Abstract:The current subgrade status monitoring for high-speed railway is mainly relied on track inspection car, and its testing data TQI (track quality index) is calculated by standard deviation. However, the flaw of this method is TQI data could only indicate the geometric status differences between the track and subgrade in the statistical scale, which couldn’t reflect the subgrade service status or the severity of the subgrade defects. Thus, supported by Shanghai-Nanjing intercity high-speed railway, on the basis of routine track inspection car, and combined with in-situ settlement observation data, the comparison between TQI and post construction settlement was conducted, aiming at revealing the correlation of their overruns value. Moreover, by verification from the detection of geological radar and wave velocity survey for subgrade defects, the studies on monitoring mechanism of subgrade service status were carried out. The testing data and analysis concluded as follows: there was no inclusion relation between the overruns of TQI and settlement, with no corresponds to respective maximum either. The TQI abnormality ratio in the settlement overrun and critical-overrun sections was 66.67% and 46.15% respectively, while the settlement overrun ratio in the TQI overrun sections was only 13.64%. Compared with the frequency and range of TQI overruns, the settlement overrun is more relevant to the frequency. Through the geological radar scanning and wave velocity detection in the testing sections, the subgrade defects were discovered with the existence of under structural cavities and hydraulic power channels. Beyond conventional track inspection car method, this study is a new approach for the comparison and filtration mechanism by organically aggregating TQI, settlement and under scanning, which is more consistent with the practical engineering status compared to the previous TQI oriented assessment method. This integrated mechanism would effectively facilitate the validity of the high-speed railway subgrade service status monitoring and provide theoretical foundation for the credible judgment of subgrade defects.