In underground construction, you pretty much expect the unexpected.
So it wasn’t a shock when an auger boring crew from Nix Construction hit … something that stopped them cold even though they’d just successfully completed a bore using a pilot tube system in the other direction on a job near Salt Lake City last December.
But it was a mystery, at least at first.
“The best way I can explain it is we ran out of jacking force,” says Jon Nix, vice president and COO of the family-owned company. “The auger boring machine operator maxed out the machine, and that piece of equipment is 200,000 pounds of jacking force. The refusal wouldn’t let the head push off from side to side. It just pinned it and held it.”
The job was for the South Valley Sewer District, which provides public sanitary sewer service in the Salt Lake City area. It called for approximately 2,300 feet of 18-inch SDR 35 PVC sewer pipe to be installed under a divided freeway. The PVC pipe was to be housed in 509 feet of 30-inch steel casing, which Nix Construction was subcontracted to install.
Utah-based Nix Construction planned for two bores, one in each direction under the road. The design slope for the sewer line was 0.3 percent.
Auger boring is a popular method for jobs like this, where precision with the line and grade are critical. It’s also a good approach when product needs to be installed under a road or railroad because there is minimal road upheaval and soil subsidence.
It turns out the Nix Construction crew hit what Jon Nix describes as a “massive amount of rock.” Once they got boring again, 6-to-8-inch chunks of cobble were coming out of the discharge on the auger boring machine.
To get through it, Nix rented an On Target steering system from McLaughlin. With it, the steering head can be kept on the proper cutting path with hydraulic actuated flaps that open and close, allowing for lateral movement. The On Target system’s active cutter head was what helped Nix Construction the most.
The cutter head fits on the lead end of the casing and is the same diameter as the pipe being installed. That allows for the ability to swallow larger stones and ingest material through the throat of the steering head, which is augered back to the bore pit. In comparison, with the pilot tube system the company typically uses, soil is displaced using thrust force on the drill string relying on material to be compacted to the sides of the head.
A positive experience with the On Target system several years before left Nix confident it’d work on this job, allowing him to kick the “expect the unexpected” maxim for that part of the bore.
“We had a pretty good feeling that even when we hit the refusal that we could get the job done and take care of the client,” Nix says.
For more information about the On Target steering system and McLaughlin, visit www.mclaughlinunderground.com or call (864) 277-5870. Check out the company on Facebook at www.facebook.com/mclunderground.