Low permeable soil is important for stabilising the tunnel face during mechanized tunnelling in sandy soils under water level. Foam is one of the most common conditioning agents to achieve low permeability of the excavated soil. This study investigates the effect of hydraulic gradient on the permeability characteristics of foam-conditioned sand in various conditioning states, according to a series of experimental permeability tests under different hydraulic gradients. With an increase in the hydraulic gradient, the duration of the initial stable stage of the permeability coefficient either gradually decreases or completely disappears. In the rapid growth period, the permeability coefficient increases greatly. The pipe flow model is introduced to analyse time-dependent permeability characteristics of conditioned sand under different hydraulic gradients. The results shows that as hydraulic gradient increases, the time required for water to penetrate throughout the foam plugs decreases and the enlargement rate of the assumed pipe diameter increases in the rapid growth period. Finally, changes in the conditioning parameters for EPB shield tunnelling and EPB pipe-jacking under the influence of hydraulic gradient are discussed. With an increase in the hydraulic gradient, the range of conditioning parameters of the soil that satisfies the shield tunnelling permeability requirement gradually narrows and moves towards a low water content (w) and high foam injection ratio (FIR). The results show that an increase in the hydraulic gradient poses a more serious challenge for foam-dominant soil conditioning during mechanized tunnelling.