Fractal model for permeability calculation from porosity and pore radius information and application to excavation damaged zones surrounding waste emplacement boreholes in opalinus clay

An excavation damaged zone (EDZ) around emplacement boreholes for radioactive waste represents a potential pathway for radionuclides, due to its increased porosity and crack permeability. As clay is one of the potential host rocks for radioactive waste disposal, a methodical approach was developed for permeability calculation of damaged opalinus clay. The approach was based on a permeability–porosity relationship and on the theory of fractals, which allows the description of rock mass properties by fractal dimensions. It was calibrated against laboratory test data and provides the possibility to calculate the permeability from the pore size parameters "effective porosity" and "pore radius" as well as from the fractal dimension. An application of this relationship was given in terms of calculating the EDZ permeability around a vertical 50m deep emplacement borehole in opalinus clay at a German generic repository site. The Particle Flow Code (PFC) was applied to simulate the fissure development in the EDZ. The results of the simulation provide the necessary pore size parameters. Considering the thermo-mechanical impact of adjacent, already filled, boreholes in an emplacement field for heat-generating radioactive waste, the crack occurrence and evolution over time in the EDZ of a newly drilled borehole was calculated on a 2D horizontal plane. A 3D calculation as well as a verification against in-situ data is still pending.