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Impact of earthquakes on geotechnical barriers

Model of the gravel column of a shaft seal. Subsidence of gravel level due to settling of gravel caused by an earthquake.

In order to prevent that fluids get in contact with the radioactive waste emplaced in an underground repository, the integrity of the geotechnical barriers under different safety-relevant scenarios needs to be demonstrated. One possible safety-relevant scenario is the impact of an earthquake on the geotechnical barrier.

Earthquakes are tectonic movements caused by fracture formations in the earth's crust. The sudden energy release of the fracturing processes generates transient, elastic waves that propagate and can cause massive tremors. Generally, the resulting damages to structures on the surface are larger than to structures underground, where the damage is usually negligible. However, according to current designs, gravel is to be implemented in several sections of shaft seals to act as porous reservoirs and as abutment for bentonite sealing elements installed on top of it. The tremors of an earthquake can further compact the gravel, which leads to a settling of the material, which in turn can compromise its effectiveness as an abutment. For a proper functioning of the bentonite sealing element it is, thus, necessary to limit the settling of the underlying gravel column to appropriately small dimensions.

The settling of granular material can be calculated analytically using the silo theory, and the additional settling due to the impact of an earthquake can be determined to some extent. However, an analytical assessment has several disadvantages. One aim of the R&D project ELSA (founded by the Project Management Agency Karlsruhe acting on behalf of the Federal Ministry of Economics and Energy) is to determine to what extent the settling of a gravel column due to an earthquake can be estimated by means of computer codes. This would allow the verification of existing estimates and the evaluation of the suitability of new technical concepts.

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