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Boring Through A Crowd
On the third day, drilling the pilot bore required only a few hours with backreaming started immediately after its completion. Backreaming continued on the fourth day, and product pullback and clean-up were wrapped up on day five, completing the HDD segment.
A Vermeer D36x50 was used to drill the 250-foot-long pilot bore and pullback a bundle of two 6-inch and one 4-inch HDPE conduits. This model drill unit is powered by a 140-horsepower diesel engine. It is rated at 38,000 pounds of pullback, 5,000 foot pounds of rotary torque and maximum spindle speed of 227 rpm. The drilling fluid pump is rated at 70 gpm. Depth of the bore ranged from 48 to 140 inches through a subsurface soil mix of clay and decomposed granite. The vacuum excavator was kept on site to remove excess drilling fluid.
“On the south side of the road,” said Hardcastle, “we had to be four feet deep so linemen could tie into a new vault. On the north side of the building -- the launch point of the bore -- we needed to be at a depth of four feet before dropping off the 15-foot downgrade. Among the utilities crossed were a 36-inch storm drain on the hill, a 12-inch water main, an 8-inch high-pressure gas line which we had to stay at least five feet away from, 34.5 kW and 115 kW power lines, a 6-inch low pressure gas line, a sewer line, two fiber optic cables and power feeder to a street light.”
A McElroy TracStar 412 fusion machine was used to assemble strings of pipe for pullback. Pipe sections were laid out and fused on a side street.
Separate passes were made to enlarge the pilot hole from 6 to 10 inches, then to 16 inches, and finally to 18 inches to ensure a good path for the 12-inch HDPE pipe bundle.
“Learning to use the new F5 locator on a challenging project was interesting,” said Hardcastle. “Because of the extreme change of grade from the launch point going down the hill, we had to recover at a point enabling crossing all utilities and to complete the job. With the locator and TensiTrak, we were able to record the bore path and provide Agilent a graph of the drill shot and record of the pullback pressures.”