From process to system understanding with multi-disciplinary investigation methods: set-up and first results of the CD-A experiment (Mont Terri rock laboratory)
Authors: Ziefle Gesa , Cajuhi Tuanny , Condamin Sebastian , Costabel Stephan , Czaikowski Oliver , Fourriére Antoine , Friedenberg Larissa , Furche Markus , Graebling Nico , Graupner Bastian , Hesser Jürgen , Jaeggi David , Jantschik Kyra , Kneuker Tilo , Kolditz Olaf , Königer Franz , Kunz Herbert , Laurich Ben , Maßmann Jobst , Ostertag-Henning Christian , Rebscher Dorothee , Rink Karsten , Rühaak Wolfram , Schefer Senecio , Schuhmann Rainer , Wengler Marc , Wieczorek Klaus 
Published: November 2021

Abstract. A potential repository site for high-level radioactive waste should ensure the
highest possible safety level over a period of one million years. In addition
to design issues, demonstrating the integrity of the barrier is essential as
it ensures the long-term containment of radioactive waste. Therefore, a
multi-disciplinary approach is necessary for the characterization of the
surrounding rock and for the understanding of the occurring physical
processes. For site selection, however, the understanding of the respective
system is essential as well: Do fault zones exist in the relevant area? Are
these active and relevant for interpreting system behavior? What is the role
of the existing heterogeneities of the claystone and how do these
site-dependent conditions influence the physical effects? To answer these
questions, the site-selection procedure requires underground exploration,
which includes geophysical and geological investigations on milli- to
decameter scales. Their results serve as the basis for numerical
modelling. This combined, multi-disciplinary interpretation requires extensive
knowledge of the various methods, their capabilities, limitations, and areas
of application. In the cyclic deformation (CD-A) experiment in the Mont Terri rock laboratory,
the hydraulic–mechanical effects due to excavation and the climatic
conditions within the rock laboratory are investigated in two niches in the
Opalinus Clay. The twin niches differ mainly with regard to the relative
humidity inside them, but are also characterized by different boundary
conditions such as existing fault zones, the technical construction of the
neighboring gallery, etc. In order to gain insights into the relevance of the
individual influences, comparative studies are being carried out on both
niches. The presented results provide a first insight into the initial
experimental years of the CD-A long-term experiment and illustrate the
benefits of multi-disciplinary investigations in terms of system understanding
and the scale dependency of physical effects. Amongst other effects, the
assessment of the impact of heterogeneities on the deformation behavior and
the evolution of pore water pressure is very complex and benefits from
geological interpretation and measurements of for example deformation, water
content, and pore pressure. The numerical modeling allows statements about the
interaction of different processes and thus enables an interpretation of the
overall system, taking into account the knowledge gained by the
multi-disciplinary investigation.