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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
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Physical Root Cause Failure Analysis
The Physical Root Cause Failure Analysis of initiating blowout sequence of the Spillway is summarized in the following:
1. Fig. B.8. Initial failure "hole" defect initiation point of the Initial Blowout Failure Area of slabs along this extensively repaired spillway seam. Hole dimension measured with forensic high resolution photographs to be near 7.9 feet along the seam by 14 inches downslope from the seam. White lines denote the drain line cracks in the slab surface. The depth of the "hole" is inferred to be three inches deep to the upper layer of rebar - as evidenced in prior concrete spalling spontaneous defect occurrences revealed in Fig. B.6. This depth was also noted by the DWR Board of Consultants Memorandum No. 2 (depth to rebar layer), page 8 [3].
2. Fig. B.10. Initiating Failure Hole location reveals a deep seam of highly erodible foundation material that is many feet deeper than the grouted 5 foot deep slab anchor bars. Angle of seam inferred by dashed line. Image reveals the nature of why the "hole" location and that full 178 foot wide seam area was a structural problem area. The upslope "soil-like" foundation material is in a transition zone of more competent rock (downslope from the dashed line). Thus the "communicating" slab forces through the load transfer bars would have experienced a differential in structural integrity or stability.
3. Fig. B.11. Blowout Failure slabs located at a transition zone of higher integrity anchorage stability slabs verses slabs emplaced on poor anchorage stability based slabs above "soil-like" erodible foundation material. Net foundation structural anchorage "differential" placed forces on problem area seam as evidenced by the extensive concrete repairs along this full 178 foot wide seam area.
4. Fig. B.21. DWR Official Final Geology Report Spec 65-09 denotes the foundation geology of the subgrade quality of foundation material that the invert concrete chute was constructed upon. The Seam (marked as a series of "S"'s) follows the dashed line seam in Fig. B.10. This drawing reveals the same foundation structural integrity transition region of the quality of the foundation material as in the blowout failure erosion images in Fig. B.10 and Fig. B.11.
5. Fig. B.20. DWR Final Construction Report FCR 65-09. Critical Design Flaw linked to blowout failure. DWR reveals that the spillway foundation will include anchor bars emplaced in "clay seams". This evidences that DWR was allowing the slab design to have anchor bars to function from the "worst foundation available". This would include poor foundation materials such areas of clay and areas of soil-like highly erodible extensively weathered rock. The blowout failure area reveals this type of material (poor foundation materials). This evidences the non-ability of the anchor bars to maintain the integrity of anchorage in these clay and soil-like foundation materials. These materials are highly erodible in subsurface slab water flow. Scouring erosion would remove these seams of materials rendering a significant loss of pounds per square inch in anchorage strength of the anchor bars.
6. Figs. B.14 and B.15. Extensive corrosion of rebar at slab drain line cracks weakened the slab into a severely degraded structural condition (little to no remaining tensile strength. Reference "Slab Structural Degradation from Rebar Corrosion at Cracks".
7. Fig. B.10. "Loss of Anchorage". Evidence of little to no ground anchorage at the blow out failure area involving 60+ anchor bars in 3 main blowout failure slabs.
8. Figs. B.8 and B.7. Multi-slab long drain line slab fracture 5.3 feet from the originating failure "hole". Construction Design Flaw of emplacing drains within the slab, thus "thinning" the slab thickness resulted in chronic slab cracking over drain lines for the entire 3000+ feet of the spillway. Three rows of slab wide drain line cracks were in the initial blowout failure slab where the failure "hole" was identified. The first drain line crack "row" was 5.3 feet from the slab "hole".
9. Stagnation Pressure in combination with hydrostatic forces fractured the slab from the seam "hole" to the nearest slab drain line downslope. Pressure force analysis: A high velocity flow near 90 feet per second, at 54,500 cubic feet per second, produces extreme uplift forces from a small offset in the slab joint alignment from Stagnation Pressure. Whether the slab alignment is offset positive or negative, these extreme forces at the high velocity flow are significant and could easily fracture a highly structurally weakened slab. Reference - U.S. Bureau of Reclamation Stagnation Pressure Mean Uplift Pressure Plot [14], with an Initiating Failure point referenced to a flow velocity at or near 90 feet per second (near Station 33+00). A half-inch offset of an upslope slab joint induces an 86.3 feet of water in uplift pressure underneath the slab. This translates to 37.4 pounds per square inch in uplift given an amount of flow to some drainage. Applying this force to a 40 foot long seam would yield uplift pressures of 53.8 tons in a simple example square footage affecting a 40 foot x 6 inch under-slab area (note: effective seam gap of 0.125 inches).
10. This First major fracture blowout failure started with the sudden collapse and/or lifting of a 5.3 foot section of the slab. This created a large hole for the high velocity 90 feet per second, 54,500 cubic feet per second, flow to penetrate under the next 20 foot slab section - along the next drain line crack - of the slab and fracturing and lifting away of the slab. The next section was to the to this next cracked drain line region, then the remaining section to the downslope seam.
11. The extreme hydraulic turbulent forces and erosion development, generated from this initial slab blowout, developed laterally and downslope in continuing to fracture and lift away adjacent slabs. The initial blowout failure dimensions, of affected slabs, determined by the strength of the anchor bars from the foundation material. Poor foundation material resulted in full lifting-removal of 9 slabs with the partial destruction of 4 additional slabs.
12. Subsequent spillway operation of higher volume flows continued the lateral, upslope and downslope destruction of the spillway.
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Main Report link - clip above from Appendix B:
https://drive.google.com/open?id=0Bz1I1mIutSEnbFJuVUJZWWNNVlU