Experimental investigation on dynamic characteristics of fiber-binder modified subgrade filler after freezing-thawing under cyclic loading

To examine the dynamic stability of fiber-binder modified subgrade filler (MSF), the dynamic triaxial test of MSF is carried out under the long-term cyclic loading after the freeze–thaw cycle. This paper describes the relationship between cumulative plastic strain (CPS) and the number of loading cycles (NLC) changing with different fiber contents, fiber lengths, additions of binder material, and freeze–thaw cycles. The critical dynamic stress under various influencing factors is determined by the numerical fitting method, and the critical dynamic stress model of MSF with multi-factor change is established. Then the normalized dynamic strength mechanical model is proposed by analyzing the normalized characteristics of dynamic strength concerning static strength. It is found that the MSF is brittle failure, the relationship between CPS and loading cycles before failure is roughly linear, and the shape of the curve after failure is approximately exponential. Compared with unmodified subgrade filler, the CPS resistance of MSF is significantly improved. The dynamic stress level required for the failure of MSF increases with fiber length and the addition of binder material increasing, and there is an optimum fiber length of 12 mm corresponding to the maximum cumulative deformation resistance. The improvement measures could improve the freeze–thaw resistance of the subgrade filler. After three and nine freeze–thaw cycles, the dynamic stress intensity of the MSF increased by 1.9 and 2.4 times compared with the unmodified filler. The proposed critical dynamic stress mechanical model of MSF provides an effective method and reference basis for controlling the long-term cumulative deformation of the railway subgrade in seasonally frozen soil regions. The normalized strength model of failure strength for MSF varying with the NLC is helpful to quickly judge the dynamic stress level required for the subgrade failure within a given number of train loads.