GPS Guides Shoreline Revetment Project
When homes recently began to be threatened on Taylor's Island, located on the eastern coast of Chesapeake Bay in Maryland, it was finally time to do something about shore erosion that had occurred for decades. A revetment that is preserving the homes for the long term was constructed in late 2007, and the contractor relied on Global Navigation Satellite System (GNSS) technology to keep the revetment within the grading specification — and complete the challenging job efficiently.
Increasingly, contractors who grade job sites are relying on GNSS to survey the sites with sub-meter accuracy, eliminating the need for time-consuming physical site preparation — and even helping construction machine operators to keep grading within specification in real time. These systems use two GNSS receivers: a stationary base station antenna/receiver located on or near the job site and a mobile "rover" antenna/receiver that can serve both a surveying role and a machine-control role. In any of these uses, the base station receives satellite signals and transmits the signals to one or more rovers. The satellite signals mark the location precisely, making site preparation more efficient because staking is not required.
GNSS machine control requires development of a three-dimensional model of the site from data compiled from the rover, which can be mounted on any machine or vehicle for surveying. Machine control necessitates outfitting of the earthmoving machine with a control box; the 3-D model is loaded into the control box via a secure digital memory card. During earthmoving, position data is processed by control box software and compared with the design grade at a given location. The control box updates positioning data several times per second and sends signals to the hydraulic valves, which position the earthmoving attachment as necessary to achieve the desired grade.
Configuring an excavator for GNSS control, as on the Taylor's Island revetment project, is a more complex undertaking than on a dozer or grader.
A dozer has a single slope (tilt) sensor mounted on the blade to measure the cross-slope of the cutting edge. Like a dozer, a grader has a slope sensor for blade tilt but adds a rotation sensor at the rotation swivel. A third mainfall sensor mounts on the mainframe of a grader and provides slope measurement in the direction of machine travel for adjustment of the blade up or down. These sensors work together to maintain cross-slope while allowing for rotation of the grader blade.
In contrast, an excavator has a swinging body and a two-piece articulating element between the machine body and the earthmoving attachment, affording it a wider range of motion and necessitating more sensors to compensate for swing, cross-slope and long slope as well as pinpointing the bucket location. A typical excavator control system must make four measurements — cab to boom, boom to stick, stick to bucket hinge, and bucket hinge to bucket teeth. The control box must take into account boom and stick articulation, and thus the distance between the bucket and cab at any given moment, bucket tilt and the level of the machine body.
Tilt sensors mounted on the stick, boom and bucket continuously calculate the distance from the cab to the bucket teeth. These non-GNSS sensors make up the excavator's 2-D system. An additional 3-D GNSS sensor, i.e., rover, pinpoints the location of the bucket teeth relative to the slope, even as the machine body rotates to a different digging position, by also taking into account the machine body's position relative to the longitudinal slope.
In the case of the Taylor's Island revetment project, the contractor, Shoreline Design, Edgewater, MD, used a Topcon HiPer Lite+ base station and rover "fourth sensor." Largely as a result of using GNSS technology, Shoreline Design completed the project ahead of the 180-day schedule and preserved a piece of prime waterfront real estate for residents.
Bob Kennedy, director of public works for Dorchester County, estimates that a narrow stretch at the tip of Taylor's Island had lost about 200 feet to erosion since the 1980s and began to threaten a handful of homes. In fact, the last three-quarters of a mile of a road along the shoreline and a bridge at the end of the road had long been things of the past by the time the revetment wall project began in mid-2007. With erosion increasing in severity by the early 1990s, Kennedy notes, the county highway department began placing assorted revetment materials of sorts such as chunks of recycled concrete along the narrow strip. "It wasn't really built to be a revetment. It was a 'dump and run,'" says Kennedy.
Plans had been made to stabilize the shoreline at Taylor's Island since 2000. Federal funding was approved in 2007. The Army Corps of Engineers financed 75 percent of the project, with Dorchester County funding the remaining 25 percent.
Shoreline Design was awarded the bid on the $1.5-million project and began work in July 2007. Company owner Wes Matheu formerly worked as a project manager for the Army Corps as well as several marine contractors before starting the company in 2003. He was already familiar with the situation on Taylor's Island.
"I actually looked at this project when I worked for the corps about 12 years ago, and the people out there were trying to get something done," Matheu notes. "The corps will spend federal dollars on a federal project as long as it's protecting a federal roadway or public infrastructure."
Over the years, Matheu had seen the erosion that the Chesapeake Bay tides were causing in the area. He also witnessed the efforts to mitigate the erosion while funding was pending. The existing shoreline stabilization material "was kind of haphazardly dumped and it was maybe a couple of feet over the high tide at best," Matheu recalls. "The tide ranges about 2 to 3 feet in this location and it's not just the tides. The wind kicks up and the fetch — which is the distance the wind blows across the water to form a wave — is very wide here. Winds out of the north or even the west create big waves and come in here and were just destroying the shoreline. It's a hard clay shoreline and that's the only thing that was keeping it from washing away in some locations."
The engineering firm on the project, Andrews Miller and Associates of Cambridge, MD, designed the new revetment wall to have a height of about 3 feet to 4 feet higher than the existing material. The toe of the slope at the bottom of the bay is located 36 feet from the shoreline and is itself 4 feet wide, says Matheu. He notes that the slope of the revetment, which is a total of about 9 feet high, is 2:1.
In a very difficult working environment characterized by high tides forcing crews to work in water depths of 4 feet and greater, the use of the Topcon HiPer Lite+ GNSS made Shoreline Design significantly more productive compared with conventional surveying methods. Additionally, the system provided Matheu with precision control from the cab of a large Hitachi excavator to easily meet the project's grading specifications.
The use of a GNSS was not new to Shoreline Design, which purchased the Topcon HiPer Lite+ more than a year before the Taylor's Island project. The revetment was actually the fifth time the company had used the system.
The first task was taking away the existing rock along the shoreline. "The process was, strip the shoreline and sort the debris," says Matheu. "We would save the bigger rock; they wanted us to save stuff that was 400 pounds on up. Everything smaller than 400 pounds was supposed to be stockpiled in the yard. We used some of the small stuff to rebuild the subgrade and we would excavate the toe and then cover everything with a layer of No. 2 stone."
Gary Williams, project manager with Andrews Miller, points out that the material recycling went a long way toward keeping the project costs down. "Taking all of that material off-site and bringing in new material would have been extremely expensive," Williams says. "We tried to utilize as much of the good quarry stone as feasible by reducing the quantity of materials that needed to be brought in as well as the quantity of materials that needed to be hauled out."
The excavator was equipped with a hydraulic thumb for stone placement as well as the bucket for excavating. The thumb was folded up during excavating, giving the machine dual-use capability.
As with four layers of rock that would be placed, construction of the toe required Matheu to lower the excavator bucket — which held the HiPer Lite+ GNSS rover antenna — to the surface of the water in order to mark the depth of the subgrade and, later, the elevation of the toe stone. Working in 4-foot-deep water, Matheu and workers in the water were "blind" but for the GNSS equipment that marked the slope and elevation of the material under water.
The abrasive material and excavating on the project required Shoreline Design to use an extra quality-control procedure. "We probably handled about 15,000 to 20,000 tons of rock on this project, so the teeth on the bucket actually wear and the teeth are the measuring point, so occasionally we'd recalibrate the bucket and check it against the handheld [field monitor] just to make sure we were as accurate as we could be," says Matheu.
Besides grade-checking accuracy, the GNSS also yielded time savings in terms of verifying the elevation of the revetment for inspectors, Matheu explains. "Normally, when you do this type of work, you have to put a lot of stakes in the ground and measure up to your 2:1 slope and figure out where the crest of your structure is going to be, which is an imaginary point," he says. "Some agencies require contractors building revetment to build a 2-by-4 template so that the inspector can look up and down the line to see if you're hitting the right elevation. It's a lot of field work to build one of these. We saved a lot of time that way."
Heather Batchelder, construction project manager for the Army Corps, says she previously had not been involved in a project that used GNSS for grade checking. She points out that the use of GNSS mainly benefited Shoreline Design in terms of productivity to a greater extent than grading accuracy. Regardless of how the grade was checked, Batchelder notes, an as-built survey was required at the conclusion of the project.
Following toe construction, the contractor placed an 8-inch revetment layer consisting of No. 2 stone with an average size of 2 inches to 3 inches. Placement of geotextile fabric on this layer turned out to be one of the most difficult tasks on the project.
"We have some tricks," says Matheu. "We had the cloth pre-sewn by one of our subcontractors into panels that were 130 feet long by 36 feet wide. We found where the toe was for the structure, and we put in 2-inch metal posts and then when we'd drag the filter cloth out. We pulled the cloth on land and then we sent two guys into the water in wetsuits. This project went into the winter, so it was cold out there. The guys in the water got out to where they could hold onto the poles, and we had some other guys on the shore who stretched out the cloth. We used 3-inch rings, and we zip-tied the rings to the filter cloth and then dropped the rings over the fence posts, and then on land we would pull that side tight."
Even with the fabric carefully deployed, more finesse was still required for the next layer — 8 inches of 4- to 8-inch average-size gabion stone, Matheu explains. "Instead of dumping the stone on it, you have to sink the cloth. That was labor-intensive; two guys would stand on the cloth and sink it. I would pass a bucket of rocks out there and they would hand-throw small rocks until we got the cloth down. That's the trick to getting the cloth down: If you just dump stuff on it, it tugs and pulls and it doesn't go where you want it. Doing it slowly gets it the most even and spread-out."
Construction of the toe required excavation of about 2 feet of soil at the bottom of the bay and placement of armor rock in the excavation. "We never really saw the really low tides," Matheu points out. "We always had at least 4 feet of water we were working with, so you could never see what you were doing when you put the toe in."
After placing the third layer and checking the grade with the HiPer Lite+, Shoreline Design placed a 2-foot layer of 400-pound to 1,200-pound secondary armor stone. The final, "primary armor" layer consists of stone weighing 800 pounds to 2,500 pounds.
The GNSS really proved its worth near the end of the project, Matheu reports. "For the last 400 feet of the project, the elevation of the toe changed; it actually came up 1 foot, so that changes the width of the slope because you're 1 foot lower in height." He adds that the company that built the 3-D model for the project, Take Off Pros of Peoria, AZ, incorporated this elevation change into the model.
Matheu adds that grade-checking the work without the GNSS would have required the use of stakes and a laser. "I would have had to have somebody in the water with a grade rod," he says. "At least one person who is a surveyor would have had to be out there.
"We're not surveyors; we would have used a laser level, but just due to limitations of my work force, I would probably have hired a surveyor to come out there," Matheu continues. "The problem with this work is that you don't put in a layer and then call somebody and have someone show up. "We probably would have put somebody on the payroll who was more skilled in surveying than any of us."