Roads & Paving
Time and Accuracy of the Essence
By Don Talend
Reconstruction of a major highway interchange in Georgia wraps up early with the aid of advanced gra
By Don Talend
Significant reconstruction of a major highway interchange near Augusta,
Ga., recently promised to impact the lives of thousands of drivers over a
period of about three years—so it was in the best interest of the
Georgia Department of Transportation (GaDOT) and the public, for the
project to be completed ahead of schedule.
Significant reconstruction of a major highway interchange near Augusta, Ga., recently promised to impact the lives of thousands of drivers over a period of about three years—so it was in the best interest of the Georgia Department of Transportation (GaDOT) and the public, for the project to be completed ahead of schedule. The grading contractor on the project knew that advanced fine-grading technology could serve as a key factor in early completion.
|Among the improvements at the I-20/I-520 interchange near Augusta, Ga., are new collector-distributor lanes and flyover ramps. (Photo: Barnett Southern Corp.)|
In April 2007, GaDOT awarded a joint venture of Scott Bridge of Opelika, Ala., and United Contractors of Great Falls, S.C., a $191.9 million contract to widen a 6.25-mile stretch of Interstate 20 and reconstruct the I-20/I-520 interchange, including the addition of two flyover ramps and a grade-separated interchange. The construction team would also add collector-distributor lanes along eastbound and westbound I-520 and eastbound I-20. These lanes, similar to a frontage road, are used on ramps that would otherwise merge into or split from the main lanes of the interstate highway—eliminating or displacing weaving from the main lanes, a problem to which cloverleaf interchanges are particularly susceptible.
Among the subcontractors that the general contractor hired for the construction team were Barnett Southern Corp, Washington, Ga., Team Excavating Co., Wrens, Ga., and Big Country Surveying, Lincolnton, Ga. Barnett Southern was responsible for all storm drainage, water distribution, and sanitary sewer installation on the project, as well as some grading of side roads. Team Excavating was responsible for the bulk of the grading. Big Country Surveying provided the grading quality assurance on the project, including setting control points and ensuring that site topography was within the specified half-inch tolerance.
The entire project was scheduled for completion in 37 months, starting in June 2007 with an end date of July 31, 2010. The use of an advanced Global Navigation Satellite System (GNSS)-laser machine control system was critical to the team’s early completion of the highway improvements.
No strings attached
Once Ames Barnett, owner and president of Barnett Southern, got involved with Team Excavating as vice president, he began adopting machine-control technology to a greater extent. Early systems included Topcon’s System 5, which uses sonic trackers to set the correct screed height for paving work, as well as the robotic LPS (Local Positioning System). In late 2007, Barnett Southern made a major leap in technological capability by purchasing a Topcon Millimeter GPS+ grading system from Roper Laser Co., Inc., Marietta, Ga., and utilized it on a highway widening project in southern Georgia immediately.
|The Millimeter GPS+ system’s combination of Global Navigation Satellite System and laser technology allows quarter-inch grading accuracy.|
In pavement grade preparation, fine grading or “blue-topping,” which has traditionally relied upon stakes with blue tips for pinpoint accuracy, increasingly is being replaced by automated fine-grading. With automation, a grader operator uses the blade, without the aid of wooden stakes and stringlines, to finish grade.
In rough or subgrade work, GNSS technology already has made a major impact. A GNSS machine-control system uses a rugged antenna mounted to a shock-absorbing, vibration-damping pole along with a GNSS receiver box mounted in a secure location on the machine. Satellites send positioning data to another antenna/receiver combination at a stationary base station. The base station then sends a three-dimensional position and 3-D corrections via radio to the mobile or machine control receiver. Positioning data is also sent to the machine. The stationary base and machine work together to provide real-time kinetic (RTK) position information, revealing the machine’s three-dimensional location on the site. Software compares the machine’s position to the design grade, which was determined using site plans, at a given location. The system also provides visual guidance for machine operators by displaying a site model on an in-cab color monitor, or it automatically adjusts the needed elevation and desired cross-slope of the blade as the operator guides the machine forward.
| Barnett Southern Corp. and Team Excavating Co., the underground utility contractor and grading contractor on the project, respectively, were able to provide positioning signals to two motor graders, two dozers, a tractor and two rovers from a single base station, pictured here alongside an antenna that transmitted signals to the equipment. (Photos: Big Country Surveying)|
The Millimeter GPS+ system is a variation on the machine-control theme, combining GNSS and laser; this “Lazer Zone” combination provides grading control accuracy to within one-quarter of an inch. The combination of the laser’s high vertical accuracy and the three-dimensionality of the GNSS 3D reportedly increases grading accuracy over existing 3-D GNSS-enabled machine-control systems, which typically achieve rough-grading precision to within one-tenth of a foot. This technology has made a major impact on highway paving in Georgia: in 2005, the Georgia DOT approved the use of similar technology and graders for grading US 1/SR 4, rather than the string line-guided trimmers that had been the standard since the 1960s. GDOT allowed grade stakes to be spaced at 200 feet instead of the usual 50, and grading time was cut in half.
In addition to a GNSS base station and rover, the system uses a PZL-1 Lazer Zone transmitter and a PZS-MC machine-control sensor or PZS-1 rover sensor that gets integrated with the contractor’s GNSS receiver. The PZL-1 transmitter sends out a wall of laser light 33 feet tall and up to 2,000 feet in diameter. The GNSS component of the system plots the location of the machine while the laser component guides the grader to position and elevates the blade precisely. The system calculates the vertical angle from the laser up to the sensor on the machine and gives a vertical correction to the GNSS system, allowing quarter-inch accuracies.
|Patrick Ward, owner of Big Country Surveying, maintained a spreadsheet containing topographical site data at numerous points to verify for the Georgia Department of Transportation that grading was within tolerance. (Photo: Big Country Surveying)|
The contractor can link up to four transmitters for a total reach of 8,000 horizontal feet and 132 vertical feet. The PZL-1 transmitter can operate multiple machines equipped to accept its signals. Also, a PZS-1 Lazer Zone sensor can be integrated with a survey rover for grade checking.
Barnett notes that the widening project was a good initial use of the new system because the areas in which it was used were not congested. Such was not the case on the I-20/I-520 project where, normally, a trimmer would have been used for fine grading. Barnett points out that his Millimeter GPS+-equipped Caterpillar 140H motor graders fit more easily in tight areas than his trimmer would have; the tight areas were created by the pave-traffic shift-grade sequencing of the project. “With the trimmer, you have to adjust your stringline and the places where you can’t get a trimmer, you’re having to grade with a grader based off of the stringline,” he explains. “With the Millimeter [GPS+], you can grade it all—you’re just using the motor grader all the time,” he says, adding that the grades from a trimmer using a stringline and the Millimeter GPS+-equipped motor grader would not have matched.
Unlike with a trimmer, once a section of a new lane was graded and then paved, restaking was not necessary on the next lane over, Barnett adds. “We could pick up right where the Millimeter stopped instead of having to reset our stringline and go back to where we were. We knew exactly where we stopped, we knew where our grades were, and we didn’t have to worry about stakes or bluetops—we could just use our rover to check the grade.”
Patrick Ward, owner of Big Country Surveying, notes that multiple surveying crews were utilized because the system was highly productive, yet GaDOT was skeptical of the system’s grading accuracy at first. Early on, as many as four or five surveying crews were involved before being reduced to two for most of the project. One crew was used during the last few months of work. The surveyor gathered topographical data every 25 feet both before and after the stone was placed and recorded the data in a spreadsheet.
“If the surface had to be a foot below finished grade, we had to put it in the spreadsheet and give it to [GaDOT] to make sure that it was within tolerance,” Ward says, adding that recording the points using conventional staking methods would have been extremely time-consuming. “If you have a Millimeter and a rod, you can check every foot if you want to and not have to compute another grade or pull another string.” Adds Barnett, “Checking grade with a rover, you could go out there and decide not to put the final layer of asphalt on until the final stage of the project and leave the pavement surrounding manholes two-tenths of an inch low. It saved on a lot of staking and a lot of setting curbs and lines to grade.”
Roper Laser helped Big Country Surveying to mount a single base station on a pole near the construction trailer and provided positioning signals to two motor graders, two dozers, a tractor and two rovers. The PZL-1 Lazer Zone transmitters were deployed along the highway, 800 feet apart.
|The use of advanced fine-grading machine-control technology allowed the use of motor graders to get into tight spaces alongside existing pavement without the need for trimmers as well. |
(Photo: Big Country Surveying)
“One thing about the system is, you’ve got to know the limits,” says Barnett. “It can’t do everything, but it can do a lot. We started the job grading with Millimeter but setting the stakes to verify what we were doing. A couple of sections were passed by the state using stringlines and conventional grade methods, and once they saw that we were matching the topographical survey, they got a comfort level. Then they would walk behind the millimeter pole and verify our grades and it ended up at the end of the job that if they said it was on grade, it was on grade. We went to the ‘show-them-don’t-tell-them’ method—we didn’t try to go onto the job on day one and tell them, ‘This is the way we’re going to do it and we know it’ll work.’ We eased them in and said, ‘we know it will work. We’re not going to tell you it’ll work—we’ll just show you it’ll work.’”
Noting that the ¼-inch concrete pavement tolerance superseded the ½-inch subgrade tolerance, Barnett says he did not take any chances in regard to accuracy. Not only were the dirt and stone grade-checked but the concrete pavement was also grade-checked after finishing to ensure that there were no discrepancies, Barnett says. “The main thing was that the Millimeter reduced our liability,” he says. “Normally, if somebody’s stake gets run over, we’d lose all of your reference points. But when we surveyed it, we had a digital file showing that this was the way we left it. There was no discrepancy there and we could always go back to it if there were any questions.”
Time is money
This project confirmed the old adage that time is money. “Your surveyor can bring your rover out and check twice as much as he can setting grades and bluetops the old way and projecting your cross slope to the edge of the pavement with your bluetops,” Barnett says. “You don’t have to do any projecting of the grades. It’s probably half the calculations and twice as fast, especially in making field calculations, where a lot of your mistakes are made.”
Barnett and the general contractor also saved money on development of a three-dimensional topographic site model. Only one file had to be developed and it could be used for both as-built surveying and machine control, Barnett explains. “We were able to split the cost with the general contractor, which saved both of us half. A lot of times, grading against an interstate, we didn’t have time to wait on staking or even have room to set a stake. I would say that the Millimeter probably saved us 30% in grading time. I know it was an investment that paid for itself.”
The fine-grading machine control system saved the entire construction team, not to mention taxpayers and the public, considerable time and money. The project was completed about nine months early, allowing the general contractor to obtain the maximum early-completion bonus. After the highway completely reopened to vehicular traffic on Oct. 29, 2009, it was expected that the Annual Average Daily Traffic volume on three impacted stretches of the road would increase by an average of about 43% by the year 2029, underscoring the importance of the project to a growing area.
Don Talend of Write Results Inc., West Dundee, Ill., is a print and e-content developer specializing in covering construction, technology and innovation.