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Flawed Maintenance Repairs Propagated and Increased the Spillway Degradation
In 2009, DWR Spillway repair bids and line item documentation, prepared by DWR engineering, noted that 240 linear feet of drains were identified to be repaired [7][11]. A 2007 photograph reveals that one section of the spillway drains (10 drains spaced 20 feet apart servicing 18,250 square feet of spillway drainage area) were non-functional revealed by the sidewall drain non- operation (Fig. B.4).
After the 2009 contract repairs, the same drain section remained non-functional as evidenced in a 2013 photograph (Reference [17] Figs. 1 and 14). 2017 photographs reveal that 1,780 linear feet of slab drains that service 36,500 square feet of spillway drainage area were non-functional (Fig. B.2). Despite the DWR drain repair identification of 240 linear feet, the drains were not repaired.
Inspections by the Division of Safety of Dams (DSOD), and the Federal Energy Regulatory Commission (FERC) should have identified such a severe non-functional drainage condition in the spillway. As the non-functioning drain state is observable from the sidewall drain & water seepage patterns, a visual inspection would have discovered this issue. The cited evidence documents that this non-functional drain state was not recognized nor repaired for close to 10 years (Nov 2007 to Jan 2017).
Had DSOD, or FERC properly recognized this issue, an investigation would have revealed the source of the widespread ‘clogging’ of the drains and remedial action could have been initiated. This Failure of Inspection from multiple agencies, and for nearly a decade, is perhaps one of the greatest failures in the critical process of insuring the safety and integrity of the spillway.
In summary:
1. DSOD, and FERC failed to recognize the significant problem of two non- functional sidewall drains which serviced 36,500 square feet of under-slab drainage for a span of 9 years 3 months.
2. DWR engineering noted 240 linear feet of drain line were to be repaired in 2009 [11]. In 2013 the drains remained non-functional (Reference [17] Fig. 14). DWR Maintenance and Engineering failed to address this issue.
3. Evidence identifies that Maintenance repairs of "deep void" filling (forming Large Concrete Slab blocks), with up to 9+ feet deep voids under the spillway, would have clogged the open ends of "dropped" drains [17] (Reference [17], Figs. 3, 4, 5, 6, 8, 10).
4. Seepage Evidence in 2013 identifies that water flow submarines below the large void block concrete filled non-functional drain area (left side view Ref [17], Fig. 14) and the seepage re-appears immediately downslope.
5. Evidence DWR failed to notify FERC on the issue of "deep void" filling repairs. Dam owners are required to notify FERC of any serious Potential Failure Modes (PFM's) [18].
6. The second non-functional drainage area is directly upslope to the blowout failure initiation location (Fig. B.5). This condition would combine a large volume of non-captured under-slab flow to the next set of downslope drains. A higher volume of pressurized water flow increases the scouring erosion of weak foundation material identified in the blowout failure region. From the evidence of the "clogging" of injection of large "void filling" from Reference [17], the same process of deep erosion (and potential injecting induced "clogging") could render drains inoperable by either "dropping" drain pipe or "clogging" to where they are unable to service flows.
The DWR Oroville Dam Spillway Incident Forensic Investigation Team recognized "plugging" or "collapse" of drains; from May 5, 2017 Memorandum [9] – items:
1. "11. Plugging or collapse of drains or collector pipes, including potential plugging by tree roots."
2. "12. Flow into the foundation that exceeded the capacity of the drain pipes, including flows from areas adjacent to the chute."
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Main Report link - clip above from Appendix B:
https://drive.google.com/open?id=0Bz1I1mIutSEnbFJuVUJZWWNNVlU
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Loss of Anchorage
The loss of the spillway foundation integrity caused a significant structural integrity degradation of slab anchorage in the spillway anchor bars. DWR engineers had not considered the design consequences that would result in the high volume of pressurized water flow under the spillway slabs. Using the experimental data generated by the U.S. Bureau of Reclamation (Fig. B.13), a single drain system (10 drains spaced 20 feet apart) with a simple example of one seam per slab (0.125 effective seam width, 1/2 inch offset, 90 feet per second flow rate) could yield up to 55 cubic feet per second of total sub-slab water flow.
This example flow rate would scour the full length of a 200 foot (18,250 square feet) slab array of eight slabs plus the sidewall slab area. As anchor bars were intended to be emplaced in the "worst foundation available”; DWR Field Engineers restricted contractors from excavating to competent rock; and DWR Final Geology Report Spec 65-09 illustrated the poor quality highly erodible foundation "wide seam area" (Fig. B.21); a significant penetrating pressurized sub-slab water flow in highly erodible foundation material, over time, would render the anchorage resistance to a dangerously degraded structural retention state.
In summary:
1. Three 40 feet by 50 feet slabs, with a combined 60 anchor bars, failed in the initial blowout failure. The fourth slab, to the left, was partially anchored on a section of "more competent rock" and it survived the initial blowout (Fig. B.10). This evidences the severe lack of structural integrity of the anchorage of the slabs as the image reveals the anchor bars were emplaced in highly weathered rock (poor) foundation material (light to brown color).
2. Non-functional drainage increased a sub-slab scouring erosion flow at the blowout failure slabs. The increased sub-slab scouring erosion on highly erodible material (noted in DWR Final Geology Report Spec 65-09 Fig. B.21) would have seriously degraded the anchorage strength in eroding at/near/around the anchor bar grout holes.
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Main Report link - clip above from Appendix B:
https://drive.google.com/open?id=0Bz1I1mIutSEnbFJuVUJZWWNNVlU