Northern Lights: Monitoring Toronto's Union Station Renovation
Professional Surveyor Magazine - June 2011
by Brad Longstreet
Downtown Toronto’s Union Station
is Canada’s busiest rail terminal, handling about 65 million passengers annually. Built from 1915 to 1920, it’s considered to be a masterpiece of Beaux Arts architecture, expressed in the ornately colonnaded front and the 250-foot by 90-foot Grand Hall, which is famous for its marble floor, soaring arched roof, and four-story windows at each end.
To cope with modern demands, Union Station is now undergoing a $650 million refurbishment, begun in early 2010. The aboveground portions of the historic building are being restored, the upper three levels of the west wing will be renovated into the head office of Metrolinx (the regional transportation agency), and the concourse is to be tripled in size to a total of 122,000 square feet.
All this expansion requires new foundation work. So, each column in the lowest basement level will, one by one, be supported by jacks, cut away to permit foundation work, slightly lengthened, and reloaded. Two factors make this difficult job even more demanding: Damage to the historic structure is simply not permissible, and train service cannot be interrupted.
Both of these factors require monitoring to ensure public safety and to prevent any settling or shifting that might damage 100-year-old architecture. With these two factors combined, and considering the extremely high-profile nature of the project, foundation contractors knew that they needed very precise, real-time monitoring of the column replacements. There aren’t many firms that can confidently take on monitoring work at this level. Fortunately, one of them is in Toronto.
Monir Precision Monitoring, Inc
., is a “daughter company” of Isherwood Associates
, a geostructural engineering firm based in Ontario. Monir has been a separate business for nine years, employs 17 people, and has been growing about 20% annually.
For the Union Station project, Monir had to combine precision and speed; onsite contractors had to know immediately if column movement was exceeding modeled predictions during the foundation work. The thresholds were tiny: movement of just 2.5 mm (.098 inches) would trigger an alarm, and movement of more that 3.0 mm (0.118 inches) would immediately suspend work. And there was a significant complicating factor: Because heavy trains would be arriving and departing directly overhead, train-caused movement had to be accounted for in the monitoring system.
Total Station Advantages
Monir began by determining the right equipment. The project’s lead contractors had assumed that strain gauges would be the only sensors that would be accurate enough, but Monir surveyor specialist Thomas Gondo, RLS, wasn’t so sure. “Strain gauges are very accurate,” he explains, “but they work by precisely measuring movement in a small area and then extrapolating. And that requires that properties be uniform throughout the monitoring area—we didn’t feel we could make that assumption.”
Instead, Gondo wanted to use a total station, which could rapidly measure the entire area of concern and surrounding areas. But Monir had to convince their clients that the total station could be sufficiently precise. The hang up was distance measurement. “In practice,” says Gondo, “the EDM actually is accurate enough. But we would be exceeding manufacturer’s specs, and no one wanted to assume the liability for that.”
Monir partnered with Leica Geosystems
to determine a method for obtaining sufficient accuracies. Leica’s Don Edgar suggested fixing the distance measurement on a few critical measurements during foundation work. For some of the prism readings during the work, a previously measured distance would be held as constant, and movement would be derived from angular readings alone. This would work because manufacturer’s specifications for angular measurement (at the distances being considered) were well within the tolerances required.
The Monir crew set 23 prisms on and around each column being worked on. The three most-critical prisms (one actually on the column and the two nearest to the column) were measured as described above with a fixed distance, and the rest were measured normally. A Leica TS30 robotic total station was used for measurement, set well away from the work in a stable area.
To process and obtain real-time results, Monir used GeoMos, Leica’s automatic deformation monitoring system, to control the TS30, guiding it through a series of automatic shots. In this case, all 23 prisms were sighted and measured automatically every seven and a half minutes, and the three most-critical prisms were measured once every minute.
To rule out sighting errors due to dust and exhaust, some critical monitoring work was scheduled for 3:00 a.m., when air was clear and the station was quiet. To account for movement caused by trains, Monir used train schedules and pre-gathered measurement data to establish baselines. GeoMos then correlated train schedules and baseline data so that train-caused movement, as opposed to movement due to foundation work, would not trigger alarms.
As it turned out, project engineers were surprised by how little movement was caused by trains. “The biggest movement we saw was 0.6 mm (.023 inches),” says Monir survey manager Colin Hope, RLS, “which the structural engineers had a hard time believing. But we confirmed that we were providing good numbers. Sometimes, we’re the first ones to really measure performance, and the results can be a surprise. In this case, the station’s slabs are much stiffer than anyone knew.”
GeoMos was also used to incorporate data from pressure transducers, which are sensors that measured pressure at the jacks that unweighted the column. This data was displayed along with TS30 data on one GeoMos screen. “Basically, what you see on screen is the change at each target and the pressure,” Chuaqui explains. “What’s neat is that we were able to read and display everything at once.”
Redundant Readings with Independent Methods
“We stress redundant readings with independent methods,” says Chuaqui. On the Union Station project, the independent methods included the pressure transducers and two additional techniques: leveling and before-and-after as-built surveys.
For leveling, Monir used a digital level with invar staffs, which helped them independently verify total-station readings before, during, and after foundation work. The results corroborated the total-station readings and reassured project engineers who were expecting to see more movement during the lifting and reinstating of column loads.
A more global approach was also applied. “There were a lot of stakeholders on this project, including the builders, architects, and Parks Canada
,” Chuaqui explains. “They all wanted data, and of course we’re giving them the very precise results as we go along. But what if, when we’re all done, someone finds a crack in the building?” In other words, how to avoid nuisance litigation and assure stakeholders that no damage has been done by major work such as foundation replacement? “The answer is to survey the whole building,” says Chuaqui. By performing a precise as-built of all areas before and after work, surveyors and builders have a very good way to demonstrate that no damage has occurred due to construction work.
When asked if the Union Station monitoring is a typical project, Chuaqui laughs and says, “It’s a typical ‘special’ project. We don’t really have typical projects—we’re usually assembling different techniques and equipment to suit particular tasks.”
Chuaqui says that the clients on this monitoring project have been “impressed with our ability to convey results immediately; they could just look at one screen and know exactly what was going on. That’s helpful to them and lets them do their work with greater confidence.”
Monir Precision Monitoring, Inc., has obviously pursued a face of land surveying very different than boundary work. Rather than measuring acres or hectares, feet or kilometers, the firm has focused on very small—ideally no—distances: measurable change that everyone involved wants to minimize. And by finding the right solution for each challenge and giving their clients the data they need, when they need it, they’ve created a very successful business. And, no doubt, they’ve done more than most to preserve Canada’s historic architectural heritage.
Brad Longstreet is a freelance writer specializing in land surveying, GIS, and laser scanning technology.
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