Oxidation Phenomena and Processes in Opalinus Clay: Evidence From the Excavation-Disturbed Zones in Hauenstein and Mt. T
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Shales (e.g., Opalinus Clay) are considered as potential host rocks for the deep disposal of radioactive waste in Switzerland and in other countries. Tunnels that intersect Opalinus Clay in the Swiss Jura Mountains with overburdens of at least 200 m form a unique opportunity to study oxidation and desaturation phenomena and other geologic and hydrogeologic processes in the excavation-disturbed zone (EDZ) under physically relevant conditions and at known time frames relevant for the operational phase of a repository (20-100 years). Such time frames also offer important constraints on bridging laboratory time scales (weeks to months) and those of natural analogues (104-107 years). The aim of this study is to evaluate constraints on oxidation processes from macroscopic and microscopic features, geochemical changes and mass balances, and inferred rates of alteration and material transport in the EDZ. This data set can be tested against independently measured transport properties and physico-geochemical models, and hence contributes to a defensible and mutually consistent scientific basis for safety-relevant issues of deep waste disposal. GEOLOGICAL SETTING Opalinus Clay is a marine sediment of Aalenian age (Early Jurassic, ca. 185 Ma) and occurs throughout Northern Switzerland and adjacent parts of Southern Germany, and is exploited for brick fabrication in several pits. The 80-120 m thick layer of Opalinus Clay disappears towards the south below the Molasse basin with a dip of ca. 4-5', and it occurs at depths in excess of 5 km close to the Alpine mountain chain. 731 Mat. Res. Soc. Symp. Proc. Vol. 506 ©1998 Materials Research Society
In the Tabular Jura and in the Molasse basin, deformation of Opalinus Clay is limited to faulting and tilting; penetrative deformation involving major tectonic thinning or thickening is absent. In the Jura Mountains, where the Mesozoic sedimentary pile is sheared off from its basement and folded to form characteristic box-shaped folds with wavelengths in the range of hundreds of meters to kilometers, the Opalinus Clay typically occurs in the core regions of the folds, where it is highly tectonized, squeezed out in some areas and accumulated in others. The Hauenstein and Mt Terri tunnels penetrate folds of the Jura Mountains and thus contain Opalinus Clay subjected to substantial deformation, whereas the Siblingen clay pit in N Switzerland is situated in the Tabular Jura where deformation is weak. Typical mineralogical compositions [1] of Opalinus Clay comprise 10 - 30 wt% quartz, 5 - 20 % calcite and 60 - 80 % clay minerals (illite and illite/smectite mixed-layer phase 20 - 40 %,chlorite 5 - 10 %,kaolinite 15 - 35 %) and accessory siderite, feldspars and organic matter. Pyrite contents are typically around 1 % (range 0.2 - 1.4 %). OLD HAUENSTEIN RAILWAY TUNNEL The EDZ within Opalinus Clay at the 140-year-old Old Hauenstein railway tunnel is characterized by an extensive fracture network parallel to tunnel walls and extends beyond a drilled depth of 1.6 m. The network consists of some a
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