Just the trim

March 12, 2008

What more could a road builder want? On a Texas tollway project, a GPS-equipped road base trimmer automatically steers itself, cuts grade to within 1/4 in. and takes the place of two motor graders.

What more could a road builder want? On a Texas tollway project, a GPS-equipped road base trimmer automatically steers itself, cuts grade to within 1/4 in. and takes the place of two motor graders.

Working on a $26.5 million earthwork-and-drainage contract to build a 61/2- mile stretch of State Highway 45 south of Austin, T.J. Lambrecht Construction is cutting the road’s subgrade and base with a GOMACO 9500 trimmer equipped with a Topcon Millimeter Global Positioning System (GPS). The total project is costing the Texas DOT $139.7 million. That is a joint venture contract held by Lambrecht with Balfour Beatty, which has the paving and bridges on the project. Lambrecht’s portion involves 1.9 million cu yd of embankment, 2.1 million cu yd of excavation and various culverts and drainage works. Construction began in early July 2007 and is scheduled for completion by May 1, 2009.

A perfect grade

Cutting a pass 16 ft 8 in. wide, the big trimmer is used at three stages of subgrade and base construction, explained Mike Wehling, Lambrecht’s regional survey/GPS machine control manager. The contractor first employs an all-Caterpillar fleet of dozers and scrapers to bring the grade to within 0.2 ft. Two of the dozers are equipped with GPS systems.

Next the trimmer swings into action. It cuts grade with a rotating drum fitted with teeth. With the aid of the Topcon Millimeter GPS system, the trimmer brings the grade to within 1/4 in. Then a Caterpillar RM 500 stabilizer mixes lime into the subgrade 6 in. deep. The grade is rolled with a pneumatic compactor and cured for four days, said Wehling. Another phase of remixing and compaction follows curing.

“Then we run the trimmer on it again, for a second time, and get it to within 1/4 in.,” said Wehling. “Next we bring in 30 in. of select fill material, rough-grade that with the GPS dozers and compact it again. The trimmer makes a final pass, once again bringing the grade to within 1/4 in. That is followed by a tack coat, a 4-in. hot-mix asphalt base and 12 in. of concrete pavement.”

“The trimmer can produce the same amount of finished grade as two GPS motor graders working together,” said Wehling. “The trimmer makes one pass and the grade is perfect, but with motor graders you have to make multiple passes. The trimmer needs to cut about 0.2 ft to keep the proper head of material in front of it. At that depth it can do up to 40 ft per minute under ideal conditions. If it has to cut more than 0.4 or 0.5 ft, we have to slow it down too much.

“We had the first two trimmers in the world equipped with Topcon Millimeter GPS,” said Wehling. “The GPS is used to control steering, and the Millimeter laser is used to control grade. At about 600-ft intervals down the road, we set up the fan-beam laser on control points. The trimmer uses two of them, and we leapfrog them one after the other down the road. The laser transmits a fan-shaped beam to the two receivers on the trimmer, one on each side.”

The Millimeter system works much like a regular Topcon GPS system. Lambrecht loads three elements—a 3-D model of the design, a localization file and a linework file—into the control box on the trimmer. The localization file fits the design to control points on the site, and the linework file controls horizontal positioning.

The Topcon Millimeter GPS system cost about $100,000, Wehling said. That buys two lasers and two receivers plus the control box and receiver/computer that mount on the machine. But, said Wehling, consider that very little surveying—and no stringline—is necessary. If you compare GPS to the cost of a three-man survey crew and a four-man stringline crew, he said, you would go through $100,000 in pretty quick order.

“We wouldn’t even own the trimmer without Millimeter GPS,” said Wehling. “It would be too costly to run the trimmer with stringline. Instead we would have to run two motor graders with GPS systems to equal the production of the trimmer.”

Lambrecht also is using Topcon Millimeter GPS systems on two Cat 14M motor graders at the project for the Texas DOT.

“We use the graders for places we can’t get into with the tracked trimmer,” said Wehling. In addition, Lambrecht has fitted a Caterpillar D6T with a Topcon GPS system for cutting slopes and ditches.

Lambrecht was an early adapter of GPS systems. Six years ago, the contractor bought a Topcon robotic total station, and used it with a Caterpillar 14H motor grader.

“We bought the GTS 2000 total station, sold our trimmer and haven’t set a stringline since,” he said.

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