## 3 Influence of NFTBT Structure's Size Parameters
### Influence of Thickness of Track Slab  
After the greening and paving are completed in the tram running stage, when the NFTBT's structure is under the combined effect of the tram load, its self-weight and the paving layer weight (the stress change curve at the unfavorable position of the track slab and adjustment layer under different thickness conditions is shown in the Figure 20), it can be concluded that:  
1. As the thickness of the track slab increases, the self-weight of the NFTBT's structure will gradually increase, which will adversely affect the maximum stress of the track slab. At this time, the stress of the grouting hole increases rapidly, and the stress in the track slab first decreases and then increases. The stress of the boss in the adjustment layer shows an increasing trend, while the stress changes in other positions are smaller.
2. The stress at the grouting hole and the stress in the track slab are the key control factors for the optimal design of the track structure at this stage. As the slab thickness increases, the local stress at the grouting hole increases and is relatively rapid; the stress in the slab first rises and then drops. There is a minimum turning point in the three working conditions, and both turning points appear near the thickness of 200 mm. At this time, the stress of the adjustment layer is basically unchanged.
3. Since the thickness of the track slab has a greater impact on construction and economy, it is not the case that the thicker the track slab, the better the overall performance. Synthesizing the law of stress change at each key position, considering the maximum stress at different positions of the track slab, when the thickness of 200 mm is adopted, the stress of the track structure is more favorable than other working conditions.  
The change in the maximum internal force of the track structure with the thickness under the temperature load is shown in Figures 21 and 22.  
Figure 20: Maximum stress change curve of track structure under different thickness conditions: (**a**) Track slab; (**b**) Adjustment layer.  
According to Figure 21, after pouring the concrete adjustment layer, when the track slab bears the temperature load, the stress at the grouting hole of the track slab is the largest under the positive temperature gradient, so the stress at the grouting hole is the control index of the optimal design. With the increase of the slab thickness, the stress value of the grouting hole first increases and then decreases; on the contrary, there is a turning point at 200 mm at negative temperature, wherein the stress in the slab increases with the increase of slab thickness.  
According to Figure 22, the stress of the adjustment layer is more unfavorable under the positive temperature gradient load. The stress at the bottom of the boss first increases and then decreases with the increase of the slab thickness and remains unchanged under the negative temperature gradient. Based on the comparison results of the stress and deformation of the track slab and adjustment layer, the stress condition of the track structure under 200 mm and 220 mm is better before the completion of the greening work.