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Piece by piece Clues to the Erosion Channels under the Green Wet Area - Clean Surface of dam with no erosion channels vs Dense Erosion channels - Linked?

DSOD had good reason to note of a "slope stability" issue in their 2105 report. Here are side-by-side images of a Clean surface of the dam and the dense erosion channels forming on the dam's surface. Same elevations, same left side of the dam. (side note - note the revealed construction layering strata lines in the Clean image).

Both of these areas of the dam have been exposed to the same weather since 1967, same heat/cold, same rains. The "normal" time weathering of the intended Zone 3 fill design is revealed in the minor weathering of finer materials, revealing minor areas of cobble rock, in the "Clean surface". The slope remains stable and resists even the strongest of rains endured over 48 years. At the Dense Erosion channel section, it too has been exposed to the same weather & rains. Yet, over 48 years, these dense erosion channels have been forming slowly, gradually.

Both of these areas were constructed in compacted layer fill at the same time in 1965 as these images are at the same elevation section. The same sands, gravels, cobbles, and boulders comprising the Zone 3 fill "shell layer" were laid in 2 ft "lifts" and compacted. So there is no evidence that a difference in construction & materials composition would be a likely mechanism tied to the "erosion channels". The "differential" key, between these two areas, is that the "erosion channels" originate at a "long established wet area" (as named/stated in DSOD inspection reports).

Yet there is a "clue" that helps answer the differences between these areas in the light of evidence to an underlying mechanism to the formation of the "erosion channels".

One comment on what you wrote. In today’s standards, it is rare to see engineered fills placed in two foot lifts or layers. Normally you see fills placed in 0.67 lifts or possibly 1.33 foot lifts but rarely in 2.0 lifts.

I will add that dam construction with massive equipment may allow lift thickness for compaction beyond what I would see in roadways, runways, and large heavy equipment pads.

Piece by piece Clues to the Erosion Channels under the Green Wet Area - Subsurface saturation? - Combined with Rain? - Causing "fines" erosion? - Wide area saturation?

EternalHope & KC Burke reveal key pieces to the formation of the "erosion channels". In order for the Zone 3 finer material to be removed there has to be a mechanism that disassociates the prior "compacted" & historical time weathered "consolidation" of the fines. As demonstrated in the "Clean Surface", the surface fines are resistant to erosion as the downward absorption of rainfall continues to slowly percolate within the Zone 3 "pervious" design, but still maintain the same "consolidation" vector to the structure. ONLY if there is a subsurface "effect", will this "consolidation vector" be disrupted. Subsurface saturation is a primary example. A "wicking" or "capillary" action will moisturize from below in the saturation, and then rainfall on the surface will percolate to meet this zone. There are actually "two" directions "fines" may thus be eroded. One is at the surface in the downhill slope vector. The other is if the "subsurface saturation" mechanism is also causing deeper percolation and evacuation of "fines". This is the same layered effect of rainfall to a saturated subsurface, except it is a saturated subsurface with another saturated subsurface - all being influenced by a downslope vector. So where would the "subsurface fines" migrate to? Or is it "subsurface fines" changing the percolation characteristics such that saturation may flow easier to deeper levels - including to the bottom horizontal "drain zone" layer?

A "wide area" saturation allows the natural formation of "erosion channels" looking to be from gravity & natural "meandering" random development vectors. However, this is very subtle.. there must be a "mechanism" to "free" the prior consolidated "fines" to allow this dense formation of erosion channels. Thus, this "wide area" subsurface saturation subtlety is overlooked, is because the focus of attention is just on single erosion channels alone.

More later…

Seeing your post regarding how the dam construction was halted every Winter brought a question to my mind.
It is as follows. Have you found any reference regarding how the structure was prepared for the Winter? Was the clay area capped with more typical fill or covered to protect it; then removed in the Spring?

It occurred to me that the could be a break in continuity of the clay barrier if the cap material(if they did cap it) was not completely removed in the Spring. Seems to be a basic simple question but in the case of this project they appear to have overlooked some very simple things.