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Generation and Self‐Sealing of the Excavation‐Damaged Zone (EDZ) Around a Subsurface Excavation in a Claystone
For over two decades, researchers have investigated and monitored the excavation‐damaged zone (EDZ) of a claystone, the Opalinus Clay in the Mont Terri rock laboratory in northwest Switzerland. Scientifically engineered investigations of the EDZ during excavations revealed a plastically deformed zone of one tunnel radius around the tunnel. This fracture network was characterized by structural, pneumatic‐hydraulic, and seismic methods. The structural investigations consisted of small‐scale mappings of newly excavated sidewall niches, and yielded a data set comprising fracture orientations, frequencies, and measurements of the lateral extent of the EDZ. Pneumatic and hydraulic investigations showed that fracture permeabilities and transmissivities are orders of magnitude higher compared to those of the undisturbed claystone. Miniseismic investigations in radial boreholes around the tunnel resulted in reduced seismic P‐wave velocities and amplitudes compared with those of undeformed claystone. We found two types of EDZ: stress‐induced and anisotropy‐induced. Stress‐induced EDZ was identified during mine‐by tests showing that formation of extensile EDZ fractures occurred mainly during and shortly after excavation. Anisotropy‐induced EDZ fractures were monitored in galleries directed parallel to the rock surface, indicating that convergences and breakouts are ongoing. Hydraulic and miniseismic characterization have yielded valuable data to assess the sealing potential of the fracture network. Both methods showed that hydraulic transmissivities decrease and the P‐wave velocity increases when moisture is provided to the fracture network. Full sealing may only be obtained after decades to centuries.
Generation and Self‐Sealing of the Excavation‐Damaged Zone (EDZ) Around a Subsurface Excavation in a Claystone
For over two decades, researchers have investigated and monitored the excavation‐damaged zone (EDZ) of a claystone, the Opalinus Clay in the Mont Terri rock laboratory in northwest Switzerland. Scientifically engineered investigations of the EDZ during excavations revealed a plastically deformed zone of one tunnel radius around the tunnel. This fracture network was characterized by structural, pneumatic‐hydraulic, and seismic methods. The structural investigations consisted of small‐scale mappings of newly excavated sidewall niches, and yielded a data set comprising fracture orientations, frequencies, and measurements of the lateral extent of the EDZ. Pneumatic and hydraulic investigations showed that fracture permeabilities and transmissivities are orders of magnitude higher compared to those of the undisturbed claystone. Miniseismic investigations in radial boreholes around the tunnel resulted in reduced seismic P‐wave velocities and amplitudes compared with those of undeformed claystone. We found two types of EDZ: stress‐induced and anisotropy‐induced. Stress‐induced EDZ was identified during mine‐by tests showing that formation of extensile EDZ fractures occurred mainly during and shortly after excavation. Anisotropy‐induced EDZ fractures were monitored in galleries directed parallel to the rock surface, indicating that convergences and breakouts are ongoing. Hydraulic and miniseismic characterization have yielded valuable data to assess the sealing potential of the fracture network. Both methods showed that hydraulic transmissivities decrease and the P‐wave velocity increases when moisture is provided to the fracture network. Full sealing may only be obtained after decades to centuries.
Generation and Self‐Sealing of the Excavation‐Damaged Zone (EDZ) Around a Subsurface Excavation in a Claystone
Dewers, Thomas (editor) / Heath, Jason (editor) / Sánchez, Marcelo (editor) / Bossart, Paul (author) / Nussbaum, Christophe (author) / Schuster, Kristof (author)
2019-09-06
19 pages
Article/Chapter (Book)
Electronic Resource
English
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