UAV-based smart rock localization for bridge scour monitoring

Since 2015, smart rocks with embedded magnets have proven promising to measure the maximum scour depth around a bridge pier. However, the field tests from a magnetometer installed on a truck-mounted crane moving across a bridge deck were time-consuming and caused a safety concern associated with traffic congestion. In this study, an unmanned aerial vehicle (UAV) is proposed as a mobile station for the measurement of magnetic fields of the Earth, a bridge, and a smart rock. The UAV was equipped with a three-axis, high-resolution magnetometer and a global positioning system (GPS). The magnetometer and the GPS were integrated to provide a synchronized measurement of magnetic fields and their corresponding coordinates. The effects of UAV speed, motor current and GPS accuracy on the accuracy of smart rock positioning were investigated in an open field. As an important parameter of the rock localization algorithm, the ambient magnetic field around a bridge site was numerically simulated under the Earth's magnetic field, taking into account the distribution of steel reinforcing bars in bridge piers. The computational results are in good agreement with experimental data with less than 3.75% in error. Six field tests at the Roubidoux Creek Bridge since 24 January 2018 has demonstrated an approximately 0.3 m accuracy of the smart rock positioning. During the February 2019 flood event, the existing scour hole was deepened by 0.4 m. The proposed UAV-based smart rock positioning method was compared favorably with the previous crane-based positioning method.