Analysis and modelling of holographic measurements of in situ stress

Abstract

Holographic in situ stress measurements were made at five azimuths at a single depth in a horizontal borehole in a mine pillar. For the first time, all six components of the stress tensor were determined by this technique. The stressmeter data indicate a vertical stress of −10.2MPa (compressive), in good agreement with the predicted vertical stress for the mine pillar depth and dimensions. The maximum horizontal stress is in ,an approximate E-W direction. The minimum principal stress is 3.2 MPa (tensile) and is oriented in an approximate N-S direction, along the axis of the borehole. The principal stresses are not strongly aligned in the horizontal and vertical directions relative to the borehole. Analysis of holographic stress data uses the analytic solution for displacements induced by drilling a small stress-relief hole in an infinite plate subjected to plane stress. Two-dimensional finite element analysis was conducted in order to compare the displacement of a thin plate to the displacements in a thicker body more closely resembling the situation in the borehole. The results suggest that the values of stress predicted by the current method of analysis may be overestimated for some states of stress of the borehole wall.

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