SCOUR DETECTION AND RIVERINE HEALTH ASSESSMENT USING INFRASOUND

Abstract

Scour and river system health monitoring are serious problems for monitoring of civil infrastructure, bridge capacity and performing military route assessments in remote regions. Knowledge of how the overall system behaves over time is crucial for assessment of bridge foundations and barge navigation. This research focuses on the use of infrasound for remote scour detection and river system health monitoring. Previous research on a steel, through truss railroad bridge at Ft. Leonard Wood, MO proved the feasibility of acoustically detecting the fundamental modes of a bridge from tactical distances (> 20 km) using infrasound monitoring without on-site inspection of the structure. While bridges are periodically inspected as per requirement by law, these inspections only provide a discrete data point without addressing the cyclical nature of scour. Scour decreases the overall stiffness of the structure by weakening the foundation and changing the vibrational modes proportionally to the degree of scour. The scour portions of this research focuses on the same steel-truss bridge and analyzes 3D and 2D substructure models coupled with the superstructure reaction loads to assess the modal deformations within the infrasound bandwidth and the correlation to scour of embedment material. The 3D models indicate that due to the modal deformations 2D modeling can accurately determine the change in low infrasonic fundamental frequency passband of the coupled superstructure/substructure. Therefore 2D plain strain models of the Interstate 20 Bridge in Vicksburg, MS and Ft. Leonard Wood, MO, steel-truss were investigated, for two different subsurface soil types, as a representative deep foundation and shallow foundation cases, respectively. These results illustrate the degree of scour correlates well with changes in low fundamental frequencies and with measured field data indicating that remote scour detection using infrasound is viable for the dominant, lower-frequency bridge modes. In addition to the scour assessment, the research illustrates the use infrasound to investigate river system health during periods of record flooding. Events of the 2011 Mississippi River flood are presented herein in which the same infrasound arrays were able to not only detect and localize a barge collision with the Interstate 20 Bridge but also identify vortex eddies and changes to the river current not observed by seismic methods. Further, regions of increased force against the eastern bank related to changes in river flow were identified with direct impact to channel navigation and embankment erosion.