Study on mechanisms of strain variation at great depth (Joint research)

Knowledge of strain variations at an excavation face is important for the prediction of rock bursts and for the application of the appropriate supplementary construction methods for excavation of shafts to great depths. With the above as objectives, JAEA (Japan Atomic Energy Agency) installed 'intelligent' strain meters (resolution in the order of 10-9), developed by TRIES (Tono Research Institute of Earthquake Science), at G.L-500m depth in pilot boreholes drilled from the 200m level in the ventilation and main shafts of the Mizunami URL (Underground Research Laboratory) in the Joint research with Nagoya University. Strain variations due to several events including the excavation itself were monitored and analyzed. Because of the breakdown of the monitoring system, the data is very short term. Nevertheless, it was possible to do some analysis for the study and important understanding derived from the analysis includes: -Ground water pressure changes due to earth tides, stress relief due to earthquakes and blasting are main factors for very small strain changes. Especially, changes to the groundwater table due to excavation are important to understanding the mechanism for long-term continuous strain change. -Strain changes due to earthquakes and blasting are instantaneous and most of the change is within the elastic limit. A total of eighteen earthquakes were observed from October 2007 to February 2008. A small residual deformation was measured from one of the observed earthquakes. -The magnitude and the direction of the strain increment over the long-term are influenced by the in-situ stress state in the geological structure. Therefore, information on the geological environment where strain is monitored is considered necessary to understand behavior. Monitoring results indicate that the intelligent type strain meter is able to measure very small fluctuations (10-9) and detect rock deformation from blasting with 300m vertical separation from the measurement section. The strain meter can detect conditions in the rock mass such as differences in deformability. On the other hand, it is very important to improve the total measurement system for stable monitoring. A CD-ROM is attached as an appendix. (J.P.N.)