Oroville 1964 - Sharp Slope of Rock - Left Abutment - Green Area - Trench to Anchor Core in Hillside Rock
An insightful early construction image reveals a wide trench cut in bedrock on the abutment hillside. This trench is designed to anchor the Zone 2 - Core (clay-clayey Zone 1) - Zone 2 sandwich design into the hillside bedrock. The orange colored core material is compacted upon the 300,000 cubic yard concrete "coreblock". The darker Zone 2 "transition zones" are observed adjacent to the center Zone 1 Core. The original Coffer Dam is observed to the left of the core region and meets at the parapet of the subsurface concrete "coreblock". Due to the construction sequencing, the downstream Zone 3 "Shell layer" was being emplaced in the center. To keep the access roads open on the hillsides, for construction of the hillside bedrock trenching formation (blasting, excavation, grout sealing), the downstream Zone 3 fill was built up in a form of a "hump shape".
This image also reveals the consistent sharp steepness of the "future" left abutment of the downstream Zone 3 fill. This sharp elevation drop goes to the "trench" boundary. Above the consistent "sharp elevation drop" is a "shallow slope", or semi-flat bench just above 600ft elevation. Thus, the future "Leaking" & "Green" wet area water flow is consistent with this "transition from a shallow bench area to a sharp hillside slope". This forms a classic differential settlement abutment zone potential. Especially since the shallow-to-sharp slope transition continues back directly to the junction of the "trench" boundary (to the Core region).
p.s. "Greetings to Boilermaker @ metabunk"
Early construction image - wide trench cut in bedrock to anchor core - Green Wet area at transition zone of shallow-steep slope that reaches back to the core zone. Classic "differential settlement" potential.
The more I look at your picture of the trench for the impervious core, ER333, the more I have to believe the original engineers made the same mistake they did on the spillway. They cut down to 'mostly' unweathered bedrock, but just filled in or ignored seams of weathered rock if they looked solid. If I look at that picture, I do not see a continuous bed of solid, contiguous bedrock suitable for the impervious layer's foundation. I see the same kind of geology that we see under the spillway and emergency spillway apron: a folded, marbled-like mixture of 'good' green and 'bad' weathered/oxidized bedrock. They may have filled in imperfections with compacted fill (like under the spillway) but that's not likely good enough
Between the solid concrete core block and pressure and settling of the clay middle of the dam, this shouldn't have been a problem. Way up the slope, to the left of your "700ft shallow slope" label would be a problem. Not to mention that the trench bed seems particularly scrappy at that level. Hard to tell from the dam blueprints that have surfaced, but I believe the impervious core is also thinner as you go up the abutment.
I have to wonder if the 'natural spring' is actually water piping under or around the impervious core at 700' or higher. This would lead to the engineer's odd observation about 'natural spring'. Sure, because it looks like the water is coming from the rock wall of the abutment. But it's reservoir water infiltrating under or around the impervious core.
This itself isn't a colossal problem. Dam engineers have been grouting imperfections in foundation rock for decades on many dams. It's fixable if you keep on top of it. The problem is if DWR engineers ignore that possibility and "monitor until just before failure" like they did with the emergency spillway.
Kind of makes one wonder about the 'contract issues' that made them let the other geologic engineer on the forensic team go. If he told them the whole dam suffered from the same geologic problems that the spillway had, then the easy fix is to replace him with someone that thinks differently, i.e., "Everything is fine - the rock is great. Let's get that half-designed spillway built NOW!"
The bigger problem that might also (or alternatively) be lurking underneath is the penstocks and diversion tunnels. If there has been erosion around the outside of those (not uncommon), then there may be voids and settlement problems inside the dam fill. If DWR has been too cheap to use ground-penetrating radar to check the spillway for hidden voids, then they sure as heck probably have never checked the penstocks and diversion tunnels for hidden voids. They don't even seem to be particularly concerned about missing monuments or thoroughly measuring settlement. And how, pray tell, does a geometry monument marker on a dam go missing?
