How Flat Can You Incline?

August 1, 2010 - By GPS World Staff

The field at Commonwealth Stadium in Edmonton, Alberta, recently received a CDN $2 million renovation. The old natural-grass 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 re-set.

Renovation required total removal of the existing medium and subgrade materials to a 1.2-meter depth. Wilco Contractors Northwest replaced the subgrade to a planarity or flatness tolerance of 3 millimeters over a 3-meter length. To achieve this precision, Wilco used a machine automation system on a Volvo G-960 motor grader fitted with a GPS receiver, and base station nearby. A second grader carried a robotic total station.

“We probably have a quarter-million dollars invested in this,” said Wilco President Art Maat. “The machine-control equipment pays for itself on an annual basis. It enables us to construct projects to tolerances that other contractors cannot match, even though they have the same big iron capabilities we do.”

Work began with removal of existing soil mixes, drainage rock, and subgrade clay. A bulldozer and the two motorgraders graded the subgrade to a 0.5 percent slope on both sides of the field’s center spine. The work included the D-shaped zone behind each goal post, created by a running track encircling the field. In all areas, the slope must be constant. “The problem is, how do you grade that half-circle?” said Maat. “Grader operators and surveyors want to work in straight lines or on rectangular grids. We use the geo-tracker, or robotic total station, to control the grader blade three-dimensionally. It is one step more accurate than a GPS system.”

Using the robotic total station involves entering a digital terrain model, called a TIN-file, into the grader’s onboard computer. The grader is 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 it back to the grader. The computer processes the differences between the actual blade location and the digital terrain model to control the blade.

The GPS-equipped grader did the rough grading at 20-millimeter accuracy, and the prism-equipped grader handled the fine grading at sub-centimeter accuracy. With final subgrade complete, Wilco dug trenches to install a drainage system, covered with a geotextile. Working in four lifts of 300 millimeters each, Wilco filled the excavation with coal bottom ash, a gritty product like playground sand. “We took the TIN file and offset the elevation by 300 millimeters at a time.”

Savings. The machine-control equipment saved Wilco $15,000–$20,000 on surveying, for 100 hours or more at $150 an hour for a crew. “The systems make our equipment 25 percent more efficient on low-tolerance sites such as fields and running tracks where grades are critical,” Maat added.

To test planarity, Wilco stretched a stringline over a 3-meter distance at many points on the field and measured with a Canadian dollar coin, a looney. If they could fit a couple of loonies under the string, they had found a low spot. If they could fit only one, the 3-millimeter tolerance had been met. “Our feedback from the consultants was that they had never seen a field prepared this well, with very little adjustment required. The slope of the field had to be 0.25 percent from the centerline spine to the sides. And the slope of the D-shaped areas behind the goal posts was exactly the same.”