This paper presents a new procedure for rock stress determination, which utilizes the strain change on a hemispherical borehole bottom surface during overcoring, and the limitation of the applicability of the hemispherical-ended borehole technique.
The observation equations using the strain coefficients analyzed by FEM at four stages during overcoring is formulated. It is clarified that a more accurate rock stress can be determined by the new procedure, comparing with the conventional one, and shown concretely that the investigation of the measurement quality can be easily carried out, analyzing the residuals between measured and theoretical strains during overcoring. Furthermore, to make clear the limitation of the applicability of the hemispherical-ended borehole technique, the relation between a rock stress state existing prior to boring and drilled direction of borehole for measurement, is analyzed taking notice of the induce stress on a hemispherical borehole bottom surface at two stages during overcoring. It is indicated that the drilled direction of a borehole for measurement may be chosen to minimize the induced stress on the borehole bottom surface during overcoring, when the borehole is drilled in arbitrary direction under a rock stress state existing prior to boring. Then, it is concluded that the investigation of the measurement quality and the calibration test of overcored rock are indispensable to determine rock stress by the hemispherical-ended borehole technique with a high reliability.
