Surveying old transportation assets

June 2011 » PROJECT CASE STUDY
Difficult conditions and complex structures present challenges for preconstruction measurements.
John Edwards
Scanning from below the Wycliffe St. Mary’s River Bridge provided the best information on the wood and concrete members.
British Columbia Ministry of Transportation and Infrastructure

While the southeastern corner of British Columbia hosts some of the world’s most spectacular scenery, the rugged, mountainous region also presents a demanding construction terrain. The environment was just one of the challenges faced by the British Columbia Ministry of Transportation and Infrastructure as it began new surveys of two venerable and vital local transportation assets — the Elko Tunnel and Wycliffe Bridge. The Elko Tunnel, constructed in the 1900s, needed a fresh survey to determine if the tunnel was large enough to allow the transport of large coal dump boxes from a nearby mine to distribution points. The Wycliffe Bridge, built in the early 1930s, needed to be rebuilt completely and modernized.

Project
Surveying the Elko Tunnel and Wycliffe Bridge, British Columbia, Canada

Participant

British Columbia Ministry of Transportation and Infrastructure

Product application

Surveyors use a Trimble VX Spatial Station and R8 GNSS system to gather data safely and efficiently on two complex structures.

Both projects demanded precise measurements to be made under highly challenging conditions. To handle the work with available manpower resources, the ministry turned to a combination of state-of-the-art techniques and surveying technology that had already proven their value during previous British Columbia surveying projects. Mike W. Skands, survey and mapping manager for the ministry’s southern interior region, said the combination of Global Navigation Satellite System (GNSS) surveying, reflectorless total stations, and spatial imaging enabled the ministry teams to collect the needed data safely and efficiently.

Multiple challenges
The Wycliffe Bridge, which crosses the St. Mary’s River, is a timber structure supported by a pair of concrete piers. It’s roughly 380 feet long, including a central span and two structures connecting the center span to the road. Skands’ team was requested to survey the existing bridge for deck replacement, removal of superstructure outside the two main piers, and work on the approaches to the structure. If using conventional methods, it would have required many days of work to create an accurate depiction of the bridge’s complex array of columns, beams, cross members, and decking. Surveyors would have needed to climb on the structure and gain access from below, a difficult proposition at best. Steep slopes, the river, and dense vegetation beneath the bridge made it difficult to see, much less reach, the bridge’s piers and support structure. Scanning the bridge was the best option. It would be faster and safer, and provide much more detail than could be obtained using conventional methods.

Survey requirements called for detailed measurements of the bridge together with topographic data on the roadways approaching it. The survey crew used static GNSS surveying techniques to establish control points around the bridge in the British Columbia coordinate system. Next, they used a Trimble VX Spatial Station to scan the bridge from four locations, resecting the instrument coordinates from the control points. The two-person team scanned the entire structure from below the bridge deck and used direct reflex measurements to capture individual points and crucial details on the main span and concrete pillars.

As part of the scanning process, they collected imagery using the instrument’s built-in high-resolution camera. The images would aid in visualizing the scanned data and in removing vegetation from the scans. After completing the scan, the surveyors used real-time kinematic (RTK) methods with a Trimble R8 GNSS system to conduct topographic surveys and to tie to nearby cadastral markers. Using the Trimble VX as a total station, they collected additional topography in areas not suited for RTK. Because the instrument can measure without using a prism reflector, the surveyors could minimize the amount of time they spent in the roadway. As a result, the entire survey was carried out without the need to close the road or bridge to vehicle traffic.

The Elko Tunnel lies on the Crowsnest Highway connecting Elko and Fernie. Originally constructed for rail traffic, the tunnel is about 330 feet long and is sized to fit the trains of the early 1900s. To survey the tunnel, a four-person survey team faced snow and February wind-chill conditions falling as low as -4 degrees Fahrenheit. The work plan was similar to the bridge, with the first day spent setting control points using post-processed, static GNSS techniques. On the second day, the crew conducted nine scans from five setup points, again using the Trimble VX. The scanner’s position was carried by resection from GNSS points established outside the tunnel. The team configured the instrument to collect evenly spaced points on the tunnel floor, walls, and ceiling. While the scans were in progress, the surveyors used a robotic total station to collect details and linework. Outside of the tunnel, RTK GNSS measurements collected topographic data along the highway corridor on both sides of the tunnel. During the scanning, the instrument could operate unattended inside the tunnel, and it was not necessary to shut down traffic.

Data processing and analysis
Work on the bridge called for the largest number of measurements, which were needed to depict the existing structure and conditions accurately. For the tunnel, the emphasis was on analysis, and the team worked to provide data to determine clearances and other information related to a possible enlargement of the roadway. Because the optical and GNSS systems use the same controller and field software, it was straightforward for the crews to complete their work and combine it into project files in a common, georeferenced coordinate system.

On both projects, the scanning data was checked and analyzed using Trimble RealWorks software, which combined the point clouds, images, and data from GNSS and the optical instruments into a single data set. Technicians created a 3D model of the tunnel and created a contour map using intervals of 0.3 feet. The contours (now in the form of 3D lines) were exported to the ministry’s CAiCE design system for cross-section analysis (individual sections were developed at 16-foot intervals) and then to AutoCAD Civil3D for plotting and further work.

The total point count for the tunnel was approximately 56,000 points, a number that was handled easily by the ministry’s engineering programs. The technicians noted that the visualization of the model is quite representative of the surface, with some shadowing and density variations related to the uneven surface of the rock.

Because of the restrictions of the tunnel’s tight confines, the surveyors noted that it would have been impossible to obtain the same resolution with a conventional total station. With manual measurements, it would be very difficult to capture dense, evenly spaced points and to measure directly above the instrument in the tunnel.

The ministry has used spatial imaging extensively for two years, measuring everything from highway intersections to rock slope mechanics and stability as well as surveying inaccessible slopes. For the British Columbia projects, Skands believes that the new technologies meant that the tunnel and bridge surveys could be completed with just a minimal crew, even under punishing conditions.

“Required manpower has been reduced to a third or a quarter of what it used to take to do these surveys, so we can use our resources more efficiently and on multiple projects,” he said. “You could get by with a single operator on each site. But for safety reasons — and the complexity of these particular projects — we had four people onsite at the tunnel and two at the bridge.”

Information collected by the Wycliffe Bridge survey allowed the reconstruction to begin on schedule in mid-2010. The project was completed in January 2011. The Elko Tunnel project was halted shortly after the survey’s completion. Officials determined that moving mine shipments through the tunnel wouldn’t be practical.

John Edwards is a freelance writer who lives and works in Gilbert, Ariz.


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