Nonlinear Viscosity of Subgrade Soils Under Cyclic Loading

Experimental investigations of nonlinear shear viscosity of subgrade soils under periodic loading have been conducted. Unsaturated subgrade soil samples were prepared to conduct triaxial shear tests under repeated loading that was used to simulate the cyclic stress induced by vehicles. Investigations included three aspects. First, a new nonlinear viscous model has been introduced to describe the relation of shear stress and shear strain rate. This relation has been reduced from the constitutive law of nonlinear poroviscosity suggested by Li (1994) as well as Li and Helm (1995 and 1997). Second, related constitutive parameters have been investigated and calibrated with experimental results from triaxial shear tests. In the present paper, nonlinear viscous parameters are assumed to be a function of deviatoric strain and loading repetitions. Third, both viscous and elastic behavior of soils associated with the deviatoric strain and the cyclic loading repetition has been studied. The correlation between shear viscosity and resilient modulus has been discussed as well. This is the first attempt to find any correlation between soil viscosity and elasticity through laboratory investigations. In contrast, in traditional numerical computations (e.g., the finite element method), the extra viscous term added to the governing equation is assumed to be related to system mass and rigidity.