Narrow trenching clamshell bucket - GIF

[Frank S]

Seems like an awful lot of superstructure to that clam, presumably to keep it aligned in the trench.

You are partially correct. Much of the superstructure is for added weight for deep penetration. The trench being cleaned out is a cement-bentonite slurry wall containment barrier. These machines can dig up to 180 feet deep designed to prevent soil liquefaction during earthquake or to contain landfills such as fly ash up to 2500 years
the excavations are generally done with an attachment called a hydromill cutter
https://en.wikipedia.org/wiki/Hydromill_trench_cutter
references
https://en.wikipedia.org/wiki/Slurry...on%20machinery.
https://9to5civil.com/slurry-wall-construction/
https://www.geo-solutions.com/resour...ell-buckets-2/