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?
"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."
Hi PavewayIV, Engineers drilled and pressurize filled grout into the continuous seam of the center of the trench (centered in the core fill). In fact, the Grout Gallery Tunnel has a purpose of being able to inspect and detect "piping" leaks from the grout curtain seal. This tunnel allows access to repair drilling and fixing to any grout curtain issues over time.
You are correct about the "weathered rock" in places. However, the image resolution is unable to determine this degree along the future core centerline.
IF there were a "piping" of the reservoir at 700ft or higher to infer a form of a "spring", fluid flow hydraulic dynamics would have a curvature of Zone 3 shell moisture if any. In other words, the water would flow along the surface of the rock abutment, with a small amount possibly in the sharper transition slope via capillary action - yet it still would be a tight downward curvature.
Yet, the Greening follows a long horizontal and upward tilting line. No spring at the abutment can cause this effect.
As far as the Forensic Team member: there could have been circumstances that came up which required the person to find themselves unable to commit to the intense work load (family, job, etc). Because the letter is vague in a reason, there is not enough information to discern. I suspect information will come out later as the press will be looking into this.
Penstocks & diversion tunnels: Proper inspections will use hammer pings for soundings in void detection. I'm not so concerned about the deeper rock tunnels. But the Palermo Outlet tunnel was inspected using an ROV. Not sure what type of instruments were on this to check for voiding. The tunnel should be in a depth area of competent rock.