Protecting Our Global Environment

Case Studies
Fiber Optic

The four-lane bridge on Indiana State Route 331, spanning the St. Joseph River near downtown Mishawaka, soon will be razed to its pier foundations and rebuilt. By then, however, Indiana Bell cables now occupy conduits in the old bridge will have been removed.

More then 10,000 working lines and special services circuits in those cables have been transferred to cables in new conduit, recently installed in solid rock far beneath the river.

The existing bridge is approximately 30 ft. in length and its deck is about 30 ft. above the riverbed. A warehouse and manufacturing facilities owned by Uniroyal Plastics Co. Inc., are situated south of the river on the west side of the highway. Uniroyal also has a seven-story power generating plant on the east side. A tunnel and a water raceway pass under the highway and link the power plant to the main complex.

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 underground conduit and cables installed only for that purpose appeared to offer the best of two worlds. The Telco's cables not only would be off the bridge forever, but the relocation could be accomplished with a single cutover. Again, the nagging question was how to install conduit under the river.

Underground conduit construction could only take place in the limited space along the east side of the bridge. Placement methods would have to contend not only with Uniroyal's tunnel and raceway but with a shortage of working room at the north end of the bridge.

Indiana Bells' engineers consulted many contractors regarding construction methods. Many visited the site, made recommendations, and submitted cost projections for constructing a conduit system beneath the river. Specific restrictions imposed because of the IDNR's environmental concerns effectively limited the method of construction to boring a large-diameter hole through bedrock under the river.

Environmental Crossings Inc., 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. Michigan Bell had used a similar approach in 1987 to install conduit under Michigan Pine River Channel.

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 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.

The skeptics had reason for their concerns. 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.

ECI began to drill near the end of June. Drilling work for the 8.5 in. pilot hole began at the south end of the job. To pass at the right depth beneath the tunnel and raceway, which were only 100 ft. from the starting point, the drill string's entrance angle had to be a steep seventeen degrees. By the time the drill bit reached the south edge of the river, it was nearly 70 ft. below street level.

From that point, the drill bit had to climb at a steeper angle of twenty degrees to hit the targeted exit spot in the city park area. This spot, which was 30 ft. south and 15 ft. west centerline of Telco's manhole where the ducts would be terminated. This location was chosen because it allowed the ducts to make graceful sweeps into the manhole. To hit the target, the pilot hole had to follow a diagonal path in the Telco's narrow easement. Along the east side of the highway right-of-way.

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. Severe magnetic interference along the entire length of the drilled path affected the accuracy of both systems, but experts from CBC Welnav, Tustin, Calif., and from ECI did a remarkable job interpreting the flawed data.

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. This was what Indiana Bell requested.

Except for the 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. Unfortunately, the crew encountered an unseen obstruction located near water level just north of the river on each reaming pass. It probably was the solid foundation of some long-forgotten structure.

The crew sweated out every inch of progress made by the largest reamer. As it bucked and jumped grounding its way through the obstruction. Fearful that each time it got hung up, the drill string might break. Finally, however, it cleared the obstruction. Compared to that, reaming the ensuing 500 ft. of bedrock was a cakewalk. Five days after the beginning of the reaming operations, the borehole was ready for conduit placement.

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. This pipe, because of a proprietary manufacturing process does not retain its coiled shape when taken off the reel. Its relative straightness makes it easy to pull through the borehole.

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.

Underground/Trenchless Technologies • Environmental Crossings

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