Chesapeake Bay shoreline revetment project is just one recent example of using Global Navigation Satellite Systems for site surveying and machine control.
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, eliminate the need for time-consuming physical site
preparation, and even help construction machine operators to keep grading within
specification in real time. These systems utilize 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 3D model is loaded into the control
box via a secure digital memory card. During earthmoving, position data are
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 the 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 the ability to pinpoint 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
machine body levelness.
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 2D system. An additional 3D 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 of 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.
Surgery, Not a
Band-Aid
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 had begun 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 US Army Corps of Engineers financed 75% of the
project, with Dorchester County funding the remaining 25%.
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 US
Army Corps of Engineers, as well as for 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 just destroy
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 about 3 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 about 9 feet high, is
2:1.
GNSS Pays Huge
Dividends
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 to 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 number two 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 offsite 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 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 were “blind” but for the GNSS equipment
that marked the slope and elevation of the material
underwater.
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. 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 two-by-four 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.”
 |
Photo: Topcon Positioning Systems |
| GNSS provides the operator with a schematic that displays how close the elevation and grade are to the specification. |
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
number two stone with an average size of 2 to 3 inches. Placement of geotextile
fabric on this layer turned out to be one of the most difficult tasks on the
project.
Construction
Challenges
“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
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; 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 sized 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- to 1,200-pound secondary armor stone. The
final “primary armor” layer consists of a stone weighing 800 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 3D 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 workforce, 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.”