Gas Main Installation: Pilchuck Contractors Proves Power of Pipe Ramming

Railroad crossings pose a challenge for any contractor that needs to install a utility line from one side of the tracks to the other. Surface slump and track settling caused by a utility line installation can make a routine project extremely costly. Trenchless technology, specifically pneumatic pipe ramming, has helped make crossing rail lines a bit easier. Pilchuck Contractors, Kirkland, WA, has been utilizing trenchless technology for over 20 years. Recently they utilized pneumatic pipe ramming on a number of gas main installation projects for Puget Sound Energy, Bellevue, WA.

Pilchuck vice president Bob Gray said trenchless technology has allowed Pilchuck crews to improve their efficiency and broaden their capabilities. He said, “We have completed ramming projects in the past and in order to complete the installation of the high pressure gas main for Puget Sound Energy, we needed to install a protective casing. We looked at several options, but ultimately pipe ramming was selected as the best option for installing the casing without disturbing the tracks or nearby utilities.”

For the ramming project in Snoqualmie, WA, Pilchuck used an 8.5-inch diameter Hercules pneumatic pipe ramming tool from trenchless equipment manufacturer TT Technologies, Aurora, Ill.

Contractor Background

Pilchuck Contractors has been servicing the needs of the telecommunications, gas, and oil industries throughout the western United States for over 20 years. Their experience in these industries has earned them an enviable reputation for quality and expertise. Pilchuck works closely with clients on a regular basis from design and material recommendations through the construction phase, all in an effort to minimize the costs of construction.

As a recognized leader in the construction of utility systems, Pilchuck firm employs a wide range of construction techniques to successfully complete projects. These techniques range from trenching in highly congested metropolitan areas, to trenchless methods such as directional drilling, pipe bursting, and pipe ramming to the plowing of cable in rural locations.

Snoqualmie Project Background

According to Gray the first ramming project was designed to help meet the growth expectations for the city of North Bend. He said, “Puget Sound Energy wanted to extend its high pressure gas line they have in Snoqualmie out to feed the community of North Bend. At the time, North Bend had an intermediate pressure line serving the area. With fast growth, the line was undersized for the community. They have cold weather issues out there as well. With that area being the end of the system, when it got cold, the pressure would drop. They were out there all the time with their CNG [compressed natural gas] trailers to reinforce the town. So the objective of the project was to get them they’re own high-pressure feed and district regulator right in town. Once in place, the town would have a 250 psi system rather than the undersized 55 psi system that currently serves the area.”

In order to reach the community, the new utility line needed to run under a set of rail lines located at the intersection of Snoqualmie Parkway and Highway 202. According to Gray the rail line is part of an area tourist attraction. He said, “The Northwest Railway Museum operates a small rail line called the Snoqualmie Valley Railroad. The museum offers small excursions on the line to places like Snoqualmie Falls, another top tourist attraction. It’s not like it’s a highly traveled rail track but, it’s railroad none the less and we needed to install the new high pressure gas line under the tracks without disturbing them.” To facilitate the installation of the new 8-inch high-pressure steel gas main under the rail tracks, the crew needed to first install a 120-foot, 12-inch steel casing to house the new line.

In addition to the tracks, Pilchuck crews needed to be as precise as possible with the installation because of adjacent utilities. Gray said, “We were having to thread a needle. We had several drainage culverts above us, and an un-located water main in the vicinity. In addition, we were required by PSE to install the casing with a 2% downward grade to allow drainage.”

Ramming Basics & Benefits

Trenchless pipe installation through ramming is a basic process that can be very effective and productive. 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.

According to TT Technologies pipe ramming specialist Scott Langfeldt, several options are available for ramming various lengths of pipe. He said, “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 ramming tool 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, an auguring system or other types of earthmoving equipment.”

Ramming tools, in general, are capable of installing 4- through 144-inch diameter pipe and steel casings. 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.

On The Job

At the location chosen for the installation the Pilchuck crew faced a series of obstacles. The Pilchuck crew began by excavating a driving pit on one side of the tracks. But being in the flood plain of the Snoqualmie River meant the ground was very saturated and needed to be dewatered. The crew dug in two dewatering wells to stabilize the flowing sand ground conditions. Once stabilized the crew dug the pit to a depth of approximately 15 feet and placed shoring boxes. After the shoring was completed the crew used 4- and 6-inch riprap to create a base to work from. Rammer set up was ready to begin.

Gray said, “For the ramming portion, we set an I-beam down on the rip-rap base. The beam acted like a sled for the pipe. Because of the limited amount of space we were only able to ram 20-foot sections of pipe at a time.”

After setting the first section of casing in the pit, the pipe rammer was brought in and connected to the casing through the soil port. Langfeldt explained, “To make the connection between the 8.5-inch diameter Grundoram Hercules and the 12-inch diameter pipe we attached a soil port which made the transition between the tool and the pipe. The soil port also helps reduce the head pressure created by the spoil inside the pipe by giving it a place to escape. For larger casing diameters, a series of segmented ram cones and sometimes ramming adapters are used to make the connection between the ramming tool and the casing.”

With the tool connected and in position, ramming proceeded without incident. Once a 20-ft section of casing was installed, the Pilchuck crew would remove the rammer, position the next 20-foot section of pipe. The new pipe section was then welded on to the end of the installed section. According to Gray the crew was required to perform complete penetration welds and welding times ranged from 20 to 30 minutes. Once welding was complete, the rammer was reconnected with the pipe and ramming continued. All totaled, the Pilchuck crew rammed in six (6) 20-foot sections of casing at a 2 percent grade.

Clean Out

Cleaning out the spoil in the pipe proved to be difficult. Gray said, “We have are own vac truck, so we tried to remove the spoil with that first. But we found that after the sand was dewatered, it was just too hard packed. So we ended up bringing in a high-pressure water jet to clean out the pipe. The unit has jets that shoot water back at a 45-degree angle. Through the force of the water the unit burrows it’s way through the sand. While it’s doing that, water jets force the sand out of the pipe. It was very effective.”

Once the spoil was cleaned out the crew was able to install the new 8-inch gas main.

Gray was pleased with the speed and success of the project. He said, “We started ramming on a Monday in the afternoon. We were able to punch in a couple of joints. Then we came the next day and knocked the rest of them through. It was so fast, I had people that were interested in coming down and seeing the process and I had to call them and say, ‘Sorry, it’s done already.’”

Pipe Ramming Powers Through

Pipe ramming was also the method of choice for another Pilchuck casing installation project, this time in Seattle. While 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.

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.

While installing a 10-inch steel casing to house a 6-inch MDPE gas main in Seattle , the Pilchuck Crew encountered some very difficult conditions. Gray said, “We were punching underneath two of Amtrak’s live rails and four future tracks. It’s an older industrial area and when we dug down our pit was full of bricks and other debris.

We ended up ramming that casing through a concrete wall and a bunch of bricks and other construction material. When we were all done, we used a directional drill rig to clean the debris out of the pipe.”

 

Pipeline & Gas Journal, May 2006