I've been digging around a little bit and I think I clarified in my own mind what's going on.
1. It appears that the weakest point in dams is usually at the sides, where it "attaches" to the surrounding geology.
2. Typically, this surrounding geology is earth rather than solid rock. Essentially, not to be flip about it, you have a concrete dam surrounded by two earthen dams.
3. In this case, it is
not the emergency spillway which is the immediate problem, it's the regular concrete-lined spillway which broke, resulting in the "crater." I think this is a point of confusion in some of these posts.
4, Water rushing into this crater and roiling around in there is washing away the earth and rocks around the sides of the dam, thus possibly weakening it. I think I've seen some rather large boulders being thrown out by the water at that point.
5. The "emergency" spillway is
not concrete lined.
6.
If they have to go to the emergency spillway (earth-lined) there are at least three dangers involved.
a. this will increase the erosion of the earthen "sidewalls" of the dam, further weakening it
b. the larger amount of water heading down will flood the levee below the dam, which also protects a fish hatchery there, where "millions" of hatchlings will die
c. the larger amount of water heading down will also overflow other levees further downstream which protect those populated areas and infrastructure, hence the evacuations --let alone if the whole thing lets go because of weakening that side of the dam
7. They are dumping material into the "crater" in the concrete spillway to help in slowing the erosion under it.
...
It will be interesting to see the forensic report on the cause of the failure.
I suspect it will be one or more, or all, of the following:
1. Somebody got stingy with the cement and aggregate mixture applied during construction of the spillway.
2. Improper compaction of the substrate material under that portion of the concrete of the regular spillway.
3. An unseen "pipeline" from the storage side of the dam weakened the substrate under the concrete at that point. Not that the water is that deep at the point of failure, but the dam itself is 770 feet high. The pressure of water at that depth would be about 360 psi*, and varies linearly with depth. A "pipeline" in terms of damming water is a sneaky little tiny leak which gradually gets bigger and bigger.
This is not intended to be "authoritative," but that's the way I understand the situation as of 13-14 February.
Terry, 230RN
* Quickly obtained from here:
http://www.calctool.org/CALC/other/games/depth_pressI'm not sure if that's for sea or fresh water, but it gives an idea of the magnitudes involved.