ABSTRACT
In-situ stress is of prime importance for the design and construction of large and deep underground structures, such as underground power station and geological disposal facility of high-level radioactive waste. Among various techniques for measuring in-situ stresses, only an overcoring method allows the determination of the complete state of stress in a single borehole without making any assumption regarding the in-situ stress field. However, most overcoring methods use boreholes that does not exceed several ten meters from existing free surface because continuous monitoring of changes in strains or displacements are made through conductor cable attached to the instrumented device in the borehole. Therefore we developed three-dimensional in-situ stress measurement equipment, which consists of strain gauge cell, glue capsule and upper cell (data loggers, magnetic compass device and clinometers are included) for 1,000 m-deep vertical boreholes filled with groundwater. The laboratory test using large sandstone blocks confirmed the stress measurement system’s reliability and its ability to determine three-dimensional stress state. The field application confirmed that the stress measurement system works in groundwater saturated 20 m-deep borehole drilled from small size horizontal drift, although the experimental subjects to be resolved were extracted for application of 200 m-deep vertical borehole.
