3.4.3 Analysis of Calculation Results
1）Surface subsidence analysis.
The variation of the maximum settlement value of each section under different working conditions is shown in Fig.3.17.
Fig.3.17 Comparison of the variation of maximum settlement value in different working conditions
Fig.3.18 Variation of soil displacement at shallow overburden excavation surface
With the gradual approaching of the excavation surface of the shield，the monitoring section shows a small amount of uplift，and with the shield excavation surface crossing the monitoring section，the settlement increases gradually，and finally reaches the stability.From the point of view of the maximum settlement，the second section（LK5+975～LK6+025）is the largest，the fourth section（LK6+075～LK6+150）still has a further settlement trend，and the first section（LK5+900～LK5+975）is close to the third section（LK6+025 and LK6+075）.From the point of view of the overall numerical value，the final subsidence value of the four sections is close to each other.
2）Stability analysis of excavation surface
From the point of view of numerical simulation，the stability analysis of excavation surface is mainly represented by the displacement of soil body at the excavation surface.The numerical results are shown in Fig.3.18.
When the shield excavation face is located in the first，second and third sections，the displacement of the excavation face gushes forward to the shield.At this time，the slurry pressure is slightly larger than the lateral pressure of soil and groundwater pressure.When the excavation face enters the fourth section，the displacement of the excavation face jumps，and the displacement of the excavation face changes from outward to inward.This is mainly due to the great changes in the overlying soil layer in the fourth section.Therefore，when the shield excavation face passes through the fourth section，the slurry pressure should be adjusted in time according to the situation of overlying soil，so as to maintain the excavation face within a certain stable range.
3）Shield gesture analysis
When the shield passes through the shallowest soil layer，its attitude is inevitably affected by the shallow soil.Therefore，the attitude of the shield machine crossing the shallowest soil layer is monitored by the simulation model.Attitude mainly has two measurement indexes，namely vertical shield head，shield tail displacement difference.Horizontal displacement difference between shield head and shield tail.The displacement difference between shield head and shield tail is shown in Fig.3.19.
In the figure，the positive value represents the shield head forward，and the negative value represents the shield head forward.From Fig.3.19a，it can be seen that when the shield passes through the shallowest soil layer，it mainly advances in a bump way.With the decrease of overburden thickness，the amount of bump increases gradually.When located in the fourth section，the amount of bump begins to decrease due to the rapid increase of overburden thickness.Figure 3.19b shows that according to the boundary conditions of the simulation model，the attitude of the shield in the horizontal direction is basically unchanged.
Fig.3.19 Shield attitude diagram