ABSTRACT:

The Compact Conical Borehole Overcoring (CCBO) method originally developed in Japan to determine stress in rock has been widely used by the Institute of Geonics, Czech Academy of Sciences. Only one borehole is required to install the CCBO instrument, measure strains and determine the full stress tensor in rock mass. To determine the pre-mining stress around the circular tunnel in undisturbed hard rock, the measurement requires a borehole of certain length to install the CCBO cell in location where the excavation does not affect the stress field. To reduce the drilling and installation costs, the Institute is developing new theoretical solutions for a short borehole strain measurement method enabling the CCBO cell installation closer to the circular tunnel. The proposed semi-analytical approach uses the short borehole strain measurement results, theoretical calculations of stress distribution in the elastic medium around the circular tunnel and numerical modelling, to back analyze the probable pre-mining stress tensor. This approach enables determination of pre-mining stress obtained from the short borehole strain measurements in excavation-induced zone around underground opening. Discussed here are the theoretical calculation methods, numerical modelling results and a practical example of the pre-mining stress determination from the short borehole strain measurements using the CCBO instrument.

1 STRESS DETERMINATION

Compact Conical Borehole Overcoring (CCBO) is a method used for stress tensor determination in rock mass. Overcoring methods are used worldwide, and like all methods of in situ stress determination, these methods also have their limits. The correct determination of the stress based on measured strains should include the characterisation of the rock mass with respect to its anisotropy (Amadei 1996, Hakala et al. 2003, Hakala 2006, Kang 2000, Zang & Stephanson 2010) to describe the most possible, true relationship between stress and strain.

The principle of the method is the stress relief of the rock core, which is formed by overcoring of the installed probe. The stress relief due to overcoring is manifested by deformation response of the rock core, which is monitored by strain gauges. The full stress tensor is then determined from strains measured in specified directions.

The measurement should be made in unfractured rock. The confidence of the strain measurement is enhanced by camera inspection in a borehole in order to avoid the probe installation that is influenced by a local anisotropy and some structural inhomogeneity. So the surroundings of the measurement probe can be considered as a homogeneous, elastically responding rock core without any cracks and discontinuities.

One of the research objectives of the Institute of Geonics, Czech Academy of Sciences, is the stress state of the rock mass and its influencing factors such as the stress strain relationship determination for the anisotropic rock and evaluation of the general error of the CCBO method.

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