Trenchless technology continues to find a home in the gas industry. Arby Construction, New Berlin, Wis. was recently contracted to install 12,000 feet of 12-inch gas pipeline for Nicor Gas, Aurora, Ill. Faced with several rail line crossings where traditional open-cut construction would not be feasible, Arby Illinois Division Manager Mario Lipira choose a trenchless installation method. Lipira decided to utilize pneumatic pipe ramming and the slick-bore process. This method allowed the Arby crews to install pipe under the rail lines effectively and efficiently.
According to Lipira, the process was simple. He said, “Basically what we’re doing is installing a bore pipe under the railway. Once the pipe is in place, we attach the product pipe to the bore pipe. Then, we remove the bore pipe with an excavator. As the bore pipe is removed the product pipe is pulled in place.”
For the Nicor project the crews used a 10-inch diameter Grundoram Gigant from trenchless equipment manufacturer TT Technologies, Aurora, IL.
According to TT Technologies Pipe Ram Specialist Bill Brennan, there are advantages to ramming in the bore pipe. He said, “The soil conditions play a big part in choosing the right installation method for the project. Ramming can be a preferable option in areas with difficult soils. This was especially true of the rocky, boulder-filled area where Mario [Lipira] and his crew were working.”
Contractor & Utility Background
Started in 1956, Arby Construction has grown from a small telephone contractor to one of the largest underground construction contractors in the United States. For the gas industry, Arby Construction offers complete installation services for all sizes of pipe for gas distribution systems.
Safety is always priority with Arby. Lipira said, “We have received numerous awards for the attention we give to safety on the job. For this particular job, job-site supervisors were required to attend a three-day pipe installation and safety course sponsored by Nicor. It’s something that we take very seriously here.”
Nicor’s roots, like Arby Construction’s, date back to the 1950s. Founded in 1954, Nicor Gas is an investor-owned natural gas utility that serves nearly two million customers in northern Illinois, excluding Chicago. Nicor Gas is the largest natural gas distribution company in Illinois and one of the largest in the United States. The company maintains a 29,000-mile distribution system that is connected to seven interstate pipelines. In addition, Nicor Gas transports and stores natural gas for customers that purchase their own gas supplies. These commercial, industrial and residential customers total 129,000.
Nicor contracted Arby Construction to complete the 12,000-foot gas system improvement near Aurora, Ill. While a majority of the project was completed through conventional construction methods, several key railway crossings were completed using the slick-bore process. Lipira said, “The railroad permits for this project specified that no directional drilling could be used under the tracks. The railroad was concerned about the potential for voids under the track. We needed an alternative construction method that would not create the potential for slumps under the tracks, so we opted for the slick-bore method.”
During the slick-bore process the product pipe is welded to the back end of an installed bore pipe. A winch, track-hoe or excavator is used to pull the casing out. As the bore pipe is removed the product pipe is pulled into place. The bore pipe is installed with a pneumatic pipe rammer.
A cutting shoe is often welded to the front of the lead casing to help reduce friction and cut through the soil. Bentonite or polymer lubrication can also be used to help reduce friction during ramming operations. For the Nicor project, the Arby crew used both a specially designed cutting-head on the lead end of the bore pipe and Bentonite lubrication to help with ramming operations.
Several options are available for ramming various lengths of pipe. An entire length of pipe can be installed at once or, for longer runs, one section at a time can be installed. In that case the ramming tool is removed after each section is in place and a new section is welded on to the end of the newly installed section. The Grundoram is connected to the new section and ramming continues. Depending on the size of the installation, spoil from inside the casing can be removed with compressed air, water, a pig system, an augering system or a combination of techniques.
Lipira described the process his crews went through for ramming in the bore pipe and installing the new 12-inch diameter steel gas pipe. He said, “We rammed a single section of bore pipe to make it to the other side of the rail line. Then we’d clean out the spoil from bore pipe. Once we have the bore pipe in place, then we’d weld on the product pipe and pull it in with a track-hoe. Once the product pipe is installed, the bore pipe can be used again for the next slick-bore.”
The Arby crew successfully completed all three of the slick-bores they attempted for the project. The first bore was approximately 120 feet long. The subsequent bores were shorter, averaging 65 feet each. The total time per slick-bore ranged from 30 minutes to one hour to complete.
Ramming & Slick-Bore Benefits
According to Brennan slick-boring and ramming are particularly conducive to transmission pipeline installation. He said, “On these types of installations, crews usually have enough room on either side of the road or rail crossing to install good sized sections of pipe. But even in tighter working areas, they are still able to ram in the bore pipe section by section and install the product pipe section by section.
“Slick-boring also limits the amount of stress placed on the product pipe, which is of concern to project owners. The bore pipe ends up taking the brunt of impact from the ramming in tough soil conditions and spoil clean out. Through ramming it can also be installed very accurately. The product pipe is pulled into its ideal position without being subjected to much of anything.”
Ramming can also overcome rock or boulder filled soils. During pipe ramming, boulders and rocks as large as the casing itself can be “swallowed up” as the casing moves through the soil and can be removed after the installation is complete. This also allows for a significant level of accuracy and makes ramming ideal for installations under roads and rail lines because it displaces the soil without creating voids or slumps.
Everyone associated with the project was pleased with the success the Arby crew had with the Grundoram and the slick-bore process. Lipira said, “If we weren’t using this ramming method, it would have been extremely difficult to complete the rail crossings. Obviously everyone was concerned about voids and slumping. We were able to avoid that issue all together by the use of ramming. We plan to use ramming and slick-boring in the future.”
by Jim Schill
Pipeline & Gas Journal, January 2003