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GNSS Aids Accuracy on Ark.’s Largest-Ever Public Project

Fri September 30, 2011 - Southeast Edition
Don Talend

As if the sheer scale of the largest-ever public construction project in Arkansas is not enough to challenge the construction team, a collision of topography and urban development is making it all the more interesting. A major part of $78.1 million improvements to the I-430/I-630 interchange is construction of a new dedicated ramp for Baptist Health Medical Center from northbound I-430 to eastbound I-630 to help alleviate increasing traffic flow on eastbound I-630. Trouble is, the new ramp was to be constructed at the site of a ridge running parallel to I-630 that was about a quarter-mile long and more than 100 ft. (30.5 m) above the highway grade. Weaver-Bailey Contractors, El Paso, Ark., needed to cut about 250,000 cu. yds. (191, 138 cu m) of dirt and sandstone in order to match the ramp and highway grades.

The original cloverleaf interchange was completed in the early 1970s and now carries more than 180,000 vehicles per day, considerably more than it was designed to handle. The interchange, though, is a key element of transportation for the west side of Little Rock, one of the fastest-growing areas in the region. In fact, I-430 is the only vehicular crossing over the Arkansas River west of downtown Little Rock for the metropolitan area. I-630 is the primary highway near Little Rock providing access to major employers including the University of Arkansas Medical Sciences and the University of Arkansas at Little Rock.

In many cases, the cloverleaf interchange design has been determined to increase congestion while decreasing motorist safety by slowing traffic flow at cloverleaf exits, often causing backups on the exited road. In addition, the design sometimes makes it difficult for cars to merge to the right for a given cloverleaf exit due to a large entry of cars from a cloverleaf entrance that comes in directly ahead of the exit. Modifications to the I-430/I-630 interchange that address these issues include four flyover ramps. Also, the existing cloverleaf is being modified to provide access to nearby streets and collector-distributor roads; westbound I-630 ends at Shackleford Road, the next north-south road located to the west of I-430. Other improvements included widening stretches of both interstates by three lanes in either direction to accommodate the flyovers and traffic volumes exceeding the original design.

The Arkansas Highway and Transportation Department awarded a $78.1 million contract to Manhattan Road & Bridge Co. of Tulsa, Okla., and Weaver-Bailey to modify the interchange, a project expected to be fully complete by 2014 and partially funded by $18.8 million under the American Recovery and Reinvestment Act of 2009. Utility relocation work began in early 2011 and delayed the large cut for the dedicated hospital ramp for several months. Jim Jolly, general superintendent of Weaver-Bailey, explained that the main electrical feed and a fiber-optic line to the hospital had to be relocated before the large cut could begin.

In late June 2011, Jolly walked near the edge of the ridge overlooking the cloverleaf ramp from eastbound I-630 to northbound I-430 and noted that several change orders were issued above and beyond the original funding outlay for Weaver-Bailey to construct a substructure under the flyover from westbound I-630 to southbound I-430. The utility relocations put the project behind by about 40 days at that point, although the change order extended the original 376-day schedule for the hospital ramp phase of the project by 120 days. At least dry conditions kept the project moving along.

About three weeks since site preparation of the new ramp site had begun, Weaver-Bailey found that Global Navigation Satellite System (GNSS) automated grade control was giving a boost to the efficiency and accuracy of its earthmoving operations.

Holding Grade

Automated grade control is not a new concept to Weaver-Bailey, which had used sonic-tracking systems on Caterpillar 12G, 140H and 12H motorgraders utilized for finish-grading subgrade on road projects for several years before adopting a GNSS-enabled system for the same application about two years ago. The company also owns a Topcon Millimeter GPS+ system, which combines GNSS positioning and laser, and installed it on a Gomaco 9500 trimmer.

For the Little Rock project, Weaver-Bailey deployed two Topcon GR-3 base stations, including one dedicated to the large cut, and installed a Topcon GR-3 satellite receiver, 9168 control box and sensors on a Caterpillar D6T dozer.

A Caterpillar D8 dozer equipped with a ripper attachment continuously rough-graded the slope, i.e., fractured pockets of sandstone and a hoe ram-equipped Link-Belt 330 LX excavator reduced the spoil. The D6T dozer fine-graded the cut to a 2:1 slope using the system for guidance and a Link-Belt 460 LX excavator loaded the reduced spoil into dump trucks coming and going along a service road constructed on a bench running the length of the ridge.

GNSS automated grading systems like Weaver-Bailey’s use 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 is 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 Weaver-Bailey 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.

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. The GR-3 receiver accommodates signals from the European Union’s Galileo constellation as well as GPS and GLONASS.

One of Weaver-Bailey’s base stations was set up at the top of the ridge. Jolly pointed out that the radio reception used to transmit satellite positioning data improved once some trees were cleared from the top of the ridge, but as the cut progressed down the slope toward I-630, it became more difficult for signals to reach the dozer.

All told, more than 100 ft. (30.5 m) of the ridge width was cut out at a 2:1 slope. A 15-ft. (4.5 m) wide bench was established between two slopes and used for trucking out the spoil.

Trucking was the biggest worry for Robert Wilson, Weaver-Bailey’s foreman; gathering black storm clouds portended the potential for deep mud that would make it difficult for trucks to get up and down the makeshift roadway while the cut was still at a high elevation. Some of the spoil would be used to fill a low elevation near one of the ramps, Wilson said.

The D6T dozer fine-graded to the specified elevation with roughly tenth-of-a-foot accuracy and by checking the elevation, helped maintain the 2:1 slope.

“The elevation and slope are basically one and the same,” Wilson said. “We just need to double-check our blade wear daily, but so far it’s been pretty accurate. When you get into this rock, you really get [blade wear] on a daily, weekly basis, but so far it’s working out really well for us.

“It is [laborious]…you don’t see a big area finished each day, but we’re getting there.”

Financial Returns

Contractors who are adopting GNSS automated grade control systems report that the technology can reduce construction costs by thousands of dollars, mainly by eliminating staking costs and allowing contractors to check grade using their own workers. At the same time, workers who otherwise would have to stake a job site can perform other tasks to keep a project — often another project entirely — moving along. Jolly noted that, once the TIN file has been loaded in the system, Weaver-Bailey’s licensed surveyor can focus on setting control points on other job sites and other tasks.

Often, contractors need to use a system on a few projects in order to compile cost data for quantification. Wilson said he was not sure how much the system was saving Weaver-Bailey on this project, but without it, “You would have to have a much more experienced surveyor standing up there on the job to [check grade and slope] in a conventional fashion. Once I program the rover and upload [triangulated irregular network] models into the system, it’s much easier to teach somebody to use the TIN model. The dozer operators catch on really quickly with the TIN models.”

How would the grade and slope get checked without the use of the system? Wilson said that stakes would probably have to be spaced every 8 ft. (2.4 m) so that a rod and level remained visible as the crew worked its way down the slope.

“The surveyor would probably be up here on a daily basis [in contrast to once every three weeks] and we’d have to take a level and a rod and an extra guy and work our way down the slope,” Wilson explained. “In the past, we would take, say, an old-style lock level and work stakes down the slope. It’s more tedious and it takes longer.”

“We basically save on labor for laying out the job site,” Jolly said of the system. “We won’t have to have grade checkers out there all the time, checking the slope. We feel like it saves one person, maybe two, on the ground. It will definitely speed up the project and save us money in the long run. You have an initial cost of getting it on all of your equipment, but after a couple of jobs, it’ll be paid for. It’ll be a cost-saving device from then on.

“Basically, your savings come in time and labor,” Jolly continued, adding that the system saves about one and three-fourths laborers in site layout.

“For sure, you would need three people for staking otherwise. [Wilson] would have to have to have a helper with him 100 percent of the time to do what he’s doing now. The system cuts labor in half, plus it’s so much quicker and more efficient. I would say that it saves several thousands of dollars just in a laborer’s time. Everyone knows that time is money in construction, so if we can save time here, that’s several thousand dollars right there.”

Working conditions tend to improve for laborers, too.

“If you were checking grade and checking slope, how would you like to walk up that slope a dozen times a day?” Jolly asked rhetorically. “This type of project just lends itself to GPS.”

Don Talend of Write Results Inc., West Dundee, Ill., is a print and e-content provider specializing in covering construction, technology and innovation.

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