Dynamic properties of granular materials at the macro and microscales

Abstract

Dynamic properties of soil, including modulus and damping, play essential roles in

evaluating the response of the soil deposit and its supporting structures when

subjected to dynamic loads induced by earthquakes, traffic, explosions, machine

foundations, and so on. It is well recognized that the dynamic properties of soil are

affected by many factors, such as strain amplitude, stress condition, void ratio,

saturation and gradation. Despite tremendous works have been done, the

macroscopic effects of several key factors on the dynamic properties of granular

material are not yet fully understood, due primarily to its particulate and multiphase

nature. Furthermore, the understanding of how the influencing factors affect the

dynamic properties of granular material or the underlying fundamental mechanism is

inadequate. This study thus is carried out to investigate the effects and the underlying

mechanisms of these important factors, including strain amplitude, stress condition,

void ratio, particle size, saturation, and initial fabric, by means of advanced

laboratory tests and numerical simulations.

To study the dynamic properties at the macro scale, a series of laboratory tests are

carried out in a state-of-art resonant column (RC) apparatus incorporating bender

element (BE) and torsional shear (TS). Test materials include artificial glass beads

with different sizes, commercially available standard sands and natural completely

decomposed granite (CDG). The specimens are prepared at various densities,

confined at different pressures, tested both in dry and saturated conditions, and

reconstituted by different preparation methods. In particular, the characteristics of

wave signals (both S-wave and P-wave) at various conditions and the associated

interpretation methods in BE tests are investigated in detail. The results obtained

from BE, RC and TS are compared to clarify the potential effect of test method.

Moreover, attempts are made to explain the test results from the viewpoint of

micromechanics.

Numerical simulations using discrete element method (DEM) are performed to study

the dynamic properties of granular materials and explore the underlying fundamental

mechanism at the micro scale. The simulations indicate that the elastic properties are

closely related to the coordination number and the distribution of normal contact

forces in the specimen. The effects of initial fabric and induced fabric, which are

respectively achieved by different specimen generation methods and the application

of anisotropic stress states, are investigated. The anisotropy of the specimen and its

evolution during shearing are also studied. The results indicate that the anisotropy is

resulted from the spatial distributions of contact force and contact number. The

modulus reduction curve and damping curve obtained from the simulations are

compared with those from laboratory tests.