The influence of Poisson"s ratio on overcoring was investigated using data from 47 tests. In these tests both the CSIRO HI triaxial cell and the triaxial cell developed by Swedish State Power Board were used. Boreholes drilled parallel to the major principal stress resulted in higher Poisson"s ratio compared to boreholes drilled at an angle to major principal stress. Numerical modelling by FLAC was used to simulate overcoring and biaxial testing. When the major principal stress is acting perpendicular to the borehole, the extensional strain is larger on the inner surface of the core during overcoring than when parallel to the borehole. Simulations of a biaxial test of a virgin core show extensional strain, which may be larger than critical extensional strain of the rock type, indicating non-elastic behaviour.
Äspö Hard Rock Laboratory (HRL) in southern Sweden is a full-scale laboratory for a future deep repository of spent nuclear fuel. In 1990 the excavation of the ramp started. The ramp is about 3600 metres long and down to 450 meters below ground surface. The first part was excavated by a conventional drill and blast method, while the last 400 meters was excavated by a Tunnel Boring Machine (TBM). A comprehensive analysis of all results from rock stress measurements performed to date at Äspö HRL has been done. The magnitudes of rock stresses obtained from overcoring were higher than those predicted by hydraulic fracturing. Statistical analyses of the elastic properties from the Äspö data showed that there was a significant difference between the mean values of Poisson"s ratio obtained from biaxial tests of cores performed with the two different cells. The analysis gave a Poisson"s ratio of 0.34 for the CSIRO HI-cell and 0.23 for the SSPB-cell (Lundholm, 2000).
