Cyclic modeling of unsaturated sands using a pore-scale hydromechanical approach

Determining proper strain-dependent shear modulus and damping ratio for soils is of utmost importance when investigating their response to cyclic loads. Experimental studies on the effect of the degree of saturation and suction on the shear modulus at large strain amplitudes are scarce due to the complexities involved in testing such soils. Furthermore, the previously presented micromechanical models for the cyclic behavior of unsaturated soils lack some of the required features of soil pore skeleton essential to model unsaturated soils' hydromechanical behavior. This paper is, thus, aimed at addressing this shortcoming by incorporating a pore-scale numerical method that couples the discrete element simulation of the solid phase and the fluid pore network to capture the cyclic behavior of unsaturated sands. To this end, the model is first calibrated to determine the micro-scale parameters using experimental results from static drained triaxial compression tests on dry and saturated sand samples. Next, the coupled model is employed to simulate the observed behavior during cyclic triaxial tests on unsaturated sands. A linear dependence of the plastic rolling coefficient and the interparticle friction angle on the degree of saturation is proposed from simulations of suction-controlled cyclic triaxial tests. Variation of shear modulus with shear strain amplitude and degree of saturation indicates a continuous increase in the shear modulus with the decrease in saturation. In contrast, the trend for the damping ratio is the opposite. Finally, closed-form relationships are proposed based on the simulation results for the shear modulus and damping ratio of unsaturated soils.

Automated summary

Determining strain-dependent shear modulus and damping ratio is crucial when investigating soil response to cyclic loads. Experimental studies on the effect of saturation and suction on shear modulus are lacking, and micromechanical models for unsaturated soils are incomplete. This paper presents a numerical method that combines discrete element simulation and fluid pore network to capture cyclic behavior of unsaturated sands. The model is calibrated using static drained triaxial tests on dry and saturated sand samples and then used to simulate cyclic tests on unsaturated sands. The results show a linear relationship between plastic rolling coefficient and saturation, as well as varying shear modulus and damping ratio with shear strain amplitude and saturation.