Feature: Making the Cut
Professional Surveyor Magazine - May 2012
Automated grading systems using precise satellite positioning can free up surveyors for other high-priority surveying tasks—and create an opportunity to provide new services.
By Don Talend
On construction projects requiring large-scale earthmoving on varying terrain, the costs of grade checking can be significant—and can result in a less-than-optimal use of a licensed surveyor’s expertise. In recent years, advancements in Global Navigation Satellite System (GNSS) technology have increasingly allowed the contractor’s existing personnel to handle this task. This results in cost savings for project owners as well as opportunities for surveyors to pursue a greater volume of higher-end, official, survey development work. With additional training, surveyors can also expand their services by developing three-dimensional site models that are loaded into automated grade control systems on equipment such as dozers and motor graders.
The Arkansas Highway and Transportation Department
awarded a $78.1 million contract to Manhattan Road & Bridge Co.
of Tulsa, and Weaver-Bailey Contractors
, El Paso, Arkansas to modify the interchange; the contract is partially funded by $18.8 million under the American Recovery and Reinvestment Act of 2009
. 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 the improvements 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 feet above the highway grade. Weaver-Bailey needed to cut about 250,000 cubic yards of earth 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 of exiting traffic on the roadway. 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 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. 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 project is expected to be fully complete by 2014. Utility relocation work began in early 2011 and delayed the large cut for the dedicated hospital ramp for several months. Jim Jolly, general superintendent for 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.
Three weeks into site-preparation work for the new ramp, Weaver-Bailey found that GNSS automated grade control was giving a boost to the efficiency and accuracy of its earthmoving operations.
For the Little Rock project, Weaver-Bailey deployed two Topcon
GR-3 base stations, with one dedicated to the large cut, and installed a GR-3 GNSS receiver, 9168 control box, and sensors on a Caterpillar
D6T dozer. A 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 mast and a dual frequency GNSS receiver mounted in a secure location on the machine. Another antenna/receiver combination at a stationary base station located at a known point is used to provide a reference for removing systematic errors in the GNSS signal. The stationary base station sends correction information via radio or cellular link to the machine to provide real-time kinetic (RTK) position information. Thus, the machine’s three-dimensional location on the site is determined to centimeter-level accuracy. Software compares the machine’s position to the design grade at a given location. The design grade information is built from site plans.
Digital data files are loaded into a machine-mounted guidance computer via a USB flash drive. The guidance computer updates positioning data and sends signals to the hydraulic valves. The blade is automatically positioned for elevation and slope. Other sensors inform the guidance computer 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” 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. GPS 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 can receive signals from the European Union’s Galileo constellation as well.
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 feet of the ridge width was cut out at a 2:1 slope. A 15-foot-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 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.”
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 jobsite can perform other tasks to keep a project—often another project entirely—moving along. Jolly notes that, once the triangulated irregular network [TIN] file has been loaded in the system, Weaver-Bailey’s licensed surveyor can focus on setting control points on other jobsites 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 TIN 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 indicated they would need to revert back to manual slope-staking methods, which are “more tedious and take longer.” It would also require additional stakeout by the surveyor. “The surveyor would probably be up here on a daily basis [in contrast to once very three weeks],” Wilson commented.
Don Talend of Write Results Inc., West Dundee, Illinois, is a print and e-content provider specializing in covering construction, technology, and innovation.
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