DIRECTINAL DRILLING STORM SEWER 1

[rcspiel]

Does anybody see any problems that could occur if I call out directional drilling an 8" polyethlene storm sewer at 1.00% under an existing concrete pedestrian tunnel under a railroad spur track? Pipe will be used for gravity draining a couple trench drains for a pedestrian walkway down to tunnel. Tracks are 20' above the pipe and tunnel floor is approximateley 4' above the top of pipe and new pipe will be approximately 10' above an existing 54" sanitary trunk line built in 1920s. Do you see any soil pressure stresses that could damage the tunnel or sanitary line?

 

[dicksewerrat]

No major problems. the RR Tracks live load should be minimal at 20 feet. You may want to look closely at what the tunnel loads on the 8 inch are. Just remember that HDPE is measured at outside diameter. An SDR 32 is only 7.5 ID and may not carry the pull in forces. Call for the bidder to design the pipe. Ask for the pulling forces on the pipe.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[LittleInch]

I would be concerned about external pressure / collapse if this line is normally empty.

PE is great stuff, but really doesn't like external pressure or even very small out of roundness and the thinner it gets the worse it is.

A conservative view would be water at the depth of the pipe then ask the pipe vendor what he will advise for long term external pressure.

The attached is a reference from a vendor I received once.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rconner]

fyi

http://www.eng-tips.com/viewthread.cfm?qid=226924

and other past threads accessible with "Search" feature (while some contractors claim they can directionally drill gravity service with even plastic pipes, from some other reports what you are proposing may prove harder to control grade than with more traditional crossing constructions. I guess that may or may not be a problem for your application, depending on your grades etc. In any case, best also check with the railroad and any other AHJs potentially involved, as they also have their own requirements based on experience etc.)

 

[bimr]

1. The typical railroad will require the storm sewer to be cased.

2. Probably need to have the railroad approve HDD.

3. Most HDD projects have a difficult time constructing a sewer grade uniform. Perhaps this can be offsite by using a steeper slope. You will have also additional problems because of the polyethylene is not ridged and will snake around a bit.

 

[ACtrafficengr]

I don't know anything about drilling, but we specify 15" minimum for storm drains. Anything under that is likely to get clogged, and is hard to clean out. Maybe a stormwater debris separator at the upstream end would work if you can't enlarge the pipe.

 

[IronDogg]

I would like to further bimr's suggestion a bit as he posted before me. Not that I am an expert, but if this was my project (and based off of the limited info in the first post), I would certainly use steel encasement and instead of HDD, use laser guided boring to get the 1% (also help to gaurantee no settlement below some expensive surface infrastructure). Also with the encasement and spacers, you can stick with pvc sdr 35. I don't know the catchment, flows, etc, but at very minimum 12" dr35 in a 24" steel encasement, but probably better to go with ACtrafficengr suggestion with 15" and then use 30" steel encasement. Add cathodic protection to the encasement and use stainless steel spacers or at least epoxy coated. As far as effect on the 54", 10' separation should be plenty with guided boring. All imho.