A Novel Unsteady Fractal Derivative Creep Model for Soft Interlayerswith Varying Water Contents

Creep properties of soft interlayers are key factors associated with the long-term stability of geological bodies. An experimental and theoretical study on the ring shear creep properties of soft interlayers collected from Esheng, Sichuan province, China are performed in this study. Ring shear creep tests of soft interlayers, which are remolded into over-consolidated samples having various water contents and the same initial dry density, are performed in laboratory, to analyze the creep deformation characteristics of samples in detail. The calculated long-term shear strength of samples is close to residual strength. By substituting the dashpot with a new unsteady fractal dashpot, a novel unsteady fractal derivative creep (UFDC) model, which can be defined in series with an improved Maxwell model and an improved viscoplastic model, is proposed based on theory of fractal derivative. The new model can efficiently explain the soft interlayers creep deformation. The results indicate that most model parameters are sensitive to the shear stress. However, at the accelerated creep stage, the fractional order of the second dashpot in the UFDC model has little effect on the fitting of experimental data.