4.2.4 Analysis of the Value of Stress Field and Mu...
The selection of support pressure on the excavation face when the shield crosses the embankment is crucial to the settlement of the embankment and the stability of the embankment.The insufficient or excessive support pressure will lead to settlement or uplift，thus endangering the upper embankment structure.Before the shield reaches the embankment，the terrain is usually flat，and the support pressure of the excavation face can be calculated from the surface at the incision.As the excavation surface gradually approaches the embankment，the stress state of the stratum becomes complex，so the support pressure becomes difficult to determine.If the calculation method of the support pressure on the flat surface（i.e.，from the surface at the notch）is still used，the difference between the obtained support pressure and the actual stress value of the stratum is large.It is a problem worthy of further study that how much influence the settlement and displacement of the embankment will be caused by the shield construction with such support pressure.
1）Analysis of the stress field of the embankment
During shield construction，with the excavation of the cutter head，the soil on the excavation face is unloaded and the initial stress balance of the stratum is destroyed.In order to ensure the stability of the excavation face，prevent the collapse of the soil and control the settlement of the stratum，a force must be applied to the excavation face to prevent the deformation of the stratum to the cement tank，that is，the support pressure，as shown in Fig.4.14.The slurry shield balances the water and soil pressure at the excavation face through the slurry pressure：the slurry pressure balances the water pressure in the stratum in the form of similar water pressure，and the slurry continuously penetrates into the stratum ahead of the excavation，forming a micro permeable or impervious mud film on the excavation face，thus transforming part of the slurry pressure into an effective stress to balance the soil pressure at the excavation face.It can be seen that in order to determine the supporting pressure，the initial stress state of the stratum should be clarified，namely，the initial geostress and the initial pore water stress.
For the case where the ground is flat and the surface is not overloaded，it can be calculated from the self-heavy stress of the stratum and the conventional static pore water stress，which is why the calculation of the excavation face support pressure can be calculated from the surface at the incision.And for the embankment this special case，this method will no longer be reliable.Fig.4.15 shows the principal stress direction of the bank model of the Wei San Road Crossing River in Nanjing.The figure shows that the principal stress is deflected at the bank slope.The most obvious.
The stratum of the Wei San Road crossing channel in Nanjing is mostly sandy stratum.The mud water support pressure is usually set by the soil moisture in the project.According to the engineering routine，the mud support pressure is calculated and the initial stress value calculated by the model is used as the mud.The water pressure is set and the difference between the two is made，as shown in Fig.4.16.
Fig.4.14 Schematic diagram of shield support surface pressure
Fig.4.15 Main stress field of the embankment slope of the Weisan Road crossing in Nanjing
It can be seen from Fig.4.16 that，at a distance away from the bank slope（y=0），the mud support pressure values obtained by the two calculation methods have a small difference of about 2.3 kPa；gradually approaching the bank support pressure gradually increases，at the bank The top center reaches the maximum value.At this time，the support pressure value obtained by the conventional support pressure calculation method is greater than the numerical calculation value，and the difference between the two is 31.1 kPa；Slope，the support pressure is gradually reduced，and the support pressure value obtained by the conventional support pressure calculation method at the foot of the slope is less than the numerical calculation value，and the difference between the two reaches the maximum（26.6 kPa）；near the bottom of the river bed，the two calculations The results are not much different.From this step analysis shows that if the special form of the bank is not taken into account，the engineering calculation method is adopted，that is，only from the surface or slope of the levee to calculate the muddy water pressure on the excavation surface may have the following consequences：①When the shield gradually approaches the levee and passes through the embankment，the mud water pressure is set too high，which will cause a large disturbance to the excavation surface and the nearby stratum，and even lead Water surface slope“extrusion”damage；②When the shield passes through the top of the embankment and enters the slope below the water surface，the mud water pressure setting is low，which will cause large settlement of the slope.
2）Analysis of support pressure value
The above two supporting stress setting methods are used for shield excavation.After the excavation is completed，the vertical displacement value of the surface above the tunnel axis，the horizontal calf displacement and the safety factor during the shield excavation are obtained as shown in Figs.4.17，4.18 and 4.19.
It can be seen from Fig.4.17 that the difference in vertical displacement calculated by the two methods is not large.Near the dam crest，using a numerical meter.The settlement of the in-situ stress value for excavation is slightly larger than the conventional support stress calculation，which is due to the position of the dam crest.The support stress obtained by the general calculation method is slightly larger than the initial stress in the formation；near the foot of the slope，the numerical calculation of the in-situ stress value for the excavation settlement is smaller than the conventional support stress calculation，which is due to the conventional calculation method.The stress value is small.
Fig.4.16 Two kinds of support stress setting values and their difference
Fig.4.17 Final settlement value of shield excavation（X=0）
Fig.4.18 Shield excavation final displacement value（X=0）
Fig.4.19 Safety factor of bank slope during shield excavation
From Fig.4.18，it can be seen that there is a big difference between the surface longitudinal displacement obtained by the two calculation methods.The surface longitudinal displacement obtained by the excavation of the support stress calculated by the initial stress is less than that calculated by the conventional support stress，which can effectively reduce the extrusion failure of the waterward slope.
It can be seen from Fig.4.19 that the safety factors obtained by the two calculation methods have little difference.
From this point of view，for this case，the setting of slurry pressure according to conventional calculation is different from the initial stress state of stratum，but has little effect on the settlement and safety factor of embankment.This is due to the deep buried depth of the shield in this case，the additional stress caused by surface fluctuation diffuses in the soil，and the transfer to the shield position has been greatly weakened.On the other hand，due to the high water level in this case，a large part of the slurry support pressure is used to resist the water pressure.If the shield passes through such embankments with shallow buried depth or low groundwater level，the effect of special topography of the embankment should be fully considered.The slurry pressure near the top of the embankment can be slightly lower than the slurry pressure value calculated by the conventional engineering，so as to avoid excessive disturbance，surface uplift or“extrusion”of the water-facing slope caused by excessive support pressure.When the shield reaches the water-facing slope，the slurry pressure can be slightly larger than the slurry pressure value calculated by the conventional engineering，so as to avoid excessive settlement at the slope toe.