We had a dam breach due to topping of the dam following a major flood a couple years ago and are in the process of repairing the dam. In the past all of our dams have been built by experienced excavators and have not been professionally engineered. For this repair we decided to do it right and have it engineered. I feel the engineer has provided a very conservative and good design but we are having difficulty meeting one spec he is unwilling to relax. The water level behind the dam is controlled to 10 ft depth by a 36 inch down tube feeding a 36 inch through tube at the base of the dam. The down tube and through tube are corrugated galvanized steel and the through tube has two bentonite seep collars over its length. The pipe is supported by a preformed concrete block at the down pipe end. The issue we have is meeting the compaction specification for the soil supporting the pipe. The engineer has specified 95% compaction beneath the pipe. The soil being used comes from offsite and has been tested to verify it can achieve 95% compaction. The first attempt at achieving 95% compaction failed due to excessive moisture. The second attempt ranged from 92% to 94% with the engineer attributing the out of spec compaction on too little moisture. I am aware of the critical part of moisture in achieving compaction and the exactness required to achieve 95%. Winter set in before a third try could be attempted. We are now a year behind in schedule and with all the work trying to achieve 95% compaction and the cost of having an engineering company come out to verify the compaction our budget is hurting. My question is: is 95% compaction necessary under the pipe and if not what is sufficient?
Any insight and/or source of data on this issue will be appreciated.
Bo, I don't know anything about % of compaction. However, I don't like the idea of through the dam pipes. Look into a siphon system. It doesn't mess with the integrity of the dam.
Welcome to PB.
I think the corrugated galvanized pipe will eventually rust through, causing major problems. Why not use the corrugated double wall plastic?
It would be hard to tell the difference between 92-94% and 95% compaction. Some professional engineers have become too liability averse to be practical IMO.
I don't like anything through the dam below normal water level.
Check into aluminized pipe or plastic pipe. NO clue on compaction.
Thanks for the replies and thoughts.
A few additional notes:
We have a total of 7 lakes using galvanized steel through pipes with some over 40 years old. Estimated life on such pipes is 40 years and the first point of failure is typically collapse of the down pipe where it enters the lake bottom and secondly rust through on the top of the through pipe, due to condensation, resulting in collapse. At this point the down pipes are looking OK. I suggested various plastic pipe solutions be considered along with the traditional galvanized steel to the engineer but he chose the galvanized steel and offered good reasoning. We recognize that at some point in the near future we will be replacing the old pipes when they fail. Hopefully, with enough time between failures to repair only one lake at a time. This new pipe should give us another 40 years on this lake.
The dam that failed did not fail due to the through pipe, in fact the through pipe was still in place after the water receded.
State water regulations for ponds of our type require they include a means of draining the lake. This essentially mandates a through pipe with a gate.
Keep your thoughts coming. Still need an answer to compaction under the through pipe.
Bo
If you have enough fall behind the dam, a siphon pipe will do the trick for you. It can be used to automatically turn on and off to keep a lake at a certain height, and if the siphon is started, then the air break is capped, it will drain the lake.
A siphon will move more water than a thru dam pipe given the same diameter, so a siphon can be downsized over a thru-dam pipe. Plus you set it up to take out the anoxic water from the bottom of the lake, helping the water quality.
Some engineers cannot think outside their box. Time to hire a new engineer?