Feature: A 3D Shift in Monitoring
Professional Surveyor Magazine - February 2012
The U.S. Army Corps of Engineers partners with a Pittsburgh-based survey and design firm for dam monitoring and bridge survey work—under challenging circumstances—using high-tech survey equipment.
By Mary Jo Wagner
hile many people talk about wanting to collaborate with colleagues or contract partners, the engineers and surveying professionals at the U.S. Army Corps of Engineers’ (USACE
) Pittsburgh District actually mean it, and they do so with regularity. In some respects, they have to—budget cuts have reduced in-house resources—but their commitment to cooperation and partnering goes beyond necessity: it is how they have been able to capitalize on enabling technologies. And, in turn, it has afforded service providers the opportunity to refine their product offerings and develop new, customized business applications.
., a specialized survey and design firm based in Pittsburgh, is one example of this cooperative effort. As a primary geodetic survey provider to USACE’s Pittsburgh District, TerraSurv has overcome many surprises in the field and, in the process, has proven the advantages and benefits of using high-accuracy digital levels, GNSS units, total stations, and 3D scanning for structural surveys. TerraSurv has, in fact, validated the technology so well that the firm has established its dam monitoring and bridge survey work as a mainstay of its business.
“We have a comprehensive range of highly productive, precise surveying technologies that allows us to select the right instrument for the job,” says John Hamilton, civil engineer and principal of TerraSurv. “That range provides us with a strong position to continue to deliver on our specialized deformation and bridge surveys and extend our service portfolio to create new business.”
TerraSurv’s acquisition of a 3D scanner, in particular, has provided both the platform to extend its services’ portfolio and the opportunity for USACE to consult with TerraSurv to collect field data and acquire business-specific, 3D-derived data for their engineering tasks.
Dam Deformation Monitoring
Known as the Headwaters District, the Pittsburgh District comprises the Ohio, Allegheny, and Monongahela rivers and covers an area of approximately 26,000 square miles (67,000 square kilometers). The District manages 16 flood control reservoirs as well as 23 navigation locks and dams, some of which are nearly 100 years old.
USACE surveyors and engineers have, every one to three years, inspected The District’s 39 reservoir and navigation structures, and they have routinely adopted technological tools and software solutions to help them monitor, inspect, and maintain the integrity of the aging infrastructure. However, when financial constraints in 2005 downsized the Pittsburgh surveying section, USACE had to begin outsourcing much of its surveying and mapping tasks to support its in-house needs. Of particular concern was ensuring continuity for its dam safety and structural monitoring program.
USACE issued an architect engineering services task order for TerraSurv to perform deformation monitoring surveys for its locks, dams, and reservoirs. “Prior to 2005, we had established baseline surveys and established collection methods, but we wanted to use the contractual opportunity to assess how we could incorporate new technologies into our deformation monitoring program,” says Steve LeBlanc, PLS. “So, as part of the contract we solicited TerraSurv’s input in creating a deformation monitoring program for the District’s 16 flood control reservoirs.”
Based on USACE’s requirements, Hamilton and the TerraSurv team began using a combination of Trimble GPS receivers, DiNi 12 digital levels, and an S6 total station to collect data and streamline the deformation survey process. “With our advanced survey technology, we can measure movement in three dimensions,” says Hamilton. “And we can definitively determine if any individual monolith is shifting upstream or downstream or left or right. Charting movement over time—no matter how small—allows you to assess the soundness of structures and develop strategies for areas needing attention.”
Scanning for Tilt
Sometimes, project strategies need to be developed faster than others, such as the one needed for the Hildebrand Lock and Dam in 2006. Operational since 1960, the Hildebrand is one of nine navigation structures on the Monongahela River between Pittsburgh and Fairmont, West Virginia. A gated dam, the Hildebrand uses a series of seven remote-controlled gates hinged into the body of the piers to control water-flow volumes.
Typically, USACE has been concerned with monitoring the dam as a whole for deformation, requiring TerraSurv crews to use their level, total station, and Trimble TSC2 Controller to collect measurements at set punch-mark pins on the downstream side of each pier, process each pin point, and plot its offset position on a spreadsheet graph. That graph of points, which shows the position of each point and its movement upstream or downstream along a reference line, enables USACE engineers to study movement over time and determine if the structural integrity of the dam is at risk.
In 2006, however, a new monitoring dimension was added to the Hildebrand: tilting. During a lift maneuver, one of the gates jammed, causing the chain mechanism to break. At issue was whether some undetected movement had caused any of the piers to lean or tilt. USACE consulted with TerraSurv to determine a survey approach to help analyze the three-dimensional alignment of the piers.
Hamilton suggested 3D scanning technology. “Though the digital level and total station are precise and efficient, this particular survey required a much denser point pattern of the whole wall, not just singular points,” says Hamilton. “With a scanner, we could collect millions of points to create a real-world view of each pier wall and the gate guides. Then we could compare one survey point cloud to another and determine if the top of the walls are leaning or moving. It’s a much better method to answer the tilting question.”
Point Clouds for Pier Walls
In July 2008, a TerraSurv crew brought its Trimble GX 3D scanner to scan each of the Hildebrand’s seven pier walls and then model any inconsistencies or geometrical anomalies in the gate openings. The crew set the scanner up on the river wall and also on each pier, occupying the same alignment pins previously measured by the S6 and enabling them to take an orientation scan and digital photos of the adjacent pier faces. The face of each adjacent pier was then selected in the imagery by creating a polyline around the edges so they could program each scan into the scanner.
Setting up on each pin, the team scanned each pier wall to collect a dense set of surface coordinates for each of the pier faces at several different epochs, as well as a denser scan of each gate guide. Within three hours, the crew had acquired a point cloud of each pier wall and gate guide, totaling 437,378 points.
Back in the office, CAD technician Todd Lubic loaded the scan data into Trimble’s RealWorks software to georeference the point clouds, remove noise and extraneous data, and extract inter-pier, half-foot distances at select elevations, providing them with an as-found view in which to compare four subsequent scans. After each scan, the team provided USACE with the point clouds and a CAD-based graph of gate-opening measurements at each half-foot elevation, which indicated that some of the openings were converging at the top of the guide. However, the 3D intelligence confirmed to USACE that the Hildebrand had not suffered significant movement and remained sound.
“Surveys that require a holistic view of a structure are ideal for scanning,” says Hamilton. “Traditional survey tools would require us to measure every feature individually to create a dense pattern of points, creating more risk that a feature will be missed. With the scanner, you collect a point cloud that shows every element you wanted plus features you may want in the future. It’s more comprehensive and will likely save you a trip back out in the future.”
Avoiding future returns to the field was a primary reason for TerraSurv to include the Trimble GX scanner in a West Virginia project last year. It required a two-fold task to survey seven bridges over the Cheat River and collect 50 stream channel cross sections, in near-freezing winter conditions.
Formed at Parsons, West Virginia, by the confluence of the Shavers and Black Forks, the 78-mile long Cheat River is a favorite of whitewater rafters and vulnerable to record-high flooding. In 2010, USACE initiated a flood-damage-reduction study to identify and prepare a flood-protection plan for the area. As part of this campaign the agency contracted TerraSurv to measure and model the bridge openings as well as acquire 50 cross sections within a 25-square-mile area.
“Our traditional techniques would have required TerraSurv to survey each bridge and its features such as bridge openings and top deck elevations and record the measurements on hard-copy data sheets,” says LeBlanc. “We determined to proceed with scanning to get the detail of the required areas we needed.”
On a face-numbing day in December 2010, Hamilton and crew traveled to Parsons with their entire toolbox of survey technologies. The team used up to four R8 GNSS receivers a day to establish the primary horizontal and vertical control network and created a secondary control network for the cross sections using a combination of static GPS, RTK-GPS, and the S6 for traverse. They also used the DiNi 12 digital level to run a line of levels along the Black Fork.
Taking advantage of low water levels, the crew first focused on scanning the bridges. Ranging in distances between 65-164 feet (20-50 meters), they established a minimum of one upstream and one downstream scan station along the riverbanks to survey each entire bridge. However, the scanner’s exposure to the frigid elements began to adversely affect its operation. The crew quickly resolved this by wrapping a standard heating pad around the instrument head, and, in two days, the team had acquired point clouds of all seven bridges.
Completing the stream channel sections, however, was not as swift due to frequent high water levels.
When it was safe to work, crews uploaded USACE-provided, pre-defined shapefiles of each section point into their TSC2 and then navigated to each line. Setting the S6 at each point along the riverbank, a team member would wade into the river with a rod, or two members would ride in an inflatable boat with one dropping the rod off the side to enable the total station to lock onto the prism and taking a measurement. It was a process they repeated until finally collecting the last cross section in May 2011.
The survey data was downloaded into Trimble Geomatics Office software for processing, and the scanning data was loaded into RealWorks. The TerraSurv team cleaned the point cloud dataset and extracted specified measurements such as the bridge length and width, waterway openings, and width, height, and top elevation of all piers and abutments. They also created a 3D surface model of each bridge, including views of the bridge piers and abutments, and formatted the models in CAD to allow USACE engineers to ingest that data with their Microstation software.
“As a vendor-neutral agency, we use many different software platforms and have struggled in the past with the proprietary data formats of 3D scanning technology,” says LeBlanc. “Our hydrologic modelers and engineers worked closely with John’s team to successfully resolve these incompatibility issues to create deliverables that we could readily work with.”
Using their HEC-RAS (Hydrologic Engineering Centers River Analysis System), engineers use the customized 3D data to build hydrologic models and produce inundation maps that show flood level limits, allowing engineers to identify flood risk areas and create effective mitigation strategies for the future.
“Previously, I could only view and study individual survey points of bridge features,” says Huan Tran, a hydraulic engineer at USACE. “Converting a point cloud to a 3D surface model allows me to view the entire structure within Microstation, and I can measure any feature I need, like elevation, the thickness of the piers, or the low chord/high chord of the bridge. That amount of detail enables me to create more precise models.”
Based on success both in the field and in the office, 3D scanning is becoming a core technology for bridge and hydrologic surveys. Partnerships such as this between the USACE and TerraSurv show us how that technology can benefit everyone involved.
Mary Jo Wagner is a Vancouver-based freelance writer with more than 15 years experience in covering geospatial technology. She can be reached at firstname.lastname@example.org.
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