Claystones in the geomechanical laboratory
Country / Region: Germany
Begin of project: July 1, 2009
End of project: December 31, 2024
Status of project: November 16, 2023
Claystone as host formation deserves detailed characterisation
Photos (and diagram) of gouge in strongly deformed Opalinus Clay
Source: BGR
Claystone differs greatly. Hardly any other natural material is equally multifaceted. However, this property makes it a good candidate as host rock. Though each specificity needs in depth characterication. In particular, the physical properties of each specific claystone need to be determined in detail. What, for example, are its strength, permeability and thermal conduction properties? How will it behave over long times of several hundred thousand years? What happens when an underground working is excavated within it?
The “Claystones in the geomechanical laboratory” work package uses modern, accredited and highly-precise methods to investigate the material properties and the deformation behaviour of various claystones. Investigations are currently focused on the Opalinus Clay in Switzerland (Mont Terri), and the Callovo-Oxfordian Claystone in France (Bure, Meuse/Haute-Marne).
Claystone is a complex material. Its many mineral phases, its anisotropic layering, and its strong hydraulic-mechanical coupling, mean that many conventional geomechanical test methods need to be modified and further developed. One of the priorities is on the development of non-destructive test methods which use mechanical and hydraulic adjustments to enable several test runs to be made on a single specimen. As a result, different pressure and temperature conditions can be tested on one and the same sample. This advantage makes it possible to assign changes in material behaviour to a single control parameter, and avoids sample-specific material differences which are associated with test runs carried out on different samples. Some of the geomechanical tests run for several years, and this in turn places strong demands on the reliability of the laboratory techniques used.
Which parameters can be determined?
Porosity, permeability, density, saturation, compactability, anisotropy, stiffness (dynamic and static e-modulus), compressive, tensile and residual strength, friction angle, cohesion, dilatancy, fatigue, plastic deformation, hydraulic-mechanical coupling parameters (Biot-α, Skempton A and B), pressure diffusion coefficients.
Which processes are being investigated?
By changing the ambient parameters in a controlled way, e.g., pressure / mechanical stress, pore water pressure, temperature or deformation rate, we can determine their influence on the material behaviour. This enables the derivation of material laws which are used to describe the material behaviour. Examples: onset of dilatancy, creep processes, and deformation mechanisms.
The knowledge gained from these investigations is used by others to create long-term models which take into consideration the parameters and processes gained in the laboratory. By involving international research locations (Mont Terri in Switzerland, as well as Bure and Tournemire in France) this work package is also closely networked with universities and other research institutes. In addition, the methods and results used in the laboratory investigations are compared with natural processes.