Calculation and Analysis of Critical Supporting Pressure for Shield Tunnel Excavation Face in Water-rich Sandy Stratum under Seepage Condition

Abstract

[Objective] Water-rich sandy stratum is a common geological formation encountered in metro shield tunnel excavation. Seepage may occur under the influence of water pressure differences inside and outside the soil chamber. Therefore, it is essential to study the accurate calculation method for the critical supporting pressure on shield tunnel excavation face in water-rich sandy stratum under seepage condition. [Method] Using the 3D (three-dimensional) finite element upper and lower bound limit analysis method and Optum G3 software, stability studies are conducted on the shield excavation face under no-seepage condition. Through sensitivity analysis of key parameters, a calculation method for the excavation face critical supporting pressure without considering seepage condition is proposed. Considering the seepage action caused by the water level difference inside and outside the soil chamber, a 3D finite element analysis method and PLAXIS 3D software are employed to investigate the instability damage modes of shield excavation face in water-rich sandy stratum under steady-state seepage and fluid-soil coupling action. Through sensitivity analysis of water level difference, tunnel buried depth ratio, and internal friction angle, based on the calculation formula for excavation face critical supporting pressure without seepage, a calculation method for excavation face critical supporting pressure considering steady-state seepage condition is further proposed. [Result & Conclusion] The proposed formula allows for a relatively straight forward determination of the excavation face pressure during shield tunneling in water-rich sandy layers. For different groundwater conditions, the critical excavation face supporting pressure is ranked as follows: critical excavation face supporting pressure without seepage>critical excavation face supporting pressure with seepage>excavation face critical pressure under dry sand conditions. In the case of burial depth ratio C/D25°, the instability damage zone in front of the excavation face is limited to a certain range above the excavation face, with a slightly larger damage area when seepage occurs compared to situations without seepage.

Keywords

• metro
• shield tunnel
• critical supporting pressure