Seems the Upper Main Spillway has Extensive crack lines in the slabs. Nearly every drain "line" shows surface repair "patching" over the cracks in the slabs. Photos from 2010, when a thin layer of water was present in the spillway chute, reveals the significant extent of the drain-emplacement-thinning slab cracking problem. The Images reveal the repaired cracks as whitish lines of the newer/fresh patch material over the cracks. Cracks nearly every 20 ft (10 crack lines across the width of the spillway for every 4 slabs) for the length of the ENTIRE spillway.
History of posts, discussion, & reference photos:
Concrete Fractured along Drain Pipe Emplacement (Upper Spillway)
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
Extensive Patched Cracks start at Gates of Upper Main spillway (first image). (Oroville's slab design only had a top layer of rebar. Oroville's slab design created up to a 60+% 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.)
Next segment: Extensive Patched Cracks of Upper Main spillway (second image)
Third segment to near Towers where existing broken section of the Upper Main Spillway.
Blowout Failure region (would be at center of image) showing the patched drain line induced cracks. notice the tree growing directly adjacent to the spillway wall just below the "blowout area". Tree is on the side where the drainage canyon formed.
History information (updated):
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, thus "cracking".
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 61% 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.
