Initial Rock Stress Measurement by Use of an Improved Conical-End Borehole Method

The present paper describes an improved Conical-End Borehole Method (ImCEBM) and its field application to initial rock stress measurement. The ImCEBM is an improved version of the Conical-End Borehole Method (CEBM). The improvements are made such ways as 1) an inspection borehole is bored ahead of the conical-end borehole, 2) the samediameter overcoring as that of the borehole is done. By use of the Fast Multipole Boundary Element Method (FMBEM), simulation of the overcoring was made and strains induced by overcoring are obtained. Using the strains, the conversion matrix from measured strains to initial rock stresses was determined. Field measurements were successfully carried out at a site of sandy shale and of fine-grained sandstone, respectively. Initial compressive principal stresses are, for example, 36, 12 and 7MPa, respectively directed in N100°E and horizontal, N10°E and 60° declined from the vertical, and N160°W and 30° declined from the vertical.