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Hi KC Burke, This new DWR zoom image illustrates what I may have fell short in wording on the "under-gravel" description. (image marked up- zoom at broken end of main spillway)

Here's the problem: A second potent underflow region in the "foundation pour" was created by putting in a gravel layer between the bedrock and foundation pour in the original construction. (note: some of the foundation pour penetrated into the gravel.)

This was NOT in the original design specifications. The drawings always show the slab poured on top of rock (the mustache wiskers markings). IF they had to backfill up to the slab level, they were to pour concrete and anchor it. Nothing was shown, discussed, or illustrated where a gravel layer was ok to be placed.

This infers that the "gravel" was intended to be a type of secondary drain system - that they did not expect any water to get below the pipe drain system. Since water WAS very profuse in the pipe drain system, and there were numerous fractures/cracks in the upper slabs, water undoubtably penetrated into this gravel & bedrock seam area. Erosion, voids, and "hydraulic jacking" effects would have created serious problems in this seam area.

This would explain the "missing water" in the failed blowout region. It all dumped into this lower seam layer, bypassing the drains, and then caused serious problems to the underlying support to the slabs. It's possible the maintenance engineers likely would not have expected such a deeper layer of flow as all of the specifications, archives, drawings, and notation did not show this "sublayer of gravel".

Only an ol' timer who was part of the construction would had to have passed this on verbally (if asked).

Oops. When Designs go awry: Plastic Sheeting wraps Perf Drain Pipe & round gravel rock shifts - making a "second bulge"

"What were the designers thinking?" pondered the construction workers.. Here is a classic case of a design that goes awry.

DWR provided video shows a Drain Pipe under a slab. The original design intent/drawings was for a nice inverted "V" shape of gravel sloping on both sides to the peak of the upper pipe curvature. Check.

Use round gravel to yield the best pocket seams for waterflow. Check.

Place a sheet of plastic over the arrangement to keep concrete from penetrating into the shaped pyramid of gravel and from into the perforated drain pipe. Check.

Result - The round gravel became like "marbles" when heavy concrete was poured. Oops #1

The round gravel & the drain pipe "shifted" such that the pyramid shape became a "double camel hump" with very little to no gravel on one side. Oops #2

The "shifting gravel" allowed the plastic to wrap around the drain pipe, on one side - restricting/blocking perforated drain pipe hole access. Oops #3

The "double camel hump" from the shifted gravel caused a significant widening of the thinning area of the main slab. Big Oops #4

Of course, there are tell-tale cracks observed above the drain pipe….

The plastic looks to be in great shape. This truly lives up to its irony - plastic tent above the drain pipe repelling waterflow from cracks caused by thinning weak point stresses above drain pipes…

Large Void created by "round rock" (marbles) being shifted during concrete pour - note the significant "hump" created to the right of the drain pipe while the other side has the plastic hugging the pipe (from the "shifting" during pour).

Original drawings, below, showing the designer's intent with a perfect "pyramid" sloping on both sides of the drain pipe.

New Evidence - Gravel bed "enabling" Water under Drain Pipe causing SubSurface Bedrock Weathering & Erosion…

New Evidence reveals a better forensic insight into How & Why the Main Spillway likely Failed. Evidence strongly indicates a substantial presence and flow of water below the under slab drain pipe system. This could cause SubSurface Bedrock Weathering (weakening) and potential Scouring Erosion - besides "missing water from some drain section regions, such as where the blowout failure occurred.

The key to this new evidence was from finding a drain pipe under the centerline of the broken Upper Main Spillway. This Pipe "aligns" with its sister "mirrored" drain pipe for the other side/half of the spillway. This "alignment" is exactly where the left half of the upper spillway fractured in the most recent damage of the Upper Main Spillway. A "top view" of the left half of the fractured spillway shows an angled line ("herringbone pattern" angle) that is straight - as would a fracture follow in a "weak" line in the concrete - such as "thinning" by a drain pipe.

What was originally believed to be a "foundation pour" of concrete (prior posting), now reveals as a unique pattern formed by the concrete impressing upon a Polyethylene sheet of plastic (giving it a "smooth" texture in contrast with the rough slab fracture). The actual slab "was" thicker and is angled and emplaced upon the rough leveled bedrock. Since Vitreous Clay Pipe is fragile, the VCP pipe was likely placed upon a layer of gravel material to keep it off of the rough leveled bedrock. The exact composition of this layer of material is not known, but early pictures of the construction of the Upper Spillway reveal a smooth and leveled layer of packed aggregate. IF so, this would enable placement of the VCP pipe upon a smooth surface (packed aggregate). Images DO reveal that there was round rock emplaced around the sides of the Drain Pipe as some of this has been captured from concrete penetration during the slab pour.

A severe shortcoming of the drainage design is that water must flow or jump "upward" from the aggregate base to enable capture within the perforated drain. This jump "upward" could be up to 2.5 inches above the packed aggregate surface if the drain pipe is supported by the larger diameter Bell Coupling connections of the pipe (including the thickness of the wall of the pipe). This means that there is a minimum of 2 to 2.5 inches of downhill waterflow that the drain pipe would miss - i.e. it would flow below the pipe & not be captured. From the start, any water penetrating under the slabs would have a notable "escape" percentage as the Drains would miss its capture.Worse, this waterflow could have penetrated & scoured the packed aggregate base. This evidence is prominent in the images as waterflow "voids" and "seams" are observable.

The sum of these conditions, combined with a single rebar layer at the top of the slab, would be susceptible to the powerful hydraulic forces of "hydraulic jacking" (Stagnation Pressure), and forcing cracks above in the spillway. The thinning of the concrete slab by the emplaced Drain Piping forms a weak zone. Many spillway concrete images show cracks that follow the drain pipe patterns ("herringbone" pattern). During spillway operation, any waterflow deeper into the SubBase rock could lead to erosion in weathered rock. Voids and scouring erosion would place even greater stress on the concrete slab above.

Given the new evidence, the safety factor of the spillway could be calculated as resting upon limiting "hydraulic jacking", by limiting water penetration, and critically depending on the slab anchor bars to retain the spillway integrity against these forces.

Concrete Fractured along Drain Pipe Emplacement. Angles from centerline of spillway. Centerline point is where sister drain pipe meets/aligns heading to the other half of the spillway side.

Right side of spillway Drain Pipe (under slab) - aligns with left side drain pipe fracture line & angles from centerline of spillway.

West Drain Pipe (missing from slab fracture failure) aligns at the center of the spillway at the junction near the East Drain Pipe (circled under the slab).

West Drain Pipe outline created by Pipe + Polyethylene Plastic + round enveloping gravel. Waterflow seams and Voids deeper under Drain Pipe line. Gravel retained from penetrating concrete during pour.

Evidence of water flow region/seam below drain pipe (possible eroded seam) with gravel layer above deeper water flow area.

Evidence of Voids deeper under Drain Pipe (possible source of "water induced" weathering of rock over 40+ years from seepage, wet weather, infrequent spillway use,…). Large pockets of prior or recent weathered base rock may have eroded easily away from recent spillway backcutting.