Effect of hydraulic gradient on the permeability characteristics of foam-conditioned sand for mechanized tunnelling

Effect of bentonite slurry on the function of foam for changing the permeability characteristics of sand under high hydraulic gradients

“…This study is a supplement to Hu et al (2020), who reported that the permeability coefficient of foam-conditioned sand with high water content was too high. A series of large-scale permeability tests were carried out in this study on foam-bentonite slurry-conditioned sand to investigate the effect of bentonite slurry on the function of foam for changing the permeability characteristics of sand under high hydraulic gradients.…”

“…Borio and Peila (2010) carried out constant head permeability tests under different water pressures to study the permeability of foam-conditioned sand and found that the permeability coefficient of foam-conditioned sand decreased with an increase in the foam injection ratio (FIR, which is defined as the ratio of the volume of foam to that of a soil specimen in the natural loose state) but increased with an increase in the foam expansion ratio (FER, which is defined as the ratio of the volume of foam to that of foam solution) and water pressure. However, because foam is a metastable multiphase system with an uneven bubble size (Pitois et al 2009;Rouyer et al 2014), the permeability of foam-conditioned sand has a certain temporal variability; that is, the permeability coefficient (k) is maintained at a low level in the initial period; however, as the foam bubbles continue to dissipate and are flushed out over time, k increases rapidly, and after a fast growth period, it finally stabilizes (Budach 2012;Hu et al 2020;Wang et al 2021). Hu et al (2020) studied the effect of hydraulic gradients on the permeability characteristics of foam-conditioned sand.…”

“…However, because foam is a metastable multiphase system with an uneven bubble size (Pitois et al 2009;Rouyer et al 2014), the permeability of foam-conditioned sand has a certain temporal variability; that is, the permeability coefficient (k) is maintained at a low level in the initial period; however, as the foam bubbles continue to dissipate and are flushed out over time, k increases rapidly, and after a fast growth period, it finally stabilizes (Budach 2012;Hu et al 2020;Wang et al 2021). Hu et al (2020) studied the effect of hydraulic gradients on the permeability characteristics of foam-conditioned sand. When the hydraulic gradient was high, the permeability coefficient of the muck did not meet the requirement of a low permeability coefficient, necessitating the selection of other agents or combined conditioning considerations.…”

“…However, at the same time, many studies have shown that using only foam as a single agent does not meet the requirements of low permeability for EPB shield tunnelling, especially when EPB shields re-thrust after a long standstill or tunnel in sandy ground under high water pressure. Adding other agents to consider combined conditioning, especially foam-bentonite slurry combinations, is necessary (Hajialilue-Bonab et al 2014;Huang et al 2019;Hu et al 2020). However, there are few studies on the permeability characteristics of soil conditioned with foam and bentonite slurry.…”

Permeability stability calculation model of foam‐conditioned soil based on the permeability constant

“…At present, although many studies have been conducted to quantify the stability of pure foam, research on the stability of foam‐conditioned sand is still limited. Until now, the permeability stability of foam‐conditioned soil was qualitatively analysed only based on experiments 1,7,29,30 . Very few studies have quantitatively calculated the permeability and its stability of conditioned soil 31 .…”