We have an office building project with garages going 4 storeys underground. The building foundation slab is about 16m deep. Should this slab still be considered for 'slab-on-grade' (even if it is rather 'slab-below-grade') and described by F-factor, or should it be modeled rather as a mass floor, with the U-value adjusted for ground (i.e. as per the EN ISO 13770)?  Thank you, Anna