The term Rettungsarbeit
is labeled as Pluraletantum in Duden Dictionary, i.e. it is used always in the plural form Rettungsarbeiten.
I am seeking for justification from a semantic point of view for this plural usage.
In contrast thereto, Rettungsaktion is used just like other common countable nouns, i.e. both singular and plural numbers are current in actual use. The case of Rettungsaktion is perfectly fine to me. My difficulty is with Rettungsarbeiten.