Innovative Arch Support
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.”
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.
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.
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.
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.