RD&D-Programme 2004 - SKB

limitations. Furthermore, for large earth-quakes, where the active zone may extend down to
depths where the properties of the rock differ considerably from those in the upper part of the
crust, uncertainties exist as to the best way to represent the movement in numerical models. The
attempts made so far indicate that the effect of large earth-quakes does not necessarily have to
be greater than the effect of earth-quakes of magnitude 6.0 for fractures at small distances from
the active zone. If the active zone is of great horizontal extent, perhaps hundreds of kilometres,
the seismic moment will be great and large quantities of strain energy will be released. For a
fracture at a distance of a few hundred metres, however, it is not the seismic moment but the
local, and presumably magnitude-independent, stress change that is crucial.
A compilation of documented earthquake-induced damage to underground structures has been
published /19-42/. The purpose is to obtain empirical evidence for understanding the concept
of respect distance. The study shows that the effects, in the form of documented permanent
deformations, are limited to the area nearest the earth-quake, even in cases where road or rail
tunnels intersect the active zone.
Possible thermohydromechanical effects of future glaciations on the repository have been
analyzed in the Decovalex project. The effects at repository level are probably small, in terms
of both impact on general mechanical stability and hydraulic fracturing /19-43/.
Programme
The validity of the general relationships that exist between the length of a fracture and the
movement that takes place along the fracture (at a given load and set of fracture parameters)
will be evaluated by means of 3DEC analyses. The general relationships are formulated for
idealized conditions, above all for elastic host rock, and are therefore conservative so that the
calculated movements are generally overestimated. The work is described in the planning report
for SR-Can /19-44/.
The movement of two or more small fractures that interact via a bridge of partially intact rock
can be underestimated if the interaction between the fractures is not taken into account. This
will be investigated with the same basic method as in the 3DEC study mentioned above (general
relationship between size and movement).
The programme of dynamic analyses of seismically induced movements of fractures that
intersect canister hole positions will continue, above all with Flac3D models. The possibility of
simulating large earth-quakes as well will be investigated. The overall objective is to arrive at
a well-defined and systematic body of data which can be used together with e.g. the results of
other types of earthquake studies /19-42/ to determine respect distance.
A programme will be carried out for analysis of the mechanisms (stress evolution of loaded
elastic crust resting on viscous mantle) behind the occurrence of postglacial fault movements.
The analyses will be performed with two- and three-dimensional Abaqus models.
The possibility that the repository, in interaction with fractures and fracture zones, may act as a
plane of weakness will be investigated numerically with the aid of 3DEC models.
19.2.8 Fracturing
Conclusions in RD&D 2001 and its review
It was stated in RD&D 2001 that compared with reactivation, fracturing is of little importance
for possible changes in canister hole geometry after closure.
The authorities point out that SKB should devote resources to the development of useful calculation
models for fracturing – both new fracturing and propagation of existing fractures. On the
other hand, they also say that the planned activity seems to provide answers to most questions.
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