Geomechanical measurements in a deep dry-drilled boreholen in Asse II rock salt. Final report

Geomechanical measurements have been performed in boreholes drilled from a gallery, 750 m below the surface in the Asse salt mine. The drilling depth varied between 300 m and 500 m and the diameter of the holes was 600 mm. Measured quantities are: The free convergence of the borehole wall, and the mechanical response of the wall to pressure load. From the latter measurements it appeared that the in-situ elastic modulus was significantly higher than the value used by ECN up to now. Based on this revised modulus of elasticity a revised set of parameters for the description of time dependent behaviour of Asse rock salt has been derived. An evaluation has been made of recent free convergence measurements in a newly developed 500 m deep borehole. Due to the relatively short period of measurements, no firm conclusions can be drawn yet, however the preliminary results point to the conclusion that the cfreep law as presented in this report may require revision a soon as more measurement data becomes available. (orig.)