Technical Papers

Critical Infrastructure Monitoring with Global Navigation Satellite Systems

Abstract

This paper documents measured deformation within four global positioning system (GPS) networks deployed on critical, heavy-engineered infrastructure in real time over a combined 5-year period. The first is an ∼2-km, four-lane floating freeway that deforms daily in response to temperature and traffic loads and seasonal lake-level variation. The second is a 6-lane elevated freeway ∼1 km in length that has subsided unevenly and discontinuously since it was damaged by the 2001 (MW 6.8) Nisqually earthquake. Two additional structures comprise ∼300-m-long, earth-filled dams forming major reservoirs (Howard A. Hanson and Tolt dams). Real-time kinematic processing of high rate (1s or 5-s epochs) GPS observations over short baselines (0.1 to ∼1 km) permits continuous deformation monitoring at centimeter-level accuracy, whereas long-term deformation was measured at subcentimeter accuracy through postprocessing of 24-h observations. The floating freeway showed 60 cm of annual vertical displacement and a 1.4-± 0.4-cm short-period oscillation that correlates with traffic and temperature but no response to bridge-perpendicular winds gusting to 74 km/h (40 knots). Along the elevated freeway, 4–7 mm/year of lateral displacement and 4.3 ± 14 mm/year of subsidence is observed. At Howard A. Hanson Dam, daily measurements show a slow response at the southernmost portion of the dam to the filling with water of a 30-m-deep pit excavated adjacent to the dam. The fourth structure, Tolt Dam, is stable but shows a vertical response to reservoir level and appears to isolate its GPS receivers from a well-documented regional hydrological signal.