(GRS 269) Code Viper – Theory and Current Status

Compacted air-dry bentonite is foreseen in many repository designs to be utilised as a geotechnical barrier since its hydrophilic and thereby swelling properties let it appear to be a material ideally suited for this purpose. The work reported here concentrates on the re-saturation of a partially saturated (air-dry) bentonite.
 
Numerical simulation of this process is presently based on a thermo-hydro-mechanically (THM) coupled approach. The underlying conceptual model, however, is not entirely consistent with the processes on a micro-scale. Thus an alternative ap-proach was developed at GRS that is more closely related to microstructural phenom-ena and fits the requirements of a bentonite buffer in a repository as realistically as possible. Based on balance equations that were especially derived for this purpose a numerical code was developed for testing the theory. The code – now called VIPER – was stepwise advanced from physically simple to the expected complex conditions in a repository.
 
This report summarises the theory behind VIPER including the conceptual model, the properties of bentonite relevant for the model, and the development of the related bal-ance equations. Also described are the transfer of the mathematical model into a nu-merical model and the steps taken to qualify the model for repository relevant condi-tions including the latest effort to extend the applicability of the model to bentonite-sand mixtures. However, since the development of VIPER is not yet finished some questions still remain to be answered.