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

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