Controlling the Sway
December 22, 2008
When the new Pitt River Bridge opens to traffic later this year, this CA$198 million (approx. US$167 million), seven-lane cable-stayed structure, which connects the British Columbia cities of Pitt Meadows and Port Coquitlam, will support more than 80,000 vehicles per day, allow up to 52 feet of vertical marine clearance and provide paths for cyclists and pedestrians. The design of the 1,660-foot-long by 160-foot-wide bridge also allows for an additional lane to be added in the future for vehicle use or light rail rapid transit. All of these features will provide transport authorities with a scalable asset to meet the growing needs of municipalities and industry.
When looking at the site of the bridge taking form about 28 miles east of Vancouver, it is difficult to visualize surveyors adequately positioning 96 cable anchors along the six 196-foot-high concrete pylon towers, staking out 189 stone reinforcement columns in the river, monitoring ground movement or raising and setting the harplike cable-stayed bridge deck without today’s modern surveying tools. “As survey technology improves, specs for construction projects are getting tighter and requiring higher accuracies,” says Jeff Zegalski, project survey manager for Peter Kiewit Sons Co., the prime design and build contractor for the new bridge. “Meeting these project requirements without the aid of high-precision total station and GPS technology would be extremely difficult. It would require significantly more work and pose more safety risks to our crews. And, most importantly for this project, our instruments give us the speed and flexibility to collaborate in real time without sacrificing accuracy.”
According to Zegalski, the design and construction of a cable-stayed bridge is not driven by the tension or length of the cables but rather by the geometry of the bridge deck. “The success of the Pitt River Bridge is heavily dependent on the measurements we take,” he says.
Monitoring on a large project is a standard procedure, but “monitoring” took on added meaning for Kiewit at Pitt River. This time, monitoring also encompassed movement-both on the ground and structurally.
Construction requirements called for the new bridge to be built between the right-of-way of two existing swing bridges to allow the heavily used bridges to remain open to traffic and for the new foundation columns to be set much deeper than the existing bridge supports. Given the unusual construction configuration and the uncertainty of how driving the deep piles would impact the existing bridges, Kiewit used Trimble S6 and Trimble S8 total stations to help them monitor the integrity of the existing Pitt River structures before construction began on the new bridge in 2007. “The S6 and S8 allowed for very precise measurements to be made quickly,” Zegalski says. “The ability of the Trimble S Series total stations to automatically aim to relatively inexpensive passive targets allowed us to place many targets onto structures of monitoring interest and to automate the aiming of the instrument to increase precision.”
Two survey crews were used for most of the project with the bridge monitoring taking only a few hours once or twice per week. Surveyors anchored two glass prism targets to the underside of each bridge pier to provide 46 monitoring targets along the two bridge spans. Setting up the instrument on a known control point, they measured to each target, recorded the precise location in their Trimble TSC2 Controller and saved the locations as a unique point list. The point list was then preloaded into the controller software each day, turning the bridge monitoring exercise into a simple “point-and-click” operation. Accuracy was maintained through regular control point verification using static GPS procedures.
“After the initial setup and recording of the monitoring targets, the whole monitoring process became automatic,” Zegalski says. “We only had to load our point file into the controller, set up our total station, choose the point list we needed and hit ‘start.’ The instrument would then automatically turn to each target and shoot the entire set in a few minutes. And because the controller works seamlessly with both total stations, we could use them interchangeably."
Surveyors monitored the existing bridge daily for six months while crews drove 26 foundation piles down 330 feet. Each new set of measurement data was electronically sent to bridge engineers to enable them to watch for any movement from their desktops. Fortunately, no alarm bells ever rang, and once the foundation columns were set, surveyors scaled their monitoring program back to the weekly schedule that will be maintained during the entire construction project.
Preparing the site for the new interchange infrastructure also required an intensive monitoring routine for Kiewit’s five-person survey crew. To prepare the ground for the new roadways, teams “preloaded” the ground with up to 20-foot-high sand piles weighing 169,000 metric tons.
“If you don’t preload the ground and then build a road on top of it, the weight of the traffic and the road construction will cause the ground to compress,” Zegalski explains. “You don’t want that settlement after construction because then you develop cracked asphalt and structures that don’t line up."
To create the preload monitoring network, Kiewit geotechnical engineers provided the survey crew with a drawing of 65 predetermined target locations. Using their Trimble 5800 GPS receiver, the crew set a 10-foot-high steel post topped with a glass-prism target in the sand at each monitoring position and then recorded its geographic coordinates into the survey controller, which enabled the Trimble total stations to measure the deflection of the sand-loaded ground automatically.
“Without the survey technology, we would have to manually turn the instrument to each point and number it,” Zegalski says. “We’d also have to know what height it is and what prism constant is being used and then manually enter that information for each point. That’s not only very time consuming, it leaves a lot of room for error because there is a lot of data.”
Autonomy at the Shore
Efficiency was the operative word for the intense work needed to reinforce the shoreline around the new bridge. Kiewit teams constructed 890 stone columns at strategic locations around the entire site for general support and reinforcement. Surveyors used both the Trimble S6 and Trimble S8 to stakeout the positions of the columns onshore, but 189 columns had to be installed in the river. Unable to place stakes in the water for drilling, they tapped into Vancouver-based Cansel’s Can-Net GPS Reference Station Network, which uses Trimble’s VRS technology and RTKNet software to stream real-time kinematic (RTK) GPS data over the Internet to subscribers.
Teams mounted a Trimble 5700 GPS receiver antenna to the top of a 100-foot crane with a long probe cable. To prepare the location grid for the 60-foot-deep columns, surveyors imported an AutoCAD design file into Trimble’s Terramodel survey software (which the firm uses for its flexibility in design data format) and plotted out the positions of the columns at 10-foot spacings. They loaded those locations as points into the TSCe Controller for the crane operator, who simply chose a column position from a display map and then allowed the GPS system to direct him to that location. Once the column was constructed, the operator then used the same system to record its position so an as-built of the structure could be completed.
“Without using GPS, we would have needed a total station sitting on the shore measuring to where the crane was and a surveyor to give him direction,” Zegalski says. “By using GPS, the operator controlled [his own positioning and surveying functions] and retrieved his own location. That freed my team to carry out other work.”
Stringing the Harp
Efficiency, cost-effectiveness, productivity and autonomy have been the main themes of the Pitt River project. But equally important, according to Zegalski, has been the ability to communicate and work in real time. “The S6 and S8 allow us to be very quick, mobile and precise,” he says. “Integrating all of our data into one controller allows us to quickly verify the quality of our work, monitor our progress and adapt to frequent changes.”
Perhaps nowhere have those capabilities been more needed than when crews began lifting the new Pitt River Bridge deck in September 2008. Acutely aware of the intricate and critical engineering feats involved in building out the cable-stayed bridge, surveyors have been using the Trimble S8 Total Station to monitor the work to help ensure every piece of the structure is where it should be.
Each time a new girder is ready to be set, the survey crew arrives onsite early in the morning to shoot the deck girder and concrete pylon targets. (The bridge design assumes an even structure temperature. However, the heat from the sun causes the bridge elements to expand unevenly, so the data must be collected and verified before sunrise while the bridge is still a uniform temperature.) Setting the Trimble S8 up at the base of the foundation, surveyors choose preset target lists for the instrument to measure locations on both the underside of the bridge and the pylon towers. The resulting location and elevation data are sent electronically to members of the design team, who analyze the girder elevations and indicate whether ironworkers need to make any adjustments. Any modifications trigger the survey crew to remeasure the section and send the data back for analysis. This iterative process continues until the design engineers are confident that the girders and, subsequently, the support cables are positioned correctly. While the manual alignment of the deck can take several hours, the survey team can complete each segment of their work in minutes.
An added benefit of the S8 is that it can distinguish targets that are set very close together. “That allows us to continue to use its auto-aiming ability even when the targets need to be placed close together,” Zegalski says. “And with [the instrument’s] auto-turn abilities, we can collect our measurements in under 30 minutes.”
The cable-stayed bridge is designed to sway during construction, so the survey crew is also using the Trimble S8 to monitor the movement of the towers and the bridge in real time as each new girder is placed. “When you start loading one side of the bridge, the whole tower will lean that way,” Zegalski explains. “At the top, it can sway up to 12 inches one way or another depending on which side is loaded. So we use our S8 to measure each girder’s position to ensure the bridge is in the correct location for each stage of construction.”
Thanks in large part to modern measurement technology, Kiewit is on target to have all lanes open to traffic in the fall of 2009 as scheduled. When asked how this type of work would be accomplished with conventional means, Zegalski sits bewildered for a few minutes as he tries to envision his crew performing a myriad of surveying tasks with theodolites and tape measures.
"I would have to say, ‘Good luck,’” he quips with a smile.