Innovative Arch Support
May 23, 2011
On the morning of December 3, 2008, the scene on the Chaudière Bridge, a major crossing linking Ottawa, Ontario and Gatineau, Quebec, resembled the typical morning commute of lines of cars, trucks and pedestrians starting their day. By nine o’clock that evening, the crossing was closed. A routine inspection had revealed structural cracks in two of the historical bridge’s masonry arches. Given the arches’ 180-year age and the Chaudière Bridge’s significant use as an important interprovincial connector, Public Works and Government Services Canada (PWGSC) issued the immediate closure to assess its safety for traffic.
One week later PWGSC determined it could re-open two lanes of the bridge, partially alleviating the crossing conundrum for commuters. However, engineers indicated that a substantial rehabilitation of both arches would be required to ensure it could bear the weight of traffic in the future. And that not only posed a significant challenge for the bridge engineers, it presented a notable puzzle for the survey team at Denis Dubois arpenteur-géomètre inc., a professional land surveying company in Saint-Bruno-de-Montarville, Quebec. “Engineers needed to build and install arch supports, but it is very challenging to survey and reproduce the precise circular shape of an arch,” explains Dubois, professional land surveyor and CEO of the firm. “And the very nature of the river-crossing bridge makes accessibility to the site very difficult--the Ottawa River in that area is quite active and produces strong currents--so we would need to survey the arches from a safe distance. Using a total station to measure the arches would be more time consuming, but more importantly, it would produce a patchwork of points rather than a real-world, cohesive view of the arches, which creates quite a challenge for designers.”
So when Dubois was presented with the project particulars, his immediate response was 3D scanning. “This kind of work was made for 3D scanning,” he explains. “Our Trimble VX Spatial Station enabled us to quickly and safely capture a very precise, holistic representation of both arches and provide a complete, intelligent dataset to engineers to help them create an accurate design of the required support arches. It was simply the perfect application for the technology and was the key to our and the project’s success.”
First constructed in 1828, the Chaudière Bridge is one of eight bridges that make up the Chaudière Crossing, one of the busiest interprovincial links crossing the Ottawa River. The oldest bridge in the region, the Chaudière Bridge connects the downtown areas of Ottawa and Gatineau and carries more than 27,000 vehicles per day. Originally constructed of wood with masonry arches, the span collapsed in 1836 and was rebuilt as a steel suspension bridge. It was rebuilt again in 1892 as a steel truss bridge. However, through each new construction, the original masonry stone arches were left intact.
The combination of time, ground surface deformation and harsh winters eventually took its toll on the bridge, and its structural integrity began to yield. The amount of deterioration that bridge inspectors discovered in 2008, however, was unexpected. Stones of arches that were originally 30 inches thick had deteriorated by up to 10 inches, and some cracks were so deep that large stone fragments had fallen into the river. Clearly, a long-term plan was needed to restore and maintain the Chaudière Bridge.
Following a competitive process, PWGSC awarded a construction contract to Peter Kiewit Sons’ (Kiewit) Infrastructure Group in October 2009. Though Kiewit is well-versed in design/build projects, the Chaudière Bridge project would pose some notable engineering and logistics challenges. Because of the historical significance of the bridge, the original structural characteristics of the span needed to be preserved. And since the heavily used crossing is one of only five bridges connecting Ottawa and Gatineau and only one of two bridges zoned for trucks, all the restoration work would need to be done without closing the bridge. In addition, construction could not impact or interrupt the Ottawa River’s flow or ecosystem.
Recognizing that the entire rehabilitation project’s success would hinge on the precise prefabrication of support arches and their installation, Kiewit turned to Dubois to produce one cohesive, design-ready model to build the concrete arches. “This job all came down to designing, prefabricating and placing the support panels,” says Robert Cornell, a project engineer with Kiewit’s Infrastructure Group in Boisbriand, Quebec. “As the panels would be prefabricated, there was absolutely no room for error in our measurements--once they were cast and delivered, we couldn’t modify them. We had to be certain that the survey data was absolute. With 3D scanning, we could acquire the complete real-world picture of both arches at one time.”
In December 2009, with winter snow fast approaching, Dubois and his colleagues Remi Loiselle and Olivier Coutu-Hébert embarked on a two-hour journey to the site with their GPS equipment and Trimble VX, an imaging sensor that integrates video capture, 3D scanning and survey-grade total station functionality. “Although land surveying is our expertise and surveying bridges is standard work for us, this project would be one of our first VX 3D scanning projects,” Dubois says. “Having only acquired the VX a month prior, it was pretty ground-breaking work for us. But the instrument was user-friendly and absolutely proved itself as the right tool in the field.”
Loiselle and Coutu–Hébert set up a Trimble 5700 RTK GPS Receiver on a known Modified Transverse Mercator (MTM) coordinate point as a base and used a Trimble 5800 GPS Receiver as a rover to locate a total of five control points using real-time kinematic (RTK) techniques along both sides of the riverbank. From these safe vantage points, they scanned arches one and three from a distance of up to 66 feet (20 m) and captured 80,000 individual 3D points at a 2-inch (5-cm) spacing in five hours. “Imagine collecting that many points with a regular total station,” Dubois says. “The VX acquires an incredible amount of detail extremely quickly.”
Given the extremely tight tolerances required to ensure the precast support panels would fit perfectly, the team also performed supplementary control using a Nikon A-25 leveling instrument while the VX ran autonomously.
Back at the office, Coutu-Hébert loaded the data from the scanner and GPS into Trimble RealWorks software and integrated the control point data from the controller and the Spatial Station 3D data to produce a complete georeferenced point cloud and 3D view of both arches. “Unlike a traditional survey, the scanner captures everything within its field of view,” Dubois says. “But you still need to georeference the points. With the automated and integrated workflows of the RealWorks software, we could easily georeference the VX point cloud to the local MTM coordinate system to ensure the designers had highly accurate, geo-located positions of both arches and had the benefit of viewing the arch symmetry in a 3D view.”
Coutu-Hébert then provided the point cloud to Kiewit for the design and prefabrication of the arch supports. After comparing the point cloud to Kiewit’s previously obtained engineering data, Cornell says his team was confident that they had the spatial and positioning intelligence to build the arch panels. “I was really surprised by how far away Denis could set up the VX and shoot the bridge with that amount of precision,” Cornell says. “And at times he was nearly surrounded by trees so you didn’t have a clear line of sight. But the precision of the point cloud was top notch. It gave us the confidence to design and build our prefabricated panels and provided our reassurance that we could complete this project successfully.”
By July 12, 2010, the 12-inch-thick (300 mm), 53-foot-long (16 m) concrete arch panels averaging 5 feet wide (1.5 m) were ready for installation.
It was one thing to successfully construct the support panels. It was another to successfully install them--a task that would also require clever engineering.
At issue was how to position the concrete supports without the benefit of an access road to the river from the shore--PWSGC would not allow any new road construction surrounding the bridge. That meant the Kiewit team would need to lower each 22-ton panel from the road level using a 330-ton crane and place it on two parallel rails. Once on the rail track, crews could slide each panel into place and seal them together.
To ensure the parallel rails were precisely positioned to install each panel, Kiewit commissioned Denis Dubois to return to the field in March to provide reference points for the rail system.
A survey team again arrived on site with the Trimble VX; however, this time they used the total station capabilities of the unit, rather than its scanning functionality.
Since a cofferdam had been constructed on the riverbed, the Denis Dubois crew could set the Trimble VX on a dry platform directly underneath the arches to measure their target points. Using the robotic total station functionality and the Trimble TSC2 Controller, the crew set control points, staked target points and then shot all target measurements directly in the local MTM coordinate system. The team then provided the survey data to Kiewit for teams to install the galvanized steel rails.
“A great benefit of the VX is that it is not designed exclusively for 3D scanning purposes, so we can really maximize the instrument to handle a variety of survey needs,” Dubois says. “Without the VX, this project would have been much more labor intensive and costly to complete. It not only made it possible for us to win this work, it has enabled us to bid on projects that we wouldn’t have considered previously.”
In only one week, seven concrete arch panels were installed on arch one and five panels on arch three. By the end of September 2010, all four lanes of the Chaudière Bridge were reopened to traffic and pedestrians, officially concluding the crossing conundrum.