[EarthResearcher333]

Piece by piece Clues to the Erosion Channels under the Green Wet Area - What is Differential Settlement? Can it crack the core?

Differential "Settlement" is an uneven rate of a compaction/consolidation of the dam from gravitational, hydraulic, and structural forces over time. "Differential settlement" effects could be from construction defects in non uniform material emplacement or from the construction compaction process itself. Earthen dams are carefully constructed to assure that the Zone fill materials and the construction compaction process are precisely controlled and monitored. However, one engineering consideration of an earthen dam is that there is a potential risk, from the undesired "differential settlement", due to sharp irregular changes in the canyon wall slopes. The ideal canyon would have even slope angles from the top of the dam to the bottom, or toe, of the dam. Earthen dams require a strong rock base in the canyon walls to form the left and right abutments where the future dam will be compacted within the canyon. (One of the reasons for having a density of the rock of a canyon base to the Zone fill density ratio is for seismic stability (liquefaction phenomenon for one)).

Over time, the Zone fill materials will "move downward" and slightly deform into the canyon in a tighter bunching of the earth dam materials. This is called "settlement". Survey markers -noted as monuments - are cemented into the surface of the dam to enable future survey measurements for monitoring the state of this normal "settlement" process. Why? This is an important safety factor to determine the "health" of earthen dams. Any sudden shift of a slope area, or an unexpected shift rate, indicates a potential threat to the dam from a possible "slope instability", or an another indicator of an internal anomaly. These survey markers allow periodic measurements to detect small changes for this reason.

One danger to Earthen dams is inducement of "Differential Settlement" from a canyon wall that has an irregular slope change. This is called a "steep rock abutment" transition slope from a "less steep, or more 'flat' slope" along the elevations in the canyon. Why is this a danger to earthen dams? It is because the steeper slope Zone fill area will experience different compression stresses than the 'flatter slope' compression. As sections of the Zone fill material respond to these different forces, the steeper area may induce an "pulling" or "dragging" shear type of force within the internal Transition layer to core layer boundaries. This effect has been known to cause horizontal seams (longitudinal) to be pulled open in the dam core (similar to a sewn clothing seam being pulled apart, leaving a rip along the sewn seam). This is a known failure mode to earthen dams and is taken very seriously [1][2][3].

Oroville dam is constructed upon a sharp slope transition boundary. This boundary is on the left abutment of the dam. The same side as where the Green Wet Area is precisely located (at this "differential settlement" transition region in a sharp slope change boundary).

So what does the survey data show? Oroville dam has "Differential Settlement" above the Green Wet Area. At the row of emplaced 750ft elevation survey markers, actual survey data from 1970 to 1975 reveals a sharp transition of "differential settlement" measurements that uncover this effect at Oroville dam. This settlement profile most likely is continuing today as the "settlement" process will continue for the life of the dam.

Is this a problem? Only if "sign" of leakage, unexplained wet spots, or unexplained "erosion sign" develop. The failure mode of internal leakage is that the erosion process within the core region could develop and escalate to where there is no ability to arrest the escalation. References below have noted that this "failure mode condition" may exist for years, until a sudden transition point to failure. Piezometers within the dam typically would measure the internal saturation or leakage from such a condition. However, DWR has sliced bundles of the hydraulic tubing to their broken Piezometers in addition to the last 3 piezometers being non-meaningful in their ability to measure correctly. Thus DWR has zero functional piezometers within the dam to monitor or detect any internal anomalies from a defective phreatic water level seeping into areas and at volumes of flow that pose a potential threat.

In normal circumstances, from a critical safety operational criteria, a dam would not be allowed to operate without these important Piezometer Sensors. An earthquake could induce a defect within the dam that would be "undetectable" without these sensors. The 1975 Oroville earthquake caused sharp pressure rise of 54 feet of piezometer pressure in one area in the central core of the dam. The dam withstood the forces, but without these Piezometers, the engineers would not have had the ability to do a stability risk assessment. Today, if an earthquake of a given size were to occur, DWR would not have the ability to assess the danger within the dam to warn or inform the public.

[1] Design and Construction of Embankment Dams - Differential Settlement Failure/Cracking Fig 2.4(a) sharp abutment change - http://aitech.ac.jp/~narita/tembankmentdam1.pdf

[2] Embankment Dams-Design Standards No. 13 - Longitudinal Cracking from Differential Settlement pg 5 - http://www.damsafety.org/media/Documents/DownloadableDocuments/ResourcesByTopic/DS13-11.pdf

[3] Lessons Learned from Dam Incidents and Failures - Steep rock abutments leading to "differential settlement, cracking, and failure by internal erosion - http://damfailures.org/wp-content/uploads/2015/07/Lessons-Learned-Complete-List.pdf

Oroville dam survey data revealing "differential settlement" directly above the "Green Wet Area" - Known failure mode to Earth Fill dams if any core shear stress induced internal defects erode to an escalation point.

DWR is operating the dam with none of its original critical safety Piezometer instruments working - either broken or non-functional. DWR is unable to detect any internal phreatic surface water flow anomalies inside the dam. FERC has been asking DWR to put in new Piezometers (phreatic surface monitoring) for many years.

[KC Burke]

differential settlement

[EarthResearcher333]

Piece by piece Clues to the Erosion Channels under the Green Wet Area - Erosion Channels, where did the "fines" go?

As evidence has been presented that the Erosion channels are slowly getting wider, yet DSOD Inspectors have not been able to observe any water in these channels when the Green Wet area is saturated, what is happening? Processed imagery data reveals that "fines" (sands, & tiny minus gravel) are being removed and causing the underlying cobble & rock to be further exposed & remain or even loosen and tumble. Yet the channels are gradually deepening and widening (Imagery data). Where are these "fines" going?

DSOD inspection reports do not reveal any discovery of piled up "sands" accumulating at the bottom seam of the hillside. With the numerous individual Erosion channels averaging from 5 feet to 7 feet wide, and the combining of these erosion channels measuring from 20.55 ft wide and greater; with the length of the channels, the depth (from shadows & cobble boulder increased exposure), surely there must be a volume of accumulation of material somewhere?

Could it be possible that the "fines" are migrating downward in a form of an "intergranular seepage"? If so, this would require a "granular" migration chain reaction from a subsurface sequence of "void" channels that these "fines" may migrate into. Then what formed these deeper "granular voids"?

Eventually rainfall percolation movement of these "fines" should eventually stack up and backfill to the surface. If so, then why do the Erosion channels keep growing? The only answer would be that a bigger subsurface condition has either already occurred or is continuing to occur. This "subsurface" condition would had to have been a large area migration of deep subsurface "fines" that were ported deeply downward & went somewhere. Looking at the full square footage of the Erosion channel area, if this whole swath was scoured in a capillary type of vertical downward migration erosion of "fines", this would allow the individual Erosion channels to form as they have - naturally from rainfall. The key difference is that the rainfall is not "pushing" the fines downhill, but is facilitating the "percolation migration" of these fines into the Zone 3 fill.

The original forces of a large hillside saturation, and likely combined with some rainfall at the time, are the only positive hydraulic pore pressure conditions that could upwardly "disassociate" the original construction consolidation of the fines in the Zone 3 fill. More so, the saturation likely was from a wide area subsurface flow. This subsurface flow, of a higher hydraulic pore pressure, would have the ability to "migrate fines" downward in a bottom-up back erosion condition where the "fines" ended up inside of the bottom horizontal Drain Zone. This Drain Zone would easily accept the migration of "fines", from a strong subsurface flow, as the Drain Zone is comprised of gravels, cobbles, and boulders - plenty of space for a large volume of "fines" to be deposited. As long as the Drain Zone is able to accept this continued "depositing", the upper strata of Zone 3 areas will be micro honeycombed with flow channels for more "fines" - simply by a mix of rainfall entering the erosion channels, eroding & migrating more surface fines into the channels + then percolation downward. This process also deepens the channels as more fines are eroded and carried downward via percolation. (note: the average "flow" of water would be thin as there would be a continuous percolation absorption compared to a normally "combined" flow in a typical channel - thus the missing DSOD information on why there are no "piles" of accumulation at the bottom of the hillside seam.. the "thinness" of a channel flow, in a heavy rainstorm, would be determined by the percolation rate absorption to sq ft deposition rate of the precipitation).

What this reveals is that there is likely a wide area "flow" condition below the Green Wet Area that is unseen. If this subsurface flow contains silts, soils, clays or clayey "fines" from the Transition Zone or the Core, this too would be "unseen" as this flow over time would be deep under the Zone 3 surface. This a key reason why there are no thick greening areas in these "Erosion channels" - there is no upward surface deposition of a vegetative soil base. All of the "fines" migration is evidenced as a downward percolation flow.

[jpal]

To EarthResearcher333: as always, I am amazed at the breath and depth of your posts.

However, I would appreciate a point of clarification:

I’m not sure I’m correctly interpreting the “differential settlement diagram” from DWR that you provided in post 3624.  
It appears that the scale on the right of this plot is in tenths of feet (of settlement, I assume).  If so, your small circled arrow on the right appears to indicate about .03’ settlement at around station 65, and the larger circled arrow to the left indicates around .18’ settlement at station 57.  That would be less than 2” differential settlement across that distance, which doesn’t seem sufficient to shear the dam and induce cracks that water could flow through.

Am I not interpreting this diagram correctly?  
What is the actual magnitude of differential strain in the dam that would cause the phenomenon that you describe?

[Jim W N]

DWR has sliced bundles of the hydraulic tubing to their broken Piezometers in addition to the last 3 piezometers being non-meaningful in their ability to measure correctly. Thus DWR has zero functional piezometers within the dam to monitor or detect any internal anomalies from a defective phreatic water level seeping into areas and at volumes of flow that pose a potential threat.

Who's in charge of DWR, Lucifer?

[Jim W N]

It would seem to me that the cost and amount of prep work needed on the canyon walls to eliminate or reasonably reduce the likelihood of “Differential Settlement” in an earthen dam would be prohibitive, especially for a dam of his magnitude. Seems like more weight on the side of choosing to build a large dam like this out of hard material like concrete.

[Jim W N]

The failure mode of internal leakage is that the erosion process within the core region could develop and escalate to where there is no ability to arrest the escalation. References below have noted that this "failure mode condition" may exist for years, until a sudden transition point to failure.

Back to a concern expressed earlier about the transition from "pre-failure" to "acceleration". It looks like DWR has fixed it so this transition could happen quicker than expected leaving the folks below the dam in sudden grave peril. Just curious if you have thought about what you would do if you were living in Oroville right now knowing what you know, but I think few in Oroville know.

This settlement profile most likely is continuing today as the "settlement" process will continue for the life of the dam...

...which may be shorter than we think...

[Jim W N]

BTW, more praise ER for such excellent and compelling analysis.