RD&D-Programme 2004 - SKB

• The gas injection phase will begin when the bentonite is considered to be sufficiently watersaturated.
Tests will be performed with constant pressure or constant flow conditions.
Lasgit started in 2003 and is projected to be finished in 2007–2008.
17.2.7 Swelling
Water uptake in the buffer and the backfill after deposition will lead to swelling, which causes
all gaps in the buffer, between rock and buffer and between canister and buffer to disappear, and
the buffer to be homogenized. However, some inhomogeneity will remain due to friction in the
bentonite. In the buffer, heating will furthermore lead to thermal expansion of the pore water.
If swelling is prevented, a swelling pressure will instead develop. With its higher clay content,
the buffer will swell more than the backfill. This leads to a mechanical interaction between
buffer and backfill whereby the buffer is expected to swell towards the backfilled tunnel. Buffer
movements can also cause the canister to move in the deposition hole. The swelling also causes
clay to penetrate into the fractures in the rock. In the long run, chemical changes in the buffer
can lead to changes in the swelling properties, see sections 17.2.16 and 17.2.17. A model for
swelling under water-saturated conditions has previously been developed for the finite element
code Abaqus. Swelling occurs during the water saturation phase as well, see section 17.2.12.
Due to the swelling properties of the bentonite, damage to the buffer – for example due to
piping and erosion, gas penetration or rock movements – will swell, shut and self-heal.
Conclusions in RD&D 2001 and its review
The process is not dealt with explicitly.
Newfound knowledge since RD&D 2001
Both theoretical and laboratory experiments have been conducted concerning the influence of
pore water composition on swelling and swelling pressure properties. These results are reported
in section 17.2.15.
During the work with KBS-3H, a model has been developed for analytical solution of
bentonite swelling through the perforated steel container surrounding a canister-buffer parcel.
The bentonite swells both through the holes in the container and in between rock and container
outside the holes. Calculations and tests show that the swollen bentonite gives rise to a density,
a swelling pressure and a hydraulic conductivity that is sufficient to effectively seal against the
rock.
Programme
The work with laboratory tests and model development to study the build-up of the swelling
pressure during the wetting phase and its influence and dependence on the negative pore
pressure will continue, see sections 17.2.12 and 17.2.15.
Laboratory tests involving measurement of swelling pressure and investigation of the influence
of pore water chemistry and density will be performed for other buffer candidates.
Swelling-out of bentonite through the holes in the perforated steel container in KBS-3H will be
studied in a test where hole geometry and gap width are simulated on a full scale (Big Bertha).
17.2.8 Mechanical interaction buffer/backfill
In the interface between the buffer and the backfill, the buffer exerts a swelling pressure
against the backfill and vice versa. Since the difference in swelling pressure is great, a net
pressure arises against the backfill whereby the buffer swells and the backfill is compressed.
208 RD&D-Programme 2004