Fiber Optics
Horizontal directional drilling has become the technology of choice for many challenging projects, including fiber optic installation. Increasingly, it is being used to save time, cut costs, and solve real-world problems. This project in Indiana is a fantastic example.
- Bypassing the Bridge -
The four-lane bridge on Indiana State Route 331, spanning the St. Joseph River near downtown Mishawaka, was razed to its foundations and rebuilt. Prior to the reconstruction, however, Indiana Bell had to remove multiple cables and find a way to again cross that particular span.
The Indiana Department of Natural Resources (IDNR) controls the river and restricts anyone from doing work that would harm the rivers environmental condition. IDNR even has salmon ladders not far from the bridge that salmon from Lake Michigan use to reach spawning grounds further upstream.
Several ideas were reviewed but engineers' concerns about the working pairs led them back to the same question each time: How can we get across the river at the existing location -- without doing temporary cable work and incurring excessive costs?
Bypassing the bridge with permanent plans, such as an underground conduit and cables installed only for that purpose, appeared to offer the best of two worlds. The Telco's cables would not only be off the bridge forever, but the relocation could be accomplished with a single cutover.
Environmental Crossings Inc., of Traverse City, Michigan, proposed a different design than what was proposed by other contractors. It called for using horizontal directional drilling methods and equipment to create a long, large-diameter borehole beneath the tunnel, the water raceway and the St. Joseph River. ECI would then snake 12 heavy-wall, high-density, 4in. polyethylene pipes that were 850 ft. long through the borehole, and encase them in cement grout as they were pulled into place.
Not convinced the design was feasible, most of the contractors declined to bid on the work. Some said, it simply couldn't be accomplished. Even the Telco hedged its bets by stipulating that conduit work at the higher end of the crossing could not begin until the river crossing was in place. Despite Indiana Bell's reservations, it awarded ECI the contract in April.
Skeptics had reason for concern. This job was, after all, far from an average conduit job. With a borehole length of 750 ft., it was 25% longer than any 12 duct/directional drilling project ever attempted, and the first one in blue shale bedrock. For any driller, it would be a job with a radical profile with a very high risk factor.
An operator at a hydraulically controlled panel directed every movement of the drill head and string. Steering was governed by a computerized in-head guidance system, located behind the drill bit. An above-ground tracking system further referenced the position of the drill bit as it progressed along the planned course.
Twenty hours after starting the bit into the ground, it emerged on the north side of the river. Its alignment was precisely on course and it was 4 ft. short of the target stake, as per Indiana Bell’s request. Except for some magnetic interference, the project went smoothly and the crew focused its efforts on enlarging the pilot hole to a diameter of 36 inches using three different sizes of reamers.
Cablecon, the annealed, heavy-wall, high-density polyethylene pipe manufactured by Integral Corp., Dallas, Texas, that ECI furnished for this job, was wound on special reels that belong to ECI. Each pipe was extruded in a continuous length that would extend from one river crossing manhole to the other. ECI positioned its 12 reels of pipe in the parking lot of a bank near the north end of the job. Workers attached the 12 ducts to a proprietary pulling head 27 inches in diameter. They then connected this apparatus to a swivel on the end of the drill string. Workers stationed at the drill rig used the machine to pull all 12 ducts through the borehole and to the south manhole location in a single pass.
During the pullback, a rig from Halliburton Services, Kalkaska, Michigan, pumped a special slurry of cement grout into the borehole in front of the pulling head to force out the bentonite drilling fluid that had been in the borehole. Then replaced it with a cement grout that filled the interstices and annular spaces between the pipes and the borehole. Once the grout was set, it provided the ducts with mechanical protection.
The grout was a special blend that would remain workable for 24 hours, although it would gel much sooner than that. Within 2-1/2 hours after pulling began, the tired but triumphant crew saw the 12-duct formation arrive at the drilling rig, proving to skeptics that they were wrong. Ferguson Michiana Inc., a utility contractor from Eau Claire, Michigan, assisted ECI with various parts of the river crossing. When the river crossing was complete, they terminated the ducts into the existing manhole on the north end and set a new manhole at the drill entrance on the south side. They also drew a test mandrel through each duct as a verification of its roundness and freedom from obstructions.
When traditional construction practices are followed, conduit projects of this magnitude tend to disrupt activities in the area and ordinarily taking months to complete. Except for a few unanticipated problems, this crossing was completed in less than three weeks, allowed Indiana Bell to remove its cables from the old bridge once and for all, and saved money by avoiding a second cutover.
John C. Ellis is an Underground Structure and Conduit Design Engineer for Indiana Bell based in Indianapolis. Douglas P. Allman is Vice President of Engineering at Environmental Crossings Inc., Traverse City, Michigan.
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