Abstract 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.

中文翻译：

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水力梯度对机械化掘进泡沫调质砂渗透特性的影响

摘要 低渗土在水位下砂土机械化掘进过程中对稳定掌子面具有重要意义。泡沫是最常见的调节剂之一，可实现开挖土壤的低渗透性。本研究通过一系列不同水力梯度下的渗透性试验，研究了水力梯度对不同调理状态下泡沫调理砂渗透特性的影响。随着水力梯度的增加，渗透系数初始稳定阶段的持续时间要么逐渐减少，要么完全消失。在快速生长期，渗透系数大大增加。引入管流模型来分析不同水力梯度下调理砂的时变渗透特性。结果表明，随着水力梯度的增加，水渗透到泡沫塞所需的时间减少，假设管径的扩大率在快速增长期增加。最后，讨论了水力梯度影响下EPB盾构掘进和EPB顶管施工条件的变化。随着水力梯度的增加，满足盾构隧道渗透率要求的土壤条件参数范围逐渐变窄，并朝着低含水量（w）和高泡沫注入比（FIR）的方向发展。