Construction is under way on a 32-mile ductile iron pipeline that will carry treated wastewater from the City of Meridian, Miss., to an industrial customer northeast of Meridian. Ductile iron pipe is routinely installed in all parts of the country using conventional open cut and cover, and since 1996, less frequently using horizontal directional drilling (HDD).
This project included installation of four segments of ductile iron pipe using HDD technology. All four directional bores were completed successfully, one of which set a new standard in terms of distance and pipe diameter for directional drilling using ductile iron pipe, according to project officials. The record-setting bore was 1,640 ft in length using 36-in. diameter ductile iron pipe.
“That length has never been done before using 36-in. ductile iron,” says Ralph Carpenter, project manager for AMERICAN Ductile Iron Pipe, a division of AMERICAN Cast Iron Pipe Co., which supplied the pipe for the project.
The HDD Project
General contractor Layne Heavy Civil, Fairborn, Ga., and its subcontractor REM Directional, Boligee, Ala., were responsible for pipe assembly and drilling the four bores, the longest one underneath the Sowashee Creek.
Layne Heavy Civil is also responsible for constructing the entire 32-mile pipeline, a new wastewater treatment plant, as well as the conversion of a small lift station. Layne’s entire portion of the project carries a fall 2012 delivery date.
The four bores — of 1,640, 1000, 880 and 880 ft in length from connection to connection — were fairly routine bores for REM Directional, which used its HRE 750 drill rig (750,000 lbs of pullback force) and Tulsa Rig Iron MCS-1000 mud cleaning system. In business since 2004, REM Directional handles large pipeline work, including HDD, auger boring, direct pipe drilling, as well as sheet pile driving. REM crews ran into some stubborn rock formations in the soils (lignite) but the pilot bores went smoothly.
Though REM Directional handled the HDD portion of the project, Layne Heavy Civil’s role was critical to the operation, specifically the pipe assembly. “During the design phase, engineers estimated the pullback load that would be required to pull the pipe back through the borepath. For three of the four pulls the actual applied load was close to the estimate, which was less than half of the allowable pull load of 310,000 lbs,” said Carpenter. The last pull was an anomaly, in terms of pull back load and bore path conditions; this resulted in a much higher pull back load that reached 304,000 lbs.
Layne Heavy Civil introduced some innovative methods of pipe assembly that allowed the project to finish ahead of schedule. Because of the limited access to easements, the pipe could not be joined in one continuous string. This forced the contractor to come up with a cost-effective and efficient assembly method. Layne Heavy Civil construction manager Bill Barron said the contractor decided to pre-assemble two standard 20-ft sections of pipe aboveground, making multiple 40-ft sections and staging them near where the pipe entered the borepath. Crews also placed a protective, double layer of polyethylene wrap tightly to the pipe to provide additional protection to its outside.
“REM Directional had sufficient space to set up its equipment but we had limited access as far as laying the pipe out. The decision early on in the design phase was that the pipeline would be all ductile iron pipe in 20-ft nominal lengths. That was the basis for our decision to pre-assemble two joints at one time,” Barron said. He noted the key work of Layne Heavy Civil site superintendent Gregg Hibbard in this process.
REM Directional president Joel Colgrove said his company had installed ductile iron pipe via directional drilling once before and that project involved successfully pulling 960 ft of ductile iron pipe in a continuous pull after it was all assembled, unlike this job. “We had never used the cartridge method of assembly before,” Colgrove said. “The pullback didn’t take too long. Once Layne Heavy Civil started putting the pipe pieces together, initially it took them about 15 to 20 minutes per section. Once they started going, they got it down to putting the pipe together every five minutes …We were able to do each of the pullbacks in 12 hours or less.”
HDD & Ductile Iron Pipe
Though ductile iron pipe is widely used in open-cut installations, very few utility owners and engineers think about its use in projects being installed using HDD, Carpenter said. Interest in its use is growing. “It’s getting more notice,” he said of HDD. “We have four more [HDD] installations coming up in Pasco County, Fla., one of which, when it goes in at the proposed length, will be a little longer than the 1,640-ft project..”
Why hasn’t ductile iron caught on for HDD projects? Carpenter believes that there are many factors in its slow acceptance, one being the contractors themselves. “They go with what they are familiar with and not a lot of them know how to put in ductile iron, when really it is very easy to do and no different than installing other pipe types,” he said. “They are used to grabbing hold of a piece of pipe and pulling it in as a single unit, but you can’t do that all the time. As in this case, where you have limited easement and right-of-way access, you have to use the cartridge assembly method where the pipe is pulled into the borepath one section at a time.”
“The ability of ductile iron pipe to handle water pressure that would tax other pipe made of substitute materials was also a consideration,” he added. “Because they have to pump the water 30-plus miles, you can get fairly high pressures that would take other pipe materials to their limits.”
Barron and Colgrove noted that ductile iron uses a rubber-gasketed joint that is not fused or welded. “For this 1,640-ft installation, we checked each joint for the proper gasket seating prior to pulling the pipe into the borepath. Our focus on quality during pipe assembly proved invaluable in that we pressure tested the line a week after it was installed, and it passed with absolutely no pressure drop,” Barron said.
Barron credited the success of the HDD installation portion of the project to the teamwork and partnership of all the parties involved, particularly early on during the pre-construction phase.
Sharon M. Bueno is managing editor of Trenchless Technology.