## 3 Field Measurements
### Overview
#### 3.1.2 Field Measurements of Ballasted Track on Serviced Railway Line  
The target section of this study is the ballasted track on a serviced railway line, and the dynamic behavior of the ballasted track is different for each railway line owing to the uncertainty of the material performance of the ballasts. Therefore, as the target section of this study is a serviced railway line, the possibilities of track irregularities and changes in the ballast state during the service of the line cannot be ruled out. Thus, the measurement of track geometry on the track under construction work is limited in terms of the reliability and accuracy of the measurements. The purpose of the measurement of the dynamic responses of the track in this study is to obtain the dynamic wheel load for comparison with the results of Wheel-Rail-interaction-force analysis through the dynamic modeling of the ballasted track. To analyze the effects of track irregularities on the dynamic wheel load, other parameters should be excluded. Therefore, to verify the adequacy of the dynamic wheel load according to the track irregularities derived from the Wheel-Rail-interaction-force analysis, representative dynamic responses obtained from a track free of irregularities and ballast degradation are required. In this study, the dynamic wheel load was measured in the newly constructed ballasted track within the service period (less than one year) after the construction of the track without irregularities, and the representative value of the dynamic wheel load of the ballasted track was calculated according to the train speed. As shown in Figure 8, the target track for field measurements was a double ballasted track on the high-speed railway line, the track shape was a straight line with a welded rail section of 60 kg/m. For dynamic-wheel-load measurements, a total of four measurement points were set at 2 m intervals. For the installation of the wheel-load sensor, a wheel-load gauge wired in eight directions was attached at an angle of 45\({}^{\circ}\) from the neutral axis of the inner and outer rail web, respectively, at a distance of 100 mm from the center, between railroad ties [2]. Figure 7: Railbed-settlement-monitoring (RM) sensor. (**a**) Overview of RM sensor installation; (**b**) Photo of a settlement pin. Most of the dynamic responses that occurred on the track by the running load of the rolling stock were high-frequency signals. Additionally, in the case of the wheel-load gauge attached to the rail, the measurement data included the effect of the current which flowed through the rail. Therefore, to prevent distortion and loss of the measurement data signals and to remove the signals of the noise components, a sufficient sampling rate (1.2 kHz) was set. For noise components, low-pass and high-pass filters were used in the fast Fourier Transform (FFT) analysis to increase the reliability of the data [2]. Figure 9 shows the results of the ballast particle-size test that was used to evaluate the state of the ballasted track after the track field measurements. The key track component that directly affected the track-support performance and the dynamic behavior of the ballasted track was the ballast. In the spring structure arranged in series that composed the ballasted track, the ballast with the softest spring stiffness had a dominant effect on the dynamic responses of the ballasted track. Therefore, to obtain dynamic responses in a ballasted track without irregularities, it is important to evaluate the state of the ballast. In the particle-size test of the ballast, which is generally used to evaluate its degradation state, the state of the ballast was determined to be acceptable if the particle size of the sample was included within the standard particle-size distribution curve. In this study, the ballast particle-size test was performed by collecting three sets of ballast samples from the target section. As a result of the particle-size test, all of the  
Figure 8: Overview of track field-measurement section and sensor installation.