The breadth of global navigation satellite system (GNSS)-enabled machine control applications continues to expand, yielding benefits in the areas of dirtmoving productivity, worker safety and project profitability. A recent bridge-widening project near Phoenix demonstrates the expanding reach of this technology in two respects: grading control for excavators and bridge pier excavations that require deep, narrow cuts with precise slopes.
In early 2009, Haydon Building Corp., Phoenix, was awarded a $7.9 million contract by the town of Gilbert, a suburb located southeast of Phoenix, for bridge widening and roadway section improvements among other work along Higley Road, a major north-south arterial. A lane was to be added in each direction over a wash in the Eastern Maricopa Floodway that passes under the existing 450-ft. bridge.
Haydon recently adopted GNSS-enabled machine control and saw an opportunity to use the technology for a cutting-edge task: verifying the location, dimensions and wall slope of 10 footing excavations of 27 ft. deep, 97 ft. long and 70 ft. wide (8.3 by 30 by 21.3 m) that support five piers on either side of the existing bridge that support the new lanes on the box-beam girder structure.
The use of GNSS eliminated the need to stake the excavations and for a surveyor to verify the location, dimensions and wall slope of the footing excavations, saving Haydon and the owner time and money. Just as importantly, use of the technology also eliminated the need for topographical verification via conventional surveying methods, and for a worker to physically enter the excavation for verification, which would have necessitated time-consuming shoring procedures and created a less safe work situation.
This project is one example of how GNSS technology is automating grading and excavation. Increasingly, contractors like Haydon are having machine control systems installed on earthmoving, grading and paving equipment such as excavators, scrapers, dozers and motorgraders, plus milling machines and asphalt and concrete pavers.
A GNSS machine-control system uses a rugged antenna mounted to a shock-absorbing, vibration-damping pole and a receiver box mounted in a secure location on the machine. Satellites send positioning data to another antenna/receiver combination at a stationary base station. Positioning data also is 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 at a given location. The design grade information was built from site plans.
The data files are loaded into a machine-mounted control box via a USB flash drive. The control box updates positioning data and sends signals to the hydraulic valves. The blade is automatically positioned for elevation and slope. Other sensors inform the control box of certain machine conditions; for example, dozers used by contractors such as Haydon are equipped with a slope (tilt) sensor on the blade to measure the cross-slope of the cutting edge. “Indicate systems” like Topcon’s 3-D systems provide visual guidance for machine operators, who manually control the machine to cut or fill to the desired grade.
During the past couple of years, these systems have evolved in terms of flexibility and reliability. An alternative to a base station — one that Haydon uses — is subscribing to a network that provides positioning corrections using cellular technology. Where available — such as through Branco Machinery of Gilbert, which also sold and services Haydon’s Topcon Positioning Systems GNSS equipment — this option provides flexibility. The contractor does not have to set up a separate base station on each job site.
“With a base station, you’re setting on a known point on that job and on that known point, that base station transmits to the machine or rover corrections of where it is,” said Todd Hermsen, Haydon’s Topcon sales representative. “When you move to a cellular communication with a network, the machine is actually dialing up to that network via cellular communication and receiving its corrections.”
He added that Branco’s TAZNet (Topcon Arizona Network), which went online in mid-2007, reaches the Phoenix and Tucson metropolitan areas.
“What it allows [Haydon] to do is take their base and rovers that they may already have and instead of tying up that base station on that job, give them the flexibility of taking that base station and converting it to a rover. So someone like Jeff [Fields, Haydon’s GPS manager] is able to take it out and use it for his survey work and not have to have that base station on the job at all times.”
Once Branco installs cables and sensors on the machine and activates the modem, the contractor accesses the network via a wireless telecommunications service provider.
GNSS has become even more reliable and accurate in recent years by adding compatibility with the Russian GLONASS satellite constellation as well as the U.S. Global Positioning System constellation. This dual-constellation capability roughly doubles the number of signals available to the GNSS antenna/receivers and provides a high degree of positioning accuracy.
Excavators Require Unique Configurations
Equipping excavators with GNSS grading technology, as on this project, requires different configurations from dozers and graders because an excavator has so many moving parts working in conjunction.
A GNSS-equipped dozer has a single-slope (tilt) sensor on the blade to measure the cross-slope of the cutting edge. Like a dozer, a grader has a slope sensor for blade tilt, in addition to a rotation sensor at the rotation swivel. A third sensor, known as a mainfall sensor, is mounted on the mainframe of a grader and provides slope measurement in the direction of machine travel for adjustment of the blade up or down. These sensors work together to maintain cross-slope while allowing for the rotation of the grader blade. A recent innovation for dozer or grader fine-grading work is Topcon’s 3-D millimeter GPS+, a combination of laser and GNSS that is designed to yield much higher precision than conventional machine control.
The excavator’s bucket can operate toward and away from the operator cab as well as up and down. It has a swinging body and a two-piece articulating element between the machine body and the earthmoving attachment, giving it a wider range of motion and requiring more sensors to account for swing, cross-slope, and long slope as well as to pinpoint the bucket location. A typical excavator control system must make four measurements: cab to boom, boom to stick, stick to bucket hinge, and bucket hinge to bucket teeth. The GNSS must account for boom-and-stick articulation (and thus the distance between the bucket and cab at any given moment), bucket tilt, and machine body levelness. Tilt sensors on the stick, boom, bucket and machine body continuously calculate the distance from the cab to the bucket teeth.
Topcon’s 2D systems for excavators comprise these non-GNSS sensors and can detect various physical references such as the existing surface, a hub, a previous cut or a rotating laser. The operator can choose a reference and enter a cut or slope depth. The systems allow the operator to create multiple elevation or slope designs and cut to the design without the need to stop and re-establish a reference.
For three-dimensional grade control, the GNSS components on the machine serve as the machine’s 3-D GNSS sensors and pinpoint the location of the bucket teeth relative to the slope. Haydon’s Caterpillar excavator was recently equipped with Topcon’s X63 grade-control system, which is specifically designed for excavators. The system consists of four temperature-compensated 360-degree CAN-based tilt sensors that measure angles from the cab, boom, stick and bucket; a GX-60 color touch screen control box; two GPS+ antennae; and a GPS+ receiver. As the pier footings were excavated, the operator viewed on the GX-60 the machine’s exact position on the site, in addition to the bucket’s constant position. The system eliminated the need for a grade checker to continuously monitor excavation depth.
Haydon had Dirt Pro LLC of Higley, Ariz., build 3-D site models and loaded the models into the X63 system control box via a flash drive. By viewing the 3-D digital site model on the monitor, the operator avoided over-excavation and minimized material waste.
“There are a few key things with an excavator that makes this type of system appealing to a contractor: safety, productivity and the elimination of over-excavation,” notes Hermsen. “It allows them to keep their employees out of the excavation. Also, they can control the bedding materials and eliminate over-excavation. So when they are supposed to put down 4 inches of bedding material, there’s actually 4 inches, not 6, 8 or 10 inches. Over time, that’s where the systems pay for themselves as well as the increase in production.”
Putting 3-D Grading to Work
Haydon has incorporated GNSS grade-control technology into its day-to-day operations, having purchased two X63 systems in 2008; the Higley Road bridge-widening project was one of the company’s first uses of the system. The company may purchase several more X63 systems in the future as the technology becomes an even bigger part of day-to-day operations, according to Fields.
“What we’re doing a little differently with the 3-D system is the holes we’re digging,” said Fields, adding that most contractors that he knows of have used 3-D machine control for applications such as grading roadway subgrades. Fields uses the GNSS equipment to verify takeoff estimates and collect location data used to build digital site models using Carlson software. Fields also troubleshoots the system with the help of Branco and Hermsen.
With dozers backfilling the newly constructed piers in the distance in early March 2009, Haydon’s Fields, Project Manager Troy Gjerde and Project Superintendent Keith Samples recalled the speed at which the 1,000-cu.-yd. (765 cu m) footing excavations were dug and graded over the previous three weeks.
“Jeff’s been bringing a lot more to our attention, what we can do with the system, so I thought, heck, we knew exactly what our excavations needed to look like,” said Gjerde. “I thought that maybe this system that Jeff was telling us about could work for them and really, the system worked just beautifully,” adding that the system provided positioning accuracy within one-tenth of a foot.
Gjerde noted that staking the footing excavations would have taken a staking crew about two hours apiece at an additional cost of several thousand dollars. Use of the system also saved several thousand dollars that otherwise would have been required to pay a surveyor to verify the location, slope and depth of each excavation, Gjerde added.
“If we wouldn’t have had this system, we would have had to have a grade checker out here the whole time, pulling stakes and checking the slope continually,” he said. “I would say it would have taken us a half a day to a day longer.”
Fields added that the use of cellular technology further enhanced productivity.
“For one, you don’t have two sets of equipment to haul around,” Fields said. “The other thing is, you don’t have [a base station] sitting out in the middle of the road; if your control point is close to traffic, you don’t have it sitting there where it can get bumped around by machines.”
By the time the bridge-widening project was under way, Haydon did not need any convincing of the X63 system’s accuracy, noted Fields, who attended a manufacturer-sponsored training course soon after he was hired.
“My main concern when I started on the network was accuracy,” he said. “I gave it probably two months of rechecking everything. I’d actually double-check the system’s accuracy with a total station, but I don’t have to anymore — I’m satisfied that the accuracy is maintained with the network.” Gjerde added that Haydon’s confidence in the system was such that the company would rely on it heavily for a project involving the location of several miles of underground utilities later in the spring.
Fortunately, the operators’ learning curve was not steep, said Gjerde. “We had three different operators — two very experienced operators and a guy who’s our universal operator — and believe it or not, the guy who’s the least experienced on the excavator did the best job with the system,” Gjerde pointed out. “I don’t even think it was a matter of hours for them to learn it. I think it took a couple of the guys a little time to get the functionality of the module — what button to push to get the views — but once they figured that out, it was really simple for them.”
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