RD&D-Programme 2004 - SKB

SSI points out that SKB should give an account of the importance of the backfill for the
long-term protective capability of the repository and what research, development and demonstration
is needed to accumulate sufficient knowledge for the needs of the safety assessment.
Kasam is of the opinion that if the different requirements on the backfill are found to be
incompatible, the function of limiting the swelling of the buffer should be prioritized.
Newfound knowledge since RD&D 2001
Studies of backfill are taking place mainly within the framework of three projects that are being
conducted in the Äspö HRL: The Backfill and Plug Test, Backfilling and Closure of Tunnels
and Rock Caverns, and the Prototype Repository. Separate studies have also been made of the
alternative backfill material Friedton.
The Backfill and Plug Test is a full-scale test of concepts A and C, see Figure 18-1. Installation
of the test was finished at the end of 1999 /18-2/.
In the Backfill and Plug Test, a 28-metre long tunnel section was backfilled and instrumented.
Half of the length of the tunnel was backfilled with a mixture of 30 percent bentonite and
70 percent crushed rock (30/70 mixture). The other half was backfilled with crushed rock
(0/100 mixture) and with bentonite blocks placed nearest the roof. The backfill was applied
and compacted layer by layer with vibrating plates developed and built for this purpose. The
technique with compaction of inclined layers was used in the entire tunnel section from floor to
roof with an inclination angle of about 35 degrees. The backfill was divided up by permeable
mats about every two metres for the purpose of artificially wetting the backfill and permitting a
hydraulic gradient to be applied between the mats for flow testing. The backfill and surrounding
rock were instrumented with about 200 sensors for measurement of water pressure, total
pressure and water content. A concrete tunnel plug with a half-metre thick bentonite O-ring
was installed at the end of the test section.
An evaluation of the installation showed that the equipment and the technique for backfilling
and compaction worked well in terms of both function and obtained density, but needed
improvement in terms of the reliability, durability and safety of the compaction equipment.
Although the dry density of the 30/70 backfill close to the roof and walls was lower than
1,650 kg/m 3 near the rock walls and the roof, it is estimated that the mean dry density was
between 1,650 and 1,700 kg/m 3 . The measured mean dry density of the 0/100 backfill was
2,170 kg/m 3 .
Water saturation of the backfill was started after installation and was predicted to be completed
in the spring of 2003, i.e. after about 3.5 years. In order to speed up the wetting of the
30/70 mixture, salt was added to the water that fed the wetting mats so that a mean salinity of
1.2 percent would be reached after full water saturation. In addition, a water pressure of 500 kPa
was applied to the wetting mats.
The results of the measurements of the water saturation process agreed well with the model
calculations obtained when the model of the unsaturated hydraulic conductivity that had been
calibrated in laboratory experiments was used. However, the hydraulic conductivity used in the
model differs from the one measured in the laboratory on water-saturated samples with constant
gradient in oedometers. This is mainly attributed to the inhomogeneity of the backfill /18-3/, but
may also be due to the difference in experimental technique.
The backfill in the Prototype Repository is very similar to the 30/70 mixture used in the Backfill
and Plug Test. The difference between the materials is that ground sodium-converted calcium
bentonite from Milos has been used instead of the natural sodium bentonite MX-80 /18-4/.
The mean density of the backfill as measured by a density meter was 1,750 kg/m 3 , while the
mean density estimated by dividing the hauled-in quantity of backfill by the available volume
was only 1,630 kg/m 3 . This is presumably due to compaction problems in portions with many
instruments and cables and to lower density near walls and roof.
RD&D-Programme 2004 231