One thing I've seen of is alot (is this still a word "alot", it gets
shot at from everywhere) is shoring here in NYC, not necessarily on
archaeology sites, though some have been driven by dockbuilders on the
old former US Assay Site. Aluminum shoring (I know it's something else
"over there") is rentable now and I was looking at a US national site
for examples fro trench safety and supplies, as I once had a HAZMAT
Health and Safety certificate, involved with EPA archaeology, it was
required. Anyway, there are site photos of shoring in use at this site
that may be kind of an answer to the problem. I imagine one could lock
these in place too, either in the design or after a purchase drilled
for a lock.

http://www.shoring.com/ushore.html#photos

George Myers