RD&D-Programme 2004 - SKB

At the Äspö HRL, full-sized tunnels have been backfilled with a mixture of bentonite and
crushed rock in the Backfill and Plug Test and the Prototype Repository experiment.
Newfound knowledge since RD&D 2001
There are reasons to further study and develop concepts for backfilling of tunnels, for example
to assure the function of the backfill even if the groundwater has high salinity at repository
depth, to satisfy the need for backfilling concepts for other rock caverns, and to develop
technology for backfilling with sufficient efficiency.
In cooperation with Posiva, within the framework of an ongoing research programme, SKB has
conducted theoretical studies to identify backfill concepts deemed to be capable of satisfying
requirements on both deposition tunnels and other rock caverns. The studies cover the following
materials: bentonite, other swelling clays, non-swelling clay, crushed rock and till, see section
18.1.6. Two different backfilling methods have been studied for the different materials: compaction
in the tunnel and emplacement of pre-compacted blocks.
Programme
SKB and Posiva will continue the ongoing research programme and three backfilling concepts
will be further studied /10-10/. The three concepts are:
• Compaction of a mixture of bentonite and crushed rock in the tunnel.
• Compaction of swelling clay in the tunnel.
• Emplacement of pre-compacted blocks in the tunnel.
The research programme comprises four stages in all. The first stage has been completed and
included technical studies for describing the prospects of the proposed backfilling concepts for
meeting SKB’s requirements.
In the second stage, geotechnical laboratory tests and a deeper analysis of the three concepts
will be performed. Fabrication of pre-compacted blocks with desired properties as regards
e.g. density and handleability is planned during this stage. Pressing of blocks of swelling clay
and a mixture of bentonite and crushed rock will be done on a laboratory scale. In addition,
the already developed technique for in-situ compaction with vibrating plates /10-11/ will be
optimized with respect to e.g. compaction technique and material properties such as grain size.
In the third stage the importance of scaling-up will be investigated. This entails technology
development plus tests to verify that the backfilling concept is technically feasible. The tests
will be performed with prototype equipment. Stage four involves full-scale tests to verify the
performance of the backfill.
10.4.3 Plugging of deposition tunnels
When all canisters in a deposition tunnel have been deposited, the tunnel is backfilled and
plugged pending the backfilling of all openings in the underground part. The temporary plug
consists of a concrete plug in the entrance of each deposition tunnel. The plug is designed to
withstand the water pressure at repository depth and the swelling pressure in the backfill. The
plugs do not have any long-term function, but will be left in place when all other openings are
backfilled.
The need for seals with permanent functions in the underground part, for example between
different deposition areas, will be studied in site-specific preliminary safety evaluations during
2005. This type of seal is foreseen to be some form of bentonite plug with long-term durability.
Performance requirements and detailed technical solutions for this type of plug will be determined
later.
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