Mechanical behavior of face mask chips-sand mixture under three-dimensional stress state

Reusing shredded face mask chips (FMC) as soil reinforcement provides a sustainable approach for geotechnical engineering, yet the behavior of FMC-sand mixtures (MSM) under true three-dimensional stress states remains poorly understood. In this study, a comprehensive series of true triaxial tests was conducted to investigate the coupled effects of intermediate principal stress ratio (b-value: 0.00, 0.25, 0.50, 0.75, and 1.00) and FMC geometry (i.e., 4 aspect ratios and 3 surface areas) on the mixtures. Results indicate that FMC inclusion enhances shear strength and effective stress ratio, while reducing shear modulus and delaying the onset of dilation. With increasing b-value, both the peak deviatoric stress and effective peak friction angle initially increased and then decreased. Crucially, the FMC geometry-dependent response under true triaxial loading is strongly influenced by intermediate principal stress state and the associated three-dimensional strain redistribution. Furthermore, increasing the individual FMC size enhances the measured peak resistance, which can be interpreted as being consistent with a more spatially continuous reinforcement effect. These findings provide an robust mechanical framework for the design and application of FMC-reinforced sand in geotechnical infrastructure.