Overcoring CSIRO Triaxial HI-CELL

Details

The CSIRO Triaxial HI-CELL overcoring test is the most accurate method for measuring the Rock Natural Stress.

On the other hand, compared to Hydraulic Fracturing, the field of application is narrower due to some inherent limitations:

  • the maximum test depth is limited to 25-30 m, in order to ensure the success of the chemical bonding of the cell to the rock;
  • the rock joint spacing must be high; with this technique, it is required a certain ease of obtaining long cores, in order to allow the overcoring operation without damaging the cell
  • the borehole can have orientation from horizontal to vertical, but it must have at least a slightly inclination upward, due to the need to clean and dry perfecltly the pilot hole where the cell is glued

This test is particulary suitable for small-medium underground tunnel, niches, etc.

It finds applications also in case of larger rock structures (quarry faces, caverns, etc.); but in these cases the stress measurement result may be a local state of stress, influenced from the excavation; it is thus necessary to execute a back-analysis with a mathematical model of the excavation, in order to obtain teh Natural State of Stress. This operation is relatively simple in the case of an unsupported excavation in elastic rock-mass. It may become very wasteful, in the case of e lined excavation in rock-mass with plastic behaviour.

   

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