Skip to comments.Emergency: California’s Oroville Dam Spillway Near Failure, Evacuations Ordered
Posted on 02/12/2017 4:26:47 PM PST by janetjanet998Edited on 02/12/2017 9:33:58 PM PST by Admin Moderator. [history]
The Oroville Dam is the highest in the nation.
water is still leaking out because the seals don’t go down all the way and this is causing erosion
they are working to tighten the seals
As I appreciate what we’ve learned so far at Metabunk, the underslab drains - vitrified clay pipe (vcp), perforated, 6” dia. - run crossways to the spillway, with a slight downhill bias. Herringbone pattern was the way it was described.
They rest in a gravel bed to allow seepage to percolate into them. Kind of like a french drain. They were covered with a polyethylene (Visqueen) sheet to prevent concrete infiltration when the slab was placed.
From the center, they extend both ways from the center to just beneath the wall footings and there connect to a 12” vcp collection drain. We think the were on 20’ centers, with about 10 each per collection drain.
The collection drain runs longitudinally along the outside of the wall footing, following the natural slope of the spillway.
At some point just past the last underslab drain, the collection pipe elbows to a horizontal run (probably slightly pitched to facilitate drainage). Then the collection pipe elbows into the spillway wall and discharges.
There are multiple systems on each side.
Thanks. Good stuff.
Thanks abb. Did I get that wrong about certain drainage pipes somehow pointing uphill?
Which ones? The underslab drains, or the 12” collection pipes that parallel the wall footing?
Dunno. If any of them run uphill, I’m curious why.
None that we over at Metabunk could ever see or find evidence of. We had a theory, since discarded, that perhaps the collection drains had vertical risers attached to the wall outlets.
Why would a drain pipe point uphill?
I think he meant to say downslope. From that original drawing that showed the drain to be horizontal, the drains were angled slightly to take advantage of the spillway slope.
By doing that, the terminal end of the drain at the spillway center would be higher in elevation than the other end that drains into the collection pipe.
Hi Jim 0216, Perhaps this image may clear this up (re: question discussion). The fractures reveal an "upward" pattern where the drain pipe is still under the slab, but angled slightly upslope to facilitate flow. Key: Pipe is angled "upslope" with respect to the resulting "downslope" flow of drainwater. If the drain pipe were constructed strictly horizontal there were design considerations discussed of "self-cleaning" and "better flow".
These series of fractures follow design information on the placement of the drains under the main spillway slabs. These fractures likely occurred after the original blowout failure. The downstream hydraulic turbulence would have pounded these slabs. As Freeper abb has provided prints on the gravel & drain pipe placement in the dimensions of the slabs, this becomes a narrowing area of the thickness of the slabs. Thus the cracks below the blowout area offer insight into many factors (stagnation pressures, thin section in slab concrete, strength of hydraulic pressures from turbulence, etc).
Of course, these insights are from an after-failure. The spillway was not designed for the after-failure - BUT - this after blowout failure cracking gives an indication of what the actual design may perform to in extreme destructive conditions (i.e. useful as a form of a destructive test to provide data in original safety factor margins & design vunerabilities).
Thanks ER. Maybe I’ll need to take an engineering class to understand how a drain pipe angled slightly upslope “facilitates flow”. My simple mind would think a pipe angled upslope would tend to back up the flow.
No, man, I'm the voice of experience—was fined thousands of $$$ by Getty images for my designer's illegal use of Getty images. And JimRob’s experiences as well.
If you look at the picture above your post, uphill is to the left and downhill is too the right.
I believe the post you are referencing suggested that the cracks that are forming above the drain pipes on the slabs-next-to-fail indicate that the pipes run “up hill”. That is up hill in the sense that if you were a mouse entering the drain at the edge, you would proceed up hill as well as across the underside of the slab. The cracking in the yet unlighted slabs angles uphill from the edge.
That, at least, was my interpretation of his meaning of “up hill” which would also be normal procedure for a sub-drain or french drain.
My auto correct turned “un-lifted” to unlighted. I am tired of that feature.
Going back to prior images before the Main pre-spillway, there are numerous sign of the "HerringBone drain pipe" alignment in Maintenance repair cracks on the surface concrete of the Main Spillway.
Why is this important? The drain pipe, & enveloping drain gravel around the pipe, reduces the thickness of the main spillway slab (see blueprint data posted by abb in post 2,132). This reduction of slab thickness would represent a weaker structural zone vs the full slab height. The very strong indicators of these Maintenance repaired cracks, in a "herringbone" type of pattern, leads to the question of if this is a leading contributor (combined with subsurface erosion, stagnation pressure, WEAK point in concrete slabs from drain design, expansion/contraction stresses, etc).
Looks like the same patches I put on my carport’s cement floor.
Of Course, I didn’t have worry about millions of gallons of water trying to escape through my patches.
There seems to be some type of irony here in that the design may have created cracks just above the drains just so the drains can remove the water from the cracks....
Not to mention each of those cracks represent a weak point in the slab.
When you drive down the interstate, you may visualize it as one continuous concrete ribbon unbroken for miles.
Not so. Concrete paving is a series of individual slabs, with “control joints” spaced about 30’ apart, and spanned by steel dowels to transfer the load from one slab to the next.
When placed, concrete eventually cracks, due to shrinkage as it cures. The trick is making it crack where YOU want it to, where provision can be made to seal the pre-planned joint, and with provision for load transfer.
Placing Joints in Concrete Flatwork: Why, How, and When
Transferring loads across joints.
When I saw detail “A” in post 2132 I said to myself I would shoot one of my civil staff for detailing something like that. It goes right back to that faulty slab on solid rock issue I have whined about in previous posts.
Detail “A” facilitates a weakening of that slab right along that drain pipe by thinning the slab. If the slab had sub-base rock below it, the sub-drain should be put in a french drain channel below the layer of sub-base rock. Having it run slightly diagonal down hill is a sensible feature that they made in the field but having no uniform sub-base with this drain below the sub-base is substandard in my thinking even for the 60s.
Here is the drawing that detail was from.
pg 172 of 193
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