Monotonic and Cyclic Shear Behaviors of EPS Geofoam: Experiments and Hypoplastic Modeling

ABSTRACT This paper investigates the stress–strain relationships and volumetric behaviors of expanded polystyrene (EPS) under compression and monotonic/cyclic shearing, considering the effects of vertical stress ( σ v ) and EPS density ( ρ EPS ). The experimental results demonstrate that under monotonic shearing, EPS exhibits strain hardening, and the resulting shear‐induced axial strain ( ε a ) significantly exceeds the creep‐induced ε a under sustained σ v . The apparent shear strength ( τ f ) is governed by equivalent cohesion ( c ), which increases linearly with ρ EPS , while the equivalent friction angle ( φ ) remains insensitive to ρ EPS . Under cyclic shearing, symmetric hysteresis loops form, with secant shear stiffness ( G ) increasing with ρ EPS and σ v but decreasing with shear strain amplitude ( γ a ), the equivalent damping ratio ( D ) shows opposite trends. Axial strain accumulates nonlinearly during cycling, stabilizing after about four cycles under fixed γ a but increasing continuously under multistage loading. An extended hypoplastic model is proposed, incorporating two key advancements: the characterization of shear‐induced volumetric contraction and a shear strain‐dependent equivalent cohesion to describe damage accumulation. The model accurately simulates the stress–strain relationships and axial strain evolution of EPS with different densities and stress levels under both monotonic and cyclic shearing, providing a robust tool for analyzing EPS in geotechnical applications.