RD&D-Programme 2004 - SKB

Newfound knowledge since RD&D 2001
Preliminary calculations have been made with 3DEC to investigate to what extent the tunnel
backfill can limit the convergence of a deposition tunnel /19-50/. The general conclusion is that
the tunnel backfill cannot contribute towards limiting convergence that is due to slow creep
movements that take place in the rock within a distance of several tunnel diameters from the
tunnel, although it may be sufficient to limit the scope of block breakout and the initiation and
development of progressive failures in the rock nearest the tunnel walls.
A literature study of creep in rock masses, and especially along fractures of different types
(filled, unfilled), has been conducted and the results reported in preliminary form. One of the
conclusions of the study is that the stress-strength ratio must exceed a certain threshold value in
order for any creep to take place.
Programme
A literature study will be conducted aimed at bounding the stress changes to which the host rock
may be exposed over time as a consequence of slow tectonic movements.
The modelling technique which was used in the preliminary modelling in the Rock-Backfill
project is being systematized with the aid of data and conclusions from the literature study and
applied generally to the host rock, above all to the various cavities. The objective is to be able to
bound the possible convergence of tunnels and deposition holes that could occur as a result of
the inherent time-dependent material properties of the rock mass.
19.2.10 Erosion
Erosion of the crystalline bedrock is judged to be limited under current climatic conditions in
the areas being studied by SKB. Weathering of exposed rock is estimated to be on the order of
a millimetre or so per thousand years /19-51/. A colder climate with permafrost can be expected
to lead to increased weathering and erosion of exposed rock. The most extensive erosion is
assumed to occur in conjunction with glaciations. The erosion in conjunction with glaciation
is strongly linked to the basal conditions of the ice sheet. An ice sheet that is frozen to its bed
preserves the bed, while an warm-based ice sheet that slides over its bed is expected to
redistribute the loose soil layers and erode the bedrock. Erosion is further dealt with in
Chapter 21.
Conclusions in RD&D 2001 and its review
Long-term erosion of the geosphere has been judged by SKB to be of subordinate importance
for the long-term performance and safety of the deep repository. In the review of SR 97, the
authorities deemed the process as potentially unfavourable. In RD&D 2001, SKB wrote that
they intended to study the process more closely.
Newfound knowledge since RD&D 2001
An estimation of the scope of glacial erosion /19-52/ has been done based on the assumption
that the sediments formed during the Quaternary Period as a result of erosion by continental ice
sheets still remain within the formerly glaciated area. Based on this assumption, glacial erosion
has been estimated with the aid of soil depth mapping.
Programme
Erosion is strongly linked to prevailing climatic conditions, and in particular the presence
of continental ice sheets. The process is therefore being studied within the framework of the
climate programme, see Chapter 21.
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