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Investigations on the functionality of bitumen/asphalt shaft sealing elements

Shear deformation and shear strain inside the simulated part of the bitumen sample at differ-ent times

DBE TECHNOLOGY GmbH together with Technical University of Freiberg are developing shaft sealing concepts for repositories for high-level radioactive waste (R&D project ELSA founded by the Project Management Agency Karlsruhe acting on behalf of the Federal Ministry of Economics and Energy). The goal of the project is to design shaft seals that are based on material investigations, site-specific boundary conditions, and the different safety concepts for salt and clay formations in Germany.

A possible sealing material used for shaft sealing is bitumen or asphalt. Parallel to the tests of TU Freiberg at laboratory scale and in situ, DBE TECHNOLOGY GmbH developed a numerical model to simulate the thermo-mechanical behavior of the material and estimated the conditions during construction of these sealing elements. The general thermo-mechanical behavior of bitumen is known from civil engineering, for instance at road construction. The Burger´s-Model describes the viscoelastic behavior of bitumen. The requirements and the loads related to shaft sealing are very different from common road construction because the main loads on sealing elements are steady, static loads that act over very long periods while dynamic loads are irrelevant. Therefore, the fluid characteristics of bitumen become more important and shear thinning has to be considered, too. The model was calibrated by means of a shear test made with a parallel-plate rheometer. In a first step, common oxidized bitumen was tested at different temperatures. A small wedge of the sample was simulated, and the simulation complied well with most of the tests. Some differences result from the fact that shear thinning has not yet been fully implemented in the model.

In a next step, DBE TECHNOLOGY GmbH will start to simulate different sealing elements. But first the thermal impact of the hot bitumen on the claystone and the Excavation Damage Zone (EDZ) had to be investigated. These simulations consider a layer by layer construction of a huge bitumen sealing element. The resulting stress and changes in stress distribution do not expand the already existing EDZ. But inside the EDZ, the high thermal impact will influence the rock properties.  

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