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Fusing and Pulling the Pipe

Launch Slideshow

Trenchless Excavation

Joining and Pulling the Pipe

Trenchless Excavation

Joining and Pulling the Pipe

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    The heavy-duty HDPE pipe used in trenchless excavation has thick walls to withstand the tensile stress of pulling the pipe into an undersized borehole.

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    While no one would mistake the welded full-length pipe for a length of garden hose, it does have enough flexibility to conform to irregularities in the borehole.

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    Once two lengths of pipe have been clamped to the pipe-fusing machine, the rotary cutter head precisely squares the ends. The area of the butt joint is then brought up to temperature with an on-board electric heating unit, and the softened ends are forced together with a hydraulic ram.

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    The boring bit initially used for drilling is replaced with the backreaming bit, which will smooth and enlarge the hole.

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    A sleeved pipe puller is secured to the pipe's downhill end; it grips the pipe like an enormous expansion bolt.

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    The pipe puller and bit are joined by a swiveling shackle that allows the bit to spin without transmitting that rotation to the pipe, and the operator backs the bit back through the borehole, drawing the pipe in after it.

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    Conventional drainage pipe was used for the short connection between the pair of catch basins that funnel runoff into the bored pipeline, the end of which lies just beyond the tee in the corrugated drain line. This part of the job required some hand digging in the heavy clay soil.

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    Compared with the amount of grading and landscaping that would have been necessary under the original plan, the actual finish work was trivial — laying a few square yards of sod over the catch basins and their connecting trench.

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    Some stone was placed at the pipe's outlet to the street.

Joining the pipe sections with fittings would greatly increase soil resistance, so instead they’re welded together with a pipe-fusing machine that heats the sections and presses them together.

First a section of pipe is loaded into each end of the welder and clamped in place, then a hydraulic ram forces the ends into a cutting blade that reminded me of a two-sided deli slicer. The “facing” blade shaves off a thin layer of material to provide a clean surface and ensure that the ends of the pipe will line up perfectly with one another. Next, the blade is replaced with a heating element, and both sides of the pipe are heated up simultaneously to about 500°F.

Finally, the two sections of pipe are pressed together with 300 pounds of force and held there until the melted plastic cools. Because 100 feet of 8-inch HDPE weighs 800 pounds, I had to use my truck to pull the pipe into position after the last weld was completed.

Back-reaming and pulling. Given the heavy clay soil, the excavators decided to smooth and enlarge the borehole with a back-reaming bit as they pulled the pipe through it. This was a matter of screwing a sleeved pipe puller to the HDPE pipe, then mounting the reaming bit ahead of it with a swivel coupling. With everything in place, the boring machine was put in reverse to draw the pipe back down the borehole. The maneuver went off without a hitch, and we were in business.

Finishing Up and Counting Costs

Installing the catch basins was easy. We cut back the 8-inch HDPE a bit in order to get the right elevation, then used 8-inch corrugated pipe to attach the basins to one another. We tried to pick the naturally lowest areas for the location of the basins, but to ensure maximum water drainage we dug back another few feet around each basin and sloped the ground toward each basin even more.

Where the pipe met the road, we put down landscaping fabric and covered the area with rock. The city engineer also required us to put 2-inch foam under the fabric and rock, because the water main was located 8 feet directly below the area, and he was worried about freezing.

The bottom line. Our final costs for the project were as follows:

  • $1,000 in engineering fees
  • $2,000 for directional boring
  • $800 for 100 feet of 8-inch HDPE pipe
  • $200 rental fee for fusing machine
  • $500 for basins and landscaping

The final tally of $4,500 turned out to be just about half what we would have spent to do the same job according to the original site plan. I wouldn’t hesitate to use this approach again if a similar situation comes along in the future.

Mark Petersen is co-owner of Access Builders Corp. in Shakopee, Minn.