The latest technology machine control systems for graders and scrapers are allowing road construction contractors to price subgrade and final finishing work on a per-square-metre basis, rather than on the more traditional hourly hire basis.
Although traditionally, bulk earthworks and final trim works have been carried out on an hourly hire basis by earthworks subcontractors, the accuracies achievable with the latest GPS-based machine control systems mean that at least one Australian earthworks contractor is looking to quote final trim subgrade work on a per square- metre basis.
The big advantage of this is that the area of the final trim work is a known quantity; the head contractor or client will know exactly what its final trim costs will be, and the contractor bidding on this basis will have a strong price advantage.
Victorian-based plant hirer Bradco Equipment Rental recently completed final trim subgrade work on a section of the Melbourne-Bendigo Freeway project, just south of Bendigo.
For this part of the job, it used two Cat 14H graders fitted with Topcon’s System 5+ machine control system, plus two 633D scrapers.
According to Bradco managing director Daryl Bradford, Topcon’s System 5+, which uses the GPS+ RTK system, has achieved accuracies to within 20 mm in grader operations.
“We were achieving ±6 mm on the first 200 mm of subgrade, and around 19 mm deviation on the second layer of subgrade, averaging 10-12 mm deviation over all,” he said.
A GPS+ receiver is mounted on the graders, while a base station at the nearby project office fine-tunes the signal and allows accuracies down to better than 20mm.
The entire GPS machine control system was supplied installed and is maintained by Victorian Topcon dealer Laser Beams.
Topcon’s GPS+ transmitters receive information from 35 GPS satellites – 24 from the US-based Navstar GPS system and a further 11 from the Russian-based Glonass system, which minimises dropouts (when signals are not available as satellites drop below the horizon).
More satellites also mean better accuracy on the ground – and that is essential to achieve the tolerances required by road authorities and other clients.
Bradco is also using GPS+ for the survey work on the project.
“Where we are going with this technology, we want to be doing final trim to a per-square-metre rate, not on an hourly rate,” said Bradford.
“There needs to be far more consistency in each layer; the pavement needs to be right from the first layer up.
“From the rock zone up, each layer has to be within fairly tight tolerances, which means a minimum three lots of subgrade trim – and all three layers have to be grade checked.
“Where we are going with pavement trim, only graders and scrapers fitted with fine-control GPS systems will be able to handle it,” said Bradford.
“This technology is the reason why we are going back to contracting after 25 years’ moving dirt on an hourly hire basis. We now want to use our expertise to make this per-square-metre rate approach economically viable.
“We can see this technology giving us substantial economic benefits through being able to trim at more than double the current industry rate.”
Bradco has been using machine-control systems since 1996, virtually since the earliest systems first came out.
In Bradford’s opinion, the technology is only now at the stage where true final trim is a reality using GPS-based systems.
He also believes that heavier graders are the key to successful machine-control operations
“Our experience with machine control systems has shown us that a 140H-sized grader is not heavy enough, not powerful enough to carry out final trim work on a large road project.
“A 14H sized machine gives you the weight and power to cut down 15-20 mm at a time. If the grader is too light, it will pigroot and buck all over the place, then your machine control system will be constantly hunting to achieve grade.
“The other mistake operators and site supervisors make is trying to cut too deep. When bulk trimming, we go to a maximum of 40 mm at a time, then in the final grades, go in 5 mm increments.”
For the Bendigo project, Bradco purchased two used 14Hs and some 633D scrapers for the bulk and final trim, and will be using them on subsequent projects.
“We are also looking at applying this system to our other equipment. For example, we are looking at the possibility of adapting scrapers for fairly detailed trim work using machine control, so we can get a better balance between them and the graders,” said Bradford.
“For example, we can get a grader to cut a windrow fairly quickly, but then we need a scraper to get that windrow cleared away quickly. The challenge has always been for the two to work together without each one screwing up the other’s work.”
Ike Wells Bradco’s project manager on the Bendigo and Ballarat rail projects, has been involved with road construction projects, specifically grader operating and
pavement construction, for over 40 years, and is a recent convert to this technology – in particular how much easier it makes his life in terms of achieving tolerances and eliminating stringlines and set out errors.
“VicRoads recently brought their own total station out to the site to do their own audit survey, and we passed on that,” he said.
“I’ve been very impressed with the system. When Darryl first told me we were getting it, I could see three weeks of purgatory, trying to get it set up. However, we were up and running with it on the first day.
“This technology also gives us greater control over our volumes. Because we know exactly what we have to fill, we can do it very precisely. Without this sort of control, your volumes of imported product can blow out,” said Wells. In late 2004, Bradco also picked up a section of the fast-rail line project between Melbourne and Ballarat – the Bungaree Deviation.
This 6 km of rail alignment includes an 8 m deep cut at the western end, then shifting 180,000 cu m down to the eastern end 6 km away for an 18 m deep fill.
Bradco is using nine or ten 50 and 85 tonne dump trucks on this job.