Oh crap. I'm sorry to see that! I agree with you. If collars were supposed to be on the pipe and they weren't, well then it's on him, not you.
I agree with bringing in good clay, but if the dam wasn't built with any, then what was the core made from?
Look at
http://ponddampiping.com/ They have anti-seep collars, and take a look at the siphon system. You may want to think about going that route to get the pipe out from being deep inside the dam. That will address the "proper compaction around the pipe" thing. You bring up a good point. If the water level showed that it didn't go over the emergency overflow, then why did the pipe wash out??
I don't recall him saying he'd put collars on the overflow pipe. He said he was putting them on the drain pipe, the 2" line through the bottom. I was ok with that at the time because the overflow pipe would never really be under hydraulic pressure. (If the pipe keeps up and water level doesn't rise over the overflow to emergency spillway) It is at full pool level on the water side, and slopes down to surface level at the back/bottom of the dam. I believe soil compaction around the overflow is the primary fault, but would think that collars around the overflow would only help.
I studied that web site shortly after it occurred. A primary reason why I thought of collars. They seem to do it for exactly such a pipe routing.
The dam was built entirely of soil scraped out of the ravine and the fields to either side of the dam. They said they used the best soil for the core. There is some clay in the soil, but looking in the channel cut through the dam, there's a good bit of rock exposed. I know water will wash soil away and leave it looking rockier than it was really. Rocks have to make water seepage worse. I was trying to find ways where we can do it better and avoid scraping yet further away into my pastures.
The siphon system is intriguing. I don't know how my guy would react to that as it's much more complex than simply running 5 lengths of pipe. So far, I've been letting the expert do it as he sees fit. He knows much more than I do.
