Damage of rock core due to tensile stresses during boring under in-situ stresses and its relation to DSCA

The maximum tensile stress experienced by an HQ core in an arbitrary horizontal cross section was accumulated in equal area stereonet for 77 stress conditions. The maximum tensile stress accumulated for a central area of the core (57.1 % of the total area) was concentrated in a certain direction, which was nearly the direction of the minimum principal stress σ3, for all stress conditions except those in which σ2 = σ3. Based on the assumption that a penny shaped crack is produced normal to the maximum tensile stress at each point of a horizontal cross section in proportion to the magnitude, the crack density in the core was analyzed by calculating strains under hydrostatic pressure as in Differential Strain Curve Analysis (DSCA). The direction of the maximum crack density was similar to that of the accumulated maximum tensile stress. Thus, the direction of the maximum crack density obtained by DSCA predicts the direction of the minimum principal stress rather than that of the maximum principal stress, if the distribution of pre-existing microcracks before stress relief is isotropic and if additional microcracks are produced by merely the tensile stresses during boring under in-situ stresses. To verify this, the crack density was measured by DSCA for two cores of quartz diorite, which were taken by overcoring when a hemispherical ended borehole technique, one of stress relief method, was applied to measure in-situ stresses at Kanetsu tunnel. The direction of the maximum crack density obtained by DSCA was nearly that of the minimum principal stress for both cores.