Multistage air-tightness degradation of shield filter cakes

The air tightness of filter cakes is critical for maintaining tunnel face stability during slurry shield tunneling, particularly in the standstill stage. However, the failure mechanisms of filter cakes under air pressure remain insufficiently understood due to limitations in existing experimental approaches. This study investigates the air tightness and failure mechanism of filter cakes through coupled computational fluid dynamics–discrete element method simulations. Numerical models were developed to replicate the airproof test, systematically exploring the micro-mechanism of the filter cake failure. A contact-based void partitioning method was adopted to quantify pore structure evolution under incremental air pressure, enabling classification of the failure process into three distinct stages: (1) overall compression stage, (2) compression limiting stage, and (3) pore network development stage. The micro-scale air-tightness threshold (airproof value) was determined via quantitative analysis of constriction size evolution, demonstrating strong agreement with macro-scale observations. Particle-scale analysis of stress distribution and displacement patterns revealed that failure initiated through tensile rupture at interparticle contacts, subsequently triggering particle rearrangement and localized pore development. These findings establish a particulate-scale framework for understanding air-tightness degradation mechanisms in filter cakes, offering theoretical basis for predicting airproof value based on pore-network analysis.