Rock-solid evidence right there. Built on a foundation of clay and weak rock. All to save the cost of concrete fill.
Hi Meyer, the revelation of the "debate" with the DWR Field Engineer and the Contractor is amazing, and the fact that this was specifically documented in the Final DWR Construction Report too.
This would reveal that the Field Engineer felt strongly about his position, even though this went completely against the smart engineering specification of using a power excavator as the "test gauge" insuring the excavation was to strong rock.
I suspect the DWR Field Engineer represented a larger group of engineering decision/thought that the clay foundation ("worst foundation available") was deemed acceptable as there were anchor bar tests performed in clay seams (to the 30,000 psi derived anchorage per bar).
Because of this, contractors - following the spec - could be viewed as seeking to get more construction revenue in back filling the soil areas with concrete.
Somewhere, this "new path" of building upon clay and intensely weathered rock (soil like), was decided going against the original specifications to power excavate to solid/strong rock. I can see financial playing a part (saving $'s), I can see faster schedules playing a part (easier/faster)... but it is not explained who and why the original designers specifications were deemed something that they "knew better" about (i.e changing the design to something different introducing a critical design flaw).
You never, ever compromise a design to a weaker integrity without fully consulting with the original engineers. The times I've seen these scenarios develop is in the "politics of engineering judgement" where people are "bypassed" or someone quietly "overrides" prior decisions.
Saving money, time, schedule, etc. can be a powerful motivator leading to this exact outcome - a series of decisions & non-considered consequences - leading to a catastrophic design combination "waiting to occur".
There is no escaping that this non-backfilling of concrete had to have been widely known in the spillway construction as Inspection crews are part of the process. Thus it wouldn't be limited to a single Field Engineer determining the "consensus".