10. EDZ along a borehole - Insitu test at Mt Terri
Record Control Number:
Quality Validation Date:
Abstract: The most important parameter for performance assessment to be measured in the EDZ is the axial transmissivity along tunnels, as this preferential flow path could act as a shortcut for radionuclides.
All tests performed so far are designed to measure the local effect of fractures, but not the change of the axial transmissivity, which is controlled by the interconnectivity of single fractures. A 1:1 scale test of the axial transmissivity along tunnels is a very difficult and expensive task. Therefore, it is planned to investigate the axial transmissivity on a much smaller scale along a borehole.
Previous laboratory and in situ tests have indicated that under special boundary conditions borehole instabilities will occur in Mont Terri clay. The main reasons for borehole instabilities are the orientation of the boreholes with respect to the direction of the material anisotropy (transversely isotropic material) or bedding, the stress field and the chemistry of the borehole fluid. The most stable conditions prevail for boreholes drilled with air perpendicular to the bedding, while the worst case results when the axis of the borehole is oriented parallel to bedding and water penetrates into the borehole.
During the in situ test, a borehole is air drilled parallel to bedding. The borehole will be instrumented with a double-packer system to perform the testing. One packer is a combined hydrotesting / dilatometer tool which is able to isolate a borehole interval with controlled pressure and at the same time to measure the deformation of the borehole wall as a function of inflation pressure.
The test is performed in the following way:
- Drilling of a test borehole with air
- Installation of the test equipment and filling of the borehole with synthetic formation water
- Inflation of the dilatometer up to the minimum pressure required to get the displacement sensors in contact with the borehole wall
- Monitoring of the borehole convergence (creep) and of the EDZ
- Hydro testing to evaluate transmissivity along borehole (packer by-pass)
- Increase of dilatometer pressure in several steps and monitoring of the associated creep (closure of EDZ) and change in transmissivity per pressure step
It is shown that an EDZ with a distinct higher transmissivity than surrounding the rock mass develops in short time around a borehole and that this transmissivity can be reduced as a function of the stepwise increase of the inflation pressure of the dilatometer from 0.5 to 5 MPa. The observed hydraulic properties of the EDZ around the borehole appear to be controlled by the load applied to the borehole wall. When the load applied to the borehole wall reaches 5 MPa, a reduction of the transmissivity of several orders of magnitude to about E-13 m2/s of the axial flow path along the dilatometer is observed.