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Satellite system makes the grade on the A120

EARTHMOVING

Balfour Beatty puts its faith in global positioning systems and says fairwell to the chainboys on the A120 upgrade in Essex.

Emma Forrest reports

STANSTED airport is a choice point of departure for many jet-setters: the airport itself is small enough to navigate without the aid of a compass and is served by many budget airlines. But reaching the terminal by car presents a distinct disadvantage, often involving a long, winding journey through a variety of picturesque, but traffic-clogged, Essex villages.

Enter the Highways Agency, with a £75 million investment into a 30-month design and construct programme for 23.5 km of replacement A120, awarded to Balfour Beatty in July 2001. By the time work finishes next year, 1.5 million cu m of earthworks will have been completed, resulting in 18.5 km of new dual carriageway, 3 km of single carriageway widened to dual status and 2 km of dual carriageway stretched to become a mammoth three-lane route.

It is a considerable and demanding job and unusual in that every inch of the grade control that shapes those earthworks is governed by a global positioning system.

'There were two principal reasons for using global positioning systems. Firstly there are savings to be made on setting-out and controls and secondly you get a better finish. We thought it would present a commercial advantage, ' says Balfour Beatty project director John Hodgkins.

Balfour Beatty's surveyor and GPS supremo is Tom Adams.

'With setting-out pins there is a chance you will get minor undulations in the ground. With GPS you get a good, planed finish, ' he says. 'Setting-out helps engineers develop their skills but there is nothing worse than having to set out for the fifth time when weather or machine movements make changes to the ground.'

Manchester-based earthmoving contractor J Jones was willing to take on the technology and a total of eight SiteVision GPS systems, manufactured by Trimble, were purchased for the job.

They communciate with the grader via an on-board display in the cab to guide the driver so the planer can be lifted or lowered to exactly the right point for the right amount of cut and fill.

'Earthworkers have never taken their use of GPS beyond machine control because the GPS companies just did everything for them, ' says Mr Adams. 'J Jones was happy to look at taking it further with us.'

The earthmover's senior engineer, Nick Cuming, admits the process involved a steep learning curve.

'Some of our drivers are quite set in their ways and we had to teach a lot of them how to use the computers and the touch-screen pads. It was intensive training and the first month was spent solving problems. But they have adapted well, ' he says with pride. 'Some have even come to me with new ideas about how to use it. They are also very competitive with each other over what their machine has done that day.'

Mr Hodgkins says he was not surprised by how quickly the drivers adapted. He stresses that using GPS does not spell the end for traditional skills, as gangers and banksmen have been moved onto other jobs.

'They are all good technicians. You would expect to get quality with these blokes, ' he says.

With the job not due for completion until 2004, Balfour Beatty says it is too early to predict if any time savings have been made. No costs savings are evident yet either and are not likely to have been calculated before the summer.

Mr Adams and Mr Hodgkins agree that GPS will dominate the earthmoving sector within a decade.

Surveyors are likely to become more like data managers.

They have to make sure that the road model is complete otherwise the machine's computer, merely following instructions, would be prone to mistakes.

Even the cards that are loaded onto computers before being inserted into the machines' cabs - each machine consequently contains the data for the entire road's design - will be made obsolete and replaced by computerised links from office to cab.

Some adjustments still need to be made. The exact thickness of blacktop required on British roads can not yet be measured by machine alone.

'We are looking at ways to measure the thickness of pavement electronically without the need for boreholes, ' says Mr Adams.

Nor is the system perfect as yet. There will always be a difference between something that has been set out on land and something that has been set out by a satellite in earth orbit. Variables are always possible because of the variety of satellites from which information is being received, so accurate placing of bases that receive the signals is crucial. Mr Adams adds that it is essential for a network to be able to accommodate both GPS and traditional total stations.

Balfour Beatty have been working with manufacturers Trimble and Caterpillar to develop adapted machines over the past few years. The team is convinced that it is not a case of 'if ', but 'when'.

'We will be happy to eliminate all need for pinning, ' says Mr Hodgkins. 'From the first excavation to the blacktop it will all be done with GPS. We haven't got there yet - but we will do.'

Project details

Project: A120 Stansted to Braintree

Value: £75 million

Contract: Design and build

Financier: Highways Agency

Client: Essex County Council

Main contractor: Balfour Beatty Major Projects

Earthmoving: J Jones

Designer: Atkins