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Pics From Today (Upper Main Spillway Slab & Inspection Work) thanks abb…

… discussion:

Looks like inspectors may be doing soundings to get readings on under-slab conditions. Percussion detonations would leave a residue. Seismic recordings from percussions could give useful information (sub voids, acoustic resonance(s), reflection times…). This would be good intel to have in this area as these slabs are closest to the last set that eroded away. The slab on the lower left corner has the most overhang of the undercut erosion. It could be coincidence, but the seams of this slab indicate they have wider gaps. I would suspect there is quite a bit of stress on this slab given its overhang at the end. This slab is also the edge slab where there was concern of hillside erosion threatening the footing of the nearest high tension electrical tower.

Blue lines (just beneath) note angled cracks in the slabs. Seems there are a similar "herringbone" pattern set of cracks here too (angled and in a pattern series).

Another note about the substructure: Close up DWR helo views of the substructure rock under the broken upper spillway edges does not show "bluish" hard bedrock. Pretty consistent of the orange/brown material that downstream erosion (near the blowout area) shows as more erodible. This substructure orange/brown rock is also very deep. I would understand concerns from lower spillway flows & undercutting further. The higher spillway flows had an inertial jump away from the back substructure area. (btw- some of the coloring could be from backsplash of the soil… however, the central area in the middle of the spillway substructure maintains this color pattern of rock. Would find it less probable that a uniform type of soil coating is covering the whole eroded face).

I was able to find a historic color picture of the Quad Cat D9 with dual rock "rippers" attached. They actually were "ripping" rock with these.

See Post 2,120 link here: SUPER QUAD CATS - OROVILLE DAM DIGGING UP HISTORY..

Article noted these "rippers" were used on the main spillway - spoke of replacing rippers due to the heavy rock work.

Very interesting picture and analysis of the visible cracks and the apparent attempt they are making to assess the condition under the spillway. Thank you!

Cracks on the surface, and an “interesting” pattern to the cracks. Lets hope there not voids in the substrate, but the pattern to the cracks certainly raises that possibility.

I wonder if those same cracks were there 2 months ago, and if they have grown since then? Likewise, I wonder if soundings have ever been done before. If so, how do they compare with NOW?

Let’s hope the analysis they are doing this time is correct. So far they have not inspired much confidence.

We already know the main spillway must be used for several months in a row during this year’s spring melt. If their current analysis shows the potential for further failure of the main spillway this spring, then they should be making contingency plans NOW.

If the main spillway reaches the point where it can no longer be used, it certainly seems possible that the “emergency” spillway could handle the whole load either.

So, in a worst case scenario, what are the other options?

I have no direct knowledge of these, but apparently there are two fingers of the lake with some sort of low dam that holds back water when the lake is high. These dams are not ever expected to actually let water out.

Could either of those small dams be intentionally breached instead of using the emergency spillway? There would certainly be downstream damage if they were, but the damage might be less than if the emergency spillway were to fail.

Modern Spillway Design vs Oroville Design - 2 Dam Failures, Drain Pipe, Rebar, "Hydraulic Jacking", "Void" Finding by Radar

The "angled cracks" in the spillway concrete slabs and the "jetting" of water out of the sidewall drains should have set off alarms years ago regarding the integrity of the Oroville Main Spillway.

Why? Both of these signs indicate a serious known failure mode in spillways from Stagnation Pressure (via cracks/voids/water). Two prominent examples are Big Sandy Dam, Wyoming, 1983, (spillway chute failure) - Dickinson Dam, North Dakota, 1954 (spillway chute failure).

Excessive water beneath the concrete slabs causes a very powerful effect dubbed "hydraulic jacking" from under the slab (think of a hydraulic car jack). Called in engineering terms: "Stagnation Pressure" & "Stagnation Pressure Failure".

How does the water get under the slabs? The most potent source is from pressurized water from the top of the spillway being forced below through cracks & non-sealed seams in the concrete chute. The more cracks, the more water, the more water, the greater ability of this water to "wash or erode away material" under the slab. Erosion leads to "voids". Voids + "hydraulic jacking" lead to greater stresses on the slab.

How do you protect from "Stagnation Pressure Failure"? Drain pipes are placed under the concrete slabs to collect & drain any leakage, thus preventing a pressurized water layer under the slab. A well sealed & healthy spillway would have very little waterflow out of the drains. The Oroville Main Spillway slab design is less than modern standards. (1) Modern slab designs have rebar in the top layer and the bottom layer. Oroville's slab design only had a top layer of rebar. (2) Modern slab designs retain the full thickness of the slab by placing drain pipe & pervious material below the slab. Oroville's slab design created a 40% reduction in thinning of the slab from the placement of intervals of drain pipes & gravel - forming a inverted "V" of non-concrete. (3) Aggravating the Oroville design was the lack of a lower layer of rebar in the slab. Thus (1)(2)(3) should have set off alarms when high waterflow was "jetting" out of the drains.

A Reclamation Dam was found to have dangerous voids & erosion under the spillway concrete (see picture below). Voids are easily identifiable using Ground Penetrating Radar (GPR) from the top surface of the slab. In the case of this dam, the engineers/maintenance authorities ended up fixing this by a complete replacement of the spillway chute and the spillway foundation.

Oroville's Main Spillway was simply an "accident waiting to happen".

History of posts, discussion, & reference photos:

Pre-failure Herringbone crack patterns in Main Spillway (drain pipes)

HerringBone Drain Pipe - Fracture pattern in Main Spillway

Construction modifications of Main Spillway Drain System

Years of Warning at Blowout Area? Missing drain water/slabs being repeatedly repaired at leakage

Alarms Raised Years Ago About Risks of Oroville Dam's Spillways

Oroville's slab design only had a top layer of rebar. Oroville's slab design created a 40% reduction in thinning of the slab from the placement of intervals of drain pipes & gravel - forming a inverted "V" of non-concrete. Aggravating the Oroville design was the lack of a lower layer of rebar in the slab.

Modern slab designs have rebar in the top layer and the bottom layer. Modern slab designs retain the full thickness of the slab by placing drain pipe & pervious material below the slab.

Oroville Main Spillway drains showing a high volume of waterflow under the concrete slabs - observable by the "jetting" of water from the drain outlets in the sidewalls.