The French national radioactive waste management agency (Andra) is in charge of studying the disposal of high level wastes (HLW) in deep geological repositories. The reference concept for HLW disposal cells consisted of a multi-barrier system: horizontal boreholes drilled in the Callovo Oxfordian (COx) claystone, cased with carbon steel (C-steel) and containing C-steel overpacks with the nuclear waste packages. Mechanical strength is required for the metallic structures to ensure safety. This study presents the work performed on C-steel to assess in situ mechanical loading, long-term mechanical behaviour based on modelling and environmentally assisted cracking (EAC) susceptibility. The results from in situ experiments have demonstrated anisotropy of the mechanical loading of the casing. Long-term calculations revealed local plastic strain after a few years to a few decades, which highlighted the need to assess the potential risk of EAC. Eventually, the results on EAC assessment in the COx claystone confirmed that the microstructure of the casing and overpack plays a key role on the mechanical resistance. This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.