.4 Epicenters
The epicenters of all registered micro tremors
during Oct. 2005 to Feb. 2008 are depicted in
Figure 8. The micro tremors are concentrated in the
rock mass to the north of the MFS Faido close to
the eastern part of the tunnel system. The accuracy
of the epicenters’ localization is less than 100 m and
less than 250 m in focal depth as determined by
relocation of the calibration shots. Within the error
ellipsoid the tremors’ sources are at tunnel level.
4.5 Impacts of rock bursts on the tunnel
The rock burst’s damage potential is illustrated in
Figure.9. The picture to the left shows the damage
of the support with a shotcrete plate ejected into the
EON. This rock burst occurred together with the
M1.9 micro tremor of July 2005. The EON’s invert
heave in the picture to the right was caused by the
M2.4 micro tremor of 25.03.2006. Due to the
seismic impact the invert heave was smaller than it
is shown in the picture taken 2 days later.

4.6 Measured particle velocities V for the design
micro tremor at station MFS-A
In Figure 10 the measured particle velocities V
registered at the station MFS-A (Figure 3 and
Figure 8) for the M2.4 micro tremor on 25.03.2006
are presented. In the diagram the velocity
components Vx, Vy and Vz are shown. The
orientations of the X-, Y-, and Z – coordinates
correspond to those of the model coordinates. The
maximum measured amplitude amounting to 0.024
m/s was in the horizontal X – direction normal to
the tunnel axis. The total measuring interval was 1
sec. The relevant vibration starts at 0.193 sec after
the measurement’s triggering. The dynamic load for
the model computations started at that same time to
reduce computation time and amount of data.
4.7 Findings form the seismic measurements
The micro tremors in the northern part of the MFS
East tube, EON
West tube, EWN
M2.4
Measuring station MFS-B
East
Measuring station MFS-A
Figure 8. Epicenters of the micro tremors from Oct. 2005 to Feb. 2008 (with courtesy form SED)
424
tend to form clusters i.e. the sources of several
tremors are located within the same area.
Considering the predominant steep west - east
dipping joint system striking sub parallel to the
tunnel axis (Figure 3 and Figure 6) shear failures
along joints are most likely. The micro tremor’s
source locations in the hard Leventina gneiss to the
east of the fault corresponds to the location of the
vertical stress concentrations resulting from the
computations of the static load case in Figure 6.
There is a general tendency of the micro tremors to
move together with the excavation of the tunnels
form the cross cavern’s area to the north. Very few
micro tremors occurred in the southern part of the
MFS Faido; these events occurred outside the
region shown in Figure 8. Many of the rock bursts
causing the support’s damages in the tunnels were
triggered by the micro tremors.
Date
01.06.2005 25.03.2006/ Photo: 27.03.2006
Figure 9. Left: Rock burst of 01.06.05: Damage of the support’s shotcrete, EON (on this date no seismic registration
was installed). Right: M2.4 micro tremor of 25.03.2006: invert – heave, EON.

5. DYNAMIC MODELLING
5.1 Aims of modelling
The aim of the numerical modeling was to provide
basic appraisals with respect to the structural safety
and usability of the tunnels during and after a
seismic event. The investigation’s result should
furthermore disclose the residual risks to be
accepted and its impact potential on the tunnels
concrete linings. The structural design has been
completed prior to the occurrence of rock bursts.
Therefore, it had to be controlled whether the lining
designed for the static load case still fulfills the
tunnels’ safety and usability requirements during
and after a micro tremor’s impact.
5.2 Computational methods and models
For the investigation 2D and 3D models were
applied using the discontinuum codes UDEC and
3DEC (ITASCA, [3]). Discontinuum models were
used to consider block movements (rigid body
movements) in the jointed rock mass. The blocks
themselves are deformable. Due to the high
computation time parametric studies were not
possible with the 3D model. For parametric studies
2D – models were used instead. The results
Particle Velocities V (m/sec)
0.03
0.02
0.01
0.00
0.0 0.2 0.4 0.6 0.8 1.0
-0.01
-0.02
-0.03
Start of Computations
Vy
Vx
Vz
Time t (s)
Figure 10.
425
presented refer solely to the dynamic load case. In a
first step the static equilibrium of the supported
tunnel system was computed. In a second step the
final lining was inserted in the model and the design
micro tremor’s load superimposed to the static case.
The properties for the rock and the two major joint
systems were specified together with the geologists.
A joint spacing of 2 m was used for the fault’s
kernel zone. The real joint spacing in the kernel is
much smaller which was accounted for by
correspondingly reducing the rock’s properties.
Weak rock properties were assigned to the layers
containing kakirites (Figure 11). For modeling
support and final lining in the 2D models block
elements and structural bar elements were used.
Special investigations have shown the deviation of
seismic waves along weak rock formations.
Therefore two different 2D models with one and
with two kakirit layers (Figure 11), respectively,
were investigated. The results from these models
reveal the influence of a tunnel’s distance from a
kakirite layer on the dynamic loading of its liner in
case of a micro tremor. With Model1 a stress drop
in the vicinity of the EON was initiated. The results
reveal the impact of such an event on the tunnel
lining.
Z
Y
X

5.3 Specification of the model design loading wave
The work group ‘Micro Tremors’ decided to
consider the M2.4 micro tremor of 25 March 2006
as the decisive design – tremor. For the seismic
model loading of the input wave corresponding to
the design tremor had first to be specified. With this
decision a conservative dynamic load was selected
in order to control the safety and stability of the
tunnels under operation in case of a micro tremor in
the area of the tunnels. The emitted wave at the
tremor’s source first had to pass the tunnel system
and the fault zone prior to arrive at measuring
station MFS-A. To determine the waves attenuation
10 times amplified measured particle velocities
Figure 11. 2D – Model1 and Model 2 with 2 and 1 layers of kakirite in the fault’s kernel
Figure 12. Left: 3D Model. Right: Vx Horizontal cut through the model with point P41 simulating approximately
the MFS-A. The curves refer to the Vx input particle velocity and the Vx at P41. ∆t is the
wave’s travel time from the model’s load boundary to Point P41.
426
were applied at the eastern boundary of the 3D
model. With this load the particle velocities in a
model point simulating the MFS-A were in
magnitude similar to those of the measured signal
(Figure 12). The dynamic design load was therefore
determined as the 10 times amplified measured
signal (Figure 10) with a peak Vx -velocity
component of 0.23 m/s. This design load covers the
strongest micro tremor identified since starting the
extended seismic monitoring in the MFS Faido
(Figure 7) and is conservative. Figure 12 shows the
3D model used. In the diagram the design load’s Vx
particle velocity is compared to the signal received
in model point P41 simulating the location of the
MFS-A.

5.4 Results form 2D Modelling
The results presented refer to the dynamic design
load only. Of particular interest were the liners’
concrete stresses and displacements as a function of
time due to the dynamic load. For this purpose the
corresponding results in the EWN’s liner were
compared for Model_1 and Model_2. For the
interpretation a liner segment at the eastern change
from invert to side wall was selected. The concrete
lining thickness of the EWN is 60 cm. Negative
stress denotes compression. In Model 1 the EWN is
embedded between two kakirite layers whereas in
Model 2 the location of the EWN is in front of a
kakirite layer and exposed to the more or less
undamped wave arriving from the right model side
(East). The concrete stresses represented in Figure
13 and Figure 14 clearly show the damping effect of
the kakirite layer in front of the EWN (Figure 13).
The concrete stresses due to the dynamic
design load are small compared to those determined
for the static case. The stress peaks occur at high
frequencies. The evaluation of the displacement’s
corner frequencies revealed a displacement’s
decrease approximately at 60 Hz. Figure 13 and
Figure 14 contain the low pass filtered stresses at 60
Hz. From the curve of the filtered stresses can be
deduced that the design stresses to be considered are
considerably smaller than the stress peaks. Due to
an irreversible shear movement along a joint in the
tunnel’s vicinity the liner stresses for Model 2
reveal a small permanent remaining compression
stress after the dynamic impact. The shear
movement in the joint was caused by the
unhindered wave’s deviation towards the tunnel
along the weak kakirite layer. Figure 15 and Figure
16 show the EWN’s displacements evaluated for the
same case and at the same liner location as used for
the concrete stresses’ evaluation. In accordance
with the stresses the deformations generally are
small. Similar to the concrete stresses, the
displacement of the EWN’s liner embedded
between two kakirite layers is smaller compared to
the case of Model 2.
427
Concrete stress in liner (MPa)
Concrete stress in liner (MPa)
Liner displacements d (mm)
2
1
0
-1
0.0 0.1 0.2 0.3 0.4 0.5
-2
-3
-4
-5
Time t (s)
EWN
Kakirite zones
unfiltered
60 Hz LP filter
Figure 13 Concrete stresses (M+N) in liner of tunnel
EWN for Mod_1 with 2 kakirite zones
2
1
0
0.0
-1
-2
-3
-4
-5
0.1 0.2 0.3 0.4 0.5
Time t (s)
EWN
Kakirite zone
unfiltered
60 Hz LP filter
Figure 14 Concrete stresses (M+N) in liner of tunnel
EWN for Mod_2 with 1 kakirite zone
2
1
0
0.0
-1
0.1 0.2 0.3 0.4 0.5
-2
-3
-4
Time t (s)
Y
dy
dx
X
EWN
Kakirite zones
Figure 15 Liner displacement of tunnel EWN for Mod_1
with 2 kakirite zones

During the dynamic computation with Model 1
a ‘rock burst’ was triggered beneath the EON
tunnel. This burst occurred due to a sudden stress
drop in a joint. Figure 17 shows the stress drop’s
Liner displacement d (mm)
2
1
0
0.0
-1
0.1 0.2 0.3 0.4 0.5
-2
-3
-4
Time t (s)
Y
EWN
Kakirite zone
dy
dx
Figure 16 Liner displacement of tunnel EWN for Mod_2
with 1 kakirite zone
development at different times represented by
particle velocities’ Vx components. The disturbance
X
Figure Figure 2 Development 17 Development of a stress of a stress drop in drop the in vicinity the vicinity of tunnel of tunnel EON. EON. Represented Represented are the are particle the particle velocities velocities Vx. Vx –
Vx. Vx Range: – Range: -0.2 ÷ -0.2 +0.2 ÷ +0.2 m/s. The m/s. represented The represented Vx - Vx interval - interval range range is 0.05 is 0.05 m/s. m/s.
Figure Figure 2 Left: 18 Left: Development Development of Vx of around Vx around the stress the stress drop drop area. area. The The represented represented Vx - Vx interval - interval range range is 0.10 is 0.10 m/s. m/s. Right:
Stress drop Right: of 17 Stress MPa drop in the of joint 17 MPa indicated in the by joint the indicated shear movement by the shear in the movement left hand picture. in the left The hand residual picture. shear stress
The residual amounts shear stress to 14.5 amounts MPa. to 14.5 MPa.
428
triggering the drop was small amounting to
0 < Vx < 0.05 m/s and occurred prior to the arrival
of the input wave’s first Vx - peak. The drop’s shear
direction and the magnitude of the drop’s stress
release of 17 MPa are shown in Figure 18 Due to
the stress drop, Vx in the tunnel area amounts to
more than 0.40 m/s (Figure 18). The disturbing
influence of the stress drop on the input wave is
depicted in Figure 17, at time t = 0.1532 s i.e.
0.0179 s after the stress drop’s triggering. From
Figure 17 the above mentioned deviation of the
input’s Vx - peak along the eastern kakirite layer is
obvious at t = 0.1532 s. The stress drop occurred
only in Model 1 with two kakirite layers.
The impact potential of the stress drop on the
lining was investigated using the particle velocities
PV according to 1) with the components arising at
the same time t.
...(1)
In Figure 19 the PV’s for a point in the rock
near the lining of the EON are plotted. Compared

are the PV’s for the cases with and without stress
drop i.e. with Model 2 and Model 1. In case of the
stress drop the peak PV in the rock near the lining
amounts to almost 900 mm/s. The dynamic loading
on the lining in this case is similar to a shock impact
[4], [5]. Without stress drop the corresponding
maximum PV amounts to 200 mm/s. The frequency
of the stress drop induced PV is bigger than 300 Hz
whereas the frequency for the peak PV without
stress drop is in the range of 50 to 60 Hz. The PV’s
of the lining segment under consideration are shown
in Figure 20 for the cases with and without stress
drop. In the models the sealing membrane between
rock and lining was simulated by means of an
interface. The transmissibility of this interface
contributes to the vibration behavior of the lining
[6]. For the case with stress drop the damping of the
lining’s peak PV is obvious. However, due to the
damping’s frequency reduction in both cases with
and without stress drop a longer duration of high
PV in the lining can be derived from Figure 20.
429
Figure 21 and Figure 22 show the stresses in
the concrete liner and the liner’s dx – displacement
respectively, both for the cases with and without
stress drop. In the case of the stress drop the
concrete stress amounts to more than 8 MPa and the
liner’s displacements to 6 mm. From the above
mentioned results must be concluded, that in case of
a stress drop as developed in Model 1 damage of the
lining cannot be excluded. The probability of the
occurrence of the design micro tremor and the
corresponding triggering of a stress drop has been
assessed by experts as very low. This assessment is
based on the fact that after termination of the
excavation in the MFS no more micro tremors
occurred and in addition a clear correlation between
excavation activity and micro tremors was
ascertained. Furthermore the seismic activity in the
area of the MFS Faido is very low. On 21.01.2008 a
M4.0 earthquake occurred at a distance of
approximately 50 km from the MFS Faido. This
earthquake was recognized by the measuring
Figure 19 Particle velocities PV of a point in the rock Figure 20. Particle velocities PV of the lining segment.
mass near the lining. With and without stress drop With and without stress drop
Figure 21 Lining’s concrete stresses (M+N) of tunnel EON. Figure 22 Lin ing displacements dx of tunnel EON.
With and without stress drom With and without stress drop

stations in the MFS. No triggering of a micro tremor
could be identified. Based on above mentioned
aspects the case of a stress drop near a tunnel in the
MFS Faido has been accepted as a residual risk.
Since a residual risk cannot be completely excluded,
the owner of the tunnel, ATG decided to install
seismic measuring equipments and vibration sensors
in the linings of the tunnels in the MFS Faido for a
permanent seismic monitoring during operation.
6. CONCLUSIONS
During tunneling at great depth in geological
conditions as encountered in the MFS Faido micro
tremors triggering rock bursts are likely to occur.
The stress redistribution due to tunneling and
the stress concentration in hard rock in combination
with an existing zone of weakness (fault) favors the
occurrence of micro tremors.
The micro tremors did clearly correlate with the
excavation activities. After terminating the excavation
no more micro tremors have been identified.
Micro tremors cannot be avoided. During
construction precaution measures such as closing of
critical sections and flexible support consisting of
flexible rock bolts and steel arches are to be applied.
A seismic wave is deviated by a weak zone.
With the orientation of the weak zones in the MFS
Faido a seismic wave is deviated towards the tunnels.
The dynamic impact on a lining of a tunnel in front of
a weak zone and exposed to a more or less unhindered
micro tremor’s wave is considerably higher compared to the
impact on a liner of a tunnel in the ‘shelter’ of a weak zone.
The specification of the design micro tremor
for the MFS Faido is conservative.
Excluding the additional loading due to a
spontaneous ‘stress drop’ in the direct vicinity of a
tunnel there has been no need for improving
(thickness, additional reinforcing) the linings
designed for the static load case in the MFS Faido.
ACKNOWLEDGEMENT
The authors thank the AlpTransit Gotthard for the
permission to publish the paper.
REFERENCES
1. Hagedorn, H., Rehbock-Sander, M., Flury,
S.,Gotthard Base Tunnel: State of the works and
Special Aspects, Proc. Int. Symposium on
430
2.
Construction and Operation of Long Tunnels,
Taipei, Taiwan, 7. - 10.11.2005
Rehbock-Sander, M., Rock Bursts Experience
gained in Mines and Deep Tunnels, Proc. 4 th Asian
Rock Mechanics Symposium (ARMS 2006),
Singapore, 8.-10.11.2006
3. ITASCA, UDEC – Manual, Theory and
Background, Itasca Consulting Group, Inc.
Minneapolis, Minnesota, USA
4. Kaiser, P.K., Vasak,P., Suorinemi, F.T. and
Thibodeau, D. (2005). New dimensions in seismic
data interpretation with 3-D virtual reality
5.
visualization in burst-prone ground, RaSiM6, Perth,
Australia, 33 – 47
Heuze, F.E., Morris, J.P., Insights into ground shock in
jointed rocks and the response of structures there-in, Int.
Journal of Rock Mechanics & Mining Sciences 44 (2007)
647-676
6. Harris, C.M., Crede, E.C., Shock and Vibration
Handbook, McGraw-Hill, Book Company, ISBN 0-
07-026799-5
BIOGRAPHICAL DETAILS OF THE AUTHORS
Heinz Hagedorn completed his study
in civil engineering at the Swiss
Federal Institute of Technology (ETH)
with a M.Sc. degree in 1969. From
1970 to 1979 he was working at ETH
with the rock mechanics institute of
Prof. K. Kovari. The activity consisted
in developing a Finite Element
program designed for underground
structures and research activity in material testing and
field measurement. Since 1979 he has been working with
Amberg Engineering Ltd., Switzerland as a geotechnical
expert. For numerous large underground constructions
such as tunnels, caverns etc. in Switzerland and abroad he
provided geotechnical expertise and consulting for design
and during construction.
Rolf Stadelmann obtained his M. Sc in
Civil Engineering from the ETH
Zurich, Switzerland in 1992. He
specializes in general stability
analyses, temporary & permanent
excavation support systems, as well as
in structural design of all types of
tunnel lining. His core competence is
also in the quality management for
geotechnical analyses as well as in the development of
Contractor’s alternatives. He has in-depth experiences in
underground space design for all kinds of rock conditions.
Since 2003 he is Head of Geotechnical Department of
Amberg Engineering Ltd in Switzerland.