Particle-scale insight into transitional behaviour of gap-graded materials – small-strain stiffness and frequency response

Abstract

This study aims to develop a fundamental understanding of the role of fine particles on the small-strain stiffness of gap-graded granular soils. Stiffness was measured using cyclic triaxial probes, which give a measure of static stiffness, and dynamic wave propagation data, from which the dynamic stiffness can be measured. Assemblies of loosely packed spherical particles were considered. In the laboratory, local deformation transducers were used to measure the static stiffness, while the dynamic stiffness was calculated from stress wave velocities, measured using planar piezoelectric elements. To relate the particle-scale responses to the overall soil stiffness, complementary discrete element method (DEM) simulations were performed in which both static and dynamic stiffnesses were measured. Both the laboratory and the DEM data indicate that at low fines contents (< 30%) the stiffness decreases with increasing fines content. When the fines content increases from 30% to 35% there is a sharp increase in stiffness with increasing fines content; this is understood to mark the transition point at which the fines start to contribute significantly to the overall behaviour. Analyses of the frequency domain response of shear wave signals revealed that the lowpass frequency increases significantly at this transition point. This observation can be used to develop experimental interpretation protocols to assess to what extent fines are contributing to the overall soil stiffness.

Using geophysical data to quantify stress transmission in gap-graded granular materials

“…increases at 40%≤FC≤50%, showing what can be termed a transitional response over these FC values. A similar transitional behaviour was observed by Otsubo et al (2019) who tested loosely packed gap-graded glass bead samples using both dynamic laboratory tests (using shear plates) and DEM simulations.…”

X-ray 3D imaging–based microunderstanding of granular mixtures: Stiffness enhancement by adding small fractions of soft particles

“…Small-strain stiffness, i.e., elastic stiffness, is a fundamental macroscale mechanical parameter for a wide range of engineering problems, and it is used for the prediction of granular response under both static and dynamic loading conditions ( 11 – 18 ). If both conditions, small amplitude and long wavelength, are fulfilled, wave measurements can be used to infer elastic stiffness ( 19 – 22 ).…”