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Photo:
Trimble
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Contractors nationwide
are discovering automatic grade-control systems as a way to do fine
grading without stakes, stringlines, or grade checkers. These new
systems offer a choice of sensors to automate blade and cutter controls
on earthmoving machines.
By Daniel Brown
Compared to setting stakes
and stringlines, the resulting time and labor savings realized with
automatic grade-control systems is nothing short of amazing. In
Bellefontaine, OH, for example, contractor E.S. Wagner Company recently
built a new 5,000-ft. airport runway. To do the fine grading and
cut the base stone for the project, Wagner used a Caterpillar 14H
motor grader coupled with a Trimble ATS-3D-600 automatic grade-control
system. "We completed the fine grading for that airport job
in half the time it would have taken using all stakes and stringlines,"
says Jon LaBriere, Wagner's equipment manager.
So pleased is Wagner
with the new system that it plans to order a second Trimble automated
control system for its CMI 4503 trimmer. The contractor will use
the trimmer to cut to grade a 15-mi., all-new section of US Highway
30. Wagner has built up the fills and used them as haul roads. The
earth is "hard as a brick," remarks LaBriere. The trimmer,
unlike a motor grader, can be used to load out excess material.
Wagner recently used
its Cat 14H with the Trimble system for the fine grading at a new
$1.2 million interchange project southeast of Findlay, OH. The job
involved building four on- and off-ramps plus the County Road 37
overpass over State Route 15, a four-lane highway.
"We used the Cat
14H to do the subgrade on the four ramps and the overpass,"
explains LaBriere. "We built the fills up conventionally with
stakes going up. When we got the fill up close to grade, we put
the last lift in and then brought the grader in and ran it half
a tenth [of a foot] high. Then we compacted and graded it back to
grade. The rough grading spec called for the grade to be within
one or two one-hundredths [of a foot]. The Trimble system gave us
that accuracy."
The Trimble system cost
Wagner about $100,000. "I would estimate that between this
interchange project and the airport job, the system paid for itself,"
LaBriere states.
How It Works
Wagner bought a computer
program with the Trimble system that enables the contractor to convert
the state's AutoCAD design to a three-dimensional (3D) model
of the job site, called a Terramodel. That model is contained on
a small card, or diskette, that's about the thickness of two
or three credit cards, says Ernie DelFavro, service manager with
Trimble's Detroit, MI, office.
The system elements onboard
the motor grader are a computer and a display unit, a prism atop
a mast, and a radio receiver. Additional systems elements include
controls that link the Trimble system to the grader's hydraulic
blade controls, a robotic total station (resembling a surveying
instrument), and a radio transmitter connected to the robotic total
station.
First, the card containing
the 3D job-site model is entered into the onboard computer. The
robotic total station, located over a base hub, is activated. As
the machine moves, the total station automatically tracks–in
real time–the prism atop the mast on the grader, so the total
station receives location information about the blade.
Next, the radio transmitter,
sitting adjacent the total station, uses a data cable to receive
grader blade coordinates from the robotic total station. The radio
transmitter sends that information to a receiver onboard the grader.
Finally, the onboard computer takes the location information from
the receiver inside the cab and computes where the blade should
run to accomplish the design elevation. The computer issues instructions
that control the blade through the grader's hydraulic controls.
"You can drive the
motor grader over the job and check for high and low spots,"
says LaBriere. "You don't have to set stringline to do
that.
"We got the subgrade
to within one- or two-hundredths, so we knew we wouldn't have
any waste on our stone. Then we applied 8 inches of stone and used
the grader to trim the stone. This system really shines on stone.
We placed the stone at 10 inches thick, then compacted it to 8.5
inches, then cut it with the grader. We calculated our taper so
that the excess stone would carry into the taper of the ramp.
"I believe the Trimble
to be a moneymaker as a construction tool," LaBriere concludes.
"You can put a below-average grader operator on this machine
and the resulting performance is as if you've got the best
operator. All he has to do is steer the machine and roll the blade
angle to get rid of the excess material. You can just plow through
this stuff, and you know that the blade is exactly where it's
supposed to be at any given point."
Automated Trimmer
Action
Robotic total station—based
systems can also be used to control fine-grading trimmers. A case
in point comes from contractor S.E. Johnson Companies in Maumee,
OH. Johnson recently built a $17 million widening project along
5 mi. of Interstate 80/90 in Ohio. The project consisted of adding
a 12-ft. lane and a 14-ft. shoulder to the inside of each side of
the four-lane divided highway. In essence, the contractor took the
56 ft. in the median and filled it with pavement.
For final subgrade trimming,
Johnson used a Spectra Precision (Trimble) BladePro 3D remote-control
system to control a CMI TR-3503 trimmer. The remote-control system
was used to set the cutter grade and slope automatically and continuously.
The driving lanes had a constant slope of 0.1875 in./ft., but the
shoulders varied in cross-slope from 0.5 in./ft. to 0.1875 in./ft.
The control system worked
very similarly to Wagner's system for a motor grader. As the
trimmer moved along, the system controlled the height and slope
of the trimmer's cutter in real time at six times per second.
No stringline was required.
Nor was any stringline
required by contractor A.S. Horner Inc. for a big concrete paving
project at Kirtland Air Force Base in Albuquerque, NM. As performed
for the United States Army Corps of Engineers, the project entailed
removing and replacing 120,000 yd.3 of concrete aprons
around three large hangers on the base.
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| Paving jobs are
made easier with stringless 3D control system. |
Horner used a stringless
3D control system developed by Gomaco and Leica Geosystems. The
new Leica system was mounted on both the Gomaco 9500 trimmer and
the Gomaco GP-2600 paver used on the project.
"We came in, broke
the existing concrete, hauled it out, and did a rough grade on the
subgrade," recalls Matts Buckland, superintendent for Horner.
"We brought the Gomaco 9500 with the 3D control system and
trimmed it down to basically perfect."
The grade was then rolled
and compacted before a base course was applied and trimmed again
to exacting specifications with the Gomaco 9500. "We set the
trimmer to exact grade because we have a thickness tolerance,"
states Buckland. "If we're below 15 inches with concrete
thickness, we have to take it out. Everything is very precise on
this job, and the 3D control system has proven itself in that aspect."
The entire job site was
surveyed with total stations early in the project phasing. The information
from that survey created the 3D digital terrain model of the project.
Next, that model was downloaded into the Command Center computer
mounted on both the paver and the trimmer. Those computers were
connected to the respective Gomaco controllers for both machines.
Three total stations
were set up along reference points on the length of the 1,231-ft.
pour. A series of shots taken with the total stations to other reference
points on the job site bring the total stations into the 3D model.
The total stations, via radio link, send coordinates for the trimmer
back and forth to the Command Center.
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| Grading and paving
accuracy is easier to achieve with today's improved technology. |
As the paver (or trimmer)
moves, it is constantly tracked by the total stations, and the information
is relayed back and forth by radio signals to the Command Center
at a rate of up to eight times per second. In milliseconds, the
Command Center takes the real-time coordinate data and compares
it to the design-plan coordinate data. The Gomaco controller uses
this information to control the paver.
"You pick points
where you want to shoot from, and they become your reference points,"
says Steve Melton, PCCP paving superintendent for Horner. "Shots
taken from the reference points tell the machine where it's
at and where it needs to be."
Stringlines were used
for some pours on the project. "It was taking us an hour and
a half to two hours to set the stringline we needed for each pour,"
notes Melton. "It's taking us 35 minutes to set up the stringless
system.
"The trimming aspect
is great. We save a lot of time moving the trimmer from one lane
to the other. As soon as we're done trimming, we just select
the next lane in the program and we're good to go. We don't
have to deal with jumping over or resetting stringline or making
sure we're at grade. The machine and computer tell us exactly
where we're at and how far we need to go to get where we need
to be."
A Preference for Trimmers
Some contractors prefer
the use of trimmers to motor graders for finish grading. "You
don't find good grader operators anymore, so we don't
mess with graders," says Jeffery R. Ardelean, division manager
of the Owosso, MI, office of Interstate Highway Construction Inc.
(IHC). "Trimmers usually get one-pass accuracy for fine grading."
An example comes from
IHC's $14 million project on Interstate 69 near Coldwater,
MI. Performed last summer, the job entailed removing 6 mi. of 9-in.-thick
concrete pavement, across four lanes, and replacing it with 11 in.
of new concrete pavement.
In March 2001, IHC took
delivery on a new CMI TR-4503 trimmer-reclaimer, which the contractor
used to trim the sand sub-base for the I-69 project. Working 10
ft. wide, the TR-4503 trimmed the sand sub-base to within plus or
minus 15 mm. The contractor also used the new trimmer to excavate
the old shoulder asphalt and base on I-69. "We loaded the asphalt
onto trucks and hauled it away," says Ardelean.
"We could have used
a motor grader to trench out the asphalt shoulders, but you don't
have a way to get rid of the material," points out Ardelean.
"You'd have to windrow it and load it out with a loader.
So a motor grader is less efficient than a trimmer for that purpose."
As for trimming the sand
sub-base, he states that a trimmer is more accurate than a motor
grader. "You don't have the accuracy [with a grader].
You'll either put on too much material or not have enough.
We need a trimmer as opposed to a grader because we can't find
grader operators. It's easier to train a trimmer operator than
a grader operator."
Accuracy and Costs
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| Jobs with graders
and trimmers are expedited with help of 3D control systems and
total stations. |
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According to CMI Terex
(CMI Corporation was recently purchased by Terex Corporation), for
fine grading, its automated trimmer-reclaimers provide extremely
accurate grade control. A crew using a CMI Terex trimmer can produce
grade accuracy of plus or minus 0.01 ft. (3 mm). Achieving accurate
grades and slopes for subgrades and base courses is critical because
base materials–and paving materials–are expensive. If
an inaccurate subgrade, for example, causes a contractor to use
too much base material, that's an extra cost. Very slight differences
in elevation can result in large amounts of excess material that
cost thousands of dollars.
"People often fail
to understand that even two one-hundredths or three one-hundredths
of a foot of paving material costs a lot of money over the length
of a grade," says Frank Clark, product sales consultant for
the John Deere Construction and Forestry Company.
Today trimmers have moved
into the realm of doing fine grading in preparation for asphalt
pavements as well as for concrete pavements, points out Chapin Sipherd,
product manager for the paver and trimmer division of CMI Terex
products. "Now, controlled depth is becoming more important
for asphalt pavement as well as for concrete." Typically a
concrete pavement contractor is paid for a certain area (square
yards) of concrete, and it must have a minimum thickness. The contractor
can be penalized for a pavement that's too thin (short cores),
but he won't get anymore money for pavement that's too
thick.
Trimmers and Motor
Graders
"It's been
my experience that grading involves both the motor grader and the
trimmer," says Deere's Clark, whose experience includes
work for four or five Midwestern construction contractors. "On
mainline highway construction, you would use both the motor grader
and the trimmer. You get within one-quarter of an inch with the
grader on your dirt or rock base, then use the trimmer for the final
grade. Now you can use a laser reference for grading with a grader
to compensate some for the lack of skilled operators," he says,
referring to systems that transmit a laser beam across a job site.
The beam is read by a mast-mounted receiver on the motor grader
and used to control the blade.
Clark points out that
a motor grader can move at 20-25 mph between sections of grade to
put each section close to final elevation. In addition, he says,
it's easier for a grader to carry material forward or backward
to balance minor cuts and fills between stations.
Or an elevating scraper
can be used to pick up a cut of material and dump it in a fill area
on grade, Clark notes. "And the elevating scraper can also
be equipped with a mast to pick up your laser signal. So you can
use the scraper with a relatively inexperienced operator and laser
controls and put the grade within, say, half a tenth of a foot."
Clark points out that
some applications, such as parking lots, have so many different
grades and slopes that the use of a trimmer becomes almost impossible.
"A motor grader must be used entirely. With a grader you can
fairly easily adjust cross-slope to a laser point of reference,
and you can build grades with highly variable surfaces.
"Contractors today
really strive to be accurate with their grades because of the high
costs of paving material if they leave a grade low." Whether
or not a contractor uses a trimmer also depends somewhat on the
sophistication of the project. Adds Clark, "Where you have
big dollars and big competition involved, you'll probably find
the contractor using trimmers."
Frequent contributor
Daniel Brown is the owner of TechniComm, a communications business
based in Des Plaines, IL.
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