Has Anyone Ever Installed DIP Water Main in Contact With RCP Storm Drain?

Has your local controlling authority for both the water line and the storm line been informed of this problem? They will usually have a solution for such situations. You'll probably not like the solution but they make the rules.

We've always tried to keep 10 feet clearance vertically and horizontally between utilities and sewers as required by the utilities. But when this isn't possible we have had to encase them in concrete spanning both the sewer and water line so many feet (usually 5 feet min) in each direction. Basically you are making a concrete bridge over the intersecting area.

I am the local controlling authority. Sometimes you can not meet your own specifications, this is one of those situations. Like I said, can not go around, can not go under, have to go over. The cover over the storm drains is the issue - 2.8' of cover and trying to place an 8" DIP water main on top of the storm drains and keep adequate cover for the water main. The concrete encasement is an idea, but I don't want to create a point load on the storm drains, and cause further problems in the future. The RCP storm drains are already 25-years old.

We routinely run into similar situations. We place HDPE or rubber pads between the two lines creating at least a half inch gap. We also always center a full stick of pipe over the crossing to maximize the distance between the storm line and the water line joints to reduce potential cross contamination risk during water main failures. Finally we also usually encase the entire mess in concrete or CDF to further reduce the risk of cross contamination. According to our overseeing regulatory authority this is acceptable. If it was a higher risk crossing, say of a water line and a discharge line from a sewer lift station, we will use a 20 foot steel casing to run the water line through at the crossing.

As an consulting inspector I have seen these situations many times, involving many utility types.

The DIP needs to be sleeved, at least 5 ft. in either direction, as SCOTCHDRKR, suggested. The sleeving, using PVC or SDR type of pipe, will assist in the prevention of contact wear to either structure. Seal the ends!

The real reason for this, however, it to prevent local disturbance of one from the other should a rupture, of either, occur.

Then, ensure thorough compaction of the soil (such as base rock, NOT sand) used to cover both. A cement slurry mixture would actually be better, placed at least to the spring line of the sleeve. This will help stabilize the now disturbed soil around the SD. Continue adding/compacting to as much cover as is reasonable over the water main.

Assuming that final cover is an issue, leave the base 1' below final grade and pour a 3500 psi or better concrete with reinforcement on 12 in centers (both directions) 6 in. thick to 'bridge' the whole thing. Do this for the whole 10 ft. and full width of the trench or disturbed area. This spreds the load well above the pipes. This elevation can be adjusted up depending on the final grade finish. Leave enough for the required thickness of pavement to be placed atop the concrete or 6 in of topsoils.

Fyi while this close crossing proximity probably for multiple reasons flies in face of codes and standards recommendations, as many of you note stuff like this "happens" and I guess most folks would probably use ductile iron pipe and encase or partially encase the area where pipes are close together to provide uniform support to both structures (and minimize or eliminate point fulcrum loading etc.?) when they do this. About only other thing I could think of would be, as this is probably going to involve long established structures next to new excavation bedding/backfill, where there might be expected subsequent/relative movements/settlements etc., you might consider also providing a couple flexible joints (a "rocker pipe") near the boundaries of any concrete encasement with new excavation etc.

All of your comments are exactly what I initially cam up with. However, there is not enough cover for the casing and the concrete. Also, to get a casing as strong as teh PC350 DIP, is quite expensive and no one stocks it. It appears the DIP manufacturer and the state would rather see the DIP installed with a rubber spacer between the twop pipes. I am going to remove the material down to the spring line of the stormdrains and fill it with concrete. Then encase the water main with concrete 1' outside and and above. Then once the concrete is set, install asphalt to grade.