# Development of Track Support Stiffness Measurement and Evaluation System for Slab Tracks
## 2 Overview of the Track Support Stiffness Evaluation System
### Development of Track Support Stiffness Measurement System
For the field test of the TSS tester, the adequacy of the TSS tester (load control ability, dynamic excitation ability, and loading rate) was evaluated, and the measurement accuracy (static and dynamic spring stiffness) was evaluated based on the measured data. Furthermore, the field test for performance evaluation of the TSS tester was conducted as an on-site test in the presence of an evaluator affiliated with an accredited test organization. The load control ability, dynamic excitation ability, and loading rates corresponding to the adequacy were evaluated according to the test standards laid out in the Korean Railroad Standard (KRS TR 0014-15 (R)) [11]. The field test view is shown in Figure 16[2]. We measured one rail at a time, as shown in Figure 16. It is possible to apply pressure to each rail using an actuator. As described in Figure 16, while the TSS tester is operated, the operator must grasp the center of the handle bar and move. The load was applied to the top surface of the rail through the wheel-shaped jig installed in the actuator. The loading test was performed by inputting static and dynamic loads, and the load and displacement measurement data acquired by the actuator sensor were collected remotely using a tablet PC. The maximum load was tested in the range of 1-68 kN considering the capacity of actuator and  
Figure 16: Photograph of field measurement. Figure 15: Photograph of the TSS tester. the Korean standard of the rail fastening system performance test (KRS TR 0014-15 (R)) [11]. The load waveform measured was shown in Figure 17[2]. Load values in the 0-90 kN range could be applied. Accordingly, a 68-kN load was considered in this study based on KRS. The applied dynamic load was coupled with a positive load measuring at least 20 kN. Figure 18b describes the load-displacement result obtained for 5-Hz loading frequency, whereas Figure 17b describes a sinusoidal loading trend. The static load modeled in this study is a linear increase function that maintains a constant loading rate from the initial load to the maximum load and checks whether the load is applied to the maximum load without loss or variation of the load through this field test. As a result of the analysis, in the repeated test condition by three times (three cycles), as shown in Figure 18a, a constant slope was obtained from the initial load to the maximum load. A linear load-time history function was implemented at a constant speed (68 kN load = 116.57 kN/min for 35 s) without any change in load. Therefore, it was experimentally proven that the system could satisfy the loading rate of the TSS tester specified in KRS TR 0014-15R (\(120\pm 10\) kN/min) [11]. The dynamic load history waveform, modeled by a sine function with an excitation frequency of 5 Hz, as shown in the dynamic test results in Figure 18b, is communicated remotely using a tablet PC during three cycles. The uniformity of the signals was experimentally verified. Accordingly, it was analyzed that the hysteresis function of the constant sinusoidal waveform could be realized without any loss of load or change of loading rate. The relationship between the load exerted on the track by the test load, and the resulting elastic displacement of the track was analyzed by the load-displacement curve. When rails are loaded using a TSS tester, the load is transferred to individual track components. The TSS can measure data to fit the load-displacement curve depicted in Figure 18b via application of the dynamic load patter depicted in Figure 17b. Accordingly, the track support stiffness of the target track can be evaluated and elastic materials and spring stiffness can be inferred. The dynamic load-displacement curve obtained  
Figure 17: Measured results of static and dynamic loads (sample). (**a**) Static load; (**b**) Dynamic load. Figure 18: Measured results of the load–displacement curve. (**a**) Static; (**b**) Dynamic.