###### Abstract
Despite the recent developments of the MDD for checking residual displacements in road pavement, subgrade layers and ballasted railway [14; 20; 21; 22], the characterization of concrete slab track layer deformation using the MDD has not been clearly analyzed yet and is still a challenging problem. In this study, the Global Navigation Satellite System (GNSS) and a Multi-Depth Deflectometer (MDD) were combined to introduce a new system that can be used to assess and monitor the slab track settlement accurately in the transition zone of railways. To achieve this, GNSS receivers were placed along the track: one for the base station placed beside the track and the other for the rover station placed on top of the slab track. The MDD was installed exactly below the rover station position on the track. The MDD was designed to measure the deflection of the track at various depths which was loaded by railway cars. This combined GNSS/MDD system provided data on the track foundation layers beneath the track with top displacement of the track at the rover station position. The most notable concept of the new system is that the MDD head is fixed at the top of the slab track, not at the bottom of the MDD as usual (Figure 1). Since the MDD head is fixed at the rover position, the MDD/GNSS system can measure the relative deflection between each track layer and the slab surface, but also the total deflection at the top of the track slab by the GNSS' rover receiver, which can evade the fixed condition problem of classic MDD. As shown in Figure 1, it is evident that the integration of MDD with GNSS technology offers greater advantages than the classic MDD measurement system. Generally, the classic MDD system is fixed at the bottom end point for measuring displacement at the surface layers of the track or the pavement layers; however, if the bottom end point moves together with the surrounding soil, the measured soil deflections become meaningless. In contrast, the new system can overcome this fatal drawback of classic MDD. In the new MDD/GNSS system, a fixed reference point is established at the slab surface for the MDD while the net settlement increase is measured by the GNSS. This combination of the MDD with GNSS enhances the capacity to assess vertical displacements within each layer of the system and provides a more comprehensive perspective on the track monitored system. The new MDD/GNSS system is installed at the transition zone between a tunnel entrance and its embankment, which experienced high settlement levels prior to repair (Figure 2). The system is used to monitor whether the repaired concrete track foundation with pressurized cement grouting is stabilized effectively and what track layer position is most unstable and contributes most to the total deflection. Figure 1: Comparison of the new MDD/GNSS system with classic MDD. Figure 2: The MDD/GNSS monitored site of the transition zone near the Sinri Tunnel in Jeonju district, Republic of Korea. Since the obtained measured data from the MDD/GNSS are somehow scattered, it is necessary to process the obtained time series settlement data using the Locally Weighted Scatterplot Smoothing (LOESS) trend model, which is a non-parametric regression method used for modeling and smoothing the relationship between two variables. This method fits a smooth curve to the data points in a scatter plot and offers more weight to the points that are close to the target value being estimated, which makes it useful in cases where the data may contain outliers or other types of noise [23]. Furthermore, OriginPro(r) 2018, a data analysis and graphing software developed by OriginLab [24], is adapted to propose prediction models for the MDD position data, which is calibrated using GNSS data. The results of this study demonstrate an optimistic potential for using a combination of redesigned MDD and GNSS technology for monitoring and predicting settlement in concrete track systems.