# Study on Dynamic Behavior of Bridge Pier by Impact Load Test Considering Scour
## 1 Introduction
Research on the bridge stability analysis and monitoring of the basis of this method has been conducted for many years [14; 15; 16; 17; 18]. In addition, many studies have been conducted to judge the state of the pier by its natural frequency. [PERSON] and [PERSON] [19] conducted research on the static measurement by using a vehicle load to estimate the ground stiffness of a bridge foundation. When a 200 kN truck passed each bridge that was built on a pile foundation and a footing, respectively, the stiffness ratio of the measured and theoretical values according to the foundation type were compared. [PERSON] [20] introduced an impact vibration test method that measures the natural frequency of a pier by using the response waveform that was obtained by exciting its head with a weight of 300 N and it determines the stability from the changes in the natural frequency. [PERSON] et al. [21] examined the possibility of estimating the natural frequency of a spread foundation pier with a microtremor measurement. They also compared the results of the impact excitation experiment and the microtremor measurement for a field bridge, and they reported that it was impossible to measure the natural frequency with a microtremor measurement. [PERSON] et al. [22], however, conducted research on a method for defining the natural frequency by measuring the microtremor of the existing bridge piers. [PERSON] et al. [23] proposed a method that can identify the natural frequency by using the tremor measurement results alone without the impact vibration test results. [PERSON] et al. [24] developed a technique for evaluating the stability of the foundation. This was achieved by measuring the vibration of the pier using hydraulic power and analyzing the natural frequency change, and they also conducted research on soundness diagnosis indicators. [PERSON] and [PERSON] [25] proposed a soundness diagnosis indicator that has a correlation with the natural frequency through a model test and the verification by a field measurement. [PERSON] [26] proposed a statistical formula based on several measurement results by using the impact vibration test method. Japan's Ministry of Land, Infrastructure, and Transport [27] specified calculation formulas for each foundation type and the foundation soil type for railway maintenance standards, and they proposed a formula for the natural frequency of spread-foundation-type single-track piers. As mentioned above, the stability of bridge substructures is closely related to the safety problem of bridges. The safety diagnosis and inspection, however, are focused on the materials for the bridge as well as structural problems, and there is no quantitative evaluation method for the substructures. In this study, an impact load test was performed to analyze the effect of the surcharge load and scouring of the pier. This paper was focused on a bridge with shallow foundation and a plate girder deck because this is the most diffused typology in Korea. The full-scale model pier was built to analyze the effect of the surcharge load and confirm the mode shape and mode number of the bridge pier. Through the impact load experiment, it was possible to determine the three mode number of the pier according to the direction of the impact load. In addition, scouring was simulated using the pier of abandoned railway. The three mode number identified in the full-scale model experiment was derived and the effects of scouring were analyzed with natural frequencies.