The LT-A experiment and the need for long-term laboratory tests

The LT-A experiment (Analysis of Properties and Physical Processes of Clay Stones by Laboratory Tests) focusses on determining the hydro-mechanical properties of Opalinus Clay from the sandy facies and Passwang marl primarily by conducting triaxial experiments. In recent years, the emphasis of the experiments has shifted from solely acquiring rock properties to also identifying constitutive equations. For this, the general concept of the LT-A experiment was changed from performing conventional tests on many samples to analysing the behaviour of a single sample under different stress conditions over a longer period of time. The underlying idea is that several mechanism occurring in clay-rich rocks can only be identified if parameters are analysed under varying conditions and with sufficient time for the rock to react by deformation. In this paper, the latest results of experiments conducted within the framework of the LT-A experiment will be discussed. To avoid ambiguity, in the remainder of the text, the LT-A experiment will only be referred to as LT-A to distinguish it from the various experiments which have been and are conducted within the framework of the LT-A. As of lately, the three subjects of interest have been studied in the LT-A: • the first one was the improvement of existing measurement techniques and the development of new ones; • the second was the investigation of the hydro-mechanical coupling within claystones especially with regard to Young’s moduli, permeability and the onset of dilatancy, which is an important parameter in the German site selection program as German law (in particular Anlage 5 to §24 Absatz 4 of the StandAG (StandAG 2017)) states that no mechanically induced secondary permeabilities outside the EDZ are allowed in a nuclear repository, which from a geomechanical point of view means that the onset of dilatancy must not be reached; therefore, in order to able to fulfil the legal requirements the onset of dilatancy has to be known, making its detailed analysis necessary; • the third one was the analysis of the self-sealing of a fracture with time and the changes in sample permeability with different applied stresses to the fracture.