Specify PowerCrete rather than Abrasion Resistant Overlay (ARO) for your pipe coating. I’ve witnessed a collapse first-hand with a now abandoned 10” pipe under the Allegany River near Olean, New York, “installed” in the late 1990’s that was too thin given the diameter. First, it will give you a better chance of not having the pipe collapse during pullback, and second, it will give you more wall thickness to compensate for gouges. The diameter-to-wall thickness ratio (D/T) needs to be less than 50. You won’t necessarily see that on the first half of joint of pipe either. If you find that the pullback force is really high even though you are in nice rock, then you might be damaging the pipe coating on the spots that are causing interference. Yes, the driller can ream out the high spots and/or ream a size larger than what would have otherwise been necessary, but a larger radius by design and a Driller with some patience during the pilot drill will pay off. As we use higher and higher yield strength and heavier wall pipe, the drill string becomes pretty stiff on pull back, and without a smooth hole it takes a lot of force to pull it into the hole. Lots of Drillers today will prefer a design with a 120 ft/1 in pipe diameter, and I believe that this is a minimum. The Driller will set the “Bent Sub” to a specific angle and can rock the drill string back and forth to create a desired radius, but my experience is it may or may not be a very smooth curve. It’s usually more of a series of short straight drills, curves of various radius, and lots of “corrections”. However, what really happens is that the pilot drill hole drill path is rarely an actual nice smooth radius. The conventional wisdom is (or was) 100 ft radius per 1-inch of pipe diameter. #2: DESIGN FOR A LARGER RADIUS THAN CONVENTIONALLY THOUGHT. Don’t forget to share the information with the bidders. If it isn’t, then the money spent on Geotech information will save many times that amount in the HDD driller’s bid price and if they know where the cobbles and solid rock is (for instance) you will save even more and have a better chance at success. If you have good reason to believe that the subsurface is consistent then you can back off the number or core drills. For a long drill, and in areas where the near surface geology changes a lot, drill even more sets. Again, drill to each side and offset from your designed path. Drill another set of core drills near the entry and exit points. Drill deeper than you think that bore path will be. Be smart and have the holes grouted in when they are done. Two as close to the feature you are crossing, and 20’ to 25’ to the right and left of your designed bore path (that’s four holes right next to the stream or road or other feature). Make it worth your while by drilling multiple holes on each side of the feature you are looking to cross. You paid a lot to have your Geotech crews mobilized. If you are looking for a technical dissertation, there are plenty of papers and books on the subject with much more engineering and experience behind them than what you will find here, and while I am interested in the subject, this is just my “Totally Biased Opinion”. So now that I am starting to turn a bit gray, I think it’s time to share my Rules of Thumb. Listening to all of the “hands” on the site that know a lot more than “those engineers” has taught me several Rules of Thumb for HDD design. Of much more educational value (and fun) is the many hours I have spent on drilling sites asking questions and listening intently to the folks running the drill. I have attended numerous seminars, purchased many texts, and read countless white papers related to the subject. Over the past 25 years I have been involved with designing and inspecting Horizontal Directional Drills (HDD) for pipeline construction.
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