Stability of foam-aggregate mixtures for EPB tunnelling

In Earth Pressure Balance (EPB) tunnelling, soil conditioning through foam injection is critical for successful excavation. However, a significant knowledge gap exists between methodologies used to assess foam stability and those employed to evaluate conditioned soil stability. This study addresses this gap by investigating the stability of foam-aggregate mixtures under controlled laboratory conditions, progressively adding solid particles of a reference soil to foam and evaluating temporal stability changes. The experimental investigation examined various particle concentrations and size distributions to understand their influence on foam stability mechanisms. Results confirm that particle inclusion significantly enhances foam stability, with stability increasing exponentially with solid fraction (mx) and decreasing according to a power law with particle diameter. These effects become significant for solid fractions exceeding 50 g per 125 g of liquid generator and for particles smaller than 0.25 mm in diameter. The study demonstrates that contributions from different granulometric fractions compound linearly, and foam-solid mixture stability can be expressed as an exponential function of total particle surface area. This relationship explains why finer particles exhibit greater stabilizing effects. While some behavioral aspects were partially anticipated in literature, previous research did not fully account for larger particle diameter effects. Despite promising findings, the study could not completely bridge the gap between foam stability and conditioned soil stability, as the minimum achievable Foam Injection Ratio (FIR) remains an order of magnitude higher than typically used for soil conditioning. Nevertheless, this research successfully narrows the existing knowledge gap and provides valuable insights into solid fraction roles in foam stabilization. The findings enhance understanding of foam-soil mixture behavior, contributing to improved soil conditioning strategies and more effective EPB tunnelling operations. This work establishes a foundation for future research aimed at completely connecting foam stability and conditioned soil stability assessment methodologies.