Design Considerations for 16" Ductile with 1.5 feet cover 3

[rocklin]

I'm trying to design a 16" DI watermain to pass above a culvert. I will only be able to get 1.5 feet of cover. I am trying to figure out what I need to consider, or if it is even possible. Any input is greatly appreciated.

 

[yogibear1]

There is nothing inherently wrong with a shallow burial of pipe IF all relevant factors and eventualities have been considered and accounted for.
Saying the pipe would have "only 1.5 ft of cover" implies your pipe is normally buried deeper - perhaps with three or more feet of cover(our minimum is 3 ft above top of pipe). That generally is a plus.
Also, you did not say that if buried deeper how the pipe ascends to be above the culvert e.g. was it deflected over several lengths of pipe or was it thru fittings. All of this leads into the question of joint restraint and the balancing of forces to keep the pipe together. The foundation guide to this is the AWWA Manual M41 Ductile Iron Pipe and Fittings. The principle behind this is that thrust forces occur wherever the direction of the waterbody changes; thus there will be thrust forces at bends, reducers, offsets, dead ends, etc. The greater the direction of change teh greater the thrust. Thrust forces if not properly balanced can cause pipe joint seperation.
Other factors enter too as the type of soil used as bedding/cover as it provides some frictional restraint, any surge pressures in your system, and normal seasonal high water level. Also, consider the likelihood of damage from other utility contractors digging in the area (assuming this is Right of Way). They could disturb the pipe/soil envelope and cause joint seperation. Our practice in this scenario would be to use a sufficient number of mechanical joint restraints (perferably the pipe manufacturer's system), though there are good after market restraint systems too. I would lay warning ribbon above the pipe, and a marker post in the ground next to the pipe.

 

[yogibear1]

In my post above, 2nd to last sentence should have been clarified as a scenario with fittings used to make the offset above/below the culvert.

 

[vzeos]

You may want to consider freezing as a potential problem if you have cold winters.

 

[BRIS]

To add to Yogibear's advice - if you are arching the pipe over the culvert you need to make sure that the cover is sufficient to prevent the pipe lifting out of the ground. I assume by mechanical joints Yogibear is referring to anchored joints - certainly worth considering if the pipe is likely to be subject to high pressures.

Also if you have a high point you may need an air release valve which you will not fit in 1.5 feet of cover. If that is the case simply provide a 1" ferrule at the rest to allow air to be released manually.

Ductile iron pipe is robust - I have seen DI pipes laid with so little cover that the crown appears through the road surfacing - they have survived.

 

[dicksewerrat]

why not go under the culvert? Or if it is real big, 60 inch, go through it. If you go over, I'd use a higher class DIP. If you normally use cl 52 go to 53 or 54

 

[rconner]

I believe the Ductile Iron Pipe Reseach Association (DIPRA) publication, "Truck Loads on Pipe Buried at Shallow Depths" contains some design information that may be of assistance and can be downloaded from

http://www.dipra.org/pdf/truckloads.pdf.