The authors investigated core discing with a center hole often encountered at the overcoring around a previously bored small hole in highly stressed rock, from observation and stress analyses. The following results were obtained:(1) Core discing with a center hole occurs under the lower in situ stress than the stress under which core discing with no hole occurs.
(2) If the thickness of a hollow core becomes the thinner, core discing occurs under the lower in situ stress. Therefore, the occurrence of core discing can be controlled by the choice of inner and outer diameter of a hollow core.
(3) Main fracture of core discing with a center hole is caused by tensile stress in rock near a boring bit. This tensile stress develops when in situ stress component in the boring direction is high.
Moreover the authors analytically determined the criterion on in situ stress condition under which core discing with a center hole occurs, and compared it with the three dimensional stress condition measured where this phenomenon really occurred. It was confirmed that both were consistent with each other