A fallback facility is another site/building that can be used when the original site/building is unusable or unavailable.
Merriam-Webster Online Dictionaryfallback (noun)
something on which one can fall back - reserve often used attributively a fallback career a fallback position
a back - falling retreat
something that back - falls the fallback from an explosion
Merriam-Webster Online Thesaurus
Fallback is a term used to describe a system or plan that is used when the primary system or plan fails. It is often used to refer to a backup plan or a contingency plan.
Alternate Locations, Alternate Site