Arbeitsbericht NAB 16-43

ENSI-Nachforderung zum Indikator "Tiefenlage im Hinblick auf bautechnische Machbarkeit" in SGT Etappe 2 – Geomechanische Unterlagen

As part of reporting for Stage 2 of the Sectoral Plan for Deep Geological Repositories (SGT), Nagra has documented the comprehensive geomechanical data base and derived reference parameters for scoping calculations to assess engineering suitability of a deep geological repository in Opalinus Clay (Nagra 2014a). In a critical review of this and supporting reports, the expert panel commissioned by ENSI concluded that the number of reliable test results was very small, despite the extensive laboratory programmes performed by Nagra (ENSI 33/460; ENSI 33/461). Accordingly, additional documentation was requested by ENSI (33/476) and Nagra was asked to reassess the geomechanical data base and to address the uncertainties arising from the smaller data base by widening the range of considered parameter values. The expert panel further criticized that the parameter values recommended by Nagra did not consider explicitly i) a depth dependency in the relevant depth range (400 – 900 m), ii) the reduction of the elastic properties of the rock mass after yielding. However, the expert panel considered the material law selected by Nagra (modified CamClay model with Hvorslev shear strength limit and a no-tension cut-off) to be appropriate.

Following the recommendations of ENSI’s expert panel, this report presents a reassessment of the geomechanical data base of Opalinus Clay, also drawing on relevant recent studies that were not available when the documentation of SGT Stage 2 (Nagra 2014a) was finalised. Particular focus is given to conceptual considerations, on the basis of which three rock models (compared to six in Nagra 2014a) are formulated to cover the range of expected material properties in Opalinus Clay using the continuum approach. In a reference rock model (GMref), Opalinus Clay is considered as a largely uncemented claystone, with geomechanical properties mainlycontrolled by the degree of overconsolidation. In alternative rock models, Opalinus Clay is either considered as containing significant diagenetic cement, with an accordingly lower porosity and higher stiffness / strength (GMmax), or as tectonically destructured Opalinus Clay with elevated porosity and a very low overconsolidation ratio (GMmin). In the chosen elastoplastic framework, diagenetic and tectonic processes can be simplified by variation in both the maximum burial depth and the volume increase during unloading.