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Horizontal Directional Drilling: Acceptable Tolerances

Horizontal Directional Drilling: Acceptable Tolerances

Horizontal Directional Drilling: Acceptable Tolerances

(OP)
Situation:
I recently had a 10-inch sanitary sewer line installed by directional bore.  The depth of the line ranged from 21 to 30 feet, so I specified a DR-9 thickness smooth-wall HDPE (which actually gives an inside-diameter of about 8 inches, but that is the design I needed for my flow-line).  The bore length was 470 feet with a plan slope of 1.50%, but the specified minimum for the pipe was 0.50%.  The pipe would also be in the water table.

During the bore, the directional bore crew had to redo their bore 3 times because they were losing the signal on their drill string in about the same location -- apparently due to a nearby radio tower. During the 3rd redo on drilling the pilot hole, we had the crew dig a hole to find the drill string to where we were going to set a manhole for the line.  An elevation shot indicated that the installed pipe would likely lay at a 0.80% slope, which was within specification.  After finding the pipe, the crew proceeded with the back-ream and pipe-pull.

After a televising was done on the newly installed pipe, 3 sags in the pipe were found:  Two sags at 1" deep and one sag at 2" deep.  Maintenance personnel examining the televising had concerns, but indicated that extra cleaning would need to be done on the line once in use and didn't feel strongly about digging down 25-30 feet to make corrections.  Another engineer and I agreed that the risks of digging the pipe up and breaking the bentonite seal after installation were not prudent given the opinion of our maintenance staff.  After further deliberations with the contractor, we finally settled on a 5-year $200,000 maintenance bond on the pipe should the pipe fail while in service.

Questions:
1) Anyone have experience with this scenario?  Have you heard of signal interferance from neighboring radio towers?
2) Is the radio tower interference considered an unforeseen condition that the engineer is responsible to disclose in the contract?  (For background, the contractor did do some signal testing on the ground prior to bidding on the job.)
3) Ideally, no sag would be the best, but what should be tolerated in terms of sags in the pipe installation?
4) Would you specify a steeper plan slope than a 1.50%?  My understanding is that a 1.50% for HDD installations is industry minimum.
5) What are your thoughts on the bond, versus corrections on the pipe?

Your opinions are appreciated!  Thanks!

RE: Horizontal Directional Drilling: Acceptable Tolerances

From my experience in directional drilling for gravity sewers, I would suggest oversizing the casing pipe and specifying a greater pitch than the carrying pipe.  The combination should allow for a less stringent tolerance in pitch of the casing pipe.  I would only use the minimum if the carrying pipe were pressurized.  Real world soil conditions make achieving the minimum unrealistic.  The drill head is going to wiggle.

RE: Horizontal Directional Drilling: Acceptable Tolerances

I don't think that it is possible to directionally bore a gravity sewer pipe. In a recent seminar with a very good Contractor, he said the same thing. The accuracy is not there.

Directional drilling is suitable only for pressure pipes.
 

RE: Horizontal Directional Drilling: Acceptable Tolerances

HDD of sewer is now availble at tight grades (.20% and up) a project is being done in high water table sand conditions in Fort Lauderdale and another starting in St. Louis using the ArrowBore HDD process.  I found the information on Google by searching the inventors name Ted Dimitroff or the company name Trenchless Flowline, Inc.

Good luck, I hope this helps.

Jofama

RE: Horizontal Directional Drilling: Acceptable Tolerances

The attitude "Well, that is close enough" is not good enough for gravity sewers.

Ted Dimitroff of that same company Trenchless Flowline, Inc. states right in this article published in April 2008 that he can not get enough accuracy.

"The Metropolitan St. Louis Sewer District (MSD) and many other sewer districts worldwide look forward to using HDD for gravity flow installations, but they want the HDD industry to match the open-excavation specifications. Engineers do not want to make design changes simply because HDD cannot consistently install tight grades below .50 percent. We must give some credit to those HDD firms that have an employee or owner who has developed the special touch for installing gravity flow sewer mainlines. The volume of work that the sewer industry offers will require many crews installing mainline sewer pipes and the construction method of using HDD must be consistent and trainable."

http://www.trenchlessonline.com/index/webapp-stories-action?id=512

 

RE: Horizontal Directional Drilling: Acceptable Tolerances

We have found only one sure way of using HDD to install sewers and it can be found at the inventors name on Google.  Try these key words for more information,
Ted Dimitroff, ArrowBore, Trenchless Flowline.

Very flat grades below .50% in water table conditions have been done many times.

I hope this helps your future projects.  

RE: Horizontal Directional Drilling: Acceptable Tolerances

you might have been better off to use a guided bore machune. there is a large Trenchless Trade show in San antonio the third week of Jan. UCT 2009. there wil be all kinds of equipment for you to see and ask about.

Richard A. Cornelius, P.E.
WWW.amlinereast.com

RE: Horizontal Directional Drilling: Acceptable Tolerances

Talk to the oil drilling companies - they're pretty good at slant drilling.   

Mike McCann
MMC Engineering

RE: Horizontal Directional Drilling: Acceptable Tolerances

I agree with bimr, the accuracy just is not there with directional drilling. Pipe jacking or micro tunneling is the way to go for trenchless sewer installation.

I don't think oil drilling companies are the ones to talk to for gravity sewer lines.

 

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