Summary |
Dynamic characteristics of soils are of significant importance for the solution of many soil dynamics problems. The shear modulus, G and the damping ratio, D are two of the most important dynamic parameters of soil. Nevertheless, geotechnical engineers always have a good understanding on the first parameter, G, but do not have enough knowledge regarding the second parameter, D. The mechanisms that govern energy dissipation in soil remain unclear or even unknown at present, especially from the micromechanics point of view. In this study, numerical simulations by the discrete element method (DEM) were carried out to examine the characteristics of energy dissipation and dynamic properties in soil under static biaxial test and cyclic shear test. In the simulations of static biaxial test of loose and dense samples, it has been found that the applied boundary work is either stored in the strain energy or dissipated by interparticle friction when viscous energy loss is not considered. At the critical state, the boundary work is used to balance the frictional loss and the strain energy ceases to increase. The release of frozen strain energy can be readily seen in the dense sample when the sample is sheared to dilate and under a strain softening response. In the simulations of cyclic shear test at small strains from 6x10-6 to 1.5x10-4 using sphere particles, both frictional and viscous energy losses were considered. The simulated damping ratio and the associated shear modulus can be obtained from two methods: traced energy and the resulting hysteresis loop. These two methods can render almost identical results. The simulation can reproduce similar responses like the experimental findings in both shear modulus and damping ratio at different confining pressures, strain levels, and strain rates. It has also been realized that not only the lost energy but also the stored energy that is related to the associated shear modulus can significantly affect the response of damping ratio. In addition, the frictional loss mainly takes place at the weak force network but the viscous energy, to a certain extent, is evenly distributed in the strong and weak force networks. In the simulations of cyclic shear test at strains from 2x10-6 to 7x10-4 using clump particles, with increasing particle numbers (i.e., decreasing particle sizes) in the sample, the shear modulus decreases whereas the damping ratio increases. Increasing particle numbers leads to decreasing associated contact normal forces among particles, which in turn lower the contact stiffness and therefore the shear modulus of the sample. The damping ratio responses at different strain levels are the net results between the stored energy that is related to the shear modulus, and the dissipated energy that mainly takes place at those weak contacts where the contact normal forces, Fn are less than the associated mean value Fnmean. The shear modulus and damping ratio can also be increased and decreased, respectively, by increasing particle aspect ratio. The simulations regarding aging effects reproduce the experimental observations. The contact normal forces among particles become more homogenized after aging. This in turn increases the shear modulus and leads to increasing contact normal forces at weak contacts, thereby reducing the frictional loss and damping ratio. Both unaged and aged samples have similar shear moduli and damping ratios as strain gradually increases because homogenized contact forces established during aging can be progressively erased by subsequent shearing and associated structural changes. |