To address the issue of muck spewing during earth pressure balance (EPB) shield tunneling in water?rich sandy strata, this study systematically investigates the evolution of permeability characteristics of conditioned muck and the critical conditions for spewing. A pressurized permeation apparatus capable of simultaneously simulating soil pressure and water pressure at various burial depths was developed. Using this setup, the evolution of muck permeability under the coupled effects of multiple factors-including permeation pressurePw, confining pressurePs, foam injection ratio (FIR), types of conditioning agents, and particle gradation-was systematically examined. The results reveal that foam conditioning exhibits distinct “critical injection zones” and “over?conditioning zones,” with an optimal FIRof approximately 5%–8%; bentonite slurry demonstrates more stable impermeability under pressurized conditions. Based on the experimental data, a response surface model incorporating the stress ratio Pw/Ps, FIR, and the permeability coefficient ratio k/k0 was established, and a critical stress ratio Pw/Ps= 0.5 was identified. Furthermore, a multi?dimensional critical criterion for spewing centered on “permeability coefficient-water head-flow rate” was proposed, leading to the development of a tiered early?warning system based on the concept of a dynamic critical permeability coefficient. Finally, a differentiated prevention and control strategy that matches “stratum–conditioning agent–parameters” was formulated. The research outcomes provide a systematic solution, from mechanism to application, for controlling spewing risks in EPB shield tunneling through water?rich sandy strata.