O’Hare International Airport, Chicago, Ill., is one of the world’s busiest airports, which made the use of trenchless technology the ideal choice for limiting disruption during a recent casing installation project. As part of a modernization program, new, upgraded utilities are being installed at locations throughout the airport grounds. The initial phase of the project called for installing two (2) 36-inch steel casings a total distance of 1,000 feet to house a watermain; and two (2) 48-inch steel casings 640 feet for the placement of multiple electrical duct banks. The casings would travel under the various taxiways of the airport itself.
L.J. Keefe Company, Mount Prospect, Ill., one of the premier underground contractors in the country, was tasked with performing the work. L.J. Keefe Co. can trace its origin back to 1892, when Luke John Keefe, Sr., grandfather of its current president, founded a plumbing business in Chicago. Since that time, construction has been the family business for the last four generations concentrating its work in trenchless construction.
Initially, the project was specified as auger boring. However, the saturated, soft silt soils proved to be too difficult for that particular method. After failing to make progress with the auger bore after 40 feet for the 48-inch casing, the decision was made to submit a change order to proceed with a combination of pilot tube guided boring and pneumatic pipe ramming.
Trenchless Specialist Rick Melvin from trenchless equipment manufacturer TT Technologies, Aurora, Ill., explained, “The use of a pilot tube guided boring machine (GBM) in combination with pipe ramming to install casings longer distances and on grade. This can be a very effective pairing of trenchless technologies in certain situations and help overcome difficult soils. Ramming can be a strong choice in challenging conditions because it is capable of displacing the soil without creating voids or slumps.”
Pipe Ramming and Guided Boring Pilot Tube
According to Melvin, pipe ramming is a very versatile installation method that can be paired with other trenchless technologies. He said, “Pneumatic pipe ramming provides a percussive power element that can really compliment other trenchless technologies. For directional drilling for example, pneumatic pipe rammers have been used in a variety of assist methods for years. The percussive action of the rammer in the back of drill string can help during difficult pullbacks. Rammers can also be used to extract stuck drill pipes and drill stems. With guided boring, the GBM establishes a very specific on line and grade path. Basically, once that path is in place, the pipe rammer is used to drive in the casing following that line and grade.”
Ramming tools on their own are capable of installing 4- through 80-inch diameter steel pipe and steel casings. Diameters up to 148 inches have been successfully installed using large scale ramming equipment. Ramming requires minimal working depths and has proven effective for horizontal, vertical, and angled applications. With the addition of the GBM pilot tube technology with its advanced guidance system, pipe ramming can be used to install casings on line and grade.
Melvin said, “The major components of this process include a line and grade control system and a hydraulic jacking frame. The line and grade control system uses a camera mounted theodolite and a pilot tube target with LED’s, which display on a monitor. As the pilot tube is pushed into place, its line and grade are continuously monitored by a theodolite optical guidance system. The operator is able to watch the advance of the pilot tube on the guidance system monitoring screen and adjust the pilot tube heading as necessary to stay on target to the receiving pit.”
The GMB utilizes a jacking frame to generate the force and rotational torque needed to push and steer the pilot tube segments and steering head through the ground. A casing adapter is connected to the first string of pilot tubes. The objective is to install the casing following the line established by the pilot tube. As each length of casing is welded together and auger-bored/rammed in place, the pilot-tubes are being retrieved at the receiving pit.
O’Hare Modernization Program
This section of the O’Hare Modernization Program included the installation of two separate casings, in two separate locations. Approximately 640 feet of 48-inch casing was being installed to house an 18-way, 5-inch diameter electric service duct package under Taxiway T and Taxiway R. Approximately 1000 feet of 36-inch steel casing was being installed to house a 20-inch ductile iron pipe (DIP) water main under Taxiway SS and Taxiway T.
According to Melvin, the project needed some additional redesign. He said, “The original plans called for each of the casing runs to be divided into two separate installations from the outer edge of each run and to a central pit. But under the revised plan, a center pit would be established for each run and the installations would travel from the center pit out to receiving pits. This made everything more efficient and really more effective. Using a central location meant we could cut the number of launch pits in half. That’s significant when you consider the amount of equipment moving and set up time that saves.”
The 48-inch casing was first to be installed. A center launch pit and two receiving pits were established. Beam and lagging shoring was used on this installation with pit depths reaching 24 feet deep. The project was divided into one segment of 260 feet and another segment of 380 feet.
The 4-inch OD pilot tubes were installed on line and grade for the entire length of the run. Steering the pilot tube in the difficult soil conditions was challenging. Once the lead pilot tube reached the receiving pit, an intermediary 24-inch casing was welded to the rear pilot tube using a spoke adapter. The 24-inch casing was required to be installed the length of the run to make installation of the final 48-inch casing easier given the poor soil conditions. At this point the transition from guided boring to pneumatic pipe ramming took place.
The guided boring machine was removed and the pipe ramming system was placed in the center pit. Crews set the tracks for a 48-inch auger-boring machine that was used in combination with the ramming process to remove spoil from the casing during installation. A 24-inch diameter pipe ramming tool was used along with two (2) 1,600 cfm air compressors. The connection between the pipe ramming tool and the casing adapter was made through standard 24-inch diameter ramming gear. As the 48-inch casing was installed, crews would remove the rammer and clean the spoil out with the auger every 120 feet.
Once the 24-inch casing was installed, a similar adapter was used to transition to the 48-inch casing and then to the pneumatic pipe rammer. Ramming times for each 20-foot section of 24-inch casing, as well as the 48-inch casing, took approximately 45 minutes to 1 hour, with welding times for each new section averaging 2.5 to 3 hours. Once the installation was completed in one direction, the process started again in the opposite direction from the same pit until both lengths of 48-inch casing were installed and then joined in the center pit.
The process was repeated for the installation of the 36-inch casings in a different section of the project area. Trench boxes were used to shore up the center launch pit and receiving pits for the 36-inch casing installation. That project was divided into one segment of 537 feet and another of 469 feet. Similar installation times were achieved on the 36-inch casing. However, a 24-inch intermediary casing was not required on that portion of the project.