RD&D-Programme 2004 - SKB

The authorities are of the opinion that SKB’s judgement that the risk in the tectonics scenario
has been overestimated is probably correct (since the judgement is based on friction-free
fractures) and that it is positive that earthquake effects on underground structures are now
being documented.
The authorities are of the opinion that it is urgent that SKB report all evidence available to
support the contention that new fracturing will not affect a tectonic lens, such as the one in
Forsmark, in the event of a future glaciation, and that the possibility that future fault movements
will take place along new lines is taken into account.
The authorities are of the opinion that SKB’s planned research activities within the area of
fracturing and reactivation appear to provide answers to most of the questions.
Newfound knowledge since RD&D 2001
The role of the fracture system for deformations in the near-field in connection with tunnel
excavation and boring of deposition holes has been dealt with in a 3DEC study /19-37/. The
fracture system, with fractures on the order of five to 15 metres, was based on a statistical DFN
model of the TBM tunnel in the Äspö HRL. The main conclusion is that fracture systems of the
type assumed in the model do not play any decisive role purely mechanically, at least not when
it comes to the occurrence of failure in intact rock blocks or the general deformation pattern.
Even if the fractures in the system are low-strength, the fracture movements will be small
except in the immediate vicinity of tunnels and deposition holes. The stress redistribution that
accompanied the movements in the fracture system was small and did not cause any part of the
modelled intact rock to come close to the rupture limit. When it comes to the risk of spalling,
the stresses and the direction of the stresses in relation to tunnels and cavities are more important
than details in the fracture geometry. It can be noted that the general conclusion that the
deformation pattern was primarily elastic agrees well with the back calculations that were done
in conjunction with the convergence measurements in the Apse experiment /19-35/.
Deformations in conjunction with the excavation of the Zedex tunnel and the TBM tunnel in
the Äspö HRL were calculated with the codes 3DEC and Flac3D /19-38/. The study was carried
out by NGI in cooperation with SKB. The calculated results were compared with the results
of measurements with extensometers during the construction phase. A main conclusion was
that the role of the fracture system may be smaller than previously assumed. Qualitatively, this
agrees with the results of the SKB study /19-37/ and with the results of the Apse experiment
/19-35/.
The general problem of translating a statistical representation of the geometry and mechanical
properties of fractures to an equivalent continuum representation is dealt with in the programme
for development of descriptive rock mechanical models /19-23, 19-39, 19-40/.
Seismically induced shear movements in fractures have been analyzed dynamically with the
codes Wave and Flac3D /19-41/. Equivalent problems were analyzed in SR 97 using a static
(non-dynamic) calculation method and statistically generated earth-quakes which induced
movements in statistically generated site-specific fracture populations containing fractures of
different lengths and orientations. The dynamic calculations have so far included earthquakes
of magnitude 6.0 with fault movements that may be representative of postglacial conditions.
The distance between the seismically active zone and the fracture with a radius of 200 metres
which is affected by the earthquake in the models has been varied between 200 metres and
two kilometres. At the least distance, 200 metres, the shear movement in the fracture was
0.065 metre, i.e. less than the movement that corresponds to canister failure according to
the present criterion. The results now available suggest that the static contribution to the
induced fracture movement dominates, at least at the short distances that will be important
for the deep repository. This might mean that for the deep repository it ought to be enough
to carry out purely static analyses (as in SR 97), which would greatly simplify the continued
work, especially for large earth-quakes, for which dynamic models may entail calculational
252 RD&D-Programme 2004