Methods for in-situ HM characterization of claystone at the 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 µm. In-situ measurements were carried out at 43 points and show a mean hydraulic aperture of 84 ± 23 µm, extending over a range from 20 to 100 µm. 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 µm with a mean value of 268 ± 276 µm. 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.