In-situ rock stress measurement at an open-pit limestone mine using the CCBO technique

Abstract

In-situ rock stress measurement is performed to estimate the state of stress at the Torigata open-pit limestone mine in Japan using the compact conica-ended borehole overcoring (CCBO) technique. The significance of the state of in-situ rock stress to evaluate the stability analysis of rock slope is then discussed. The CCBO technique was applied to the two horizontal boreholes, Borehole BH-1 at location I and Borehole BH-2 at location II. As a result of in-situ rock stress measured from the two horizontal boreholes using the CCBO technique, the maximum horizontal principal stress is in the NNE-SSW direction at present. The measured average vertical normal stress (−6.50 MPa) for Boreholes BH-1 and BH-2 is in good agreement with the estimated average overburden stress (−6.60 MPa). The horizontal normal stresses σxx (−5.10 MPa) and σyy (−6.00 MPa) are 78.5–92.3% of the vertical normal stress σzz (−6.50 MPa) suggesting that large horizontal stresses exist in Mount Torigata. The ratio of the horizontal normal stress to the vertical normal stress is relatively large at the measured location. Accordingly, for an appropriate evaluation of the mining-induced rock slope stability, not only the vertical stress components but also the horizontal stress components component should be considered.