**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 piers 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 [1–4]. 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 on

the 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]. Cooley and Tukey [13] developed a fast Fourier transform (FFT) algorithm. Research on the bridge stability analysis and monitoring of the basis of this method has been conducted for many years [14–18]. In addition, many studies have been conducted to judge the state of the pier by its natural frequency. Sanayei and Maser [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. Nishimura [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. Haya 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. Samizo et al. [22], however, conducted research on a method for defining the natural frequency by measuring the microtremor of the existing bridge piers. Keyaki 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. Samizo 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. Abe and Nozue [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. Masahiro [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. 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

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problem of bridges. The safety diagnosis and inspection, however, are focused on the materials for

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. 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. **2. Test Set-Up** 

### **2. Test Set-Up**

#### *2.1. Specifications of the Full-Scale Model Pier and the Cheongnyangcheon Bridge Pier 2.1. Specifications of the Full-Scale Model Pier and the Cheongnyangcheon Bridge Pier*  The full-scale model pier used the spread foundation type to evaluate the aged foundations.

The full-scale model pier used the spread foundation type to evaluate the aged foundations. The pier foundation slab's dimensions were 5150 mm × 2420 mm × 50 mm (length × width × height), and the pier, which had a height of 4500 mm, was fabricated by repeating concrete pouring and curing with a height of 1500 mm for three times. The length and width of the pier were 4150 mm and 1420 mm, respectively. The pier foundation slab's dimensions were 5150 mm × 2420 mm × 50 mm (length × width × height), and the pier, which had a height of 4500 mm, was fabricated by repeating concrete pouring and curing with a height of 1500 mm for three times. The length and width of the pier were 4150 mm and 1420 mm, respectively. The target pier of the field test was a shallow foundation type and an unreinforced concrete

The target pier of the field test was a shallow foundation type and an unreinforced concrete structure. The length and width of the top of the pier were 3900 and 1350 mm, respectively, and those of the bottom of the pier were 4680 and 2130 mm. The total length of the pier was 7800 mm; 3000 mm of the pier's length was embedded under the ground. structure. The length and width of the top of the pier were 3900 and 1350 mm, respectively, and those of the bottom of the pier were 4680 and 2130 mm. The total length of the pier was 7800 mm; 3000 mm of the pier's length was embedded under the ground. Figure 1 illustrates schematic view of the pier and Figure 2 shows the target pier for the impact

Figure 1 illustrates schematic view of the pier and Figure 2 shows the target pier for the impact load test. load test.

**Figure 1.** Schematic view of the pier: (**a**) full-scale model pier and (**b**) field bridge pier. **Figure 1.** Schematic view of the pier: (**a**) full-scale model pier and (**b**) field bridge pier.

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**Figure 2.** Target pier for the impact load test. (**a**) Full-scale model pier and (**b**) field bridge pier. **Figure 2.** Target pier for the impact load test. (**a**) Full-scale model pier and (**b**) field bridge pier. natural frequency of a sound pier by using the pier height, the girder weight, and the earth covering

#### *2.2. Non-Destructive Impact Vibration Test Method* based on the natural frequency results of the piers that were derived through a series of tests. Eight accelerometers were used to evaluate the stability of the pier through the impact load test,

*2.2. Non-Destructive Impact Vibration Test Method*  The impact vibration test method can be used to evaluate the stability of the piers. The stability was evaluated based on the natural frequency that was derived when an impact was applied to the top of the pier in the pier length direction with a weight of approximately 0.3 kN. The impact vibration test method was proposed by Nishimura [20], who proposed a simple formula for the natural frequency of a sound pier by using the pier height, the girder weight, and the earth covering The impact vibration test method can be used to evaluate the stability of the piers. The stability was evaluated based on the natural frequency that was derived when an impact was applied to the top of the pier in the pier length direction with a weight of approximately 0.3 kN. The impact vibration test method was proposed by Nishimura [20], who proposed a simple formula for the natural frequency of a sound pier by using the pier height, the girder weight, and the earth covering based on the natural frequency results of the piers that were derived through a series of tests. and a weight of 0.3 kN was used to apply an impact load. Figure 3 presents the measuring equipment and the weight that were used in the experiment. In the full-scale pier model test, the surcharge load slowly increased from 0 to 250 kN by 25 kN (a total of 11 loads) to analyze the influence of the surcharge load. Here, the surcharge load simulated the weight of the girder. The field test was conducted for cases with or without a girder on the pier (Figure 4). In addition, the impact load test was conducted by simulating scour on one side of the ground that was

based on the natural frequency results of the piers that were derived through a series of tests. Eight accelerometers were used to evaluate the stability of the pier through the impact load test, and a weight of 0.3 kN was used to apply an impact load. Figure 3 presents the measuring equipment and the weight that were used in the experiment. In the full-scale pier model test, the surcharge load slowly increased from 0 to 250 kN by 25 kN (a total of 11 loads) to analyze the influence of the surcharge load. Here, the surcharge load simulated the weight of the girder. Eight accelerometers were used to evaluate the stability of the pier through the impact load test, and a weight of 0.3 kN was used to apply an impact load. Figure 3 presents the measuring equipment and the weight that were used in the experiment. In the full-scale pier model test, the surcharge load slowly increased from 0 to 250 kN by 25 kN (a total of 11 loads) to analyze the influence of the surcharge load. Here, the surcharge load simulated the weight of the girder. adjacent to the pier to analyze the effect of the scour on the pier (Figure 5). The bridge pier in lab tests simulated the embedded in bedrock condition, and bridge pier in-situ condition simulated the embedded in weathered soil. The weather soil's SPT N value ranged 6–8. The full-scale model pier simulated a shallow foundation embedded weathered rock, in addition, therefore, the tests were performed without scour case.

**Figure 3.** Equipment used in the experiment. (**a**) Data logger and (**b**) weight. **Figure 3.** Equipment used in the experiment. (**a**) Data logger and (**b**) weight.

**Figure 3.** Equipment used in the experiment. (**a**) Data logger and (**b**) weight. The field test was conducted for cases with or without a girder on the pier (Figure 4). In addition, the impact load test was conducted by simulating scour on one side of the ground that was adjacent to the pier to analyze the effect of the scour on the pier (Figure 5). The bridge pier in lab tests simulated the embedded in bedrock condition, and bridge pier in-situ condition simulated the embedded in weathered soil. The weather soil's SPT N value ranged 6–8. The full-scale model pier simulated a shallow foundation embedded weathered rock, in addition, therefore, the tests were performed without scour case.

summarizes the test cases.

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**Figure 4.** Pier of field experiment (**a**) With girder on the pier and (**b**) without girder on the pier. **Figure 4.** Pier of field experiment (**a**) With girder on the pier and (**b**) without girder on the pier.

**Figure 4.** Pier of field experiment (**a**) With girder on the pier and (**b**) without girder on the pier.

**Figure 5.** Simulated ground scour (scour depth = 1000 mm). **Figure 5.** Simulated ground scour (scour depth = 1000 mm). **Figure 5.** Simulated ground scour (scour depth = 1000 mm).

Figure 6 illustrates the test cases according to the impact direction. The accelerometers were attached to points 50 cm away from the top of the pier, a point 50 cm away from the bottom of the pier, and the center of the pier. Two accelerometers were attached to three points (a total of six accelerometers) to measure the acceleration in the bridge axis and pier length directions. Two accelerometers were attached to the outer surface of the pier to measure the acceleration in the pier length direction. An impact was applied to three points. The full-scale pier model test was repeated 33 times while considering the surcharge load, and the field test was repeated nine times. Table 1 Figure 6 illustrates the test cases according to the impact direction. The accelerometers were attached to points 50 cm away from the top of the pier, a point 50 cm away from the bottom of the pier, and the center of the pier. Two accelerometers were attached to three points (a total of six accelerometers) to measure the acceleration in the bridge axis and pier length directions. Two accelerometers were attached to the outer surface of the pier to measure the acceleration in the pier length direction. An impact was applied to three points. The full-scale pier model test was repeated 33 times while considering the surcharge load, and the field test was repeated nine times. Table 1 summarizes the test cases. Figure 6 illustrates the test cases according to the impact direction. The accelerometers were attached to points 50 cm away from the top of the pier, a point 50 cm away from the bottom of the pier, and the center of the pier. Two accelerometers were attached to three points (a total of six accelerometers) to measure the acceleration in the bridge axis and pier length directions. Two accelerometers were attached to the outer surface of the pier to measure the acceleration in the pier length direction. An impact was applied to three points. The full-scale pier model test was repeated 33 times while considering the surcharge load, and the field test was repeated nine times. Table 1 summarizes the test cases.

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**Figure 6.** Conceptual diagram for the impact and measurement directions. **Figure 6.** Conceptual diagram for the impact and measurement directions.



direction (outside)

#### **3. Results**

#### **3. Results**  *3.1. Mode Number Analysis for the Analysis of the Test Results*

Case-3

*3.1. Mode Number Analysis for the Analysis of the Test Results*  Prior to the full-scale pier model test and the field test, the eigenvalue of the pier was analyzed using Diana [28], which is a finite element software program, to analyze the behavior of the pier according to the impact direction. The finite element analysis only analyzed the mode number according to the direction of the impact load. Therefore, in order to reduce the variables in numerical analysis, the boundary condition between the pier bottom and the ground was set as a fixed condition. The size of the pier that was used for the analysis was the same as the full-scale pier. It was possible to analyze the behavior of the pier that corresponded to the first, second, and third Prior to the full-scale pier model test and the field test, the eigenvalue of the pier was analyzed using Diana [28], which is a finite element software program, to analyze the behavior of the pier according to the impact direction. The finite element analysis only analyzed the mode number according to the direction of the impact load. Therefore, in order to reduce the variables in numerical analysis, the boundary condition between the pier bottom and the ground was set as a fixed condition. The size of the pier that was used for the analysis was the same as the full-scale pier. It was possible to analyze the behavior of the pier that corresponded to the first, second, and third modes according to the eigenvalue as shown in Figure 7. The pier exhibited displacement in the bridge axis direction in the first mode and in the pier length direction in the second mode. In the third mode, the torsional

modes according to the eigenvalue as shown in Figure 7. The pier exhibited displacement in the

behavior of the pier was observed. According to the mode number analysis, the natural frequency corresponding to the second mode was caused by the impact in the pier length direction. In addition, the natural frequencies corresponding to the first and third modes were caused by the impact in the bridge axis direction. By analyzing the eigenvalue of the pier, the impact directions to derive the mode numbers of the full-scale model pier and the field pier could be selected. Based on this result, it was possible to determine the applicable impact load direction in the test, and full scale pier tests was establishment of the impact load test method to implement the pier behavior in the 1st–3rd modes. analysis, the natural frequency corresponding to the second mode was caused by the impact in the pier length direction. In addition, the natural frequencies corresponding to the first and third modes were caused by the impact in the bridge axis direction. By analyzing the eigenvalue of the pier, the impact directions to derive the mode numbers of the full-scale model pier and the field pier could be selected. Based on this result, it was possible to determine the applicable impact load direction in the test, and full scale pier tests was establishment of the impact load test method to implement the pier behavior in the 1st–3rd modes.

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third mode, the torsional behavior of the pier was observed. According to the mode number

**Figure 7.** Mode number analysis. (**a**) First mode; (**b**) second mode, and (**c**) third mode. **Figure 7.** Mode number analysis. (**a**) First mode; (**b**) second mode, and (**c**) third mode.
