3D Scanning: An enabling technology

April 2013 » Features » PROJECT CASE STUDY
Adding a third dimension to bridge surveys and dam monitoring helps firm expand its services portfolio.
Mary Jo Wagner
3D scanning surveys provide a holistic view of a structure by collecting a point cloud that shows every element plus features that may be wanted in the future.

While many 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 do it with regularity, and for good reason: It is how they have been able to capitalize on enabling technologies. And in turn, it has afforded service providers the opportunity to both refine their product offerings and develop new, customized business applications.

Project
Hildebrand Lock and Dam Survey, Morgantown, W.Va.

Participants

U.S. Army Corps of Engineers TerraSurv Inc.

Product application

TerraSurv uses a Trimble GX 3D scanner and RealWorks software to scan each of the dam's seven pier walls and model geometrical anomalies in the gate openings.

TerraSurv Inc., a specialized survey and design firm based in Pittsburgh, is one such example of this cooperative effort. As a primary geodetic survey provider to USACE's Pittsburgh District, TerraSurv has successfully used advanced survey technology to establish dam monitoring and bridge survey work as a mainstay of its business.

TerraSurv's acquisition of a 3D scanner, in particular, 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.

Displacement of a different kind
USACE surveyors and engineers have routinely inspected the District's 39 reservoir and navigation structures every one to three years, and they have routinely adopted technological tools and software solutions to help them monitor, inspect, and maintain the integrity of the aging infrastructure. However, a 2005 downsizing of the surveying section pushed the district to begin outsourcing much of its surveying and mapping tasks.

USACE issued an architect engineering (AE) services tender and awarded TerraSurv the task of collecting deformation surveys for its locks, dams, and reservoirs. Although a TerraSurv team had been using a combination of Trimble GNSS receivers, total stations, digital levels, and controllers to collect and streamline the deformation survey process, one particular dam required a different approach.

A new monitoring question
In 2006, one of the seven remote-controlled gates of the Hildebrand Lock and Dam (Hildebrand) on the Monongahela River southwest of Morgantown, W.Va., suffered a gate malfunction during a lift maneuver, causing a chain mechanism to break. USACE engineers wanted to confirm if any undetected movement had caused any of the piers to lean or tilt.

Hildebrand Lock and Dam, constructed between 1950 and 1960, is one of nine navigation structures on the Monongahela River between Pittsburgh and Fairmont, W.Va.

Initially, Terrasurv used a total station to take discrete shots of the pier wall in 2007. After review of the initial results, USACE decided that further monitoring would be desirable and consulted with John Hamilton, civil engineer and principal of Terrasurv, to establish a monitoring plan. Hamilton suggested using 3D scanning technology.

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. Setting up on previously measured alignment pins, the team scanned each of the dam's seven pier walls to collect a dense set of surface coordinates for each of the pier faces at several different epochs, and acquire 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.

The team used Trimble's RealWorks software to georeference the point clouds, remove noise and extraneous data, and extract inter-pier half-foot distances at select elevations to produce an as-found view in which to compare four subsequent scans. After each scan, a 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.

To ensure it had remained sound, USACE requested TerraSurv to scan the Hildebrand at set intervals three more times – once in 2009 and twice in 2010; the last scan was completed in July 2010. Each time, the data has confirmed that the Hildebrand remains structurally sound.

"Surveys that require a holistic view of a structure are ideal for scanning," said 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. That will likely save you a trip back out in the future."

Floating a new technique
Avoiding future returns to the field was a primary reason for including the scanner in TerraSurv's two-fold task in December 2010 to survey seven bridges over the Cheat River in Parsons, W.Va., and collect 50 stream channel cross sections. USACE contracted TerraSurv to perform the work as part of a flood damage reduction study.

The crew first used a combination of Trimble R8 GNSS receivers, a Trimble S6 total station, and Trimble Dini 12 digital levels to establish all the necessary control, and then focused on scanning the bridges. Ranging in distances between 65 to 164 feet, they established a minimum of one upstream and one downstream scan station along the riverbanks to survey each entire bridge, and in two days, the team had acquired point clouds of all seven bridges.

For the cross sections, the team set the total station at each predefined section point along the riverbank, and a team member either waded into the river with a rod or two members worked from an inflatable boat, dropping the rod off the side to enable the total station to lock onto the prism and take a measurement.

After the Hildebrand Lock and Dam suffered a gate malfunction during a lift maneuver, USACE engineers wanted to confirm if any undetected movement had caused any of the piers to lean or tilt.

The survey data was downloaded into Trimble Geomatics Office Software for processing and the scanning data was loaded into Trimble RealWorks. Using 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 into their Microstation software.

Setting up on previously measured alignment pins, the TerraSurv team scanned each of the dam's seven pier walls to collect a dense set of surface coordinates for each of the pier faces.

"As a vendor-neutral agency, we use many different software systems, and have struggled in the past with the proprietary data formats of 3D scanning technology," said Steve LeBlanc, PLS. "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 HEC-RAS (Hydrologic Engineering Centers River Analysis System), USACE 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," said 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 the success both in the field and in the office, 3D scanning is becoming a core technology for bridge and hydrologic surveys.

 

Mary Jo Wagner is a Vancouver-based freelance writer with 20 years of experience covering geospatial technology.

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