Anisotropy in small-strain shear modulus of granular materials: Effects of particle properties and experimental conditions

Abstract

Granular soils display directional diversity in stiffness at small strains, which is essential for geotechnical engineering design and can be reflected by the anisotropy of small-strain shear modulus G0. However, the understanding of the stiffness anisotropy of soils is still limited. This contribution presents a series of shear (S-) wave propagation tests using cubical specimens and planer piezoelectric transducers, and discusses the effects of the intrinsic particle properties and experimental conditions on the G0 anisotropy. Four granular materials: spherical glass beads, Toyoura sand, basmati rice, and wild rice are prepared densely in a dry condition and tested under both K0 and isotropic stress states. The experimental results reveal that the particle shape and particle orientation after deposition can affect the anisotropy of the S-wave velocities (Vs), and thus, G0. Tests with different granular materials show that a more elongated particle shape can induce larger anisotropy in G0. From the results of tests on the wild rice specimens, Vs is found to be higher when the major axes of the particles coincide with the direction of the wave propagation, compared to the situation when the major axes are parallel to the oscillation direction of the S-wave. Among the six components in the principal directions, the S-waves neither propagating nor oscillating along the particle orientation are the slowest. Considering the experimental conditions, a mix of flexible (membrane) and rigid boundaries tends to induce larger anisotropy in G0 and a measurable difference between symmetric shear moduli Ghv and Gvh, while the effect of the specimen aspect ratio is less significant.

Keywords

• Shear wave
• Shear modulus
• Granular materials
• Anisotropy
• Particle shape
• Particle orientation