05 Feb 2021

05 Feb 2021

Review status: this preprint is currently under review for the journal SE.

Mechanical and hydraulic properties of the excavation damaged zone (EDZ) in the Opalinus Clay of the Mont Terri Rock Laboratory, Switzerland

Sina Hale1, Xavier Ries1, David Jaeggi2, and Philipp Blum1 Sina Hale et al.
  • 1Karlsruhe Institute of Technology (KIT), Institute of Applied Geosciences (AGW), Kaiserstr. 12, 76131 Karlsruhe, Germany
  • 2Federal Office of Topography (swisstopo), Seftigenstr. 264, 3084 Wabern, Switzerland

Abstract. Construction of cavities in the subsurface is always accompanied by excavation damage. Especially in the context of deep geological nuclear waste disposal, the evolving excavation damaged zone (EDZ) in the near field of emplacement tunnels is of utmost importance concerning safety aspects. As the EDZ differs from the intact host rock due to enhanced hydraulic transmissivity and altered geomechanical behavior, reasonable and location-dependent input data on hydraulic and mechanical properties is crucial. Thus in this study, a hydro-mechanical characterization of an EDZ in the Mont Terri underground rock laboratory, Switzerland, was performed using three different handheld devices: (1) air permeameter, (2) microscopic camera and (3) needle penetration test. The discrete fracture network (DFN), consisting of artificially induced unloading joints and reactivated natural discontinuities, was investigated by a portable air permeameter as well as combined microscopic imaging with automatic evaluation. Geomechanical and geophysical characterization of the claystone was conducted based on needle penetrometer testing at the exposed rock surface. Within the EDZ, permeable fractures with a mean hydraulic aperture of 84 ± 23 µm are present. Under open conditions, self-sealing of fractures is suppressed and cyclic long-term fracture aperture oscillations in combination with closure resulting from convergence processes, is observed. Based on measured needle penetration indices, a uniaxial compressive strength of 30 ± 13 MPa (normal to bedding) and 18 ± 8 MPa (parallel to bedding) was determined. Enhanced strength and stiffness is directly related to near-surface desaturation of the claystone and a sharp decrease in water content from 6.6 wt.-% to 3.7 wt.-%. The presented methodological approach is particularly suitable for time-dependent monitoring of EDZs since measurements are nondestructive and do not change the actual state of the rock mass. This allows for a spatially resolved investigation of hydraulic and mechanical fracture apertures, fracture surface roughness as well as physico-mechanical rock parameters and their intra-facies variability.

Sina Hale et al.

Status: open (until 19 Mar 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on se-2021-4', Zeynal Abiddin Erguler, 06 Feb 2021 reply

Sina Hale et al.

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Mechanical and hydraulic properties of the excavation damaged zone (EDZ) in the Opalinus Clay of the Mont Terri Rock Laboratory, Switzerland Sina Hale, Xavier Ries, David Jaeggi, and Philipp Blum

Sina Hale et al.


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Short summary
The construction of tunnels leads to substantial alterations of the surrounding rock which can be critical concerning safety aspects. Here, we use different mobile methods to assess the hydro-mechanical properties of an excavation damaged zone (EDZ) in a claystone. We show that long-term exposure and dehydration preserves a notable fracture permeability and significantly increases strength and stiffness. The methods are suitable for on-site monitoring without any further disturbance of the rock.