Manhole Size for Parallel Pipe

[Dozer03]

I have a double run of 36" RCP for storm drainage and need an interim junction. My supervisor wants me to use a circular manhole, rather than a rectangular junction box thinking it would be more economical (not sure about that, but I guess we'll see). Assuming this junction only has two parallel 36" RCP's coming in and going out (i.e., no other pipe penetrations), what is the minimum diameter manhole required? Design references or resources would be appreciated. Also, what is the minimum gap required between the parallel 36" pipes...I was assuming 12-18" in order to properly compact. I suppose I could separate the lines a little more and have 2 manholes side by side, but I'm not sure if that would be economical. And before you may ask...no, I can't change the two 36" RCP's into a larger single pipe (round, arch, elliptical, etc).

The following link was pretty close to having a solution, but not quite there:

http://www.dot.state.fl.us/rddesign/DS/06/IDx/200.pdf

Thanks in advance!

 

[cvg]

so your minimum manhole diameter assuming 18 inch separation would be about 106" ID. assuming about a 10 inch wall thickness that would be about 126" OD (10'-6"). That is a large piece of pipe and they don't typically stock those. Plus you will need a big slab on top to mount to mount the riser. That will cost you an arm and a leg and hydraulics will suck. However, your option to build a cast in place rectangular structure is expensive also. Two separate 36" manholes would be far cheaper in my opinion.

 

[ceelias]

Would not two 48" manholes be preferable to two 36" manholes? That would provide some meat to hold the manholes together during construction.

 

[cvg]

for 48" and smaller pipes, typically use just a concrete base with a 48" riser on top. this is a standard detail.

 

[gbam]

Following up on cvg's idea the two manholes could be offset as to not conflict with each other. I have handled parallel pipes in this manner. I also like the junction box option as well. The large manhole will most likely be costly.

 

[cvg]

one issue to consider with twin storm drain pipes is flow equalization. assuming you have drainage inlets connected to either one side or the other, the flow may be different in the two pipes. you might need to connect them together every so often to try and equalize the flow, if that is a concern.

 

[Twinkie]

Check with local ordinances. We can not put anything larger than a 30" into a round, we have to use a modified box, with the cone section on top.

But like the others suggested, go with the dual junctions if you can.

 

[dicksewerrat]

If the pipes don't have to mix flows, use RCP Manhole Tee sections.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com