RD&D-Programme 2004 - SKB

10.1.3 Sealing of the rock by grouting
The host rock must be sealed to reduce groundwater inflow for construction and operation, but
the KBS-3 system does not require any long-term permanent sealing measures. Heavy inflow
causes poorer working conditions and environmental impact. Groundwater drawdown increases
the risk of intrusion of deep groundwater with high salinity (upconing), which can affect the
performance of the repository. Furthermore, local seepage must be limited to permit buffer and
backfill to be emplaced under controlled conditions. It is assumed that the deep repository will
be sealed primarily by injection grouting of fractures. The design work includes determining
how much grouting work is needed and estimating the quantity of grouting material needed. The
high water pressure prevailing at repository depth creates different conditions from those that
are normal in the excavation of Swedish road tunnels. Even small fractures can conduct a great
deal of water, and SKB’s work is therefore focused on improving the means for sealing small
fractures. Furthermore, requirements regarding the chemical environment in the deep repository,
for example the pH of the groundwater, may require adaption of the grout. For example, grouts
that give rise to leachate with a pH of less than 11 may be required.
Conclusions in RD&D 2001 and its review
In the mid-1990s, SKB initiated research and development concerning rock grouting based
on an expert assessment of the state of knowledge and development needs /10-2/. Among other
things, work was begun on development of models to describe the rock and the grouting process
and to characterize grouting materials, as well as to develop materials for various situations. The
possible need for a cementitious material in the deep repository that would give leachates with
a pH below 11 led to the start of a special project aimed at developing and qualifying low-pH
grouts.
Kasam suggests that SKB should study various methods for grouting of fractures aimed at
achieving a permanent reduction of permeability in the rock surrounding a deep repository,
without adversely affecting the chemical environment in the repository.
A permanent reduction of permeability is not required in the KBS-3 system, nor is it possible to
prove a permanent reduction in permeability, since methods are not even available for proving
a durability of 100 years for available grouting materials. Accordingly, SKB does not share
Kasam’s view.
Newfound knowledge since RD&D 2001
A pre-study, followed up by a feasibility study, was conducted in cooperation with Posiva and
Numo in Japan to develop and qualify low-pH cement for various purposes in the deep repository,
including grouting fractures in the rock. The goal of the development work is to develop
materials that produce leachates with a pH lower than 11, see Figure 10-3. The studies have
so far not resulted in a formulation for a low-pH injection grout that will also penetrate into
small fractures. The conclusion was that alternative, non-cementitious materials should also
be investigated, at the same time as the development work on cementitious grout continues. In
the project that has been under way since 2003, efforts are being focused on development of
formulations in which Portland cement, slag cement, silica, high alumina cement and gypsum
are principal components. A non-cementitious material that is being investigated is silica sol.
Since RD&D 2001, two doctoral theses have been published dealing with a methodology
for rock characterization prior to grouting /10-3/ linked to models for grout spread /10-4/.
The methodology was tested during the construction of the Apse tunnel at the Äspö HRL.
This meant that systematic pre-grouting could be avoided thanks to the fact that detailed rock
characterization and early assessment of a suitable grouting method could be performed. Even
though investigations led to changes in the scope of grouting along a portion of the tunnel, a
combination of inflow measurements and pressure build-up tests provided a good basis for the
choice of grout and a preparedness for handling more extreme conditions. The quantity of grout
used could be limited while the sealing effect was good /10-5/.
RD&D-Programme 2004 117