RD&D-Programme 2004 - SKB

Conclusions in RD&D 2001 and its review
Not dealt with.
Newfound knowledge since RD&D 2001
Water uptake in the backfill is being measured in the Äspö HRL in the Backfill and Plug Test
(water saturation achieved) and the Prototype Repository. Modelling and laboratory experiments
have yielded good information on the parameters that drive this water flow, see section 18.2.2.
Programme
The resaturation phase in the backfill will be modelled within the framework of SR-Can.
The results of this work will provide boundary conditions for the calculations of the buffer’s
resaturation.
Studies of unsaturated water transport will take place within the framework of the projects that
deal with backfilling according to section 18.2.2.
18.2.6 Water transport under saturated conditions
The permeability of the backfill after water saturation is determined by the content of montmorillonite
and other clay minerals, the composition of the pore water and the density. In contrast
to the buffer, the backfill of crushed rock mixed with bentonite cannot be made highly homogeneous,
partly because the mixing procedure does not produce a uniform distribution of bentonite
and crushed rock, and partly because application and compaction cannot be done as effectively
over the entire cross-section and length of the backfill.
Conclusions in RD&D 2001 and its review
The conclusion is unchanged that there are a number of uncertainties for bentonite-mixed
backfill. The properties are measured directly after mixing. The risk and effect of possible
homogenization of the bentonite density in the aggregate pores are not known. The measured
hydraulic conductivities presume that there are no channels or gaps. The effects of very high
salinities also require further study.
Newfound knowledge since RD&D 2001
The properties of saturated backfill are being investigated for bentonite-aggregate mixtures
mainly in the Backfill and Plug Test, and for other backfill concepts mainly in the project
“Backfill and Closure of Tunnels and Rock Caverns”, see section 18.2.2.
For backfilling with a mixture of bentonite and crushed rock, homogeneity is an important
factor for the hydraulic conductivity achieved. The experimental conditions and thereby the
stress state in the backfill in the tunnel are also important. A comparison between measured and
theoretically calculated hydraulic conductivity, the latter assuming that the bentonite is evenly
distributed in the pore space between the particles of crushed rock, shows that the measured
permeability is generally higher. The lower the bentonite content and the higher the salinity of
the added water, the greater the difference /18-3/. Homogenization with time would thus reduce
the permeability.
Programme
Investigations of saturated hydraulic conductivity of mixtures will continue and are mainly
taking place within the Backfill and Plug Test, while the investigations for other concepts are
primarily being done within the project “Backfill and Closure of Tunnels and Rock Caverns”,
see section 18.2.2.
234 RD&D-Programme 2004