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Both foam and bentonite slurry are commonly employed in soil conditioning to prevent water spewing during earth pressure balance (EPB) shield tunnelling. A novel calculation model to estimate the permeability coefficient of soils conditioned with foam and bentonite slurry is developed. In this model, bentonite particles are assumed to adhere to the foam film in a monolayer of bridging particles, and the permeation channels within the conditioned soil are updated using the Kozeny-Carman equation. A series of permeability tests on conditioned soil, varying conditioning parameters and water heads, validate the accuracy of the model. The measured results that the value of permeability coefficient can be well captured by the calculation results, which indicates that the model can accurately predict the permeability of soil conditioning by foam and bentonite slurry. While there is a slight reduction in accuracy for conditioning states with high foam and bentonite slurry injection ratios, the model remains conservative for tunnelling engineering safety due to calculated values consistently exceeding measured ones. Furthermore, the model performs well in cases of foam-conditioned soil from previous studies. Additionally, the model underscores the significant influence of bentonite slurry on the permeability coefficient by altering soil grain gradation. [ABSTRACT FROM AUTHOR]