Creating a level playing field

Modern technology helps create the almost perfect playing surface for Edmonton football 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 very well prepared piece of real estate.

That's because the field at Commonwealth - site of the 2010 Grey Cup and home to the Eskimos - recently underwent an estimated $2.6-million renovation. The former field was expensive to maintain because of the natural grass playing surface, and it was the last stadium in the Canadian Football League to convert to artificial turf.

The renovation was extensive. It required total removal of the existing growing medium and subgrade materials to a depth of 1.2 m. And precision became the order of the day. The project's subcontractor, Wilco Contractors Northwest Inc. of Edmonton, finished the subgrade to near-perfect planarity, or flatness - a tolerance of 3 mm over a 3 m length.

To achieve all of this precision, Wilco used a Leica PowerGrade GPS/GNSS Machine Automation system. One Volvo G-960 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-centimetre accuracy.

"We probably have a quarter-million 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.

EXPERIENCE COUNTS

"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.

Gordon Butler, VP of Butler Survey Supplies Ltd., is Wilco's Leica dealer. "Wilco's strategy of using leading machine automation technology to deliver premier results is extremely impressive," he says.

The project began during the 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 graders graded the subgrade to a .5 per cent slope on both sides of the field's center spine - a longitudinal centerline between the goal posts.

Because a running track encircles the field and runs behind the goal posts, the excavation and grading process extended to the D-shaped zone behind each goal post. The centre point of each half-circle was very close to the goal posts; the slope of the field from the half-circle's centre point down to the track edge must be constant.

"So the problem is, how do you grade that half-circle?" Maat asks. "From a grader perspective, grader operators and surveyors want to grade in straight lines or on rectangular grids."

The answer: the team used "the Leica Geo-Tracker, or robotic total station, to control the grader blade, three dimensionally," he says. "It is one step more accurate than a GPS system."

Using the robotic total station involved entering a digital terrain model, which Maat calls a "TIN-file," into the computer on-board 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 3-D location and communicate that location back to the grader.

A MATTER OF CONTROL

The on-board 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 graders divided grading chores for the final subgrade. The GPS-equipped grader did the rough grading. 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," Maat explains. "One has a GPS receiver and the other has a prism. The Leica Power Grade 3-D system gave us sub-centimetre accuracy with the robotic total station, and about 20-millimetre 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 eight-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 mm 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," Maat says. "We use it because we get 100 per cent 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," Maat says.

When the bulldozer and the two graders spread the coal bottom ash, the ease of using the Leica systems made it quite simple to get the 300 mm lifts, Maat says. "We simply take the TIN file and offset the elevation by 300 mm at a time." The ash required a steady application of water to achieve the required density.

While there was no need for sub-centimetre accuracy in spreading the material in the lower lifts, it was critical that compaction tests for each layer be performed on 300 mm - 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 mm lifts and the consultants were quite happy with that."

Maat says the machine control equipment saved $15,000 to $20,000 on surveying. (He figured that cost over 100 hours or more at $150 an hour for a surveying crew.)

To test the planarity of the coal ash fill, Wilco stretched a stringline over a three-metre distance at many points on the field. Maat says if you could fit a couple of loonies under the string, you've got a low spot. If you cannot fit them under the strings, the tolerance of 3 mm has been met. Once ready for inspection, engineers test the grade using a 20 m string with more difficult tolerances.

"In our case, the consultant's stringline testing proved that the grading we did using the Leica equipment was absolutely perfect," Maat continues. The slope of the field had to be 0.25 per cent from the centreline 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 mm thick shock pad. Maat describes it as 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," Maat says. "So it walks and feels like real grass."

All ready for the next touchdown - or tackle.


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