RD&D-Programme 2004 - SKB

Programme
See section 18.2.2.
18.1.12 Pore water composition
The water in the backfill is a blend of water from the crushed rock (if concept A is chosen), the
bentonite’s original water and water added during mixing. Its composition is mainly dependent
on which water is added.
18.1.13 Impurity levels
The quantity and composition of impurities is mainly dependent on which backfill concept is
chosen. The consequences of these impurities must be evaluated for each concept.
A related question is the importance of foreign engineering materials and materials forgotten in
the tunnels, see section 18.2.16 (general), 17.2.17 (iron and cement) and 19.2.19 (cement).
Programme
See sections 18.2.16, 17.2.17 and 19.2.19.
18.2 Processes in the backfill
18.2.1 Overview of processes
Basically the same processes take place in the backfill as in the buffer, although sometimes
on a different scale. Moreover, the crushed rock, if any (defined as an impurity in the backfill),
plays a somewhat different role than the impurities in the buffer, for example by contributing to
sorption. The research programme for the various processes in the backfill is dealt with in the
following sections.
18.2.2 Integrated studies – composition and function
The properties of the backfill are determined not only by its composition, but by the salinity
of the groundwater as well. Salinity is of great importance for these properties, due to the fact
that both swelling pressure and hydraulic conductivity are determined by the properties of the
bentonite in the spaces between the crushed rock (concept A and possibly concept D). Since
the bentonite density is low, the properties are very sensitive to salinity in certain concepts.
As a result, the original composition of 15 percent bentonite and 85 percent crushed rock may
be suitable for a site with a non-saline groundwater, while a site with a more saline water may
require a higher proportion of bentonite. The exact composition of the backfill will be chosen
when the necessary data are available from the site in question.
The function of the backfill in general was identified as a priority research area in RD&D 2001.
The requirements on the function of the backfill (swelling pressure, conductivity) are strongly
related to its content of smectite or other swelling minerals.
Conclusions in RD&D 2001 and its review
SKI comments that SKB should make sure how useful the results from the Backfill and Plug
Test in the Äspö HRL are for choosing backfill materials and for investigating the interaction
between backfill and buffer in the deposition holes.
230 RD&D-Programme 2004