Mechanical and hydraulic properties of the excavation damaged zone (EDZ) in the Opalinus Clay of the Mont Terri rock laboratory, 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 are crucial. Thus, in this study, a hydromechanical 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 penetrometer. The discrete fracture network (DFN), consisting of artificially induced unloading joints and reactivated natural discontinuities, was investigated by a portable air permeameter and 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 are 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, and physico-mechanical rock parameters and their intra-facies variability.

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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

10.5194/se-2021-4 ◽

2021 ◽

Author(s):

Sina Hale ◽

Xavier Ries ◽

David Jaeggi ◽

Philipp Blum

Keyword(s):

Actual State ◽

Near Surface ◽

Mechanical Fracture ◽

Excavation Damaged Zone ◽

Needle Penetration ◽

Rock Laboratory ◽

Hydraulic Aperture ◽

Damaged Zone ◽

Mont Terri

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.

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Methods for in-situ HM characterization of claystone at the Mont Terri Rock Laboratory

10.5194/egusphere-egu2020-20570 ◽

2020 ◽

Author(s):

Sina Hale ◽

Xavier Ries ◽

David Jaeggi ◽

Philipp Blum

Keyword(s):

Opalinus Clay ◽

Mechanical Fracture ◽

Needle Penetration ◽

Rock Laboratory ◽

Hydraulic Aperture ◽

Fracture Apertures ◽

Mont Terri ◽

Needle Penetration Test ◽

Mont Terri Rock Laboratory

Claystones are considered to represent an important barrier rock in the context of nuclear waste storage. When cavities are opened underground, the rock mass in the near vicinity of the constructed repository is strongly affected by unloading, which is generally referred to as the Excavation Disturbed Zone (EDZ). This area is primarily characterized by newly formed unloading fractures, leading to an enhanced hydraulic transmissivity of the EDZ in comparison to the intact host rock. This phenomenon can affect the integrity of a geologic barrier as open fractures provide possible flow paths and endanger the long-term safety of underground storage facilities. A precise characterization of the EDZ is therefore essential for risk assessment and strategy development in terms of radioactive waste disposal.In this study the Excavation Disturbed Zone (EDZ) of the Mont Terri Rock Laboratory is investigated with regard to hydraulic, mechanical and geophysical properties by using three simple field measuring devices, (1) portable permeameter, (2) microscope camera and (3) needle penetration test (NPT). The hydraulic aperture of accessible joints within the Opalinus Clay formation in the EZ-B niche is measured by a portable transient-airflow permeameter. The instrument was validated by flow-through experiments and is able to accurately determine hydraulic fracture apertures down to about 10&#160;&#181;m. In-situ measurements were carried out at 43 points and show a mean hydraulic aperture of 84&#160;&#177;&#160;23&#160;&#181;m, extending over a range from 20 to 100&#160;&#181;m. Fracture apertures do not change with increasing distance to the gallery in the accessible area of uncovered claystone.For the same set of measuring points, the mechanical fracture aperture was determined by a digital microscope camera. Mechanical fracture apertures in the EZ-B niche ranged between 16 and 1400 &#181;m with a mean value of 268 &#177; 276 &#181;m. As comparable hydraulic apertures can be derived from the measured mechanical aperture by using empirical relations based on estimated joint surface roughness, the microscope camera represents a valuable alternative besides the air permeameter. The hydraulic characterization of the EDZ proves the existence of accessible fluid pathways within the Opalinus Clay of the Mont Terri Rock Laboratory, even about 15 years after tunnel excavation.The mechanical and geophysical properties of the EDZ are investigated by a needle penetration test (NPT). Whereas the needle penetration index (NPI) is strongly influenced by bedding anisotropy, the influence of the EDZ is negligible. The NPT proves to be a suitable tool for estimating mechanical properties by using different empirical relations. Especially for the uniaxial compressive strength, a high correlation with literature values is observed. In contrast, geophysical parameters such as P-wave velocity cannot be reliably determined with this method. The obtained field data could be used as a reasonable input for numerical models that aim at investigating swelling and shrinking behavior of the Opalinus Clay with regard to self-sealing processes within the EDZ.

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Anisotropy of electrical conductivity of the excavation damaged zone in the Mont Terri Underground Rock Laboratory

Geophysical Journal International ◽

10.1111/j.1365-246x.2010.04517.x ◽

2010 ◽

Vol 181 (1) ◽

pp. 303-320 ◽

Cited By ~ 15

Author(s):

Florence Nicollin ◽

Dominique Gibert ◽

Nolwenn Lesparre ◽

Christophe Nussbaum

Keyword(s):

Electrical Conductivity ◽

Excavation Damaged Zone ◽

Rock Laboratory ◽

Damaged Zone ◽

Mont Terri

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Large-scale testing of a sandwich shaft-sealing system at the Mont Terri rock laboratory

Safety of Nuclear Waste Disposal ◽

10.5194/sand-1-133-2021 ◽

2021 ◽

Vol 1 ◽

pp. 133-135

Author(s):

Klaus Wieczorek ◽

Katja Emmerich ◽

Rainer Schuhmann ◽

Jürgen Hesser ◽

Markus Furche ◽

...

Keyword(s):

Pore Pressure ◽

Host Rock ◽

Large Scale ◽

Excavation Damaged Zone ◽

Rock Laboratory ◽

Custom Made ◽

Damaged Zone ◽

Mont Terri ◽

Mont Terri Rock Laboratory

Abstract. Shaft-sealing systems for nuclear waste repositories are constructed to limit fluid inflow from the adjacent rock during the early stage after closure of the repository and to delay the release of possibly contaminated fluids from the repository at later stages. Current German concepts of shaft seals contain the hydraulic sandwich sealing system as a component of the lower seal in host rock (Kudla and Herold, 2021). The KIT-developed sandwich sealing system consists of alternating sealing segments (DS) of bentonite and equipotential segments (ES) that are characterized by a high hydraulic conductivity. Within the ES, fluid is evenly distributed over the cross section of the seal. Water bypassing the seal via the excavation-damaged zone or penetrating the seal inhomogeneously is contained, and a more homogeneous hydration and swelling of the DS is obtained. The functionality of such a system was proven in laboratory and semi-technical-scale experiments (Schuhmann et al., 2009). After a joint international pre-project (Emmerich et al., 2019) dedicated to the planning of a large-scale in situ test that demonstrates the feasibility and effectiveness of the sandwich shaft-sealing system in interaction with the host rock, the large-scale experiment was launched at the Mont Terri rock laboratory in July 2019 with partners from Germany, Switzerland, Spain, UK, and Canada. It consists of two experimental shafts of 1.18 m diameter and 10–12.6 m depth, constructed using a core drilling technique with a custom-made drill rig in a new niche in the sandy facies of the Opalinus Clay. The seal in shaft 1 consists of four DS (calcigel) of 1 m thickness and five ES (fine-grained quartz sand), each 30 cm thick (Fig. 1). Shaft sinking began in August 2020 and was completed in November 2020. In the following months, the sealing system and instrumentation of shaft 1 were installed. The sealing system is saturated from a pressure chamber located at the shaft bottom via an inclined lateral feeding borehole. Hydration of the system started in May 2021. Shaft 2 will host a slightly modified system emplaced 1–1.5 years later, in order to integrate experience obtained during the early operation phase of shaft 1. In contrast to shaft 1, the excavation-damaged zone around shaft 2 will have had time to develop. The seals and the surrounding rock are intensely monitored. Measurements in the rock (geophysics, pore pressure, and total stress) were started between August 2019 and March 2020. Characterization of the excavation-damaged zone along the wall of shaft 1 was performed by geophysical and surface packer measurements prior to seal emplacement. Measurements inside the shaft comprise water content, relative humidity, and temperature, pore pressure, stress, and displacements. The in situ work is backed by laboratory testing and model simulation. Data and experience obtained to date will be presented. The sandwich experiment is funded by the German Federal Ministry for Economic Affairs and Energy under contract 02E11799.

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