Background

The long-term international research project DEvelopment of COupled models and their VALidation against EXperiments (DECOVALEX) aims at the further development of coupled thermo-hydro-mechanical-chemical models to support the development of geological disposal options for radioactive waste, geothermal energy or other geotechnologies. The focus is on gaining knowledge of the relevant physical and chemical processes, their relative influence and their interdependencies. The core of the initiative is the comparison of the results of various numerical process models with elaborate experiments from a broad spectrum ranging from laboratory to large-scale field tests for the purpose of model development and validation (for further information please visit decovalex.org).

Here at the Technische Universität Bergakademie Freiberg we are participating in this project by further developing and using the open-source software OpenGeoSys, for example to model the HTV experiments performed within the Mt. Terri Sandwich project. This project investigates the Sandwich shaft sealing system that consists of alternating sealing (e.g. bentonite) and equipotential segments (e.g. sand that has a higher hydraulic conductivity). The layered structure distributes penetrating fluid evenly over the cross section of the shaft. This leads to homogeneous hydration and swelling of the sealing segments, which in turn limit the fluid flow between host rock or surface and repository. Mimicking the Sandwich sealing system, the HTV setup bridges the gap between small-scale lab tests and the in-situ tests performed in the Swiss underground research laboratory in Mt. Terri by applying a semi-technical scale while maintaining controlled boundary conditions.

The aim of these experiments and their modelling is in particular to understand which multiphysical approaches can be used to predict the pore pressure development and stress redistribution due to the bentonite swelling in a Sandwich sealing system. The main objective of simulating the HTV tests is to study swelling pressure increase under confinement, which compacts the material, reduces pore space and thus decreases the flow of liquid. Of particular interest in scaling from lab to field data are the influence of wall friction and inhomogeneous irreversible compaction effects. To this end, we are developing models that can describe the hydraulic and mechanical behaviour of bentonite under unsaturated conditions. Not only the description of the individual processes by means of balance relations and constitutive relationships, but especially their coupling poses a challenge, since, among other aspects, different spatial and temporal scales must be taken into account. The modelling results are compared to measurements of liquid pressure and saturation as well as radial and axial stresses throughout the setup. By modelling such semi-technical scale experiments, the understanding and model capability of the involved processes is tested, improved and demonstrated.

The project is funded by the federal company for radioactive waste disposal (Bundesgesellschaft für Endlagerung BGE).