Shear characteristics of foamed flowable stabilized soil under triaxial compression conditions

Foamed Flowable Stabilized Soil (FFSS) is widely used in backfill engineering. However, its performance evaluation is mostly based on unconfined compressive strength, with limited studies addressing the influence of lateral confinement. In this study, a series of Unconsolidated Undrained (UU) triaxial shear tests were conducted to systematically investigate the effects of wet density, silt content, and confining pressure on the mechanical behavior of FFSS. The results show that the stress–strain curves of the specimens exhibit distinct elastoplastic characteristics and conform to the Mohr–Coulomb failure criterion. Both increased confining pressure and wet density contribute to enhanced triaxial compressive strength. Specifically, when wet density increased from 600 kg/m³ to 1200 kg/m³, cohesion increased by over 250%, while the internal friction angle decreased with increasing density. Confining pressure was also found to delay post-peak strength degradation, reducing the brittleness index by 40%~60%. A critical silt content of 35% was identified, at which the material achieved optimal strength and deformation performance. Moreover, a multiple regression model for shear strength was developed based on the test data, offering valuable support for the engineering design and performance prediction of FFSS.