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

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.
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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.