Determining the thermo-mechanical and hydraulic behaviour of evaporitic rocks (THM evaporites)

Country / Region: Germany

Begin of project: January 1, 2005

End of project: December 31, 2026

Status of project: July 21, 2023

German Version

Indirect tensile test (Brazilian test) being carried out on a rock salt specimen from a flat-bedded evaporite formation (Middle Triassic, South Germany) Source: BGR

As a potential host rock for the long-term disposal of radioactive waste in a geologic repository, rock salt has been the subject of scientific investigations by BGR for several decades.

Various properties of intact rock salt as well as on damaged rock salt are under investigation. These investigations include the long-term deformation behaviour (creep) at low deviatoric stresses far below the damage limit (dilatancy limit), the disaggregation and fracturing (dilatancy and strength limits) that can occur close to the shafts and drifts, and the elastic properties.

The investigations in this project also include a range of laboratory experiments, including uniaxial, triaxial and true triaxial strength and creep tests, as well as tensile tests.

The rock physical (especially rock mechanical) laboratory investigations are carried out at the Hanover site. The "true triaxial" testing machine for rock mechanical tests is located in Grubenhagen near Einbeck (Lower Saxony).

These laboratory investigations are carried out with the following objectives in mind:

1. Determination of specific petrophysical parameters as a basis for site characterisation and numerical modelling e.g. for safety analysis for radioactive waste repositories.
2. Determination of principles of the thermo-mechanical and hydraulic behaviour of evaporitic rocks and the refinement of the mathematical formulation of material laws (creep laws; stress-dependent descriptions of strength and dilatancy boundaries).
3. The determination of the dependencies, e.g. on the stress, moisture, macro- and micro-structure as e.g. grain size, sub-grain structure and the number of fluid inclusions in the respective salt examined from different types of deposits.

Rock samples after loading in the "real triaxial" testing machine used in in our rock mechanics laboratory Source: BGR

In the first years of the project, the focus was on domal rock salt, such as the Gorleben salt dome and the Morsleben salt structure. Since 2016, the focus of the project lies on investigations of rock salt from flat-bedded deposits, e.g. from the WIPP site in New Mexico (USA), as well as from various German salt mines. Rock salt from domal or flat-bedded formations need to be looked at separately, because there are considerable structural differences between the two material types. These differences in turn may lead to differences in thermo-mechanical behaviour.

For the description of the behaviour of salt glaciers, for calculations in cavern construction or for the description of processes in the far field of artificial structures such as repositories, the long-term deformation behaviour (in short: creep) of salt at small deviatoric stresses is of relevance. The aim of the current project phase is to be able to carry out long-term experiments with low-maintenance equipment in a climatically advantageous environment in the medium term.