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Three Point Construction Completes 1st U.S. Front Steer Microtunneling Project
Three Point Construction Inc., Mechanicsville, MD, has successfully completed the first front steer microtunneling project in the United States using a Bohrtec BM400L machine from Icon Tunnel Systems.
Three Point Construction was contracted by C.J. Miller of Hampsted, MD, as the tunneling subcontractor for a difficult utility project located at the Applied Physics Laboratory of Johns Hopkins University. As part of the work, Three Point had to install a 200 linear foot, 42-inch diameter steel casing that would accommodate a large duct bank with 24 individual conduits for a new high speed communications system. Having already completed work on this project gave the contractor a direct knowledge of the soil conditions that would be encountered. At the location of the bore the conditions were going to be very hard schist with a high content of silica and hard quartz.
“This bore had to be extremely accurate because we have to maintain a minimum distance beneath existing utilities such as two water lines, a gas main, numerous communications duct banks, the main electrical feed to the campus, two causeway footings and the root systems of two oak trees,” said Mike Boyd, president of Three Point Construction. “With only inches to spare on either side of the 42-inch steel casing in the receiving pit due to an existing duct bank and building footing, accuracy and precision on line and grade were paramount.”
Due to the difficult nature of the project, Three Point decided to do some homework and research a variety of trenchless alternatives to conventional auger boring. Three Points had past experience on several jobs with Pilot Tube Microtunneling Systems rented from Icon, but they also knew it was only designed for use in soft, displaceable soil and not hard ground conditions.
“The pilot tube system would not have penetrated the soil due to its density and the presence of quartz,” says Boyd. “In addition, the manual steering and lag head would not have been precise enough to hit the target. We then considered using a tunnel boring attachment as the only other viable alternative.”
However, it was ultimately decided that the tunnel boring attachment would not be feasible due to the amount of time to set-up and dismantle. The size of the tunnel boring attachment would also prove to be an issue as Three Point would need a crane for the installation and dismantle process. “The crane required to make the lift would have been costly and there was also no room to set it up,” observed Boyd.