# Study on Dynamic Behavior of Bridge Pier by Impact Load Test Considering Scour
## 1 Introduction  
The maintenance of railway bridges is required due to their special structures and socioeconomic roles. Therefore, technologies that can facilitate maintenance of the target performance based on reasonable inspections, measurements, evaluations, decision-making, repairs, and reinforcement procedures are required. However, the number of bridges that are damaged not only by structural problems but also by various environmental factors is gradually increasing. In particular, when flooding occurs, scouring occurs due to runoff in the ground adjacent to the bridge pier's crossing the river, causing the bridge to collapse. This not only seriously affects the safety of people, but can also cause enormous losses to society and economy over a long period of time. Additionally, it is reported that the first cause of the bridge failure is not the structural defect of the bridge, but the destruction of the foundation due to scouring around the pier during flooding [1; 2; 3; 4]. In general, railway bridges do not suddenly collapse; warnings are usually provided in advance. It is difficult to identify these abnormal signs via personnel-oriented irregular inspections or regular inspections at long-term intervals performed with inspection vehicles. Worldwide, studies have actively investigated the development of technologies for detecting the collapse of bridge piers in advance. In April 1987, a bridge collapsed at the Schoharie creek in New York, USA owing to the scour that occurred onthe bottom surface of the pier footing. This accident resulted in more than ten human casualties as well as significant economic damage, and a research fund of 11 million USD was supported for the scour alone. Since then, research has been supported on a national level, led by the National Cooperation Highway Research Program (NCHRP). In addition, the Federal Highway Administration (FHWA) prepared the technical manuals of Hydraulic Engineering Circular (HEC)-18 [5], HEC-20 [6], and HEC-23 [7] for bridge scour, river stability, and countermeasures, respectively, due to active research and evaluation programs since 1987. These manuals are being used for the analysis and design of the bridge scour. Most of these studies, however, are focused on bridges that are built on sandy soils; thus, the characteristics of scour on soils other than sandy soils have not been considered. For scour analysis, the formula proposed by HEC-18 has been widely used. It is difficult to apply the formula to soils other than sand because the formula was obtained on the basis of experiments conducted on sand. Recently, a method that considered the scour rate and the influence of time [8] was proposed for clayey soils, and a new approach that used the erosion index [9] was attempted for rocky soils. In the Netherlands, systematic and comprehensive research on the scour pattern has been conducted as a national project by Dutch Delta Works since 1953. This research project was led by the Ministry of Transport, Public Works, and Water Management as well as Delft Hydraulics. Delft Hydraulics derived a semi-empirical scour formula as a function of time and location. This was achieved by performing numerous laboratory experiments while considering a variety of variables that are related to the hydraulic properties of the flow and the scour materials. They prepared a comprehensive technical manual that is referred to as the Breusers-equilibrium method on the scour phenomenon. This is based on the average flow velocity and the relative turbulence intensity of the flow and the dominant characteristics of time for the maximum scour depth [10]. As mentioned above, many projects have been conducted on the stability evaluation for piers and many studies have also been conducted. In previous research, the scour effect was simulated where the actual scour occurred by numerical analysis [11; 12]. [PERSON] and [PERSON] [13] developed a fast Fourier transform (FFT) algorithm.