Seepage failure of foam-conditioned gap-graded soils: an experimental study

This study investigates the seepage failure characteristics of foam-conditioned gap-graded soils by examining the effects of fines content and foam injection ratio through a series of upward seepage hydraulic tests. The results demonstrate that a high fines content is essential for enhancing internal stability; otherwise, the effectiveness of foam conditioning may be limited. However, an excessively high foam injection ratio can reduce the soil’s resistance to hydraulic pressure. Piping failure progresses through stabilisation, channel formation and erosion, with foam migration channels typically forming alongside a sudden drop in pore pressure. Heave failure is characterised by the formation of horizontal cracks within the soil, leading to a rapid increase in pore pressure. Furthermore, the calculation of critical hydraulic gradients for foam-conditioned soils is discussed, considering the limitations of Terzaghi’s equation. A modified model for estimating critical hydraulic gradients in heave failure is proposed, yielding results closely aligned with observed data. Finally, the mechanisms by which foam alters seepage failure patterns are explored from a particle-scale perspective. These findings provide valuable insights into the seepage failure mechanisms of foam-conditioned soils, which are crucial for optimising soil conditioning and seepage control strategies in shield tunnelling.