Paper contribution

Long-term monitoring of strain changes around a circular tunnel at the Grimsel Test Site

Alice Petrlíková; Lubomír Staš

Proceedings
EUROCK 2026
Pages
1095-1102
Submitted
1 June 2026
Accepted
15 June 2026
Published
12 August 2026

Summary

Abstract

The Compact Conical Borehole Monitoring (CCBM) strain-gauge probe is derived from the Compact Conical Borehole Overcoring (CCBO) technique, both employing strain gauges mounted on a conical borehole bottom to measure the deformation response of the surrounding rock. While CCBO determines the in situ stress state from stress relief during overcoring, the CCBM configuration enables long-term monitoring of strain changes without induced stress relief, allowing the observation of time-dependent stress changes in the surrounding rock mass. This study presents long-term strain monitoring within the secondary stress field around the circular tunnel, conducted using the CCBM probe at the Grimsel Test Site (GTS), Switzerland, operated by Nagra (National Cooperative for the Disposal of Radioactive Waste). The primary objective was to assess whether stress changes related to pumped-storage power plant operation could be identified in the recorded strain responses. The analysis, however, does not reveal a clear causal relationship between the operational cycles and the measured strain signals, indicating that other mechanisms dominate the observed response. To complement borehole monitoring, Compact Flat Wall Monitoring (CFWM) devices were installed directly on the tunnel surface to record strain changes in the near-field rock mass. A joint analysis of strain data from both monitoring systems confirms the presence of time-dependent variations in deformation in the vicinity of the circular tunnel, which are discussed in terms of their possible origin.

Contributors

Authors

  1. Alice Petrlíková Institute of Geonics, Czech Republic
  2. Lubomír Staš Institute of Geonics, Czech Republic

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References

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