For geologists and engineers, the host rock is the body of rock immediately surrounding the repository; it is characterised by a low hydraulic permeability.
Together with the adjacent low-permeability rock layers (so-called confining units), the host rock forms the geological barrier of a repository. Host rocks being considered in European disposal programmes include crystalline, salt and clay.
In Switzerland, four clay-rich sedimentary rocks fulfil the strict requirements placed on host rocks:
- Opalinus Clay
- 'Brauner Dogger' clay sequence
- Effingen Member
- Helvetic Marls
A first selection that fulfilled the minimum requirements was further narrowed down by applying stricter requirements.
The rocks listed below are either placed in reserve or do not come into question in Switzerland.
- The clay-rich rocks of the Lower and Upper Freshwater Molasse fulfil the minimum requirements. They have been placed in reserve due to their increased horizontal permeability.
- The crystalline rocks of Northern Switzerland and the Alps also fulfil the minimum requirements. They were also placed in reserve because water flow often occurs in narrow channels (joints) that are difficult to explore from the surface. This makes predictions difficult.
- The salt deposits in Switzerland are not suitable as a host rock. The salt layers are too thin to accommodate the waste and are also too close to the surface. They are partly extracted as a raw material using a leaching process.
The Opalinus Clay is suitable as a host rock for a deep geological repository for radioactive waste in Switzerland.
The Opalinus Clay occurs extensively in Northern Switzerland and neighbouring countries. The layer package is around 110 metres thick and has a uniform structure. It originates from the Jurassic period some 175 million years ago. At that time, Northern Switzerland was covered by a shallow sea. Fine clay muds were deposited on the seabed and were then solidified to form the Opalinus Clay. The name comes from the ammonite “Leioceras opalinum” that is often found as fossils in the clay.
Opalinus Clay has a very good sealing and isolation properties due to microscopically small plate-like clay minerals.
The requirements relating to thickness and depth of the host rock are met in a section extending from Olten to Schaffhausen. The engineering and chemical properties of the Opalinus Clay are well understood through laboratory experiments, tests in the Mont Terri Rock Laboratory and from investigations in boreholes and tunnels.
Open tunnel wall at the Mont Terri Rock Laboratory (Canton Jura). Image: Comet Photoshopping, Dieter Enz
The 'Brauner Dogger' clay sequence comprises a stack of clay-rich, low-permeability rock layers that were deposited around 175 to 160 million years ago between the Opalinus Clay and the Effingen Member.
The clay sequence extends east of the lower course of the river Aare to the northern margin of the Molasse Basin in the vicinity of Schaffhausen. As with all marl-rich rocks, it is difficult to determine the extent and distribution of sandy and limy zones underground from the surface. The pores of the sandstones are cemented with limestone over large areas, making them less permeable. The clay-rich units have a self-sealing capacity very similar to that of the Opalinus Clay.
Two core sections from the 'Brauner Dogger' clay sequence from a depth of around 500 metres in the Benken borehole. The left core contains fossilised shells (white) filled with clay mud. Image: Nagra
The Effingen Member was formed around 160 million years ago from marine deposits. It consists of clay-rich marl layers with local limestone beds. The rock composition and its thickness are variable, with the latter reaching up to 300 metres. A suitable deposit can be found extending from the west of the lower course of the Aare river to the vicinity of Olten.
The Effingen Member has a low permeability if it is covered by at least 200 metres of overburden. The swelling capability allows self-sealing of small fissures. The engineering properties are known from underground construction projects.
The fine layering of the Effingen Member can be seen in this quarry. The height of the rock wall is around 15 metres. Image: Dr. Heinrich Jäckli AG
The marl formations of the Helvetic Zone at Wellenberg consist of fine sandy, shaly clay and limestone marls and were formed around 135 million years ago (lowermost Cretaceous) in the ancient Tethys sea.
During the course of the alpine orogeny they were displaced far to the north together with the adjacent limestones, where they now form the thick rock packages along the northern margin of the Alps, the so-called Helvetic nappes.
These marls were already investigated in detail in an earlier phase of the search for a site for a low- and intermediate-level waste repository.
The large volume of rock offers a high element of flexibility when designing the repository geometry. The hydraulic conductivity of the clay-rich rocks is low
Section of a core with marls from exploratory borehole 4 at Wellenberg. Image: U. Gerber