Grading and Excavation Contractor

Washington Dulles International Airport is undergoing a massive $3.4 billion improvement program, a major component of which is the construction of an underground train system. The new system will replace the familiar “mobile lounges” that have been used for decades to transport passengers between the terminal and concourses. The tunnel construction work is being done by Clark Construction LLC and its subsidiary Guy F. Atkinson Inc. and joint-venture partner J.H. Shea. To protect the safety of structures, equipment, and—most importantly—people working in and around the excavation, the contractors installed a comprehensive deformation monitoring system, which was supplied by Leica Geosystems through its dealer Loyola Spatial Systems.

The 3.11 miles of tunnels for the underground train system are being built using three different methods. Cut-and-cover techniques are being used to excavate approximately 7,700 track-feet in areas close to existing facilities and where aboveground construction will not affect ongoing airport operations. The New Austrian Tunneling Method is being used in areas where the tunnels curve. The Tunnel Boring Method is being used to bore approximately 4,300 track-feet for straight tunnel runs. A 23-foot-diameter tunneling machine is boring through solid rock approximately 55 feet below grade, with precast concrete walling lining segments mechanically put into place by the machine as it moves forward.

Deformation Monitoring Requirement
In mid-2004 Clark/Shea/Atkinson was awarded a contract for the West Automated People Mover (WAPM) construction job, followed in early 2005 by the East Automated People Mover (EAPM) contract.

Dieter Agate, manager with Clark Field Engineering, says, “The nature of the job meant that tunneling work would take place in close proximity to airport buildings and active taxiways, and it was imperative that the work be completed safely and on time without disrupting airport operations. This meant a very atypically extreme monitoring operation.

“From the beginning, it was made clear to us that real-time deformation monitoring was a top safety priority for the Metropolitan Washington Airport Authority,” says Agate. “They issued very precise and detailed specifications calling for an automated, multi-tiered system that would use motorized total stations to take continuous, real-time measurements to multiple reflectors at strategic locations, networked through reliable radio data links to locations where the results could be monitored by Clark/Shea personnel as well as the customer’s resident engineer.”

The specification called for high-quality, precision optical monitoring targets mounted on buildings and structures, support of excavation, and tunnel linings, with fully automated motorized total stations under computer control to monitor remotely the three components of movement with a measurement precision of 1 millimeter for sight distances up to 100 meters, with wireless data links to the control site. The robotic total stations would have to be totally automatic and operate unattended 24/7 under all weather conditions, and also be insensitive to refraction effects caused by temperature or pressure variations. Each target would have to be “hit” at least every 30 minutes. “The spec also stated that bicycle-style reflectors were not to be used; they had to be high-quality, precision optical targets,” says Agate.

Agate and the Clark/Shea WAPM and the Clark/Shea/Atkinson EAPM teams realized that this would involve a large-scale commitment to acquire the necessary hardware and software systems. Their research indicated that the only instrument that would meet the specification was the Leica TCA1800 robotic total station. Accordingly, in the summer of 2004, Agate contacted Bill Murphy at Loyola Spatial Systems, a major Leica Geosystems dealer in Virginia and Maryland.

“We determined that it was important to bring this technology in-house rather than contracting it out,” says Agate. “It was also important to us that we have a single-source supplier who could provide a turnkey solution with a high level of technical support, and that’s why we turned to the Loyola and Leica Geosystems team.”

Murphy put together a proposal that included multiple TCA1800 robotic total stations, located where they could monitor hundreds of target prisms to be affixed at predefined intervals to the structures to be monitored. The measurement data would be transmitted at specified intervals via radio data links to a central location, situated 3 miles from the actual instrument monitoring stations. The total network would be tied together and controlled by Leica’s GeoMoS (Geodetic Monitoring Software) system.

“I realized from the outset that training would be a critical element in the success of this project,” says Murphy, who made arrangements for Mike Eason, who would be responsible for managing the monitoring system for Clark/Shea and Clark/Shea/Atkinson to attend a GeoMoS training session in Salt Lake City in March 2005.

“The training program was fantastic,” says Eason. “It enabled me to get up and running very quickly as soon as we completed installing the monitoring instruments.”

Installation Challenges
Installation of the monitoring system was completed in April 2005 for the West Side, and August for the East Side. There were two TCA1800 total station sites on each side, for a total of four. The total stations were permanently mounted on 10-foot concrete and steel pillars with vented, heavy-duty glass enclosures to protect the system from the elements. Each installation included an Intuicom radio and modem with directional antenna to transmit data from the site to the controller. Band pass filters were added to overcome the high levels of RF activity in the airport environment. The pedestals were isolated to eliminate vibration and movement.

Photo: Marc Cheves

Leica TPS mounted in tunnel lining

“On the West Side, there was no permanent power, so we had to use battery power instead,” says Eason. “We decided to use deep-cycle marine batteries. Experience has shown that we can typically go for about one week between charges. On the East Side, we were able to tie into permanent power, but we added battery backup for redundancy.”

The GeoMoS software was installed on the computer network in the Clark/Shea trailer, and was configured so that data can be accessed via a secure IP link by authorized Clark/Shea personnel and the resident engineer.

“The remote total stations were mounted in large, square, glass enclosures,” he says. “They worked perfectly. They have hinged access doors so we can get to the total station easily. They are vented to avoid moisture and heat buildup inside and are sturdy enough to stand up to the extreme weather conditions year-round. Most importantly, there is almost no refraction of the EDM.”

The radio modems are enclosed in watertight, pelican-style plastic gun cases.

Monitoring Software
“The GeoMoS software is a powerful tool for controlling the network of the remote sites, as well as collecting data, providing alarms, post-processing, reporting, and visualizing data,” says Murphy. “The software represents the data and results in graphical or numerical format. You can select a timeline graph showing the trends of movement over selected time periods. Multiple points can be viewed simultaneously in the same graph. Alternatively, you can select a vector view that shows displacement for a selected area, to easily see where the greatest movement has occurred.

“Senior-level project engineers and other authorized personnel can access the GeoMoS data through a real-time Web-based portal from any PC or laptop,” adds Murphy. “They can log onto the secure GeoMoS site and download system status and reports.

“Measurement tolerances are established and loaded into the GeoMoS system,” says Murphy. “If any of these tolerances are exceeded, an automatic alarm is activated to notify the appropriate engineers.”

After getting the system up and running, Eason was transferred to another Clark construction project at the Quantico US Marine Corps base, and Aquiles Torres took over as the GeoMoS control engineer for the Dulles job.

“It takes awhile to become accustomed to the natural, daily fluctuation cycles revealed by GeoMoS,” says Torres. “Most of the variances are caused by natural occurrences, such as local weather conditions. We can clearly see daily and seasonal variations. The daytime environment is noisier, but it settles down considerably at night.

“We are really more concerned about trends than isolated spikes, but we’re careful to investigate any unusual spikes with a visual onsite inspection,” he adds.

“Fortunately, we have seen very few serious deformation events since installing the system,” says Torres. “We have never had to shut down work due to detected variances.”

“There was one incident in which we detected movement in one of the steel columns supporting an overhead pedestrian walkway that was in active use,” notes Eason. “It was close to a cut-and-cover excavation site. We strengthened the support and added more prisms so as to be able to monitor that area more closely.”

“We could not be more pleased with the results we have achieved on this job with the automatic deformation monitoring technology,” says Agate. “We are convinced that this is the wave of the future, and we expect to see similar specifications from customers for other large civil engineering construction projects. We expect to get a good return on our investment, not only in hardware and software, but in on-the-job experience, which will give us an important competitive edge in bidding on future work.”

GEC - November/December 2006

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