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through pressure in the saturated state enabling gases to migrate out of the disposal cell at reasonably
low pressure without any damage of the host rock.
The objective of the test programme is to test and demonstrate that the sealing properties of
clay/sand mixtures determined preliminarily in the laboratory can be technically realized and maintained
under repository relevant in-situ conditions. The test programme consists of three steps: (1)
laboratory investigations/mock-up tests for selection of suitable seal material mixtures and for testing
material installation techniques, (2) scoping calculations for in-situ test preparation with an assessment
of seal saturation under in-situ conditions, and (3) in-situ tests at the Mont Terri Rock
Laboratory (MTRL).
Laboratory Investigations for the Selection of Suitable Material Mixtures
In the GRS-laboratory in Braunschweig, different clay/sand mixtures with mixing ratios between
35clay/65sand and 70clay/30sand have been investigated with regard to their sealing performance
[7]. These investigations have shown that the functional requirements given above are best met by
35clay/65sand and/or a 50clay / 50sand mixtures (see Table 3.4-1) which therefore have been selected
for further testing under in-situ conditions at the MTRL.
Large-Scale Mock-up at GRS’s Laboratory in Braunschweig/Germany
Before going in situ, both the installation techniques and the required saturation time for the material
mixtures being considered were to be investigated and optimized in large-scale mock-up tests in
GRS’s laboratory in Braunschweig.
Table 3.4-1. Measured material parameters at installation conditions (averages in parentheses)
Sample
Gas permeability
under dry
conditions
[m 2 ]
35/65 1.2E-13
50/50 7.5E-14
Initial water permeability
at full saturation
[m 2 ]
3.3E-17 - 9E-18
(5.2E-18)
1.1E-18 - 4.3E-18
(2.2E-18)
Gas
breakthrough
pressure
[MPa]
0.4 - 1.1
(0.75)
0.4 - 2.8
(1.83)
251
Gas permeability
after gas
break-through
[m 2 ]
1.1E-17 - 1.6E-17
(1.4E-17)
5.5E-18 - 6.2E-18
(5,9E-18)
Swelling
pressure
[MPa]
0.2 - 0.4
(0.28)
0.3 - 0.5
(0.35)
The mock-up tests (Figure 3.4.1) are performed in vertically arranged steel tubes and they are designed
as a full-scale replica of the envisaged in the in-situ experiments (Figure 3.4.2). The lower
fluid injection volume is filled with a porous material (stone chips or sand). At top of the porous
medium a filter frit is placed for ensuring a homogeneous distribution of the injected water over the
entire borehole cross section. Above the filter frit, the clay/sand-seal is installed in several layers to
a height of 1 m. The predetermined installation density of about 1.9 g/cm 3 for the 35/65 clay/sand
mixture has been realized by using an electric vibrator for material compaction. A further filter frit
is installed above the seal for water and gas collection. The whole test set-up is sealed against the
ambient atmosphere by a gastight packer on top of the upper filter frit. In contrast to the in-situ experiments
where instruments are not installed in the seal itself to avoid any negative impact on the
sealing properties, the mock-up is equipped with sensors in the test tube wall to monitor the pore
and total pressure evolution at three levels along the seal. Additionally, two total pressure sensors
are installed at the bottom of the upper frit. Furthermore, the mock-up is equipped at the inlet and