Study on the Stability of Shield Excavation Face in Coastal "Boulder" Stratum

It is very easy for shield machine to encounter boulders in the process of tunneling in completely weathered granite stratum, and there is little research on the stability of shield tunnel excavation face in completely weathered granite stratum. PFC2D numerical analysis is used to obtain the failure mode of shield tunnel excavation face in boulder stratum. Based on the limit equilibrium method and silo theory, a calculation model of the ultimate support force of the shield tunnel excavation surface is established. The theoretical calculation formula for the ultimate support force of the excavation face is obtained, and the influencing factors of the ultimate support stress are analyzed by parameters. The results show that the fully weathered granite stratum containing boulder has the properties of clay strata. The theoretical calculations and numerical analysis results in this paper are in good agreement, which proves the necessity of considering stratification when boulders exist in the excavation face. The limit support stress value decreases as the inclination angle of the sliding surface increases. The lower the position of the excavation face where the boulder is located. The lower location of the boulder which is needed excavated, the lower the ultimate support stress. The relevant research results have reference significance for shield construction in coastal boulder stratum.