Influence of Isolation Pile Modeling and Crown Beam Reinforcement on Tunnel Deformation

The excavation of deep and large foundation pits poses significant risks to adjacent existing subway tunnels, and isolation piles are widely used as a protective measure. However, the combined effects of isolation pile modeling methods (element type/constitutive model) and crown beam reinforcement on tunnel deformation remain insufficiently clarified, especially the quantitative mechanism of stiffness degradation induced by pile damage and the conditional effectiveness of crown beams. This study, based on a case study of Hangzhou Metro Line 1, provides novel quantitative insights for optimizing isolation pile design through systematic finite element analysis. The key findings, with direct practical implications, are: (1) solid-element models outperform beam-element models in deformation control, with the efficiency difference narrowing from 25% (s = 2b) to < 1.5 mm deformation difference at s=16b; (2) damaged isolation piles (CDP model) exhibit “concave-type” deformation, and their horizontal displacement control ability decreases by 30%-45% compared to undamaged elastic piles; (3) crown beams have no significant effect on densely spaced elastic piles, but their enhancement effect increases with pile spacing; for densely spaced CDP piles, crown beams slightly weaken control effect (≤ 5%), while achieving optimal enhancement (reducing tunnel deformation by 18.2%) at a pile spacing of 16 times the pile diameter (s=16b). The study demonstrates that the selection of modeling techniques is not merely a numerical exercise but has direct consequences for the accuracy and conservatism of engineering predictions, thereby providing quantitative guidelines for both simulation and design. These findings also provide quantitative design guidelines for selecting appropriate modeling techniques and reinforcement strategies in tunnel protection projects.