Study on the formation mechanism of the hard-shell layer of liquefied silty soil

Abstract Hard-shell layers often form during the migration and segregation of fine-grained sediments and play a critical role in influencing the mechanical behavior of seabed soils, with implications for marine geological hazards. Understanding the mechanisms that govern the formation and evolution of these layers is therefore essential for coastal and estuarine geotechnical engineering. In this study, laboratory experiments were conducted to investigate the formation mechanisms of hard-shell layers in saturated silty soils subjected to two types of controlled physical disturbances: vertical hammering and horizontal rotating wheel-induced shear. Using high-speed particle image velocimetry, pore pressure sensors, and earth pressure measurements, the study analyzed the coupled evolution of the force field, particle migration field, and liquefaction behavior under different disturbance modes. Results show that liquefaction is a prerequisite for hard-shell layer formation, allowing fine particles to migrate upward while coarse particles settle, leading to effective particle separation. Hammering produced high pore pressure and strong vertical particle mobility, resulting in a thin but dense hard-shell layer with high strength. In contrast, the rotating wheel disturbance generated moderate liquefaction and vortex-driven particle segregation, forming a thicker hard-shell layer with relatively lower mechanical strength. The findings highlight a trade-off between shell thickness and strength depending on the disturbance type and suggest that the mechanism of fine–coarse particle separation under liquefaction conditions is key to shell formation. This research provides a theoretical and experimental basis for understanding internal liquefaction processes in silty coastal slopes, particularly under the influence of waves or seismic activity following landslides.