Pneumatic pipe ramming continues to be used in a wide range of applications, both for primary pipe installation and as a method to help facilitate the installation of other pipes or products. The method is proven effective for horizontal, vertical, and angled applications. Design engineers with the help of contractors and equipment manufacturers are finding new and unique ways to utilize ramming technologies.
For a recent project for natural gas utility Nicor, Naperville, Ill., several 36-inch steel natural gas transmission lines, located 25 feet below Interstate 88 in neighboring Aurora, Ill., needed to be extracted and replaced with new mains. The contractor, KS Energy Services, LLC., New Berlin, WI., attempted to remove the lines using static pulling force. The pipes, installed decades ago, were housed inside a 42-inch steel casing with plastic spacers and grouted in place with a tar resin. Because the condition and consistency of the tar resin prevented a static pull option, KS Energy Services contact trenchless equipment manufacturer TT Technologies, Aurora, Ill., to help develop a solution to extract the mains and install new ones.
TT Technologies pipe ramming specialist Scott Kneip said, “Initial attempts to extract the pipe by static pull force proved unsuccessful due primarily to the hardened tar resin that surrounded the casing. The tar resin, used for cathodic protection, cures over time, effectively immobilizing the pipe. The decision was made to attempt to push the pipe out with the assistance of a pneumatic pipe rammer.”
The Pipe Ramming Process
During trenchless pipe ramming, a pneumatic hammer is attached to the rear of the casing or pipe. The ramming tool, which is basically an encased piston, drives the pipe through the ground with repeated percussive blows. A cutting shoe is often welded to the front of the lead casing to help reduce friction and cut through the soil. The use of Bentonite or polymer lubrication can also be used to help reduce friction during ramming operations.
Kneip said, “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 the latter case, the ramming tool is removed after each section of new pipe is in place and a new section is welded on to the end of the newly installed section. The ramming tool is connected to the new section and ramming continues. Depending on the size of the installation, spoils from inside the casing can be removed with compressed air, water, an augering system or with a mini backhoe.”
Some casing installation methods are impaired or even rendered inoperable by rock or boulder filled soils. Pipe ramming is different. 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.
Ramming tools are capable of installing 4- through 80-inch diameter pipe and steel casings. Diameters up to 148 inches have been successfully installed using large scale ramming equipment. Ramming requires minimal working depths and is proven effective for horizontal, vertical, and angled applications. Ramming is also ideal for installations under roads and rail lines because it displaces the soil without creating voids or slumps.
Typical pneumatic ramming tools require a compressor outlet pressure of 85-100 psi. If two compressors are used in parallel, non-return valves should be placed between the outlets to ensure that air is not being pushed from one compressor back into the other. Install the non-return valves directly to the T-connector that joins the two lines together, and make sure that the non-return valves are facing in the proper direction.
Proper lubrication is essential to the optimum performance of the pipe rammer during operations. Synthetic lubricant is often used. The lubricant is specially formulated for pneumatic tools and will not deteriorate air hoses or plastic pipe. The lubricant typically used is biodegradable and non-toxic. It is water-based, water-soluble and does not contain ethylene glycol. It contains no environmentally hazardous materials and is nonflammable and non-corrosive. It is an all-season lubricant available in summer grade and winter grade formulas.
The lubricant is fed into the pipe rammer through an inline lubricator. The lubricator is connected between the compressor and the ramming tool. It atomizes the lubricant and feeds it into the rammer through via compressed air. Lubricant flow can be regulated by adjusting the lubricator settings.
Pipe Ramming Pit Construction
Pipe ramming bore pits do not typically require the use of reinforced back abutments and are not restricted in the length or diameter of the product pipe sections that can be used. Bore pit preparations will, in part, be dictated by job site conditions and will vary by project. Generally, the bore pit needs to have a minimum length equal to the sum of the measurements of the ramming tool, the ram cone, soil removal cone, the length of the product pipe sections being used, plus 5 feet of working space behind the tool.
For the Nicor project the ramming pit measured approximately 65 feet in length and almost 40 feet deep. The pit was thoroughly shored and a face wall for the pit was constructed. A workable ramp was constructed to allow for the transport equipment, materials and new sections of pipe to the ram pit. A 24-inch diameter Grundoram Taurus pipe rammer was selected for the ramming operations.
On The Job
The replacement of the existing 36-inch gas transmission main was part of larger infrastructure rehab and replacement program along the I-88 corridor west of Aurora, Ill. This particular 280-foot pipe section was targeted for replacement as part of Nicor’s program because of its age and condition. The program includes the replacement of approximately 5-8 miles of transmission pipe annually.
A section of new 36-inch pipe was attached to the end of the exiting pipe. The rammer was suspended with side booms and attached to the end of the new 36-inch pipe standard 36-inch diameter ramming gear. A single 36-inch ram cone made the connection between the 24-inch diameter tool and the new pipe.
Kneip said, “In order make pipe ramming an effect option, static tension was incorporate to limit the amount “bounce-back” effect created by the percussive rammer. The ramming pit measured almost 65 feet in length, but the exit pit was much smaller. Because the exit pit was too small to facilitate an effect pull tension set up, a block and tackle configuration was established on the face wall of the ramming pit. An eye was welding on the top of the 36-inch pipe and another on the 42-inch casing at the front of the pit. A sheave was attached at that point.”
A winch line was run from the top of the 36-inch casing, to the bottom of the pit, around a sheave attached to the 42-inch sleeve casing and back over the top of the pit and the rammer to a D-8 dozer winch. As ramming commenced, the dozer winch provided tension on the pipe to limit the amount of bounce back.
New 40-foot sections of 36-inch pipe were rammed into place one section at a time while the existing main was simultaneously pushed out. Ramming would stop every 16 feet so that crews could cut off sections of the old main with welding torches and remove them as the existing pipe was pushed into the exit pit. It took approximately 20 minutes to ram out a 16-foot section of the old pipe. After the percussive force of the pneumatic pipe rammer was introduced, the resin began to break down and liquefy to an extent, releasing the bond with the pipe. At that point it was hoped that the pipe would be able to be removed quickly. However, in the time it took to extract and remove the first section, the resin began to reset and it was evident that ramming would need to continue.
During the completion of this particular project, the same situation was encountered at another section of pipe in different location. A second ramming tool and crew was dispatched to that pipe and the same process employed. KS Energy Services will continue with the Nicor infrastructure upgrade project in 2014. After the success of the ramming application for the extraction of the tar resin encapsulated main, the process will be utilized in the future and provides KS Energy Services with an effective option for pipe extraction and replacement.