Sanitary sewer and storm sewer crossing clearance? 1

[hydroponder]

How close would you cross a sanitary sewer with a storm sewer. The sanitary sewer is an 8" PVC and the storm sewer is a 12" RCP. Normally, I try to keep 18" between the crossings, but I cannot make this happen. Basically, the way it is designed right now, the sanitary is sitting right on top of the storm sewer, with the sanitary being about 8 feet below grade. I know this is less than ideal, but the sanitary is an existing pipe and the storm sewer cannot be lowered (or raised above) the sanitary. Any comments are welcome. Thanks.

 

[rconner]

fyi I think there are many past similar discussions, searchable e.g. with Advanced Search feature, such as

http://eng-tips.com/viewthread.cfm?qid=149195

 

[avane]

I don't think there is a general standard. The 18" vertical clearance comes from the 10 states standards for sewer and water lines. You should check your state's requirements. If there is no code you're ok on that distance. However, good engineering judgment would be to make the sanitary line DIP or some pressure-related pipe and to ease the bearing of one on another, pour a concrete saddle/cradle between them in the field. This eliminates a point load on one pipe from the other. One of my clients actually has a standard detail of a sanitary line going through a stormwater manhole!

 

[CPENG78]

The clearance between storm and sewer lines varies depending on the agency or agencies overlooking the construction of one or both of the lines. I have come across clerances taken down as low as 3 inches. However, anything less than 1 ft would require a concrete saddle to avoid those loads like Avane mentioned. In some cases, I have even come across concrete piers to prevent the sewer line from settling. The other design consideration that I have come across is that if such clearances are less than 1 ft, then pipe material is not only changed to DIP, but there are no connection and/or fittings allowed within 10 ft in both directions of the crossing. I would think that anything over 1 ft should be fine but verify with your local/state agency as to their requirements.

 

[Crystalct1]

In Delaware, we are designing a storm system that crosses an existing sanitary (72x24 RC Box over 8" Terra Cotta) with 6" clearance. We were directed by DelDOT that this was minimum. This also mandates concrete encasing for the sanitary line.