Reliable in situ rock stress measurement from the excavation surface

摘要

Measuring rock stress has always been a challenging task. Although borehole overcoring stress methods produce a 3D stress state if multiple measurements at different orientations are conducted, the variation in the result is commonly high and only representative of a small area that can be affected by local geology. Furthermore, these methods suffer from low success rates because of glue-related problems or core disking in highly stressed rocks. To overcome these problems, a new method was developed to measure the induced stresses in the vicinity of an excavated surface and further to use these results to interpret the in situ state of stress at the excavation-scale. The new linear variable differential transformer (LVDT) cell method described in this paper produces a 3D stress state while being less expensive than traditional methods, fast, and produces more repeatable and reliable results. Key advances are glueless mounting, utilisation of a compact drill rig, and side coring technique for high-stress conditions to overcome core disking. The reliability of the solution has. been proven by comparing against known rock stress conditions at the Aspo Hard Rock Laboratory in Sweden. Since 2009, in situ stress measurements have been carried out at over 10 sites in Scandinavia in underground laboratories, hydropower excavations and mines from 15 m up to 1.4 km below surface. Also, the method is in production use in several mines, proving it to be a reliable source of data for the changing stress field around the mine during production.