A Deeper Look at Excavating Underwater
by Daniel C. Brown
January 5, 2012
A Caterpillar excavator with a 4-cubic-yard clamshell performs some of the heavy-duty digging on the river bottom.
Any
job is easier when you can see what you’re doing, but that can be tricky when
it comes to excavating underwater.
This past summer, contractor M. Bowling, of Henderson, Ky., found a way to
successfully “see” underwater on a project that required the firm to widen and
deepen the upstream approach to the John T. Myers Locks and Dam on the Ohio
River near Mt. Vernon, Ind. The U.S. Army Corps of Engineers is preparing the
river for a possible extension of one lock chamber from 600 feet to 1,200 feet
long to accommodate the long barge tows that ply the river.
Working under a $3-plus-million subcontract to Semper Tek-Intersteel Joint
Venture, of Lexington, Ky., the Bowling firm started their portion of the job
in June 2011. The project called for excavating 130,000 cubic yards of mud from
the river. Two large excavating machines—an American 9270 crane and a
Caterpillar 385B long-boom excavator, both fitted with 4-cubic-yard
clamshells—handled the heavy-duty digging work in the river. But the key
machine on the project for fine-grading and checking grade on the river bottom
was a Case CX240 excavator with a 60-foot boom, rented from JOB Rentals and
Sales, of Jeffersonville, Ind.
Muck is towed ashore in a material barge and off-loaded into articulated dump trucks by a Komatsu PC300 LC excavator.
Knowing that they needed to be able to “see” underwater, Bowling decided to have the Case excavator fitted with a PowerDigger 3D GPS machine control system from Leica Geosystems, which JOB Rentals installed on the machine. They also purchased a Leica base station and rover from JOB Rentals.
Cory Page, service manager with JOB Rentals and Sales, says the PowerDigger 3D allows the operator to use a full-color, touch screen display in the cab to see where the bucket is digging relative to the design grade of the river bottom. The contractor can configure multiple bucket types and store them in the cradle, which is permanently mounted to the machine. The XC16 display fits into the cradle, giving the contractor the flexibility to move the display from machine to machine without worrying about losing the machine configuration—it all stays in the cradle. This allows the contractor to wire up one, two or 10 machines relatively inexpensively and purchase only a couple of the display panels, which account for most of the investment, Page says.
To pinpoint the bucket location, Page says the PowerDigger relies on four sensors and a GPS receiver mounted on the Case excavator. One sensor is located on the bucket itself, one on the dipper stick, another on the boom, and the fourth is on the counterweight.
Signals from the GPS satellites are received by the Leica PowerBox receiver on the excavator and by a Leica PowerBox base station. By using radio signals, the base station sends location corrections to the excavator. A Leica service representative loaded a digital terrain model—the design grades—into the computer on the excavator. Software in the PowerDigger then compares the excavator bucket’s actual position to the digital terrain files in the computer and displays the results on the screen.
“There’s no guesswork involved,” says Brandon Bowling, the project superintendent. “And there’s no over-excavation—that’s a big advantage to the GPS system.”
Dirty Work
A Case excavator fitted with a Leica Geosystems PowerDigger 3D GPS system fine-grades and checks grades along the Ohio River bottom.
“We dug a pit on the land side of the bank and left a berm in the river to hold the water back,” Brandon Bowling says.
Next, the American crane and the big Cat excavator, each working from a barge, dug out the berm and placed the spoil in a material barge. Tugboats pushed the material barge to shore, where a Komatsu PC300 excavator off-loaded the barge into the two articulated trucks.
Bowling says the PowerDigger was especially handy in grooming the required 3:1 slope on the river bank. And when it came time to place rip-rap on the bank and in the river, the PowerDigger helped Bowling gauge the exact amount of material needed. The project called for placing 40,000 tons of rip-rap along a 1,500-foot stretch of river bank and on two underwater dikes in the river, which are used for current diversion.
“We used the Case excavator with the Leica GPS system to fine grade the river bottom and to give us the exact 3:1 slope we needed on the river bank,” Brandon Bowling says. “The Case worked from its own barge.”
Beneficial System
Brandon Bowling estimates the PowerDigger system sliced two weeks from the 26-week project schedule—by providing one-time assurance of grades achieved and eliminating wasted movement of work barges from place to place.
Another benefit of the PowerDigger is that because it can check final grades, no survey crew was needed to continually measure the newly excavated depth of the river bottom, Brandon Bowling says. That can be a major savings for firms, by avoiding extra contracts for survey crews, because the “as-built” can be completed by using the machine. And it may not always be easy to find crews who own the equipment to accurately map the river bottom, which means that delays could occur while waiting to get into the schedule of an engineering firm.
In early October, Brandon Bowling says the firm planned to finish the heavy excavation by the middle of the month. Brandon Bowling says he was impressed with the benefits he saw from using the Leica GPS system.
“It’s a good system,” he says. “It’s the first one I’ve ever used ... we need to be sure that when we leave an area that we have completely excavated it—and the Leica GPS gives us that assurance.”
Daniel C. Brown
Daniel
C. Brown is the owner of TechniComm, a Des Plaines, Ill., communications
business specializing in engineering and construction topics.
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