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!
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
RE: Horizontal Directional Drilling: Acceptable Tolerances
RE: Horizontal Directional Drilling: Acceptable Tolerances
Directional drilling is suitable only for pressure pipes.
RE: Horizontal Directional Drilling: Acceptable Tolerances
Good luck, I hope this helps.
Jofama
RE: Horizontal Directional Drilling: Acceptable Tolerances
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:/
RE: Horizontal Directional Drilling: Acceptable Tolerances
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
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Horizontal Directional Drilling: Acceptable Tolerances
Mike McCann
MMC Engineering
RE: Horizontal Directional Drilling: Acceptable Tolerances
I don't think oil drilling companies are the ones to talk to for gravity sewer lines.