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Gravity Sewer and Stormwater Conflict

Gravity Sewer and Stormwater Conflict

Gravity Sewer and Stormwater Conflict

Has anyone allowed or designed a gravity sewer to "penetrate" and pass through a stormwater culvert?

We are constructing a new building in a previously developed area with existing underground utilities.  The new sewer (6") will be gravity (pvc) and will discharge into an existing sewer cleanout located south of the building.  In order to reach said cleanout, the new sewer line must cross (perpendicular) an existing corrugated trench drain.  The trench drain is a 12" diameter culvert with a 3" slotted opening (8" depth).  In order to avoid a conflict manhole, the contractor is proposing to simply penetrate the slotted trench drain (upper portion of storm drain) and seal around said sewer line.  

We are trying to avoid using a pressurized system (pump) due to other existing utilities in the area that would have to be crossed in order to reach a proper discharge point.  Would a penetration like this be ok to use if the portion of the sewer line conflicting with the slotted drain be called out as ductile iron and sealed appropriately to avoid any leakage?

RE: Gravity Sewer and Stormwater Conflict

How deep is the sanitary main where this 'cleanout' line taps the main? this is not a good way to go. You are causing a problem for the trench drain and when it overflows, the wtaer will probably enter the building.

Richard A. Cornelius, P.E.

RE: Gravity Sewer and Stormwater Conflict

The existing cleanout discharges into an existing sewer manhole roughly 100' south of the building.  The existing sewer manhole depth is approximately 4'.  Not sure at what depth the cleanout ties into the manhole.  

If the new 6" pvc sewer line were allowed to "pass thru" the existing slotted drain, then roughly 1/2 of the upper portion of the slotted drain (culvert portion) will be taken up.  I'm about to run a model to determine what the depth of flow in the slotted drain is without obstruction.  

I'm leaning more to the resolution of cutting out the portion of existing slotted drain, installing a small conflict manhole (concrete), and using ductile iron for the portion of gravity sewer that passes thru the conflict manhole structure.  

I was just curious if anyone has seen or specified this "pass thru" option without using a conflict box.

RE: Gravity Sewer and Stormwater Conflict

conflict structure is the way to go

RE: Gravity Sewer and Stormwater Conflict

All you are asking for are problems. Dig up the 'cleanout line 15 feet past the trench drain. Then shoot the invert. Lay the sanitary with minimum slope to stay out of the trench drain. I can't believe that the inspector or owner want the line laid as you are suggesting.

Richard A. Cornelius, P.E.

RE: Gravity Sewer and Stormwater Conflict

What are you refering to as a "conflict structure".

I have ran across a residential street widening / greenway project with limited fall across the length of the street which made reworking driveway culverts very difficult.  Also we had no information on sanitary service lines.  When we did have a conflict with a san. service and storm culverts we specified a siphon type structure for the storm to go under the sanitary since a possible storm backup/overflow is less detrimental for public health.

RE: Gravity Sewer and Stormwater Conflict

By conflict structure, I'm referring to a concrete or brick inlet/manhole in which the drainage culverts tie into.  The sewer line then "passes thru" the manhole without actually discharging into the manhole.

I've sent a detail to the contractor for a conflict box.  I understand that the cost is more, but I'd rather sleep easier at night.

RE: Gravity Sewer and Stormwater Conflict

Google search:
"conflict structure" sewer detail

Agree on conflict structures being the way to go, but some municipalities don't like them.  Talk to your reviewer first before you go too far down that rabbit hole.  If you do go with a conflict structure, or some other kind of system where one pipe goes through the other, go DIP for the sanitary sewer, not PVC.

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