When rivers swell, the effects of attrition on bridge pier foundations can lead to instability, but it is impossible to visually check the state of foundations under flood conditions. It is also virtually impossible to carry out such checks by applying the method of determining natural frequency from the impact vibration caused by a weight, since impact vibration tests are generally difficult in terms of safety when water levels are high. By observing the minor vibration of bridge piers, therefore, the RTRI performed site surveys and tests to prove that the natural frequency of bridge piers can be determined with high precision from their minor vibration (Fig. 1).
   Based on this idea, the RTRI has developed a simple system using the minor vibration of a bridge pier to accurately evaluate the soundness of its foundation when the river level stands high. The system is composed of various portable components set at the bridge crown, including a vibration sensor, a processing unit to control measurement and to save, evaluate and transmit data, a water level gauge and a unit to receive and display evaluation results.
   The system automatically calculates the natural frequency of the bridge pier based on minor vibration data, and compares it with the standard natural frequency set in advance according to the conditions of the bridge pier, thus evaluating its soundness on a real-time basis (Fig. 2). The evaluation results are stored in the measurement-processing unit, and are shown on display units carried by maintenance workers, thereby decreasing work on bridges over swollen river water.
   This study was promoted with a subsidy from the Ministry of Land, Infrastructure and Transport.