Hi Ray76, KC Burke, Mariner, Repeal The 17th,…..I'm a bit behind in questions asked.
Other Engineering Forums have been covering/discussing these same good questions, plus some new interesting points.
>>Q: "Given the amount of erosion what are your thoughts regarding the feasibility of the spillway remaining at this location?"
..I would say the first obstacle to this is the Main Gate orientation regarding laminar flow. The whole design of the chute & Main Gate Structure is straight. It is aligned to a massive concrete structure (that still operates fine) that contains meticulously designed shapes to prevent vortices, turbulence, under all flow rate specifications. This concrete structure is also anchored into bedrock. At the very bottom of the spillway, the rock has shown itself to be time proven & solid. If a choice ($costly$) were made to move the spillway you lose two immediate solid gold starting & ending points.
There are modern technologies that could provide deeper core anchoring to overcome any new "cut line" from the severe erosions/damage.
>>Q/discussion: "It also appears there were slab areas where competent rock was not available to provide the grade needed for slab placement."
"This means that water that got under the slab under pressure would have wedged under that portion placed on rock and lifted it."
=== For slab under pressure water: see: VI-1. Stagnation Pressure Failure of Spillway Chutes
http://www.usbr.gov/ssle/damsafety/risk/BestPractices/Chapters/VI-1-20150610.pdf
Quote: "Stagnation pressure refers to two conditions that can result in damage and/or failure of the spillway: (1) High velocity, high pressure flows enter cracks or open joints in the spillway flow surface (such as a chute), which results in uplift pressure that lifts (displaces) portions of the spillway conveyance feature; and (2) High velocity, high pressure flows enter the foundation through cracks or open joints in the spillway flow surface, which results in internal erosion of the foundation and loss of support of portions of the spillway conveyance feature [26]."
===
..Yes to water under the slab. But to which effect was key? (1)(2)? The best information, to what was developing as a pre-failure to the blowout, would be from human intelligence information on the recurring repairs. There should have been sign of "uplift" cracking or "loss of support" cracking. Images show that drains in the area of the failure were not draining equally (or zero) compared to the neighboring sections. So to what degree each element was in play gets back to any fore knowledge by the construction workers & inspectors on what they were observing in the surface fracturing via repairs.
(discussion..more re: in lifting the slab (not enough hydraulic pressure?) - Keep in mind the 15 inch thick slabs were heavily anchored with steel (rebar) into the bedrock. The tremendous weight of the waterflow has a much higher weight compression force than a small waterflow under the slab(s).
However, photographs of the early construction show a "compressed" gravel layer prepared for the concrete slab pour. This gravel is also described as a medium to capture any water for the under-slab drain piping. So you have a ready made layer for scouring erosion, besides any deeper fractured rock anomaly.
Regarding "competent rock": Yes, they found areas that had to be restored to grade level using concrete (see image above- construction blueprint). The prints denote "Details are typical for case where 'accept. rock' is below cut grade". What is not in these prints, but are found in construction commentary archives, is that they encountered seams of "less than acceptable rock".
Archives Construction Report- direct quote: “proved to be of a lower quality rock than anticipated. There were several large seams running parallel with the chute. The planned anchor bars were replaced with grouted rock bolts, pigtail anchors, and chain- link surface covering in that area”
This means that they took extra measures to shore up these areas they thought were not as complete as good bedrock.
KEY Point: There may be a human error element to stress fracturing in the concrete slabs…
The Oroville dam "operating rules" are very specific on water release rates of change. In Engineering it is known issue where differential expansion rates cause stress fracturing/cracking in anchored concrete.
== reference:
Quote: "Severe problems develop in massive structures where heat cannot be dissipated. Thermal contraction on the concrete’s surface without a corresponding change in its interior temperature will cause a thermal differential and potentially lead to cracking. Temperature changes that result in shortening will crack concrete members that are held in place or restrained by another part of the structure, internal reinforcement or by the ground. For example, a long restrained concrete section is allowed to drop in temperature. As the temperature drops, the concrete tends to shorten, but cannot as it is restrained along its base length. This causes the concrete to be stressed, and eventually crack." http://www.engr.psu.edu/ce/courses/ce584/concrete/library/cracking/thermalexpansioncontraction/thermalexpcontr.htm === Human error (if rules not followed):
Oroville Operating rules quote (re:''Limitation on releases") "C. Releases from Oroville dam are not to be increased more than 10,000 c.f.s nor decreased more than 5,000 c.f.s in any 2-hour period."
http://www.water.ca.gov/orovillerelicensing/docs/FEIR_080722/AppendixA/Extracted_Comments/C0005_SutterCty_YubaCity_Levee%20Dst1_Appendices_Pages_10-177.pdf
=== These rules mean that going from 50,000cfs to 100,000cfs should not occur faster than 10 hours -even rate. And, going from 100,000 to zero should not occur faster than 40 hours.
I don't have any data on past history of rate operation - just that these thermal effects on an anchored concrete slab spillway is a real effect on structural damage (cracking of concrete).
>>Q/discussion: Layers of concrete, Dowel transfer support, Stepped seams, heavy rebar & metal mesh, mud/soil underneath.
Images show that there was heavy rebar used to anchor the 15 inch concrete slabs to bedrock. AS noted above, there were areas that additional anchoring means were employed in areas where they found lesser quality rock, including additional chain link surface covering.
Re: soil or mud: There is no evidence in any of the construction archives where the main spillway where soils existed underneath the slabs. The construction blueprint shows that excavation was done to insure that all surface areas were excavated down to rock.
Large steel dowels were used as load transfer to the main spillway slabs at the edges of the sidewall gates. Other Images (if you zoom) show rows of holes where the dowels were present - before the damage & concrete pulled them out.
The impression of sub-layers in the concrete: The slab horizontal seams used multiple staggered steps to overlap the adjacent slab (see image). Dowel pin holes have not been observed in these "stepped" slab edge images. The sub-layer look may have been from the method of pouring/construction to create the "staggered interlocking steps".
This is all of the time I have for now.