Compression and extension behaviour of three-dimensionally printed metastructure–sand composites under triaxial loading

A three-dimensionally printed metastructure featuring a re-entrant honeycomb topology with negative Poisson’s ratio (NPR) effect is introduced in this study for material reinforcement. To systematically decouple the effects of structural topology and constituent material stiffness, the structures were fabricated using flexible thermoplastic polyurethane (TPU) and rigid polylactic acid (PLA) polymers. Then, a series of undrained and drained shear tests were conducted on the metastructure-enhanced sand composites under triaxial compression and extension. It is found that the NPR topology fundamentally transforms the deformation behaviour of reinforced loose sand composites, promoting a more ductile and stable post-yield response through nodal rotation. Crucially, the NPR composite mitigates the inherent tension–compression asymmetry of reinforced sand composite; its auxetic expansion provides counter-deformation radial support during extension, effectively suppressing localised necking instability. Furthermore, material stiffness dictates the reinforcement mechanism: flexible TPU ensures kinematic compatibility and sustains substantial negative pore water pressure, whereas the rigid PLA enhances peak initial confinement. NPR inclusions expand the state boundary surface and induce a positive intercept shift of the critical state line in the e – (p'/pa)0.7 plane exclusively under compression, highlighting a distinct strength anisotropy.