## 3 Influence of NFTBT Structure's Size Parameters  
### Influence of Length of Track Slab  
After the greening and paving are completed, in the tram running stage, the NFTBT's structure has the combined effect of the tram load, its self-weight, and the paving layer's weight, the track slab displacement under different slab length conditions is shown in Figure 12, and the displacement measurement points are shown in Figure 8.  
According to Figure 12, when uneven settlement occurs in the subgrade part of the track structure, under the combined action of the tram load and the uneven settlement, as the slab length increases, the displacement of the middle part of the NFTBT is similar. The surface displacement is different under different slab lengths at the main settlement of the subgrade. However, as the length of the track slab increases, the displacement of the edge of the NFTBT will gradually decrease, and the reduction from the minimum slab length to the maximum slab length can reach 1 mm. The reason for this is that the track's structure is not strong when the slab length is short, which in turn causes the track's geometry to be more affected by the settlement of the roadbed, which is not conducive to maintaining the stability of the track's geometry. At the same time, considering the load diversity of tram operations, the minimum slab length of the NFTBT should be limited to ensure the stability of tram operations. To compare and analyze the structural stress of the NFTBT under different track slab lengths, according to the key monitoring positions of the track structure determined in Section 2.3, the unfavorable stress at the key positions is analyzed. The stress change curve is shown in Figure 13, which shows:  
1. As the length of the NFTBT increases, the stress at the center of the track slab and the adjustment layer first increases and then decreases, reaching the minimum point at 5400 mm, while the stress at the end of the track slab does not change much. With each slab under long-term conditions, the track slab stress changes little and shows a downward trend, but the stress changes in the adjustment layer are larger and increase further. 2. With the increase in the length of the NFTBT, the stress at the grouting hole fluctuates greatly, and the stress value at the grouting hole of the track slab and the adjustment layer is the smallest at 5000 mm. In order to achieve the NFTBT's optimal construction, the area of the grouting hole of the track slab must inevitably increase, and the stress state at the boss of the adjustment layer will change from compression to tension. Therefore, the length of the track slab needs to be restricted. 3. The stress difference in key positions of the NFTBT's structure under different slab length conditions is small (0.25 Mpa), and the stress in the track slab has an opposite trend to the stress of the grouting hole. The stress amplitude of the adjustment layer is smaller than that of the track slab. Since the grouting hole is often a weak link in the construction process, considering the impact of later maintenance, it is recommended that the length of the NFTBT be 5000 mm. Figure 12: Track slab displacement curve under different length conditions. Figure 13: Maximum stress change curve of track structure under different length conditions: (**a**) Track slab; (**b**) Adjustment layer. The stress at the grouting hole of the track slab under a positive temperature gradient load is the control factor of the track slab's design, and the stress in the adjustment layer under a negative temperature gradient is more unfavorable. Therefore, the change in the maximum internal force of the track structure with the length under the temperature load is shown in Figures 14 and 15. According to Figure 14, the stress of the track slab increases gradually with the increase in the slab length. The stress trend of the slab under negative temperature is opposite, and there is an extreme value at 5000 mm.