Online Exclusive

Microtunneling Makes Quick Work of Tough Boring Job

Over the past five decades, Bancker Construction of Islandia, NY, has completed numerous sewer line installs. The installation of new sewer pipes under the PepsiCo bottling plant foundation in Queens, NY, however, was not your typical installation. The PepsiCo plant had a failing 40 year-old cast iron sewer line located under the plant floor that needed to be replaced. Project engineers determined that two 6-in. HDPE pipes would be used to replace the sewer line, since the size and strength of the pipe met the project's specifications.

In addition to the plant floor, the two new sewer lines would run under a number of other rooms, such as a storage room, refrigeration room and lab, which happens to be one of the most sensitive areas in the plant. Throughout the construction, the plant was required to stay operational.

"There were many factors to take into consideration while planning this project," says Charlie Madsen, Vice President of Bancker Construction. "In addition to the new sewer lines needing to be installed in a timely manner without disrupting plant operations, we had to pay close attention to air quality due to the sensitive nature of the bottling process."

Attachment expands installation options

Bancker determined that open cut construction to install the new sewer line was not an option, since it would cause too many disruptions and create too much dust and debris inside the plant. Directional drilling was ruled out due to its lack of accuracy, large site layout and the vibration from tooling that could potentially disrupt the lab and bottling equipment. The contractor then turned to ICON Tunnel Systems' pilot tube microtunneling technology.

"Pilot tube guided auger boring has proven itself over the years to be one of the most accurate methods for installing gravity sewer pipes on line and grade," states David Crandall, Vice President of ICON Tunnel Systems. "Typically, pilot tube guided auger boring machines jack clay or steel pipe once the initial drive is made. Since this project called for HDPE pipe, we recommended our new Pull Back Expander attachment."

The attachment would be used to make the initial bore on line and grade, then pull the HDPE pipe back through the bore instead of jacking it. "Jacking would potentially damage or even destroy the pipe," Crandall notes.

"Another aspect of the project that made pilot tube microtunneling and the Pull Back Expander ideal were the soil conditions and pipe size," says Dan Paster, National Guided Auger Boring Consultant for ICON Tunnel Systems. "The soil was soft and sandy with trace amounts of gravel and small cobbles, which allowed for further displacement of the soil by the Pull Back Expander after the initial pilot tube install. What this meant was no spoils had to be removed or dealt with during the installation process."

On line and on grade

The microtunnel drives would not be an easy task for Bancker and ICON. They would first excavate the jacking pits outside of the building, next to the foundation walls. The jacking pits were 20' long x 10' wide x 8' deep and shored with Bancker's ICON steel trench box. A Bohrtec BM600LS guided auger boring machine was then placed in the jacking pit with the theodolite guidance system.

For the first drive, Bancker hand excavated the 4' x 4' tie-in pit, which was 107LF inside the plant from the jacking pit. "The tie-in point was located in the actual bottling section of the plant," says Madsen. "There was very little room in this particular section of the plant, so storing equipment, HDPE pipe and creating a true receiving pit was not an option."

To solve the tie-in point issue, Bancker was permitted to excavate a receiving pit in the refrigeration room, which was located between the jacking pit and tie-in point.

Bancker made the initial drive on line and grade from the jacking pit, through the receiving pit and into the tie-in point. Once the pilot tube reached the tie-in point, the pilot tubewas reversed back to the receiving pit. The next step of the drive called for the removal of the pilot tube head and attaching the Pull Back Expander to the front of the pilot rod.

"The Pull Back Expander is threaded at the attachment end like a pilot tube head," says Paster. "Removal of the pilot head and attaching the Pull Back Expander only takes a few minutes."

Ashort portion of the HDPE pipe was then jacked between the receiving pit and tie-in point. "Because the length from the receiving pit to the tie-in point was under 20', we were able to successfully jack the HDPE pipe into place," says Madsen.

The team then attached another HDPE pipe to the head of the Pull Back Expander and successfully pulled the HDPE pipe to the jacking pit. Bancker then fused both pipes together in the receiving pit.

The second drive also used the Pull Back Expander but was located at a different section of the building with a much shorter distance of 76LF.

With both drives, there were a number of existing utilities in the way such as a main water line, roof drainage lines, electrical lines and old brick and cement foundation piles.

"The accuracy of the pilot tube system was crucial as it kept the bores on line and grade thoughout the project," says Madsen. "It also allowed us to stay exactly 3' under the concrete foundation, which eliminated vibrations and heaves in the concrete while boring. In addition, both the pilot tube and Pull Back Expander attachment displaced very little soil during the project. This allowed us to speed up the installation process and finish ahead of schedule as we did not have to deal with removing spoils."