RD&D-Programme 2004 - SKB

The size of the upswelling depends on the original densities of the buffer and the backfill and
their associated expansion and compression properties, as well as the friction against the rock.
Calculation models, both analytical and numerical, exist for analysis of this interaction.
Conclusions in RD&D 2001 and its review
SKI points out that SKB must make sure that the density of the buffer is not reduced too much
when it swells up and displaces the backfill in the overlying tunnel (i.e. provide design premises
for the backfill). In SKI’s view, studies of the buffer’s mechanical and rheological properties are
of great importance, especially in view of the buffer-canister-rock interaction and the interaction
with the backfill in the deposition tunnel.
Newfound knowledge since RD&D 2001
The upswelling of the buffer against the backfill has been investigated for an alternative backfill
material (Friedland clay). Calculations have been carried out using the same methodology as
before and with new measurements of the compression properties of Friedland clay. The results
show that with the low dry density of 1,400 kg/m 3 that was achieved in the field tests, the swelling
is 0.3 m, which requires a cover of 2.7 m buffer above the canister (instead of 1.5 m, which
is the current concept) so that the buffer around the canister will not lose in density /17-10/, see
also Chapter 18.
Programme
The consequences of this interaction for other types of backfill will be studied in a similar
manner as for Friedland clay.
When the Prototype Repository is dismantled, the consequences will be able to be measured
and comparisons made with calculations.
17.2.9 Mechanical interaction buffer/canister
Mechanical interaction between buffer and canister is caused by the buffer’s clay matrix, which
generates both compressive stresses and shear stresses, by the pore water, which generates only
compressive stresses, and by gas in the buffer, which also generates only compressive stresses.
Changes in these three variables take place during the water saturation process, and can also
occur in response to external forces. The weight of the canister acts on the buffer, while the
influence of the weight of the buffer on the canister is negligible. Rock movements that arise
in the fracture plane, for example after earthquakes, give rise to stresses on the canister, which
are transmitted from the rock through the buffer. The processes associated with the mechanical
interaction between buffer and canister after water saturation are relatively well understood. The
uncertainty mainly concerns the evenness of the wetting and the pressure build-up caused by
possible gas formation.
An important process is the movement of the canister in the buffer after water saturation. The
weight of the canister results in a creep movement, caused mainly by shear stresses in the buffer.
Extensive laboratory tests have been performed to study these creep movements. An established
model for creep in clay has proved to work for the buffer, and calculations with this model show
that the movement is only a few millimetres. An uncertainty in the model is the extrapolation to
a long period of time.
Conclusions in RD&D 2001 and its review
In SKI’s view, studies of the buffer’s mechanical and rheological properties are of great
importance, especially in view of the buffer-canister-rock interaction and the interaction with
the backfill in the deposition tunnel.
RD&D-Programme 2004 209