The successful implementation of safe deep geological disposal of nuclear and other long-lived waste is one of the most pressing environmental challenges in several countries worldwide. Site investigation and selection are primarily geoscientific tasks that require interdisciplinary collaboration among, e.g., geophysics, hydrogeology, (hydro-)geochemistry, mineralogy, geomechanics, material science, and geological as well as THMC modelling. The geoscientific information will then, together with other lines of evidence, be used in order to investigate performance and safety of disposal facilities.
As for other subsurface technologies such as the storage of thermal energy and other energy carriers, or the deposition of chemotoxic waste or carbon dioxide, barrier integrity is a crucial aspect. Numerical simulations, in conjunction with experimental studies are an integral part of safety and environmental-impact assessment concepts involving barrier integrity as a key component. Reliable comparative analyses of potential technological options require coupled THMC models capturing the particularities of each rock type and associated repository concept to a comparable level of sophistication. Structural as well as process complexity are often met by data scarcity and variability, necessitating the treatment of uncertainties and variability.
The session provides a platform for the exchange on the following topics:

• The THMC characterization of materials in natural or engineered barriers
• Hydro-mechanical/hydro-chemical behaviour of materials with extreme hydraulic properties (e.g. low permeability, high suction) and ranging from ductile viscopolastic to quasibrittle fractured rock masses
• Computational methods, models and uncertainty quantification for barrier integrity assessment in the multi-barrier system under repository-induced (excavation, emplacement, heat) and geological (tectonic, glaciation) loads
• Geotechnical aspects of repository construction, operation, and post-closure, such as monitoring methods, excavation and support, retrieval/recovery, etc.
• The minimally invasive characterization of geology and underground installations using geophysical and geohydrological methods

Contributions on the above topics can include all aspects covering lab-scale experimentation, large-scale experiments in underground research laboratories, observation of natural analogues, physics- and data-driven modelling and code development.