RD&D-Programme 2004 - SKB

18.2.7 Gas transport/dissolution
Gas transport in the backfill is not judged to be an important process. If gas can find its way
through the buffer, the transport capacity of the rock is sufficient for the pathway through the
backfill to be uninteresting.
18.2.8 Swelling and compression
It is important that the backfill can swell to prevent the formation of conductive transport
pathways in the boundary zone between backfill and rock, see sections 17.2.4 and 18.2.17.
Compression properties are important. They may not be such that the swelling of the buffer
against the backfill in the deposition holes is too great. Compression properties are mainly a
function of the density of the backfill. Determining the compression and swelling properties and
the density that can be achieved in compaction of the backfill is an important part of the studies
of different backfilling concepts and materials, see section 18.2.2.
18.2.9 Mechanical interaction buffer/near-field rock
The permeability of the backfill is determined by its material composition. The mechanical
properties of both the backfill and the near-field rock determine the backfill’s stabilizing
capacity. An understanding of the interplay between rock and backfill is of great importance
in both design and site-specific safety assessment in conjunction with the site investigations.
The following factors are of importance for the interaction between backfill and near-field rock:
• Swelling pressure and weight of the backfill.
• Indirect effects of the buffer’s upswelling.
• Creep movements in the rock around the tunnel.
Conclusions in RD&D 2001 and its review
The conclusion is unchanged that the swelling pressure against the roof is dependent on what
density can be achieved in compaction of the clay phase. The difficulties of compaction against
the roof make it difficult to guarantee a given swelling pressure. The swelling pressure is caused
by the bentonite in the space between the particles of crushed rock and is dependent not only on
the mean density of the bentonite but also on its homogeneity. An inhomogeneous mixture can
give rise to a high swelling pressure to start with in certain spots. It is not known whether the
swelling pressure subsequently decreases due to homogenization because the bentonite swells
out from pores with high bentonite density.
Newfound knowledge since RD&D 2001
General aspects of the problem of spalling and the stability of the walls of the deposition holes
are discussed in /18-7/, and particularly the importance of a light support pressure from the
buffer to prevent initiation and propagation of failure that can lead to spalling. Background
material is the theory of brittle failure developed at AECL’s Underground Rock Laboratory
(URL) in Canada. The general conclusion, that a small support pressure is enough to improve
stability, also applies to other types of failure, for example breakout of rock wedges in the tunnel
periphery.
Experience from the URL Mine-by Test Tunnel in Canada confirms the importance of light
support. Removal of floor muck led to rock breakout /18-8/.
For slow creep movements that take place over a long period of time and lead to stress
relaxation in large rock volumes, swelling pressures on the order of several hundred kPa
are of no importance /18-9/. To limit such a slow creep-induced convergence, the tunnel
RD&D-Programme 2004 235