I had some plumbing work done to relocate some drains, broke the 4" concrete
slab and the trench was 2' x 8', but the main drain was located in a
different location and in order to reconfigure properly, the plumber
eventually made a trench 4' x 12' in size. This is a pretty big trench and
I don't think I can just put back the sand, compact it, throw in a layer of
moisture barrier and pour new concrete.

Do I need some sort of rebar or wire mesh? If so any recommendation on
procedures?

Thanks,

MC