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When California’s Bay Area Rapid Transit (BART) first opened to passenger traffic in 1972, it was the first new rail rapid transit system built in the U.S. in over 60 years. Traversing complex geography and varied population densities, BART used innovative technologies and construction methods to provide the region with high-speed commuter rail service. As it approaches its 45th anniversary, BART has expanded to serve millions of people in communities along both sides of the San Francisco Bay. Today the system includes 167 km (104 mi) of track and carries roughly 122 million passengers each year. It’s still growing.
In 2009, BART began work to extend service along the southeast side of the bay, adding 8.7 km (5.4 mi) of railway and a new station to bring service to the Warm Springs district in the city of Fremont. Expected to open for passenger service in late 2016, the $890 million project features both subway and surface track, including a tunnel beneath Fremont’s Lake Elizabeth and several vehicular overpasses. In order to ensure safe operations, BART required its construction contractor, Kiewit Corporation, to conduct precise measurements to verify that the more than 18 km (11 mi) of new track closely matched the design alignment. The track, which is rated to speeds of 180 kph (110 mph), could not vary from design by more than 1.3 cm (0.5 inch). Although Kiewit was capable of handling much of the work, the contract called for verification of the rails by an independent surveyor.
Kiewit managers contacted Jim Dickey, President of Cinquini & Passarino, Inc. (C&P), a Northern California-based surveying firm with expertise in railway surveying. The two companies had worked together previously and Kiewit was confident with the technologies and processes that C&P would use on the BART extension.
C&P planned to measure the rails using a pair of Trimble GEDO trolleys. The trolleys are placed on the tracks; each trolley can be pushed and operated by one person. The instrument trolley carries a Trimble S8 1" total station. The prism trolley carries a small retroreflector for the total station together with a tilt sensor to measure transverse inclination and a gauge sensor to measure distance between the rails. Radio data links connect the total station to a Trimble TSC3® controller on the prism trolley, enabling the instrument to operate unattended during measurements. The controller uses GEDO Vorsys software to manage the sensors and record the data.
One of the key aspects of the survey– control–was quickly addressed. "Kiewit provided points about every 500 feet along the project corridor," Dickey recalled. "They had used good procedures for the horizontal component and they had done digital leveling for all the vertical components, so the control fit very well."
To measure the track, a C&P surveyor placed the instrument trolley on the track adjacent to a control point. The operator then oriented the total station to the trolley and measured to the control point before pushing the instrument trolley ahead to the next control point, roughly 150 m (500 ft) along the track. Then the total station measured to that point and back to the prism trolley, which had been placed on the track next to the first control point.
With the instrument oriented to the track and control, an operator could then push the prism trolley along the track, stopping for measurements at 3-ft (1 m) intervals. When the prism trolley reached the total station, a second operator moved the instrument trolley ahead to the next control point and repeated the process. The crew could collect complete data–coordinates and elevation on both rails along with gauge and superelevation–in a single pass.
The trolley enabled C&P to provide a much denser dataset than conventional track surveys. "We collected points every 1 m (3 ft) along the track," Dickey said. "When Kiewit was doing track inspection they were measuring the track every 15 to 30 m (50 to 100 feet). We discovered some places in between their checks that needed attention."
The team needed roughly twelve days to collect as-built data on more than 11 miles (18 km) of track. They made regular quality checks on each day’s data, waiting to do final processing until all of the track measurements were complete. The deliverables consisted of electronic data and hardcopy reports, including charts showing the design and measured track alignments in horizontal and vertical axes. C&P used output from GEDO office software and reformatted it to meet the client’s requirements. Kiewit used the information to adjust the track as needed, using either a large track tamping machine or manual methods.
After the initial surveys were complete, C&P performed additional measurement on sections of the track. We’d re-survey 3,000 feet (900 m) in a couple of locations," Dickey said. "We would process that data in Kiewit’s on-site office before we left for the day. We could give them as close to real-time results as possible." C&P also performed post-tamping measurements to confirm that the track adjustment produced the needed results.
Working on newly constructed track made the entire process faster and easier. Because train and test operations had not yet begun, there was no need to coordinate measurement with rail traffic; construction activities were managed to minimize impact on the measurement team. The surveys came in on schedule and within the planned budget.
C&P’s decision to work in railway surveying has paid off. The company has used its capabilities to add new clients and expand to projects across the country. Dickey said the success comes from focusing on their clients’ needs. "Using the GEDO we’re getting more and better data in less time," he said. "It’s a big benefit. We’re giving track geometry reports that the clients and owners can understand and use. Everybody needs to keep an open eye for new technologies. Just because you’ve been doing something one way for 20 or 30 years doesn’t mean that there’s not a better way to take data or measure how track is installed. The amount of data that we’re able to get out for the track geometry is very impressive."
Erik Dahlberg is a writer specializing in the geomatics, civil engineering and construction industries. Drawing on extensive training and industry experience, Dahlberg focuses on applications and innovation in equipment, software and techniques.
A 6.550Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE