Study On Liquefaction Potential And Control Technology Of Water-rich Fine Sand Layer Under Cutterhead Vibration During Shield Tunneling

In the process of shield tunnel excavation,the cutterhead cutting soil layer will inevitably produce dynamic disturbance.When the soil layer is a kind of saturated weakly cemented soil such as water-rich fine sand layer and silt layer,the pore water pressure in the stratum rises rapidly under the action of dynamic load,resulting in a high liquefaction risk in the stratum.Based on the shield tunnel project of No.1-2contact line of Taiyuan rail transit,this thesis adopts the methods of indoor test,numerical simulation and field monitoring.Firstly,the dynamic triaxial test of saturated water-rich fine sand is carried out to explore its dynamic characteristics.Then,the vibration velocity time history curve of shield tunneling in water-rich silty fine sand layer is extracted by field monitoring method for this project,and the location with the highest liquefaction risk is analyzed theoretically.Dynamic triaxial test and field monitoring provide relevant parameters for subsequent numerical simulation.The liquefaction of water-rich fine sand layer under shield disturbance load is simulated by FLAC3D software,and the improvement effect of surface grouting method on liquefied fine sand layer is further studied.The main conclusions are as follows:（1）The dynamic parameters of dynamic strength,dynamic shear strength and dynamic elastic modulus of fine sand were obtained by dynamic triaxial test.The effects of confining pressure,frequency and dynamic stress amplitude on the liquefaction law of fine sand samples were analyzed.（2）Based on the actual project,the vibration load during tunnel excavation is monitored,and the time domain and frequency domain characteristics of the cutterhead vibration when the shield is tunneling in the fine sand layer are analyzed in detail.It is considered that the vibration velocity of the cutterhead is concentrated in 0.1～1.2mm·s^-1when the cutterhead is tunneling in the fine sand layer.The vibration frequency is concentrated in 0～30 Hz,which is a typical low-frequency vibration.The vibration frequency in different directions has frequency mutations at individual positions and there are multiple peaks in the velocity spectrum.The vibration load is most disturbed along the heading direction of the tunnel face,and the liquefaction risk is the highest.The vibration energy is concentrated in the low frequency range,and the energy proportion is the highest in the frequency range of 0～10 Hz.（3）When the shield is driving in the fine sand stratum,the stratum parameters and construction parameters have a significant influence on the vibration response of the shield.The influence degree between each factor and the vibration velocity is mathematically fitted,and the influence degree of each factor on the vibration velocity is obtained from large to small:the dynamic elastic modulus of the stratum,the buried depth of the tunnel,the thrust and the torque.（4）Through FLAC3D numerical simulation,the applicability of Finn liquefaction constitutive to fine sand stratum is verified,and the liquefaction dynamic response of fine sand layer without reinforcement measures is studied.The liquefaction law of water-rich fine sand layer is analyzed with the indexes of excess pore pressure ratio,excess pore water pressure and related displacement.It is considered that the liquefaction risk of fine sand layer under shield disturbance load without reinforcement measures is concentrated in the space range of 0.5～1 m around the excavation tunnel.（5）Through FLAC3D numerical simulation,the improvement of liquefied fine sand stratum by surface grouting method is explored.The simulation results show that the grouting method can improve the liquefaction of the fine sand stratum and reduce the displacement of the stratum.When the elastic modulus of the slurry is 600 MPa and the thickness of the grouting layer is 1 m,the better improvement effect can be achieved by using the half-ring area grouting reinforcement method.