Different test sites at the Grimsel Test Site (GTS) have been considered and investigated in 2006 and early 2007 to find a suitable site for the planned field experiments.
A site characterisation report of the BK area was documented in an NAB (NAB06-17) and distributed among the partners. The report consists of information about the existing boreholes as well as their present status, detailed geological mapping of fracture zones and an overview of the actual water inflow points of the west-northern part of the BK area.
Though the area would provide the right conditions for grouting experiments (i.e. high flow rates) it was found to be unsuitable for further investigations because the site was heavily used in past projects, particularly with artificial tracers. A further argument against this site was the fact that potential long-term experiments with radionuclides could not be performed on the same shear zone.
In order to enable a comparison with the HPF results and to facilitate later tests with radionuclides, two investigation boreholes were drilled into the VE (or GMT) shear zone between the VE tunnel and the GMT cavern. The sub-horizontal borehole (LCS06-001) revealed unsaturated conditions in this area and pressures of only about 2 bars. Both points disqualified this site as being suitable for long term experiments. The second borehole (LCS07-001) was drilled 42° upward and found a surrounding pressure of about 4 bars. This is seen to be a suitable site to set up the first experiment (grout injection).
Radionuclide behaviour could be tested on the same shear zone but in the controlled zone in a later phase of the project. The experiences on long-term monitoring developed in the first experiments could then be applied for this more demanding experiment.
Figure 1. Geological map in the vicinity of the VE tunnel
On the 12th December 2006 the shear zone was photographed and the groundwater inflow into the VE tunnel was mapped. Figure 2 shows the photograph of the shear zone at the west side of the tunnel (towards the GMT silo). Water bearing fracture planes are partly wet (dark). The yellow “V” indicate groundwater outflow points that were in use during the active pressure phase of GMT.
Long-Term Cement Studies (LCS) Experiment