Topcon GPS lets Macmahon go stringless on major Perth freeway project

Topcon GPS systems are playing a key role for Macmahon Contractors in the stringless construction of Perth’s 4.5 km Mitchell Freeway extension, which serves the city’s rapidly growing northern suburbs.

The project, which started in November 2006, and is due for completion in August, was well into the pavement construction works as of early 2008 – with the entire project constructed without the need for stringlines or pegging.

These include two Millimetre GPS™ on a John Deere 770D and a grader for the pavement works, and a 3D indicate system on a Cat D7 dozer for bulk excavation and levelling. Additionally, a number of Hyperlite rover systems have been used for survey and grade checking throughout the project.

According to Leith Collins, Macmahon’s project manager on the $113 million Mitchell Freeway extension, in addition to the 4.5 km of freeway construction, the project includes the realignment of 1.5 km of rail line, three bridges, five pedestrian underpasses, a 300 m rail tunnel, 52 hectares of land clearing and 24 hectares of revegetation.

“So far we’ve moved over a million cubic metres, both on site and off. We’ve still got about 300,000 cubic metres to go,” Collins said when interviewed in December 2007.

John Ransom, road works construction manager, Macmahon, said the great advantage of having Topcon GPS equipment on the project was the ability to run the entire operation stringless.

“It’s been a great relief on resources, and now on the pavement operations. We are able to get plus or minus 10 mm, which is well within our specification tolerances,” he said.

“It makes it possible to run a really lean crew, without the need for stringers, so the cost of the operation is reduced greatly and the efficiency, therefore, goes up.”

Scott Pollard, senior surveyor on the project, said the Topcon Hyperlite systems had proved very versatile for survey work throughout.

“On this site, we’ve got one base station set up near the main office, so we’re able to take our GPS rovers pretty much anywhere on the job site,” he said.

“We’ve got full radio range over the whole job site, so we don’t have to set up our bases. They are a nice, small, compact unit.”

Pollard said the Topcon Millimetre GPS system has been giving very consistent results in the paving operations.

“The tolerances that we have on subgrade are -35mm to +5mm and we get that without any problems at all. Even on base course – where we’ve only got a 20 mm tolerance – we’re having no problems at all,” he said.

“Within McMahon, we’ve been using the Hyperlites for about five years now; we’ve found they’ve been very consistent and save the surveyors a lot of time.

“In addition, the as-cons (as-constructeds) have been coming up very consistent. They’ve minimised our rework on all our pavement layers. It saved us a lot of pegging on large batters,” said Pollard.

Subcontractor Dan Cox of Roadtrim Contracting, who has one of the Topcon Millimetre GPS systems on the project, fitted to his JD 770D grader, found it has greatly increased efficiency on the project.

“On this project, we’ve been carrying out bulk earthworks right through to final trim on roadbase, limestone and subgrade,” he said.

“We’ve been running Topcon systems on our graders for quite a few years. It’s only the past three to four months we’ve switched over to Millimetre GPS. Which has greatly improved our efficiency and operating ease.

“From an operator’s point of view, we’re not relying on labourers to string. Which holds the grader up and we are not waiting for surveyors to change pegs or replace pegs. It just speeds the whole system up greatly,” said Cox.

“We find now when we are balancing out, it’s just taken all the guesswork out. We can get our quantities right there, within a semitrailer load. We know exactly what’s going on, how much dirt we need for that area – it’s a major improvement,” he said.

“From the experience we’ve had here so far, I’d recommend using Topcon grade control systems on future projects,” said Ransom.

“I think that it will become the standard procedure for this company from now on. The benefits are too great to walk away from.”

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