ABSTRACT
Excavated soil generated during Earth Pressure Balance (EPB) shield tunneling often contains a significant amount of foam. Understanding the seepage failure characteristics of foam-soil mixtures is crucial for comprehending the spewing mechanism of shield muck. A series of upward seepage tests were conducted to investigate the effect of foam on the seepage failure patterns of gap-graded soils. The results showed that for gap-graded soils with low fines content (FC), foam bubbles could not remain stable within the pores, leading to piping failure. However, when the FC was higher (FC ≥ 20%), foam enhanced the internal stability of the gap-graded soil, resulting in heave failure. The critical hydraulic gradient values for heave failure approach 1, whereas those for piping failure are notably below 1. Additionally, the long-term hydraulic performance of foam-conditioned soils with relatively high FC was examined, revealing that under a water pressure of 5 kPa, the internal stability of these soils gradually deteriorated over time, eventually leading to piping failure. These findings provide important insights into the muck spewing mechanism in EPB tunneling.
