Effect and Mechanism of Freeze–Thaw Cycles on Static and Dynamic Characteristics of Expandable Polystyrene Lightweight Soil

We study the static and dynamic characteristics of expanded polystyrene (EPS) lightweight soil (SCS) under freeze–thaw cycles (NF-T) through unconfined compressive strength (UCS) and dynamic triaxial tests. The stress–strain curve, UCS, static elastic modulus (E), dynamic strength (σdmax), dynamic elastic modulus (Ed), and damping ratio (λ) of SCS with different EPS contents and NF-T are discussed. From an energy perspective, the change rules of the SCS failure strain energy density and hysteresis loop energy dissipation after NF-T are analyzed. The results are as follows: (1) the UCS and E of the SCS decreased with increasing NF-T; the UCS decreased significantly after NF-T = 1 and then tended to stabilize. EPS particles can reduce the thermal conductivity and volumetric water content of SCS, and enhance the deformation ability of particles inside SCS, thereby improving the overall frost resistance of SCS; (2) σdmax and Ed decrease rapidly and then tend to be stable with an increase in NF-T. The SCS backbone curve and the curve of λ against dynamic strain have an obvious intersection when the dynamic strain is approximately 1%; and (3) the total work, elastic strain energy, and dissipated energy of the SCS decreased with increasing NF-T, which was significant after NF-T = 1. The dissipative energy of the SCS increased with increasing dynamic strain. When the dynamic strain was constant, the dissipative energy increased with NF-T. Accordingly, the relationships between the UCS and E and σdmax and UCS of SCS under different values of NF-T are established to provide a theoretical reference for the application of SCS to subgrade engineering.