Three-dimensional in situ stress evaluation using a new under-coring technique: the Tseng-Wen Reservoir Transbasin water tunnel

The Tseng-Wen Reservoir Transbasin water tunnel has broken the historical record of the maximum overburden of a tunnel within Taiwan. The three-dimensional in situ stresses at the tunnel were evaluated using acoustic emission (AE) and deformation rate analysis (DRA) with an under-coring technique. This is the first time that the AE and DRA have been used in evaluating an in situ stress in Taiwan. As cores drilled in different directions are required for the three-dimensional stress evaluation, but the in situ borehole drilling is usually limited to the vertical direction, small samples 2.28 cm in diameter and 4.56 cm in length were under-cored from the borehole cores in six directions to evaluate the in situ stresses. Cyclic uniaxial compressive loadings were artificially applied to the Changchikeng sandstones under-cored from the outcrops. The experimental results validate the definitiveness of using under-cored samples at different pre-stresses with AE and DRA. Then, cores taken from the Tseng-Wen Reservoir Transbasin water tunnel site at a great depth of 416–739 m underground were under-cored. The experimental results indicate that the major principle in situ stress is from 30.8 to 39 MPa and is in the directions of SE–NW and SW–NE at different depths.