4.2.3 Calculation Results Analysis
4.2.3.1 Computational Rationality Verification
Numerical simulation requires reasonable calculation to have reference value.The usual verification method is to compare the calculated value with the actual monitoring data.The shield starts from the launching shaft in the north of the Yangtze River and enters the bottom of the Yangtze River through the north bank of the Yangtze River.In order to monitor the ground subsidence and optimize the control of shield tunneling parameters according to the ground subsidence，multiple ground monitoring sections are set up on the surface between the levee and the working well in Jiangbei for subsidence monitoring，numbered S1-S3，L1-Ln，M1-M2，arranged as shown in Fig.4.7.
The S1-S3 monitoring section is located in the reinforcement area of the originating well，and the stratum has been reinforced.The settlement law cannot reflect the influence of shield tunneling on undisturbed soil，so it is not analyzed here.The surface of L1-Ln and M1-M2 sections is farmland and outside the reinforcement range of the originating well，there is no disturbance before construction.M1 and M2 are located in the above model，so the settlement values of these two monitoring sections are compared with the settlement values obtained by numerical simulation，and the results are shown in Fig.4.8.
It can be seen from Fig.4.8 that the measured settlement trough caused by shield construction is slightly wider than the calculation result.The measured settlement above the tunnel axis is slightly smaller than the calculation value，while the actual settlement on both sides is slightly larger，but the difference in settlement value is 2-3 mm.The settlement curve calculated by the model is generally consistent with the measured settlement curve，and the width and depth of the settlement trough are not significantly different，indicating that the selection of the above calculation model is reasonable.
Fig.4.7 Schematic diagram of monitoring band distribution
Fig.4.8 Settlement calculation and measured curve of surface observation zone
4.2.3.2 Analysis of Surface Settlement and Horizontal Displacement
According to the excavation steps，the shield construction is simulated.After the tunnel runs through the model，the final vertical settlement contour and the final horizontal and vertical displacement contour（along the horizontal axis of the tunnel）are obtained，as shown in Fig.4.9.In the figure，X=0 corresponds to the surface above the tunnel axis，and X=100 corresponds to the surface on the right boundary of the model in Fig.4.5.The location of Y=0 corresponds to the mileage pile number SDK4+350，which is the starting point of the tunnel excavation model.The location of Y=500 corresponds to the mileage pile number SDK4+850，which is the location when the tunnel excavation is completed.
Fig.4.9a shows that shield construction causes ground settlement，the settle
ment above the tunnel axis is larger，and the ground settlement far from the tunnelaxis is smaller.At the central position at the top of the embankment（Y=150 m），the settlement is the largest，reaching 28.5 mm，and the settlement trough is the widest.The width of the settlement trough at the bottom of the slope facing the water surface gradually decreases，and the settlement gradually decreases.Fig.4.9b shows that after the completion of the shield construction，the slope produces longitudinal displacement along the tunnel axis，and the slope facing the water shows the outward extrusion.For example，the horizontal displacement at the midpoint of the slope（Y=300 m）reaches 8.4 mm.Therefore，when shield tunneling under embankments with thick overburden，it is necessary to ensure stable posture and set reasonable supporting stress，so as to prevent excessive surface settlement at the top of embankment and threaten the safety of embankment.When the shield passes through the water-facing slope section，the slurry pressure should be controlled to avoid the“extrusion”damage of the water-facing slope.
Fig.4.9 Final settlement and longitudinal displacement contours of the surface
4.2.3.3 Coastal Slope Stability Analysis
For the embankment slope structure，besides the settlement and horizontal displacement，the stability of the slope is also crucial.It is necessary to extract each construction step file of the shield crossing the embankment，and use the strength reduction method to analyze the stability of the slope during the shield crossing the embankment.The shear strength of stratum soil is reduced step by step，and the calculation convergence criterion is selected as the judgment basis of slope instability combined with slope displacement nephogram and plastic zone.When the shear strength of soil is reduced to a certain extent，the calculation is no longer convergent，and the plastic zone is penetrated and the displacement of soil in the circular arc is significantly increased，the slope is judged as instability，and the initial strength of soil and the resistance at this time are judged.The ratio of shear strength is defined as safety factor.Fig.4.10 shows the variation curve of slope safety factor during shield construction.
It can be seen from Fig.4.10 that before the shield construction，the safety factor of the initial slope of the embankment is about 1.63.When the shield excavation face advances to the Y=100 m，it begins to affect the embankment slope，so that the safety factor continues to decrease，and remains unchanged after the shield advances to the Y=400 m，maintaining at about 1.48.At the same time，it can be seen that the stability of the embankment slope is closely related to the position of the shield：when the shield is far from the slope，the disturbance influence area of the excavation surface has not yet reached the embankment slope，and the shield tunneling has little effect on the safety factor of the slope facing the water surface.When the shield excavation surface is gradually close to the slope section，the disturbance of shield construction on the water-facing slope is increasing，and the safety factor begins to decrease slowly.With the advance of the shield，the shield reaches the front half of the waterward slope.At this time，the disturbance of the cutterhead excavation，the shear friction of the shield shell on the soil，the deformation of the shield tail hole and the grouting body behind the wall，and the“extrusion”effect of the shield on the waterward side of the levee reach the maximum，and the safety factor of the slope is significantly reduced.When the shield enters the rear half of the slope，the influence area of the excavation face gradually moves out of the bank slope，and the change of the safety factor gradually tends to be gentle.As the shield continues to advance，the safety factor will not change when the excavation face is away from the slope.
Fig.4.10 Change of safety factor of slope during the process of shield crossing the bank
4.2.3.4 Analysis of Different Operating Conditions
The above analysis is only for the specific working conditions of Nanjing Wei San Road Project，for different construction parameters，different bank slopes and the settlement of the stratum and the stability of the bank slope under burying depth need to be further studied.This section selects the three conditions of grouting effect，embankment slope and tunnel depth to analyze.
1）Influence of different grouting effects
The settlement when the shield tail is detached is the most important settlement during shield construction，accounting for 25％-40％of the total settlement deformation of shield construction，which is the main source of shield construction settlement.Therefore，this construction parameter is selected for analysis.This settlement is mainly due to the formation loss caused by the shield tail gap produced by the shield tail out of the segment.In the project，the back wall grouting method is used to fill the shield tail gap to reduce the formation settlement.In general，the post-wall grouting effect is determined by the filling rate of the slurry，the consolidation and cementation characteristics of the grouting body（elastic modulus is often considered in the numerical analysis）and the grouting pressure.In order to simplify the calculation，the filling rate of slurry is used to approximately characterize the post-wall grouting effect.In order to consider the influence of different grouting effects，the post-wall grouting model which can consider the filling rate is adopted.It is assumed that the filling rate of the slurry is 40％，50％，60％，70％，80％，90％and 100％，respectively.The slurry is considered as a“three-stage consolidation-cementation”elastic equivalent layer.The grouting pressure is 0.1 MPa higher than the formation pressure at the corresponding position.The final safety factor of the slope and the maximum surface settlement under different grouting effects are calculated as shown in Fig.4.11.
Fig.4.11 Maximum settlement value and final safety factor of surface filling under different wall grouting filling rates
It can be seen from Fig.4.11 that with the increase of grouting filling rate，the maximum ground settlement gradually decreases，and the safety factor of embankment slope increases.When the filling rate is less than 60％，the vertical settlement increases obviously and the safety factor changes little.When the filling rate is greater than 60％，the trend of vertical settlement change slows down，and the safety factor increases significantly.Therefore，in practical engineering，grouting filling rate must be controlled more than 60％.Due to the complex grouting process，the maximum filling rate can only reach about 90％.Even if the grouting amount is large，it is difficult to achieve full filling of the shield tail gap.Excessive grouting amount or excessive grouting pressure is easy to cause surface uplift，threatening the safety of the embankment，and grouting at the bottom of the river.Based on this，it is suggested that the filling rate should be controlled at 70％-90％in the construction of the river-crossing channel project of Wei San Road in Nanjing.According to the engineering experience，the empirical over-excavation coefficient α=1.21 and the slurry loss coefficient δ=0.42 can be taken to obtain the corresponding grouting rate of 146.0％-187.8％，so the grouting amount of 16.9-21.65 m3/ring can be used in the construction.
2）Influence of different bank slopes
Large-scale river crossing tunnels at home and abroad are frequently constructed，and crossing various complicated forms of dikes has occurred from time to time.Because it is different In the case of the bank form（especially the slope of the bank），the shield construction has different effects on the settlement and stability of the dike.In order to study this difference，on the basis of the above calculation model，change the slope coefficient of the bank's waterfront slope（6，5.5，5，4.5，4，3.5，3）without changing other parameters（after wall grouting）“Three-stage consolidationcementation”equivalent model，assuming a fill rate（70％），the simulated shield crosses the dike different slopes.
From Fig.4.12，it can be seen that with the increase of slope coefficient（i.e.，the slope gradient decreases），the decrease of surface settlement at the top of the dam is not obvious，but the safety factor increases significantly.When the slope coefficient is 3.0，the water-facing slope is steep，and the safety factor is only 1.35.As the slope slows down（that is，the slope coefficient increases），the safety factor increases.When the slope coefficient is 5.0，the safety factor is 1.43.When the slope coefficient exceeds 5.0，the slope has become very flat and the safety factor increases more significantly.
3）Influence of different tunnel depths
In different river crossing tunnel projects，the buried depth of shield passing through the embankment is different.Under different buried depth，the influence of shield construction disturbance on the embankment settlement and bank slope stability is different，which needs further research.In order to consider the different buried depth of the tunnel，the thinnest overburden thickness at the riverbed is assumed to be 40，35，30，25，20，15 and 10 m without changing other parameters（the“three-stage consolidation-cementation”model is adopted for grouting behind the wall，and the filling rate is assumed to be 70％）.The results are shown in Fig.4.13.
Fig.4.12 Maximum settlement value and final safety factor of different surface slopes
Fig.4.13 Maximum settlement value and final safety factor of different surface thicknesses
It can be seen from Fig.4.13 that with the increase of shield depth（i.e.，the minimum overburden thickness in the figure increases），the vertical settlement value of the embankment top surface decreases.The safety stability coefficient of embankment slope increases approximately linearly，but the increase is small.The influence of shield depth on the surface settlement of embankment is more significant than that on the safety factor of embankment slope.
4）Sensitivity analysis of influencing factors
Fig.4.11 shows that when the filling rate is increased from 40 to 10％，the settlement value of the surface of the embankment is reduced from 72.0 to 13 mm，and the reduction value is 59 mm.Meanwhile，the safety factor of the levee increases from 1.45 to 1.52.Only 0.06 is increased.Fig.4.12 shows that when the slope coefficient of the surface of the water increased from 3 to 6，the settlement value of the surface of the embankment was reduced from about 35.4 to 34.7 mm，which was reduced by only about 0.7 mm，while the safety factor of the embankment increased from 1.35 to 1.70，increasing by 0.35.Fig.4.13 shows that when the minimum soil cover thickness is increased from 10 to 40 m，the sedimentation value of the surface of the embankment is reduced from 49 to 30.5 mm，with a reduction of 18.5 mm，while the safety factor is increased from 1.40 to 1.60，with an increase of 0.20.In order to visually represent the effects of the above three factors，the range of changes in the parameters is unified to 10％，and the corresponding settlement and safety coefficient changes caused by them are listed as percentages in the form of a percentage of Table 4.2.
Table 4.2 Comparison of influencing factors
Table 4.2 shows that the effects of grouting effect，bank slope and tunnel depth on bank settlement and slope stability are different：for stratigraphic settlement，grouting effect has the greatest impact，followed by tunnel burial depth.Impact of bank slope，it is almost negligible；for the safety factor of the bank's waterfront slope，the slope of the bank has the greatest impact，followed by the tunnel.The depth of the road is buried，and the effect of grouting is minimal.
So in different working conditions should be selected according to the actual situation of embankment settlement and embankment slope stability as the key control index：when the embankment slope is small，shallow tunnel depth，should be mainly to the embankment surface settlement as the main control target，to prevent the embankment structure due to uneven settlement and damage.When the embankment facing the water slope is large and the tunnel depth is deep，the control of settlement is no longer the key control target，and the slope stability control of the embankment becomes very important.