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Minimal Trench Separation for Utilities

Minimal Trench Separation for Utilities

Minimal Trench Separation for Utilities

(OP)
Can anyone tell me what they have spec'd out for storm and sanitary sewers that will have overlapping trenches? Recently, the stormwater department changed their design criteria and one of the repercussions is that there will be more (large 30"+) storm pipe in the ground. There are several instances where the proposed storm alignment is really close horizontally to the existing sanitary sewer (not necessarily close vertically). A lot of the sanitary sewer collection system is VCP and will eventually need to be rehabilitated. I am concerned (because it has happened) that when it is time for the sanitary sewer to be replaced the bedding from the storm pipe will fall into the sanitary sewer trench and cause the storm pipe trench to fail and and damage the storm pipe. Costly repairs outside of the scope of work.

Ideas already considered:
*Current VCP condition for these areas do not warrant the sanitary sewer pipe to be removed at this time.
*CIPP or other trenchless methods to rehab the VCP is an option but we have encountered issues with that as well. A thread for another day. We would prefer to keep our rehab options open rather than pigeon hole to one method.
*In these instances there are not any other locations to install storm pipe due to elevation constraints or conflicts with other utilities. Unfortunately, sanitary sewer is typically the deepest so it doesn't conflict with the storm construction - until sanitary sewer mains need to be dug up.
*Placing flowfill (CLSM) in lieu of backfill material to be stabilized when the sanitary sewer trench is excavated. Concerns with constructability, how to actually accomplish - does a flow fill truck come daily during construction (will that get really costly?), protecting pipe, if ADS or other plastic material, from floating, etc.

I have to believe that someone else has encountered this issue and has come up with a brilliant solution. Thanks for your time.

RE: Minimal Trench Separation for Utilities



This is our spec

RE: Minimal Trench Separation for Utilities

(OP)
Thank you swazimatt!
Is there any additional work you do when the clearances are not met to protect either utility?

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