# Pressure-Dependent Meso-Scale Evolution of Dispersed Foam and Field-Application Clogging Mitigation Strategies for EPB Shield Tunnelling Through Highly Cohesive Clay

## TL;DR

This paper introduces a new foam agent to reduce clogging during tunneling in clay, improving construction efficiency.

## Contribution

A pressure-dependent dispersed foam agent combining AES and HDT is developed to mitigate clogging in EPB tunnelling.

## Key findings

- HDT content affects foam characteristics in a pressure-dependent manner, influencing bubble size and half-life.
- Conditioned clay cohesion dropped to 1.8–4.3 kPa, and adhesion amounts to 10–15 g, reducing clogging risks.
- The dispersed foam agent successfully resolved clogging in an EPB shield tunnelling project in Jinan Metro.

## Abstract

Clogging in earth pressure balance (EPB) shield tunnelling through highly cohesive strata critically undermines construction efficiency. Conventional foam agents exhibit limited conditioning effectiveness, even with increased dosage. This study developed a dispersed foam agent by combining anionic surfactant (AES) with nonionic dispersant (HDT). The effects of air pressure (0–2 bar) and HDT content (0–10%) on macro-meso characteristics of foam and adhesion characteristics of conditioned soil were quantified through an evolutionary mechanism investigation of the bubble size distribution of foam, half-life measurements, and mechanical tests on conditioned soils. Results demonstrated that the influence of HDT content on foam exhibited pressure-dependent behavior. Under 0 bar within 0–10 min, HDT increased the proportion of small bubbles while marginally reducing the mean radii. Although HDT accelerated the degradation of small bubbles, it extended the foam half-life. Conversely, under 1 or 2 bar, HDT demonstrated opposite effects on these parameters. The cohesion of conditioned clays was reduced to 1.8–4.3 kPa, and adhesion amounts decreased to 10–15 g, significantly mitigating clogging risks. The optimal injection ratio of dispersed foam was determined for different pressures and clays. Engineering application in an EPB shield tunnelling section of Jinan Metro successfully resolved clogging issues, demonstrating the effectiveness of dispersed foam agent.