Borehole Sealing Project at the Grimsel Test Site

Within the context of the phase IV (1994–1996) research and development activities at the Grimsel Test Site (GTS), Nagra developed, in collaboration with the Agence Nationale pour la Gestion des Déchets Radioactifs (Andra), an investigation project for the sealing of boreholes drilled from underground. The project had the following goals:

–	sealing of boreholes drilled from underground facilities with a length of up to 500 m,
–	sealing of boreholes with mainly irregular shape (e.g. breakouts of borehole wall),
–	ensuring a hydraulic conductivity of 10−11–10−12 m/s for the seal,
–	ensuring reliable quality control in routine production.

The new concept developed in this project was to use highly compacted bentonite pellets only. The two techniques tested were

(1)	pneumatic injection of pellets into a borehole using a grain size distribution of 4–10 mm,
(2)	emplacement using a modified core barrel for transport and compaction of the pellets.

Both techniques were tested in situ at the GTS to estimate their performance under realistic field conditions. The swelling pressures were monitored for 4 months after seal emplacement until an almost constant value was attained. Finally the hydraulic and mechanical performance of the seal was tested. It was found that the conductivities measured across the seal were equivalent to the matrix properties of the surrounding rock (2–5 × 10⁻¹² m/s). The hydraulic testing also showed no linear preferential flow.