The Concrete and the Clay

August 1, 2002 By: Matteo Luccio GPS World

Monitoring Large-Structure Deformation

Every structure moves, if only imperceptibly to the unaided human eye. Its “natural frequency” — the frequency at which it naturally wants to oscillate — depends on such factors as its geometry, the materials with which it was built, and the soil on which it rests. However, when a structure moves by an abnormal amount (for example, in an earthquake), those responsible for it need to know, especially if it is a very large one — say, a dam, a highway bridge, or a a skyscraper. To monitor the deformation of such structures engineers increasingly use GPS-based systems.

Baseline Performance. According to Jeff Behr, president of Orion Monitoring Systems, Inc., of Salt Lake City, Utah, his company often utilizes pre-existing data from structural engineers and facilities managers to assess baseline structural performance and to help identify optimal locations for GPS monitoring stations. Common data sources are traditional surveying techniques as well as extensometers, inclinometers, tiltmeters, and other geotechnical instrumentation. Although these techniques can be more accurate, Behr believes the great strength of GPS monitoring is its continuous nature, which allows real-time positioning as well as detailed indications of long-term performance. This is especially so when the alternative is manual surveying done at annual, semi-annual or monthly intervals. “With continuous GPS, the data are there when needed.”

Similarly, Dr. Barry Hillman, president of Condor Earth Technologies, Inc., in Sonora, California, points out that non-GPS-based measurement techniques and mathematical models used to help approximate the baseline behavior of a structure only provide periodic information on its deformation, which can be inconsistent or inadequate for establishing the true motion taking place. For this reason, his customers choose real-time, GPS-based monitoring to independently confirm the motion.

Structures such as dams, buildings, and bridges, he adds, are constructed to design parameters that have been established through engineering and mathematical models. Typically, some motion detected by other instrumentation has led the owner to want an independent assessment for confirmation and to determine whether remediation is necessary. “The fact that GPS is very accurate and all solid state and does not require calibration often provides a much greater degree of confidence in the actual reported motion than any other instrumentation,” says Hillman.

Seasonal Effects. Once long-term, continuous GPS-based deformation monitoring of a structure begins, diurnal or seasonal effects readily appear as background noise in the numerical charts, tables, and graphics displayed by the system, according to Hillman. He says that true motion is apparent as a departure from this background noise. Behr, however, on the basis of his experience with crustal deformation monitoring, believes that the signal of interest may be masked by large noise signatures. He adds that seasonal effects, often in the form of thermoelastic deformation, can be removed through spectral methods.

As an example, Behr points to a 3-year project to monitor Pacoima Dam, in California. The data indicated that the dam was experiencing an annual cycle of upstream–downstream deformation of approximately 18 millimeters peak-to-peak amplitude. Although the nearest temperature record his team could find was from a location nearly 20 kilometers to the south of the dam, a strong coherence existed between this daily, “regional” temperature and daily dam displacement.

An impulse–response function was derived and then convolved with the temperature record to provide a time series of predicted displacement that was a strong fit to the real displacement data. Behr and his colleagues then subtracted the predicted displacement from the GPS-measured displacement to produce a record of residual displacement with thermoelastic signals removed. This allowed the recognition of millimeter-level displacements in response to reservoir loading.

Crustal Motion. Depending on its structure and intent, the monitoring program can also include crustal motion. For example, in the case of a dam located near a fault, the owner will be interested in assessing the true movement of the dam versus the crustal motion. This can be accomplished by establishing multiple base stations and then simultaneously monitoring baselines to the target structure as well as between base stations. It is then possible, according to Hillman, to determine the relative motion of the base stations and the true motion of the structure. Behr adds that processing which includes regional reference stations (CORS) can be effective in resolving regional crustal deformation, and that this will become more valuable as these stations migrate to real-time in the coming years. This is often a solution he proposes for monitoring base station stability.

Multiple Receivers. Depending on the structure it is monitoring and its owners’ concerns, Hillman’s company uses multiple receivers on the target structure as well as multiple base stations. The multiple receivers on the target structure give them greater assurance that they are accurately monitoring its motion. Establishing two or more base stations off the structure and then running multiple baselines to the target structure, as well as between base stations, ensures that motion is isolated to the target structure as opposed to movement of a base station. All but one of his company’s 14 installed systems uses a multiple-receiver network.

Best of Both Worlds. Most monitoring systems are designed to enable both real-time monitoring and post-processing of data. Typically, owners are interested only in real-time data unless some unusual movement outside of user-defined parameters triggers alarms. When this occurs, the monitoring engineers can “play back” over any period of time the data collected by the instruments and post-process it.

“Our real-time accuracy, both horizontal and vertical,” Hillman says, “is of a few millimeters, a typical requirement. We can confirm this by post-processing if necessary.” Behr’s company’s software is enabled for real-time and/or static positioning, with intervals from a few minutes to 24 hours. His company also provides post-processing services using software that allows for improved orbits and earth-orientation parameters, particularly suited for high-precision long- baseline monitoring.

As for the trade-off between the immediacy of real-time data and the higher accuracy of post-processed data, Behr’s company tunes its systems to its customers’ monitoring needs. “For tall buildings, the critical need is for high-frequency data that will describe a building’s performance during and after periods of strong deformation, usually seismic shaking,” he says. Ultimately, he adds, his company’s software’s ability to sub-sample the real-time data stream for simultaneous short-period static processing allows his customers both real-time peace-of-mind as well as a long-term, higher-accuracy history of baseline structural performance.

Sampling Rate. For buildings, says Hillman, the solution rate must be very high (20 Hertz), but for volcanoes it can be much lower (5 minutes). Orion has effectively instrumented three ~40-story buildings using real-time GPS with 10 Hertz positioning rates. High frequencies, are best suited to capture the dynamic deformation of long-period structures, such as tall buildings and long-span bridges, while low frequencies are best suited for slowly or impulsively deforming structures, such as earth-fill dams and hazards such as landslides.

Manufacturers
The receiver at Libby Dam was manufactured by Ashtech Precision Products (now Thales Navigation) and installed by Condor Earth Technologies. However, Condor recently signed an agreement with Trimble to use exclusively Trimble products from now on. Condor deploys its own, proprietary 3D Tracker software. Orion Monitoring Systems’ proprietary monitoring software package is called InteTrak. The software currently handles receivers from Thales, CMC, Trimble, Javad, and Novatel. Orion is presently working with Thales-Ashtech for its GPS hardware needs.

Matteo Luccio is GPS World’s managing editor.

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