# Ballasted Track Status Evaluation Based on Apparent Track Stiffness Index
## 5 Conclusions  
In this study, the apparent track stiffness measured using LFWD and a track stiffness index combining the mean and standard deviation of the apparent track stiffness were introduced to propose a new track status evaluation standard. The correlations between the track stiffness index, TQI, and cumulative tamping length were analyzed. The conclusions are as follows:  
1. The investigation of the apparent stiffness distribution for each substructure confirmed that not only the rail, sleeper, and track stiffness but also substructure characteristics were reflected in the apparent stiffness. Therefore, it is necessary to classify sections by substructure to evaluate the track status based on apparent stiffness. 2. The correlation among the average apparent track stiffness, TQI, and cumulative tamping length was found to be very poor, whereas the correlation with the track stiffness index combining average and standard deviation was very high. Therefore, the average apparent track stiffness and standard deviation must be considered simultaneously to properly evaluate the status of the track. 3. Based on the track stiffness index, TQI, and cumulative tamping length correlation, the standards for evaluating the track status in terms of five levels using the track stiffness index were presented. The standards vary with the substructure type. For bridges, the stiffness index values for each level are 184, 128, 81, and 51 kN/mm. For tunnels, the stiffness index values for each level are 137, 87, 50, and 28 kN/mm. For earthwork, the stiffness index values for each level are 90, 67, 47, and 33 kN/mm. 4. Based on the verification of the evaluation standard presented on the basis of additional field surveys, it was found that the status of the track based on the track stiffness index and that based on the TQI showed consistency for the earthwork sections. For the bridges, the correlation between the track stiffness index and TQI was relatively low, and the evaluated track status was also inconsistent. 5. On the other hand, the status of the track based on the track stiffness index and that based on the cumulative tamping length showed consistency for one of the earthwork sections and a bridge section. This means that the stiffness index can reflect not only TQI but also the cumulative tamping length. Besides this, it is thought that inconsistency can be overcome when the correlation increases by accumulating more data through additional field surveys. The extent to which the characteristics of the substructures are reflected in the apparent stiffness has not been clearly defined in this study. Therefore, there is a disadvantage in that evaluation has to be performed for each substructure to obtain the status of the track. Further studies are need to clarify the mechanism of apparent stiffness so that a unified track status evaluation standard can be formulated for all substructures. More statistical analysis is needed to understand the nonlinearity and low correlation. Conceptualization, Y.-T.C., S.H.H. ; data collection, B.P., Y.-T.C., S.H.H. ; data analysis, B.P., Y.-T.C. ; manuscript writing, B.P. ; manuscript review and editing, Y.-T.C. ; funding acquisition, S.H.H. All authors have read and agreed to the published version of the manuscript. This research was supported by the Ministry of Science and Information and Communications Technologies, Korea.