Robotic total stations and 3D laser scanners improve safety of construction and maintenance operations.
|Three robotic total stations automatically monitor a series of prisms positioned at key locations along the San Pablo Dam during a seismic upgrade construction project.|
The ability to conduct precision surveys using robotic total stations and 3D laser scanners—in conjunction with powerful processing and analysis software—is broadening the application of geomatics technology in the civil infrastructure arena. The following three examples demonstrate how such technology can improve safety during dam rehabilitation and tunnel construction and provide high levels of detail for effective operation and maintenance of bridges and roads.
Earthen dam monitoring
An advanced robotic total station monitoring system is helping East Bay Municipal District (EBMUD) monitor the almost 90-year old San Pablo Dam in California as part of a $55 million seismic upgrade. EBMUD supplies water and provides wastewater treatment for parts of Alameda and Contra Costa counties on the eastern side of San Francisco Bay in northern California. The San Pablo Dam is an earthen dam constructed in 1920 that impounds an 834-acre reservoir that can store 38,600 acre feet of raw water. The reservoir serves several important water supply functions including emergency standby storage, regulation of an aqueduct supply, and conservation/storage of local runoff from Bear and San Pablo Creeks.
According to EBMUD, a study completed in October 2004 showed that some of the soils and foundation that make up the dam are susceptible to liquefaction. If a 7.5 earthquake occurred on the Hayward Fault, the study predicted the dam would slump and decrease in height, allowing water to flow over the top, resulting in flooding downstream. The seismic upgrade includes expansion of the downstream buttress for the dam with a series of shear walls using a cement-deep-soil-mixing process.
Construction on the seismic upgrade started in August 2008 and is expected to take two years. Three robotic total stations are used for proactive monitoring and risk assessment of key points along the dam during construction. The system includes Leica Geosystems TCA1201 robotic total stations set up in permanent, environmentally protected huts at the project site. These total stations automatically monitor a series of prisms positioned at key locations along the dam. Data are relayed in real-time via a wireless connection from the total stations to Leica GeoMoS processing and analysis software. Data are easily accessed by EBMUD personnel and provide comprehensive information on displacement.
Leica GeoMoS is a multi-purpose, automatic deformation-monitoring software that, according to the company, can be used for structural deformation monitoring (dams, tunnels, bridges, high-rise buildings, construction), landslide and settlement detection (mining, rock falls, volcano slopes, subsidence), and automated surveys (continuous, automated measurements). The program stores all measurements and results in an open SQL database and allows local and remote access.
Leica GeoMoS software comprises two main applications called Monitor and Analyzer. Additionally, Leica GeoMoS Adjustment is an add-on software that allows users to make decisions based on statistically optimized and validated data. Monitor is an online application responsible for sensor control, data collection, computation, and event management. Analyzer is an offline application responsible for analysis, visualizations and post-processing of data. Adjustment is an application responsible for network adjustment, deformation analysis, and network simulation.
|A point cloud of the interior of a tunnel beneath Devil’s Slide made with a tunnel laser scanning system is visualized with the geotechnical Visualization Tool—gVT.
CREDIT: Jeramy Decker, Kiewit Corp.
Developed as part of a National Science Foundation Information Technology Research Initiative (ITR) project, the software, called "geotechnical Visualization Tool" (gVT), converts imagery of millions of rock-surface points—collected at a safe distance by a laser scanner—into an easily manipulated web of information. The data become a permanent digital record of the newly exposed material.
The scan data, at a resolution of 5 millimeters, provides information that the software program packages into visualizations incorporating as many as 10 meters of excavated tunnel. Engineers then use gVT to spot potential hazards to both the tunnel and the construction crews before weaknesses in the rock have a chance to trigger a collapse.
The information is so detailed that researchers can observe where rock layers are separating and how fractures are oriented. Researchers can even recreate sections of rock after they have fallen, providing a critical asset for determining where and how to safely drill. Because the data is portable, engineers can conduct all of the analyses from their home base at any time, far from the danger of the tunnel.
"Geologic maps have traditionally been made using manual measurements taken by geologists directly on the rock," said Joseph Dove, the lead developer of gVT at Virginia Tech and co-principal investigator on the ITR project. "Laser scanning is revolutionary for underground mapping because it allows direct collection of digital data in three dimensions at high resolution."
After a careful analysis of the scanned data, the engineers can take manual follow-up measurements to confirm their results.
"These 3-D visualizations enhance geological documentation and an engineer’s ability to make decisions," added Jeramy Decker, Ph.D., a graduate of Virginia Tech and co-developer of gVT working with Kiewitt Pacific Company, the construction contractor excavating the tunnels.
In use as part of a suite of private industry engineering tools and software critical to the tunneling beneath Devil’s Slide, gVT is the product of a two-year collaboration between civil engineers and computer scientists. The Devil’s Slide application is the first use of gVT in a true construction environment.
CalTrans manages more than 45,000 miles of freeway and highway lanes. A 3D laser scanner helps field crews collect detailed information quickly and safely.
3D scanning of bridges and roads
In 2008, CalTrans purchased its third high-definition laser scanner. The agency previously purchased two Leica ScanStation 2 instruments after conducting a study in conjunction with the Advanced Highway Maintenance and Construction Technology Research Center at the University of California Davis that aimed to develop standards for using laser scanning technology in CalTrans survey applications. The in-depth research project tested the features and abilities of four laser-scanning vendors’ technology during critical transport survey scenarios such as long-range surveys of dark asphalt and bridge spans.
"We have had some experience with laser scanning in the past and knew that it would be very beneficial for our work, but first we needed to have a solid idea of how the technology would perform under specific conditions," said Kevin Akin, a licensed surveyor and senior transportation surveyor with CalTrans’ Office of Land Surveying. "There has been little research done in the transportation field for laser scanning. We conducted our own research to develop a foundation for wide-scale adoption of laser scanning in CalTrans operations. We put several laser scanners through their paces to see how they would perform under our field conditions—particularly in high-heat conditions over 100° F."
According to Akin, the Leica ScanStation 2’s 270-degree vertical field of view helped sell CalTrans on the technology. "The larger vertical field of view with Leica’s scanner was one of the deciding factors for purchasing it," said Akin. "With that degree of view we can collect areas directly above the scanner like the under deck of a bridge or over ground utility lines.
"One of the most significant benefits of the Leica laser scanning technology is the ability to capture a level of detail we haven’t been able to collect before," said Akin. "That increased detail will benefit everyone during the planning, construction, and maintenance cycle. The ability to represent reality with point cloud data should allow everyone to make better decisions and communicate more effectively."
This article is based on information provided by Leica Geosystems (www.leica-geosystems.com), East Bay Municipal District (www.ebmud.com), and the National Science Foundation (www.nsf.gov).