DSOD 1/12/2000 Inspection report:
Found the "Drummy" soundings on the concrete in the Spillway. note: "drummy" soundings are from either dragging chains over the spillway slabs & listening for "hollow" types of sounds that occurs from open voids below. This is from a 1/12/2000 DSOD report.
Amazingly, the Inspector writes that "No treatment is proposed until they are damaged by a heavy flow". Talk about prophetic. This statement could be the "poster child" sign hanging from Engineering => "NO TREATMENT IS PROPOSED UNTIL THEY ARE DAMAGED BY HEAVY FLOW"
Another astounding finding is that DSOD discovered the cause of all of the "chipping" "spalling" on the bridge abutment ends. The original design never put in sliding bearing plates. Thus the bridge slab will rub "concrete on concrete" in the expansion overlap seat. Inspectors measured the size of the expansion seat as 11 inches. Worst section losses of "chipped" away spalling of the abutment support concrete was measured at 5.5 inches. The inspector calculated the required support overlap is 6 inches. This would be -0.5 inches below the required support. (note the Inspector's math of 6.5 inches of bearing left is in error - should be 5.5 inches - skipped a digit). DWR should not be using this bridge. It is a highest level Category 1 Potential Failure Mode (PFM). THIS WAS 17 YEARS AGO. THEY ARE STILL PAINTING THE SPALL AREA TO WATCH THE CHIPPING CONTINUE. Amazing.
Note: PFM in this case would be from a heavy construction vehicle inducing a collapse of either end of the bridge at the chipped away abutment support. The heavy vehicle would crash down directly on top of the Radial Trunnion Gate anchorage, the Radial Gate Trunnions, and the Anchor Tendon assemblies.
ER333: you always have interesting, well researched, and thought-provoking posts. Again, many thanks.
Regarding the concrete chunk you reference, Bill Crowle from DWR did say that the spillway slab was 6 foot thick in sections, and that chunk does appear to be from one of those extra thick sections.
On one hand, I’ve worked enough construction to know that no contractor will lay down 6 feet of concrete if they can get away with a 1 foot slab on top of whatever type of (erodible) base material they were using. The contractor pays for every truck load of concrete, and that comes out of their profit margin, which they’re generally rather concerned about. This would be an argument for there being a 1 foot base slab, with the rest mud jacked in years later, as you suggest.
On the other hand, from the picture, the entire chunk of concrete appears monolithic with the same mix design, and I couldn’t see two lifts of concrete placed a couple decades apart tumbling 1000 feet downstream as a monolith. I think we’d need a closer examination of that chunk of concrete to make the call.
If they did mudjack the slabs, it seems plausible this could plug up some of the diagonal under-slab drains. However, I couldn’t see this plugging up the longitudinal side collector drains. This would be too easy to check for, even in the pre-borehole camera era. You just pour water down the upstream vent tube, and if it doesn’t come out the downstream sidewall port at the same rate, then you know you have a clog between the two. DWR may not be the sharpest crayon in the coloring box, but you’d have to be a total idiot to let a spillway contractor walk away from the job with a paycheck without checking for a clogged drain.
If a tree root was a contributing factor to the drain clog which precipitated the spillway failure, and we know the drains were under high pressure, then it seems plausible that the pressurized water could escape along the root pathway through the fill to the side of the spillway. This would provide a high-volume escape route for the water, taking more more base material with it. Some pictures appear to indicate the side of the spillway was blown out before the major slab failure.
If the pressurized water had no escape, it would push the slab up, “hydraulically jacking” it. If the water had an escape route, it would go longer be pressurized and spurting out the sidewall, but eroding underneath it, and the slab would fail downward. Up or down, the slab failed one way or the other.
A contributing factor to both the spillway failure and FCO structure problems which hasn’t been examined in this thread (to the best of my knowledge) is concrete shrinkage. Some old mix designs would shrink quite a bit. With the extra large slabs they were using on the spillway, the expansion joints could pull apart, letting more water under the slabs than could be caulked out. This could also contribute to why FCO is now 5 1/2 inches shy of the fixed foundation at the abutment, and cracking at the seams.
To build a R/C structure and that needs to function reliably for a 100 years or so, I’d go with an expansive mix design, using a type K cement. This has Ettringite (calcium aluminum sulfate) added. There are type K slabs which are approaching 50 years old, the same age as Oroville, that are still in almost perfect shape with no cracks or joint issues.
A plausible hypothesis as to why a tree root may have grown adjacent and then into the drain at the blowout failure location is that there was a natural “Water Percolation Seam” at this location. The native geology of the area appears to contain a great deal of fractured and erodible rock with many fissures that water could percolate through. It is also plausible that such a seasonal “Water Percolation Seam” could be injecting moisture into the side of the dam, creating the (apparently) seasonal “Green Spot”.