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"The pilot hole of first crossing was enlarged first. The 6 inch pilot hole was enlarged in 4 inch increments to 18 inches in diameter," said Bosch. "It took nine days to pre ream and pull the first crossing pipe. The rig was moved to the adjacent crossing site and the process was repeated. Rubble and building materials on entry and exit caused issues with circulation and reamed hole stability. Therefore, a second swab pass was implemented to ensure successful pullback. Eight days was required for completion."
Both bores were surface launched with the drill set up in one lane of Illinois Street and right of way belonging to the city park department, he added..
"Just a few feet from the entry point there was a buried foundation of an old foundry building," Bosch said. "There was sheet piling about 55-feet deep near the river's edge. We encountered a large amount of buried steel, tracks and miscellaneous building rubble. There were so many voids, it was impossible to maintain circulation because drilling fluid would fill the voids in the rubble until we excavated and put in the conductor barrel."
Both pilot holes exited at the surface in the park on DuSable Island.
A relatively compact Vermeer D36x50 drilling unit developing 36,000 pounds of pullback and 4,995 foot pounds of spindle torque was used for both installations. Ten foot rod stems were used.
"We would have preferred to use one of our two larger machines," Bosch said, "but the small footprint was better suited to the space available for set up, and a smaller diameter drill stem facilitated making the 300 foot vertical radius. However, opening such a large hole with a tight radius with the smaller machine was a challenge.
A wireline system was used to guide the path of the pilot bores, and GPS equipment was used to plot tie in points.
In addition to installing conduits under the river, the project posed other challenges.
"Obtaining permitting was time consuming, cutting into valuable work time needed to finish the installations before the deadline to close temporary excavations and reopen streets," said Bosch. "We had the responsibility of acquiring the numerous permits needed before work could begin. Our contractors, the city, and the power company helped expedite the process. Our plan allowed seven and a half weeks to complete both installations. Work started the first week of October to pour new sidewalks and complete other surface restoration the day before the Nov. 15 moratorium on street closings."