Pipe Crossing Help 4

[raydefan]

I have an 8" PVC sewer that crosses perpendicularly OVER an 18" RCP storm drain. The vertical clearance is less than 1'. I think I should encase the storm drain in concrete because it is the lower pipe, but the plan check thinks it should be the sewer. What do you guys think?

 

[rconner]

You might want to check Ten States standards that could be argued at least briefly addresses situations that sound like this for their jurisdictions [I think the basic inference is that if the new sewer is closer than say 18", it should be made of watermain quality (for the jurisdiction) pipe and/or encased, and there also might be guidance on location of joints, etc.) At least if ductile iron pipe is used you could also refer to some arguably applicable verbiage in ANSI/AWWA C600 as well.

 

[Maury]

I don't see a need to encase either pipe in concrete unless the storm sewer is in poor condition. Ten-States Standards doesn't have any requirement for minimum separation or using watermain pipe for sewer/sewer crossings, only watermain/sewer. Require that a full length of the upper pvc pipe be centered over the lower pipe and ensure that it is properly bedded. If the storm sewer is an existing pipe in bad shape, then replace it 10' or so on each side of the crossing.

 

[bimr]

Maury is pointing out that the separation requirements are generally called out to protect potable water from contact with sewage. Rconner is pointing out that this requirement generally means that you should have watermain quality piping within 10' of the crossing with a water main.

In the original question, you have not addressed the need for protection of the storm water, so you could go either way. Concrete encasement is probably not necessary, but the installation of water main quality piping for the sewer is not going to break the bank and would probably be justified.

 

[ccor]

bimr is correct that the water-sewer separation isn't an issue here. With crossings you must determine if either pipe will impart a critical load on the other pipe. Assuming you have a small sewer over relatively larger RCP that is in good condition, you probably dont need to do anything other than ensure proper bedding at the crossing. To reiterate Maury's comments it is worth detailing a full length stick of sewer to be centered over the crossing. In addition to getting the joints as far away as possible for leaking reasons, it is the strongest system if the RCP ever has to be dug up. This is one that is worth some field presence to make sure it gets done correctly.

 

[raydefan]

Thanks for your help.

 

[gbam]

Have you given any thought to using 8" DIP in the vicinity of the storm drain?

 

[LCruiser]

I've seen DIP called out for that situation. If you want to encase, the sewer would certainly be the easier one to do.

 

[cvg]

if you are installing the storm drain underneath an existing sewer - then you might want to consider encasing the sewer to prevent it from collapsing into the trench or to prevent future settlement within the trench backfill from damaging the sewerline. If this is a new sewer over an existing storm drain, encasement isn't necessary.

 

[rconner]

The presence of a crossing pipeline below the proposed route of a new pipeline of course means probable previous substantial excavation or greater effective undercut far below the new pipeline bedding at that specific locale. Depending I guess on when the lower existing pipeline was installed, and the quality of the bedding/backfill of same that was accomplished at that specific location, I would think there could be opportunity for differential bedding and settlement conditions of the new upper pipeline at this crossing. While I think I understand the niceties of minimum code requirements (the risk of cross-contamination is arguably greater if the existing pipeline is a watermain instead of a "storm drain"), I'm not sure the fact that the existing pipeline is a "storm drain" much helps the new pipeline situation e.g. particularly if the storm drain is not made of watermain quality pipe and joints, and when fine grain soils are around or over this piping. Is it possible future infiltration of water and any supporting fines carried by same into the storm drain (and there to be carried away) might even in sort of sinkhole fashion exacerbate future settlement of/loads on the upper pipe (beam) that is in effect spanning the old lower pipe trench?? Personally, I don't degrudge the intent of a new pipeline designer who is wanting to use great beam strength pipe and/or casing pipe or encasement designed to strengthen the new upper pipe beam at this location.

 

[saruman26]

Fairfax County, Virginia requires in this case that the sanitary sewer line be encased in concrete to ensure the safety of the sewer line.

 

[Sharrick]

Every state has their own codes for sewer seperation from other utilities. In Michigan, If a sanitary sewer is crossing any other sewer or watermain, the sanitary must be enclosed in cement, not so much for support, but just to stop any leakage from the sanitary that could follow the water main or seep into a storm sewer. Best to check state and local requirements.