Imagine a virgin motorway trace, snaking its way across open countryside, with not a setting out or levelling pin in sight.
Imagine a conventional-looking paving machine laying base and wearing courses with seemingly no control points; but instead bristling with on-board radio antennae and computer screens plus boasting a blacktop gang 30% smaller than usual.
Imagine also a resulting 180mm thick limestone aggregate base course, laid across a full 11.8m wide carriageway to 5mm level accuracy and 0.5 degree crossfall tolerance, without a site-worn ruler or sagging fish-gut stringline to worry about.
Imagine all that and you are hopefully imagining the latest state of the art three dimensional automated paving control system manufactured by US specialist Trimble.
Carl McCoid, a paving engineer with 24 years site experience is pleased with his recent acquisition despite initial reservations. “I was initially very wary of the new system, having worked with setting out pins and stringlines for decades,” he says. McCoid is works manager for Bardon Composite Pavements (BCP), the first UK paving contractor to use Trimble system on a major road project. “I was far from convinced it could achieve the tolerances claimed,” he admits.
Eight months down the line, and with a 52km run of cement bound granular material (CBM) sub-base laid as part of the Irish Republic’s new N18 Gort to Crusheen motorway, McCoid has become an enthusiastic supporter of the PCS900 paver.
“I am a total convert and never want to use any other system again,” he asserts. “Old style pinning out now seems expensive and time consuming.”
“Pinning out now seems expensive and time consuming. The system hgas paid for itself within our 36 week contract”
The company’s other half dozen pavers are currently all employed on major surfacing contracts and McCoid “definitely” wants a second Trimble package in time for the next new project. “At Gort the system has totally paid for itself within our 36 week contract,” he says.
Level and slope accuracy, plus surface smoothness, are all excellent, he says. Using a smaller paving crew has reduced labour costs by around £50,000, and laying up to 50% more sub-base in a day than on a comparable contract has saved further time and substantial cost.
At Gort it would not have been BCP’s role to set out any levelling pins, but McCoid estimates that if there had been, it would have cost an extra £50,000 in labour costs. “Main contractors are today tending to ask us pavement subcontractors to do the setting out, so in future, these considerable labour savings could also come our way,” says McCoid.
The up to £100,000 PCS system, sold to BCP by UK distributor Korec, centres on a computer control panel mounted in front of the paver operator. Into this is plugged a memory stick containing the road’s entire pavement design – thickness, profile and slope for every millimetre of chainage – all computed in three dimensional coordinates accurate to 0.05mm.
“What you design, you get on the ground, with the software ordering the exact profiles to be reproduced by the paver,” explains Korec machine control sales manager Peter Brooks.
“If necessary we can be accurate to 1mm level and one second crossfall.”
At Gort, three total station survey points were repeatedly set up alongside the carriageway, leapfrogging each other about 200m apart. These enabled the software to know the paver’s exact chainage position and were also used for a follow-on “as built” survey.
The stations would triangulate between fixed, known-position survey pins located every 50m along the route and a reflective prism mounted on the side of the moving paver. This circular prism, with its ring of reflecting diodes, is one of Trimble’s patented clever bits. “For the total station to get an accurate fix, it needs an uninterrupted sighting of the prism,” says Brooks.
“If necessary we can be accurate to 1mm level and one second crossfall”
“This can be disrupted by the reflective number plates of passing materials lorries or even operatives wearing high visibility vests; both of which can reflect the total station beam, giving false readings.”
But the Trimble total stations and prism diodes offer controlled frequencies allowing only their own interaction to be recorded, so avoiding inaccurate measurements.
Radio beacons on the paver receive its exact chainage coordinates from the total station, allowing the central control box to issue precise height and level orders to the machine’s hydraulics.
On either side of the paver arms, a small box fires out ultrasonic vertical waves, which bounce off the surface beneath. This determines the existing height of the sub-base being laid and, once the design height is calculated by the onboard computer, its software signals the main screed hydraulics to adjust its level accordingly.
The next clever bit is that in the time it takes to read this paragraph, paver positions can be computed, design heights transmitted and the screed movements ordered 300 times. This is read and calculated up to 20 times every second.
Similar 3D measurements are transmitted to a central slope inclinometer - an instrument likened to an electronic spirit level - located mid point along the screed that allows crossfalls to be set, if necessary, to a one second accuracy. In practice, with up to 7˚crossfalls needed at Gort, tolerances could be relaxed to 0.5˚. BCP’s third total station survey point was located immediately behind the paver. This recorded exact as-laid levels in real time.
These levels could be inputted to the control box software and any distribution adjustments ordered instantly as the paver advanced. The same level results then also became the client-ordered “as-built” survey which could be checked and signed off daily if required. McCoid would admit to only two drawbacks. The Trimble system was particularly susceptible to bad weather, a factor that delays any paving operation. Both fine rain and fog led to blurred sightings between the total stations and the paver-mounted prism.
Additionally, accurate levels are dependant on a high specification CBM limestone aggregate being used to ensure consistent moisture content and good grading control. But neither challenges hindered the Gort operation, with the contractor estimating only two days lost through instrument readings.
The Irish motorway contract was the first full scale use of the PCS900 system so far. If McCoid has his way it will not be the last.
Pegless paving control
On a conventional motorway surfacing contract, levelling pins are needed roughly every 10m along the trace, hammered in just outside the line of both carriageways.
But at Gort this was not an option, leading surfacing contractor Bardon Composite Pavements to seek out Trimble’s “pegless” paving control system.
A high water table, just 1m below the occasionally boggy terrain, led to the National Roads Authority demanding a sealed surface drainage system.
Its design incorporated horizontal waterproofing membranes to avoid leaking vehicle pollutants entering the watercourse.
This ruled out the use of any levelling pins that could have punctured the waterproofing membranes.
The £84M dual carriageway Nl8 runs 22km south from Gort in Galway to Crusheen in County Clare. Main contracting joint venture Siac Wills is also building 14 bridges, two junctions and is realigning 10km of local roads. Completion of the two year contract is expected by Christmas.
Over a year in development, Trimble’s PCS900 3D system is the successor to its long established PCS400 two dimensional automatic grading software.
This, and a suite of other Trimble 2D tools for computer control of backfill, formation stone and track ballast, gave Korec its best year yet for such systems in 2009, with turnover up 30%.
“The recessionary need to save on materials and labour triggered this success,” says the firm’s UK machine control sales manager Brooks.
Korec markets the PCS900 for sale at around £100,000 or hire at up to £2,000 a week. And eight months ago Bardon Composite Pavements took delivery of the only one so far sold in the UK specifically for the N18 in Ireland.
The next development, expected in 2011, will be a self steering system designed for laying slipformed concrete road base.
Existing paving control packages rely on the machine being steered manually along prelaid drive lines. The new software’s expected ability to include automatic steering should, claims Korec, offer further time and cost savings.