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.
Running out of ideas, Three Point contacted Icon Tunnel Systems for possible alternative solutions. Due to the hard soil conditions and extremely tight line and grade tolerances required, Icon recommended the front steer microtunneling system from Bohrtec. The front steer system makes it possible for the contractor to excavate the soil while establishing the line and grade instead of displacing it like the pilot tube method. Employing a full cutting face, a pilot tube theodolite guidance system with CCTV monitor, and an illuminated LED target directly behind the cutting head, the operator maintains constant control of line and grade.
As the front steer boring unit is advanced in the ground, the operator can see the position of the cutting head with respect to the intended line and grade at all times and make any necessary corrections to maintain that line and grade. On this particular project, Icon recommended that the 16-inch diameter front steer system be used to establish the exact line and grade and then the contractor would follow with his conventional American Augers Boring Machine to install the required 42-inch diameter steel casing.
Several successful front steer microtunneling projects had already been completed in Europe in varied ground conditions but not yet in the United States. This was a concern of the contractor, but with the proven track record abroad, specifications that met the project’s needs and the backing of Icon, Three Point quickly approved the front steer system for use on this project. According to Boyd, “This was the only system that we believed could deliver the accuracy required for the job in these soil conditions.”
Icon mobilized the required equipment and coordinated with all parties on every step of the delivery, setup and operation. “Because of the critical nature of the project, we flew in Bohrtec’s Lead Operator/Field Engineer for the front steer system, Stefan Janssen, from Germany to provide training and support,” says David Crandall, vice president of Icon Tunnel Systems. “We also had Dan Paster, our national guided auger boring consultant and operator, as well as Rob Langenbach, operations manager and chief operator for Icon on site with Three Point.”
By the time Stefan arrived, the Bohrtec BM400L and front steer system had been delivered and assembled, taking Icon and the Three Point crew only one day.
From the beginning, crews worked seamlessly throughout the entire operation. The system was powered and driven through the ground by one of Bohrtec’s smaller long frame machines, the BM400L, with 100 tons of jacking force and approximately 14,500 foot-pounds of torque. Roughly halfway through the drive, the system encountered two separate locations containing solid quartz rock ranging from six to 8 feet in length each time.
“The front steer went right through the rock,” claimed Boyd. “Typically when you hit rock like that with another boring system, it will completely destroy the head. The front steer was in great shape when it arrived at the receiving pit.”
The front steer exited at the receiving pit/existing manhole within 1/10 of one inch of line and grade, without disrupting or damaging the various utilities and other obstacles that were near the bore path. Production and excavating rates reached an average of 13.5 feet per hour, keeping Three Point ahead of schedule.
“Our operators were able to constantly monitor the exact position of the front steer boring unit using an illuminated LED target, theodolite camera and CCTV monitor. This technology allows the operator to make corrections to the intended line and grade as needed in real time similar to conventional computer-controlled, laser-guided microtunneling with half the headaches and half the equipment,” says Crandall.
Three Point then upsized and completed the bore by following the line and grade established by the front steer with their boring machine pushing 42-inch steel casings with large augers inside.
Three Point was able to successfully finish the casing bore within a matter of two days including setup of the machine. The total project, including the front steer operation, was completed two weeks ahead of schedule and under budget.
Icon Tunnel Systems has steadily grown in technology and expertise to become a full-service company and industry-leader in Pilot Tube Guided Auger Boring and Front Steer Microtunneling for the trenchless construction of pipelines, sewers, and other various underground utilities.
FOR MORE INFORMATION:
Three Point Construction, (301) 884-2755
Icon Tunnel Systems, (800) 836-5011, www.icontunnelsystems.com