The international Stripa Project, executive summary of phase 2

The Second Phase of the Stripa Project included the continued development of methods and techniques for repository site investigations. The crosshole investigations demonstrated that it is possible to characterize fractures in crystalline rock with a reliability and realism not obtained before. At the investigated site at Stripa, it was shown that groundwater flow is concentrated within a few major fractures. The main features were considered to be broadly planar, containing patches of high and low hydraulic conductivity. The migration experiment demonstrated that the groundwater flow could be very unevenly distributed in the rock. Together with the tritium measurements it also gave strong support to the notion that a non-negligible portion of the flow takes place in channels which have little contact with other main channels. It is indicated that a new type of solute source must be considered - fluid inclusions in the host rock. At Stripa, the age of the solutes is likely to be hundreds of millions of years older than the groundwaters. Furthermore, this source contributes the largest portion of the total porosity. Although fluid inclusions are considered to be a residual or non-flow porosity, it could become part of the flow porosity through microfracturing brought about by changing stress fields. Sealing and redirection of the groundwater flow away from man made openings in the rock was tested at Stripa and found to be feasible as shown in the various plugging and sealing experiments. The use of Na bentonite in the form of suitably shaped blocks of highly compacted powder has been found to be very practical for sealing off boreholes, shafts and tunnels in repositories. The clay forms a tight, integrated contact with the rock, so that water flow along the rock contact is hindered. The compressibility and expandability of the clay means that this tight contact is preserved even if slight rock displacements occur. (J.P.N.)