Vol 2 - No 1

Guidance System Can See Underwater

Written by Daniel C. Brown

Created on Monday, 30 January 2012 19:55

A GPS system takes the guesswork out of underwater excavation

A 607Kb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

Any job is easier when you can see what you're doing, and so it is with excavating underwater. That's what happened last summer when contractor M. Bowling Inc., Henderson, KY, found a way to "see" underwater as the firm widened and deepened the upstream approach to the John T. Myers Locks and Dam on the Ohio River near Mt. Vernon, IN. 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, Lexington, KY, the Bowling firm began work 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 used to fine-grade the river bottom and check its grade was a Case CX240 excavator with a 60-foot boom.

To "see" underwater, Bowling had the Case excavator fitted with a PowerDigger 3D GPS machine control system from Leica Geosystems. JOB Rentals and Sales LLC, Jeffersonville, IN, installed the 3D PowerDigger system on the Case excavator. Bowling rented the excavator and bought a Leica base station and rover from JOB Rentals, one of the leading suppliers of rental GPS machines and equipment in the Midwest.

Working in 3D mode, the operator uses a full-color, touch screen display screen 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. That gives 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. That way the contractor can wire up one, two, or ten machines relatively inexpensively and purchase only a couple of the display panels, which account for most of the investment.

"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." Bowling estimates that 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, says Bowling. Without the GPS system, a survey crew would have to take a john boat into the river and use a tape measure or sounding equipment to check grades.

That is a major savings in extra contracts for survey crews, because the "as-built" can be completed by using the machine. It is not always easy to find crews who own the equipment to accurately map the river bottom, so another delay can be built into the project while waiting to get into the schedule of an engineering firm.

What's more, the PowerDigger was especially handy in grooming the required 3:1 slope on the river bank, Bowling said. And when it came time to place rip-rap on the bank and in the river, once again 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. The dikes are used for current diversion.

To pinpoint the bucket location, 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, explains Cory Page, service manager with Job Rentals and Sales. Job Rentals rented the Case excavator to Bowling.

Signals from the Global Positioning System (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. Then software in the PowerDigger compares the excavator bucket's actual position to the digital terrain files in the computer and displays the results on the screen.

Bowling began excavation by cutting back a 1,500-foot-long section of the river bank by about 150 feet. All of that excavation could be handled from land. A Caterpillar 330 excavator provided by JOB Rentals and a John Deere 330 loaded earth into two articulated dump trucks, a Volvo A35 and a 40-ton Komatsu. The two trucks hauled the spoil to a nearby basin for disposal.

"We dug a pit on the land side of the bank and left a berm in the river to hold the water back," says Brandon Bowling. 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.

"We put a 3:1 slope on the river bank, and the bottom of the river has to be 22 feet below summer pool level," says Brandon. "When we started, the depth was 8 to 10 feet deep."

Starting on July 23, the American crane and the Cat excavator, each working from a barge, began excavating the river bottom. In plan view, the area they cut down was about 180 feet at the widest point and 1,500 feet long. "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," says Brandon. "The Case worked from its own barge."

In early October, Bowling was planning to finish the heavy excavation by the middle of the month. "That's when the GPS excavator will start working every day," Brandon said. "Right now we're getting anywhere from 1,200 to 1,700 cubic yards per day. Our material barge hauls about 1,300 tons, but we can't load it up completely because the bottom of the barge would hit the river bank and our excavator couldn't reach it. We get 75 to 100 truck loads of material every day."

The American crane has a paint mark on the clamshell cable so that the operator can tell approximately when he has reached design grade. Simnilarly the barge-mounted Cat excavator has a mark on its stick to indicate design depth.

We asked Bowling how he likes the Leica GPS system. "It's a good system," he said. "It's the first one I've ever used. It saves us from hiring a survey crew to do grade checking. "We do have to get an outside surveyor, so that we can tell the Corps of Engineers how much we took out of the river and the amount of rock we put into the river. So 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 is the owner of TechniComm, a communications business based in Des Plaines, Illinois. You can contact Dan at This email address is being protected from spambots. You need JavaScript enabled to view it. .

A 607Kb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

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