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 an central area of the core (57.1 % or the total area) was concentrated in a certain direction, which was nearly the direction of the minimum principal stress б3, for all stress conditions except those in which б2=б3. 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 10 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 overcoming 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.
DSCA(Differential Strain Curve Analysis) is a method to evaluate in-situ stress by measuring the distribution of cracks in rock specimens (Strickland and Ren (1980), Ren et al. (1983), Dey et al. (1986), Oikawa et al. (1993), Fujikawa et al. (1997), Ito et al. (1997). The method is based on the premise that underground rocks have no effective microcracks and assumes that, with stress relief, microcracks that are perpendicular to the principal stress are produced in proportion to the magnitude of the principal stress. This assumption has not, however, been sufficiently verified and the reliability of DSCA is not very high. To evaluate the reliability of DSCA in in-situ stress measurement, it is necessary to quantitatively determine core damage due to the tensile stress concentration. Evaluating the core damage is also important in understanding how the mechanical properties of the core of the rock, sampled from deep underground, differ from those in-situ. Previously, we analyzed the stresses inside the core of the rock produced by boring in the general stale of stresses. In this study, we applied the results to quantitatively estimate the tensile stress concentration inside the core and clarify the relationship between the initial in-situ stresses. Then, based on the assumption that the tensile stress that the rock has experienced before complete stress relief causes penny-shaped cracks, we analyzed crack density obtained from DSCA. Lastly, using a specimen where the in-situ stresses was evaluated with the hemispherical-ended borehole technique.
