4.1.1 The Analysis of the Causes and Processes of ...
1）Analysis of the causes of formation settlement caused by shield construction
There are many factors of ground deformation caused by shield construction，and settlement results are often caused by multiple factors.Based on the existing research at home and abroad，the main causes of stratum deformation caused by shield construction are：①stratum loss caused by shield construction.②deformation of segment lining structure.③Consolidation and secondary consolidation of soil around the tunnel.
©Shanghai Scientific and Technical Publishers 2022 95
J.Chen et al.，Construction Technology of Large Diameter Underwater Shield Tunnel，Key Technologies for Tunnel Construction under Complex Geological and Environmental Conditions，https://doi.org/10.1007/978-981-16-5896-9_4
Fig.4.1 Schematic diagram of soil loss
The stratum loss caused by shield tunneling is the main reason for stratum settlement deformation.The stratum loss is equal to the volume of soil excavated during shield construction minus the volume occupied after the completion of the tunnel（the volume occupied after the completion of the tunnel includes the volume of the internal space of the tunnel，the lining structure and the volume of the post-wall grouting slurry wrapped on the outer surface），as shown in Fig.4.1.Due to overexcavation of shield cutterhead，thickness of shield body and assembly clearance，the excavated soil volume during shield construction is often much larger than the excavation volume（theoretical excavation volume）calculated according to the designed section area of tunnel.
The formation loss rate is represented by the percentage of formation loss in the theoretical excavation volume.Stratum loss can usually be divided into three categories：①Normal ground loss in the case of a person's operation process in full compliance with the operating rules，which is all attributable to the objective conditions of the site.②Abnormal stratum loss caused by improper operation of people in the construction process，such as shield machine body retreat，shield over-excavation，insufficient support pressure and insufficient grouting amount.③Hazardous stratum loss caused by special geological conditions or unpredictable disaster reasons，such as sudden large amount of soil flow in front of shield excavation.
Lining deformation is also one of the reasons for ground settlement.The lining segments of the shield tunnel are assembled by multiple blocks.The stiffness of the segment lining block，the block type of the lining ring，the structural type of the indirect head of the ring and the fineness of the segment assembly will affect the stress and deformation of the structure.When the shield passes through，because the original stratum is replaced by segment lining and tunnel，the surrounding stratum squeezes the lining，and the lining is subjected to stratum soil pressure and deforms under force，resulting in a certain displacement of the stratum.However，in practical engineering，as the main structure of tunnel stress，the segment needs to meet the specific strength and stiffness requirements in design，and the allowable deformation range is small.Therefore，the influence of lining deformation is often not considered in the analysis of stratum settlement.
The consolidation and secondary consolidation of soil around the tunnel are also important causes of formation deformation.The over-excavation，pushing，friction，shear and post-wall grouting of shield in the process of propulsion will disturb the surrounding soil，so that the surrounding strata will produce excess pore water pressure.When the shield tail passes through，the excess pore water pressure reaches the peak value，and then dissipates.The consolidation of the soil causes the formation settlement，which is called consolidation settlement.The excess pore water pressure generated by shield construction and its dissipation distribution are shown in Fig.4.2.In addition，the soil skeleton around the tunnel will also generate creep due to shield disturbance，which is called secondary consolidation settlement.Generally speaking，the consolidation settlement caused by shield construction accounts for the main part，and its duration is relatively long.The secondary consolidation settlement is relatively small in value，and it occurs when the consolidation settlement occurs.
2）Analysis of formation settlement caused by shield construction
During shield construction，stratum will inevitably be disturbed.With the advance of shield，the settlement of stratum will gradually increase.Based on the relevant literature at home and abroad，the stratum settlement caused by shield construction is divided into five stages shown in Fig.4.3.
（1）Pre-settlement：the settlement of the soil outside the sliding surface in front of the shield before the excavation surface reaches，and the settlement range depends on the formation conditions.For sandy soil，the main reason is that shield tunneling decreases the groundwater level and increases the effective stress of the stratum，which leads to the consolidation settlement of the stratum.For ultra-soft clayey soil，the pre-settlement is caused by over-excavation of excavation face.In general，the initial settlement is less than 5％-10％of the total settlement.
（2）The settlement in front of excavation：refers to the surface subsidence before the shield cutterhead is about to arrive.It generally refers to the settlement generated during the whole process from the observation point several meters away from the excavation face to the shield excavation face reaching this point.The main reason is the imbalance of water and soil pressure on the excavation face.If the shield support pressure is too small，the soil in front of the excavation unloading deformation，resulting in settlement.If the supporting pressure is too large，the stress state of the soil in front of the excavation will change，resulting in surface uplift.This part of the settlement accounts for 15％-20％of the total settlement.
Fig.4.2 Schematic diagram of excess pore water pressure distribution
Fig.4.3 Ground deformation law caused by shield tunnel
（3）Settlement generated when passing：that is，stratum settlement generated when passing from the incision position to the shield tail.The main reason is the friction between the outer surface of the shield and the stratum，or the formation disturbance caused by over-excavation，and the weakening of the threedimensional support effect，resulting in the formation stress release.In addition，over-excavation will inevitably occur when the shield curve advances，resulting in ground settlement.This part of the settlement accounts for 15％-25％of the total settlement.
（4）Settlement when the tail of shield is out：when the tail of shield is out，the gap between the outer diameter of shield and the outer diameter of tunnel lining causes formation loss，ground stress release and elastic-plastic deformation of the stratum due to the delayed grouting behind the wall or insufficient grouting amount.When the lining segment is separated from the tail of the shield，the earth water pressure acting on the segment will squeeze the lining ring and cause the deformation of the lining ring，which will also lead to small ground settlement.This part of settlement deformation accounts for 30％-40％of the total settlement deformation of shield construction，which is the most important settlement during shield construction.
（5）Subsequent consolidation settlement：after the shield tail passes，due to the gradual dissipation of excess pore water pressure caused by the previous shield disturbance，the settlement caused by the continuous consolidation and secondary consolidation of soil mass accounts for 5％-25％of the total settlement of shield construction.The settlement at this stage depends on the stratum properties and the disturbance degree of shield to the surrounding soil，which is more obvious in clay stratum than in sand stratum.
Shield construction inevitably disturbs the surrounding soil，causing the movement of stratum and surface settlement.The stratum is more complex when largediameter slurry shield is constructed at the bottom of the river，and the embankment structure also has a certain impact on tunnel construction，thereby increasing the construction difficulty.If the construction control is improper，it will cause excessive ground displacement and endanger the stability of embankment structure.During the construction of many cross-river tunnels in China，there have been major risks such as deformation of embankment gates，sand gushing and seepage of flood control walls.Therefore，it is very important to control construction to reduce the influence of shield on embankment.