The employer isn't necessarily negligent here, it's the main fault of the EOR and his allowance that work be continued despite a premature failure in a stressed member. At that point it should have been shut down.
I read through the big report on the bridge over the weekend. Ugh.
They had cold joints where the truss members met the bridge deck, because they forgot to grind/abrade/whatever the surface off the deck before casting the truss members on the second pour. The drawings didn't show it being necessary, apparently the builder was supposed to know they were supposed to do it to meet some government regulation, but in any case it didn't get done and nobody noticed it didn't get done.
End result was a joint that couldn't handle the shear force that the truss member was putting on the joint, and the crack began at that joint. The rebar in the joint was probably the only thing keeping the joint from shearing apart the moment they laid the bridge on its supports. When they re-tensioned the joint in an attempt to close the crack (yes, because if a major failure isn't visible, it goes away? what the fuck) that put even more shear force on the compromised joint and it let go.
Again, pure hubris on the part of the engineers (or EOR, or whatever) who didn't want to admit the bridge was fucked because it would hurt their pride or some shit.
Most of the engineering failures that are shown in engineering ethics courses (Hyatt walkway collapse, Therac-25, Quebec Bridge, etc) are the result of 'incompetent failure' - a bad decision got made on the design side, and a failure happened as a result. This engineering failure is worse than all of those, IMO - they had an obvious failure sitting there right in front of them, and decided to carry out steps to hide that failure.
