[EarthResearcher333]

Subsurface Seam at Blowout Failure location gives Strong Clue

A 1967 postcard image reveals a natural "Water Percolation Seam" at/near the Blowout Failure location. (note: the postcard was digitally enhanced to filter the "texture dots" to provide this clearer forensic image).

As this photograph was taken prior to the pour of the spillway concrete slabs, the image reveals the "grade" construction of which the slabs were to be emplaced upon. This forensic "seam" presence provides geologic insight, especially regarding the question of "Where's the Missing Water?" from the spillway drains in this pre-blowout failure location. The photograph also provides insight into how "un-captured" waterflow in the sub-par drain design could "wash" a void layer between the bedrock and the concrete slab pour. The spillway "chute" design has embankment fill. There were no signs of embankment erosion near this failure area. The only sign was a preference of large tree growth (roots + growth = subsurface water). The "Seam" answers the question of a "deeper" subsurface flow capability. Over time, the erosion of this flow could increase the underslab voiding.

Given these conditions, the deep waterflow + erosion could have created substantial voids (note: persistent spillway "missing water" at equal levels of the "working drains" observed flow rates = a high capacity deep flow "seam" likely was created in time; exploiting this location's natural formation). Each time the spillway was operated, it eroded and enlarged a void area. Until a higher structural stress condition was applied - such as a higher flow rate - did the right conditions of the voiding stress the failed slab to its limits. Part of this equation is the other dynamic stress condition of hydraulic jacking (one is up the other is down).

Subsurface Seam at Blowout Failure location. Photograph provides insight into how "un-captured" waterflow in the sub-par drain design could "wash" a void layer between the bedrock and the concrete slab pour.

[abb]

Bottom line, a naturally weak sub-grade area, prior to construction.

Old time highway pavers have told me that the areas that give them the most long-term problem are natural streambeds (normally dry) and fence lines.

[mad_as_he$$]

Nice find!

[meyer]

Good information!

I think that the erosion that took place after the spillway blowout has uncovered the location and presence of hard “bedrock” below that section of the spillway. But the evidence was there long before it was completed.

[Ray76]

To see how the dam fit the landscape a current topographic map was superimposed with one pre-dating the dam. The older map showed a stream originating on the hill adjacent to the spillway, in the vicinity of "841" near the center of the map, the stream does not appear on the current map. I doesn't really matter, I just thought it was an interesting feature.

fig 1

 

Flowing water cuts a "v" shaped profile into hillsides. Some of these profiles are highlighted in blue on this map:

fig 2

 

Here is an overall view of the complex prior to the emergency. The profiles highlighted in figure 2 are visible in this photo.

fig 3

 

This photo shows the heavily eroded area downstream from the emergency spillway.

fig 4

 

Several channels were cut.

fig 5

 

The channels in figure 5 correlate with the flows illustrated in figure 2 (channels A and C), and with a road (channel B-blue).

fig 6

So...

1) the topography was known, stream cuts are obvious
2) the geology was known, the weathered "rotten" rock was known
3) the emergency spillway is expected to be used

Water followed known paths. The erosion was foreseeable. It was foreseeable yet not mitigated. Why not?

Another question is, why is the erosion most severe at the head of channel B?