Fault hydromechanical response to a remote gallery excavation in Opalinus Clay (Mont Terri, Switzerland)
Abstract
Opalinus clay, as present in the Mont Terri rock laboratory, is considered a potential host rock formation for the long-term storage of radioactive waste due to its very low hydraulic conductivity and self-sealing of fractures. However, gallery excavations are known to create an excavation damage zone (EDZ) which extends about two gallery radii into the intact rock. EDZ failure mechanisms correspond to mixed effects of slippage on bedding planes associated to extensional fracturing induced by rock mass deconfinement. Here we report on the continuous monitoring of a fault reactivation that was triggered 30 to 50 m away from the excavation of a 5 m radius gallery, thus well beyond the currently expected EDZ extension. Several instruments were set across the fault zone, chiefly a SIMFIP (Step-Rate Injection Method for Fracture In-Situ Properties) probe which was able to capture the movement of the fault in all six degrees of freedom and a chain extensometer measuring the 1-dimensional extension of several segments of a borehole drilled perpendicularly through the fault. Other instruments included pressure temperature gauges in multiple boreholes at several depth levels, optical fibers for distributed strain sensing (DSS) and platform-tiltmeters at the tunnel floor.
We observed short term reverse-strike-slip during excavation followed by long term pure opening after excavation. Displacements magnitudes are consistent between instruments, being hundreds of micrometers. The extensometer shows that displacements are localized at the fault zone and close to the gallery floor, but no damage is detected in the rock mass between. During excavation, a fault pore pressure increase is measured followed by a decrease after excavation. Months after, the pore pressure still did not recover to its initial state, indicating an irreversible fault permeability change. These measurements thus show that an excavation conducted in a host rock can remotely cause irreversible hydromechanical changes to a fault zone, at distances much larger than expected on the basis of current scientific knowledge, highlighting potential poroelastic stress transfer effects.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMMR005..04S
- Keywords:
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- 1009 Geochemical modeling;
- GEOCHEMISTRY;
- 1011 Thermodynamics;
- GEOCHEMISTRY;
- 1822 Geomechanics;
- HYDROLOGY;
- 1829 Groundwater hydrology;
- HYDROLOGY