A Level Playing Field
The next time a defensive safety for the Edmonton Eskimos football team smacks down an opposing wide receiver at Commonwealth Stadium, both of them will land on a well-prepared piece of real estate.
That’s because the field at Commonwealth Stadium, the home of the Edmonton Eskimos professional football team in Edmonton, AB, recently received a renovation to the tune of about $2 million Canadian. The former field was natural grass, and it was the last stadium in the Canadian Football League to convert to artificial turf. The old field had become expensive to maintain properly. And the Grey Cup game, Canada’s Super Bowl, will be played at Commonwealth Stadium this year. The stage needed to be reset.
|Photo: Wilco Contractors
A Volvo G930 grader, controlled by a robotic total station, is able to achieve sub-centimeter accuracy.
The renovation was extensive. It required total removal of the existing growing medium and subgrade materials to a depth of 1.2 meters (4 feet). And precision became the order of the day. The project’s subcontractor, Edmonton-based Wilco Contractors Northwest Inc., finished the subgrade to near-perfect planarity, or flatness—a tolerance of 3 millimeters over a 3-meter length.
To achieve all of this precision, Wilco turned to a Leica PowerGrade GPS/GNSS Machine Automation system. One Volvo G-960 motor grader was fitted with a Leica GPS receiver, and Wilco set up a Leica Redline GPS/GNSS Base Station. A second Volvo grader, a G-930, was controlled by a Leica Redline Power Tracker Robotic Total Station. That grader could achieve sub-centimeter accuracy.
“We probably have a quarter-million [Canadian] dollars invested with Leica Geosystems,” says Wilco president Art Maat. “The machine-control equipment pays for itself on an annual basis. And that equipment gives us the ability to construct projects to tolerances that other contractors cannot, even though they have the same big iron capabilities we do.
“Wilco was selected to work with the Edmonton Eskimos solely based on our experience with projects like this and because of our surveying abilities and machine control systems,” says Maat. “That is not a sales pitch; it is a reality. We, The Wilco Group, are the primary sports field contractors in Western Canada.”
Gordon Butler, vice president of Butler Survey Supplies Ltd., is Wilco’s Leica dealer. “We are very pleased to be working with Art’s team at Wilco, the preeminent leader in fine-grading for sports fields in our area,” says Butler. “Wilco’s strategy of using leading machine automation technology to deliver premier results is extremely impressive. The entire Wilco organization, from ownership to design staff to the field staff, should be commended.”
The stadium project began this spring with snow removal. Then large excavators and dump trucks excavated the existing soil mixes, drainage rock, and the subgrade clay. Maat says one Caterpillar D6N dozer and the two Volvo motorgraders graded the subgrade to a 0.5% slope on both sides of the field’s center spine—a longitudinal centerline between the goal posts.
|Photo: Wilco Contractors
The coal bottom ash required water to achieve optimum density levels.
That’s not all. A running track encircles the football field and runs behind the goal posts. So the excavation and grading process extended to the D-shaped zone behind each goal post. The center or radius point of each half-circle was very close to the goal posts. And the slope of the field from the half-circle’s radius point down to the track edge must be constant.
“So the problem is, how do you grade that half-circle?” asks Maat. “From a grader perspective, grader operators and surveyors want to grade in straight lines or on rectangular grids.
“So we use a Leica Geosystems tracker, or robotic total station, to control the grader blade, three dimensionally” says Maat. “It is one step more accurate than a GPS system.” Using the robotic total station involves entering a digital terrain model, which Maat calls a “TIN-file,” into the computer onboard the grader. The grader is also fitted with a mast and prism, which has a fixed relation to the grader blade. The robotic total station can “see” the prism, read its 3D location, and communicate that location back to the grader.
The onboard computer then processes the differences between the actual blade location and the digital terrain model. Knowing those differences, the computer can control the grader blade.
The two Volvo motor graders divided grading chores for the final subgrade. The GPS-equipped grader did the rough grading and the prism-equipped grader handled the fine-grading. “Both the systems are fundamentally the same except for the receiving unit on the grader and some minor hardware inside the grader cab,” says Maat. “One has a GPS receiver and the other has a prism. The Leica PowerGrade 3D system gave us sub-centimeter accuracy with the robotic total station, and about 20-millimeter accuracy with the GPS-equipped motor grader.”
With the final subgrade complete, Wilco installed a drainage system in the floor of the excavation. The contractor excavated eight trenches running the length of the field, then placed 8-inch-diameter perforated pipe, surrounded by washed rock, into each trench. The perforated pipes feed into a collector system to drain the field. Next, all of the drain trenches and subgrade were covered with a geotextile.
Then, working in four lifts of 300 millimeters (12 inches) each, Wilco filled in the excavation with a product called coal bottom ash. “It’s a product like playground sand, but it’s very gritty,” says Maat. “We use it because we get 100% compaction without much effort. We get incredible compaction and incredible vertical drainage. And it insulates against frost very well.”
When the coal bottom ash had to be placed, Wilco could not have access to the field with trucks due to other conflicting contractors’ schedules working on the building facility. So the contractor used a series of electric conveyors to move the ash to the field. “We dumped it up near the top of the bleachers at street level and brought the material down onto the field with conveyors,” says Maat.
When the Cat D6N dozer and the two motor graders spread the coal bottom ash, the ease of using the Leica systems made it quite simple to get the 300-millimeter lifts, says Maat. “We simply take the TIN file (terrain model) and offset the elevation by 300 millimeters at a time.” Two HyPac smooth double-drum vibratory rollers handled compaction, and the ash required a steady application of water to achieve the required density.
While there was no need for sub-centimeter accuracy in spreading the material in the lower lifts it was critical that compaction tests for each layer be performed on 300 millimeter—no more and no less. “And because of the Leica systems, we did not have to go out there and do any staking with a survey crew,” Maat says. “We were able to spread it in just 300-millimeter lifts, and the consultants were quite happy with that.”
Savings on Surveying
Maat said the machine-control equipment saved $15,000 to $20,000 on surveying. He’s figuring that cost over 100 hours or more at $150 an hour for a surveying crew. “And the machine-control systems probably make our equipment 25% more efficient on low-tolerance sites such as fields and running tracks, where the grades are very critical,” says Maat.
To test the planarity of the coal ash fill, Wilco stretched a stringline over a 3-meter distance at many points on the field. Maat says that if you could fit a couple of Canadian $1 coins under the string, you’ve got a low spot. “If you cannot fit them under the strings, the tolerance of 3 millimeters has been met. Once ready for inspection, engineers test the grade using a 20-meter string with more difficult tolerances. Our feedback from the consultants was that they had never seen a field prepared this well with very little adjustment required.
“In our case, the consultant’s stringline testing proved that the grading we did using the Leica equipment was absolutely perfect,” said Maat. The slope of the field had to be 0.25% from the centerline spine down to the sides of the field, at the track. And the slope of the D-shaped areas behind the goal posts was exactly the same. In three dimensions, each D-shaped area formed an inverted shallow cone.
Atop the bottom ash, crews placed a 23-millimeter-thick shock pad. (It was close to 1 inch thick.) Maat said it is similar to a carpet underlayment, but firmer. Next the turf goes in like long shag carpeting. Granulated rubber and sand are swept into the shag threads with large power brooms. “At the end you have about a quarter- to a half-inch of carpet, or thread, showing at the surface,” says Maat. “So it walks and feels like real grass.”
Author's Bio: Daniel C. Brown writes on safety and technology in the construction industry.