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Results of non-intrusive monitoring experiments to date at Mont Terri
The results of seismic investigations on the HG-A experiment at Mont Terri suggest that cross-hole
travel-times do not enable information to be gathered about the state of a 1 m diameter micro-tunnel
that lies midway between source and receiver boreholes separated by distances of 5-30 m. In contrast,
data recorded on geophones mounted within the micro-tunnel provide diagnostic information
about the micro-tunnel fill and the state of the micro-tunnel EDZ. Changes in the micro-tunnel fill
and EDZ result in marked variations in seismic wave arrival times, polarities and waveforms. A full
waveform inversion of the combined geophone and hydrophone data will be undertaken as part of
this study to fully assess the imaging capabilities of seismic tomography for this type of application.
3.6 Hydraulic seal material selection and installation test (EURIDICE)
At the Mol Underground Research Facility (named “HADES”), EURIDICE is currently installing
the different components for the PRACLAY Heater Test, in which the thermal behavior of a disposal
gallery will be simulated at near real scale. A gallery ("PRACLAY Gallery") has been constructed
to serve as a dummy disposal gallery. To obtain the undrained initial and boundary conditions
– to which the real disposal galleries will also be subjected – an annular seal composed of
compacted bentonite will be installed between the heated zone and the access gallery.
Figure 3.6.1 provides an illustration of this.
Figure 3.6.1 The seal will be installed between the heated and saturated part of the dummy disposal
gallery ("PRACLAY Gallery"), and the accessible part (linked to the Connecting Gallery)
The objective to obtain the undrained experimental condition requires the seal to be as impermeable
as possible at its location at the interface Boom Clay/seal. This annular seal also provides an interesting
opportunity to study the feasibility of a hydraulic cut-off of any preferential pathway to the
main gallery through the EDZ and the liner with a seal in a horizontal drift (horizontal seal). This
implies minimizing the longitudinal hydraulic gradient along the repository gallery.
A swelling bentonite will be used for the seal material, which should provide:
• a permeability one or two orders of magnitude lower than that of the Boom Clay itself, i.e.
not larger than 10 -14 m/s (~ 10 -21 m²) at its saturated state;
• a suitable swelling pressure, but not larger than the in-situ lithostatic pressure at long term;
the target value for the PRACLAY seal is 4.5 – 5 MPa.
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