Dynamic behavior of clay modified with polypropylene fiber under freeze-thaw cycles

In order to explore the behavior of clay reinforced by polypropylene (PP) fiber, a series of dynamic triaxial tests were conducted on samples with multi-stage dynamic loading process under different confining pressures of 0.3 MPa, 0.4 MPa and 0.5 MPa. The soil physical properties were significantly affected by the seasonal freezing of the active layer. To simulate this effect, all samples were subjected to the freeze-thaw process of up to 15 cycles. Samples reinforced with 0.75% PP fiber showed the greatest dynamic strength, and a similar regularity was found for samples after 15 freeze-thaw cycles. The dynamic shear modulus decreased with the increase of confining pressure, but decreases with the increasing strain and number of freeze-thaw cycle. The dynamic shear modulus of samples with PP fiber is relatively larger than that of unreinforced samples. After 15 freeze-thaw cycles, the dynamic shear muduli of samples with 0.25%, 0.5% and 0.75% fiber increased by 27.3%, 50.1% and 63.6%, respectively, compared with the one of unreinforced samples at gamma = 0.3% and sigma(c) = 0.3 MPa. The dynamic shear moduli of unreinforced sample and samples with 0.25%, 0.5% and 0.75% fiber content after 15 freeze-thaw cycles decreased by 7.9%, 7.6%, 5.4% and 6.3%, respectively, compared with the one before freeze-thaw cycle at y = 0.05%. The damping ratio shows an increasing tendency with the increases of shear strain and fiber content. The damping ratio of samples reinforced with 0.25%, 0.5% and 0.75% fiber increased by 11.11%, 8.89% and 15.56%, respectively, compared with the one of unreinforced sample at gamma = 0.005%. The Davidenkov model was adopted to describe the dynamic behavior of reinforced soil after freeze-thaw cycles at various confining pressures. The dependences of the normalized shear modulus on the shear strain amplitude are described. The behavior of the fiber after exposure to freeze-thaw cycles was analyzed by using Scanning Electron Microscope (SEM) method.