Influence of Pea-Gravel Layer on Segment Ring Support Performance in Shield TBM Tunnels: A Discrete-Continuous Coupling Simulation and Model Test

Abstract

In shield tunnel boring machine (TBM) tunnel construction, grouted pea gravel is used to fill gaps between the surrounding rock and segment rings, acting as a connecting layer that significantly impacts the support performance of the segment ring. This study first obtained the mechanical parameters of pea gravel through experiments, followed by an in-situ investigation to analyze the stress and surface settlement law of the segment ring during TBM construction. A discrete-continuous coupling model was then employed to examine the load transfer process at the micro level between the surrounding rock, pea gravel, and segment ring. Finally, physical model tests were conducted to compare the stress characteristics of the segment ring under different states of pea gravel during continuous loading. The findings reveal that loose pea gravel fails to sufficiently transfer the surrounding rock load to the segment ring, resulting in inadequate support and continuous surface settling until the pea gravel is consolidated. The degree of compression of loose pea gravel directly affects the contact force acting on the segment ring and its supporting effect on the surrounding rock. Moreover, the ultimate bearing capacity of the segment decreases when the pea gravel is loose. The state change of pea gravel resulting from grouting plays a crucial role in the supporting effect of the segment ring; thus, simultaneous grouting is necessary during tunnel construction in unstable strata or insufficient pea gravel filling.