Xing under AC Main

[BernieH]

Anyone know the feasibility of crossing under a 1970's 8" AC Water main at right angles to install a sewer - without causing the AC main to leak in the process?
Thanks

 

[coloeng]

It is done in our system a lot. We always require that they backfill up to the spring line of the water main with what we call flowable fill, others use other names, but it is basically very low strength concrete (40-60 psi). You also need to ensure that the contractor provides support for the existing AC line while they have it exposed. We have had good success with this method. Any other method of backfill under the water main that has been used in our system has usually resulted in the main beam breaking.

 

[rconner]

You are perceptive in sensing risk, as an incidentally concurrent thread at

http://eng-tips.com/viewthread.cfm?qid=301348&page=1

(as well as near 36 years of other accounts I've heard from being around piping systems, before flowable fill was a rage) indicates that even worse happenings than leaks can and do occur, and response suggests alternative. A weak pipe that might look adequate in simplistic two-dimensional design simply does not look as good in three-dimensional beam loading scenarios spanning excavations with uncertain support, and you have been handed a quite difficult assignment here.
Who would have thunk anyone would ever try to cross a pipeline they were designing and selecting pipe materials for back in the 1970's(it appears you have been dealt a poor hand from a few years back)?

 

[Shawner]

Our municipality has always required the installation of a "flex joint" wherever there is a passage under an AC main.

Line is shut down, snapped and a piece of C900 is installed with two couplings.