## 4 Conclusions  
1. The location of the maximum transverse convergence value of the existing shield tunnel is related to the location of the surcharge, because the location of the load on the two sides of the tunnel is different, resulting in the combined force of the two sides of the load on the tunnel being different. With the surcharge on two sides of the tunnel, as the left surcharge's offset position increases from 0.2 m to 0.6 m, the tunnel's transverse convergence value will first increase and then become smaller, and the transverse convergence value will decrease rapidly after the surcharge's offset reaches a certain degree.
2. In dry sand soil, as the left surcharge's offset position increases from 0.2 m to 0.6 m, the additional pressure of the tunnel decreases sharply; after one side of the tunnel is surcharged, if the other side continues to surcharge, it will lead to a continued increase in the surrounding pressure, and when the two-sided surcharge reaches 172 kg, the local additional pressure of the tunnel increases by more than 100%. After unloading, the additional pressure in most of the tunnel will still exist to some extent, and the residual proportion of additional pressure will be related to the offset position of the surcharge--the more distant the surcharge position, the higher the residual proportion of additional pressure.
3. Under the same test conditions, compared with dry sand, the maximum value of lateral convergence of the tunnel in wet sand is significantly reduced, the peak value of ground settlement on both sides is not significant, the amount of ground settlement caused by the surcharge is significantly smaller, and the mitigating effect of unloading on ground settlement and the reduction in additional pressure are not obvious.
4. When the left surcharge is offset by 0.4 m, the additional envelope pressure of the tunnel in wet sand is generally significantly larger than that of the tunnel in dry sand under the same conditions because, due to the differences in soil properties, the stress \(\sigma\) contours are flatter when the soil is wet sand, and the stress decreases slowly with the increase in the offset distance D. When the soil is dry sand, the stress \(\sigma\) contours are relatively long and thin, and the stress \(\sigma\) decreases rapidly with the increase in the deflection distance D.
5. In the wet sand soil, the additional earth pressure of the tunnel's subsoil presents three peaks when the surcharge to 172 kg on both sides, one of which occurs at measurement point 4 at the bottom of the tunnel, which may be due to the stress concentration occurring here, resulting in a high additional earth pressure. The other two occur at measurement point 2 and measurement point 6 directly below the surcharge, and the range of the two-sided surcharge is 0.4 m \(\times\) 0.4 m, but the accumulated additional soil pressure of the tunnel subsoil directly below the surcharge and the additional soil  
Figure 25: The base (the bottom of the pressure plate) of the reaction force distribution diagram.  
pressure of the tunnel subsoil below the edge of the surcharge are very different, due to the characteristics of the sand soil itself.  
Only one kind of wet sand with a specific water content was used in this model test, and the effect of water content in the sand soil was not considered, which still needs further investigation in future tests and research.  
Methodology and guidance, G.W.; data curation, editing, and analysis, P.X.; analysis and project administration, H.J.; supervision and interpretation of data, D.S.; review and revision, F.X.; supervision and translation, H.G. All authors have read and agreed to the published version of the manuscript.  
Not applicable.  
The authors declare no conflict of interest.