Particle scale analysis of soil stiffness and elastic wave propagation

Abstract

Soils are granular materials consisting of many particles, and the overall response of a soil can be considered to be a complex accumulation of the inter-particle responses. Small-strain soil stiffness is important to predict the ground deformation in situ and in practice and is often deduced from elastic wave velocity in laboratory experiments. The dynamic properties of soils are also important for dynamic analyses including site response analysis. Stress waves propagate through soil via the grain contact network, thus the actual particle-scale mechanics differ from those assumed in continuum mechanics which is often used to simulate and analyse stress wave propagation. Thus the particle properties including surface characteristics should have a direct impact on the overall response of soil to stress wave disturbances. Surface roughness effects on the inter-particle response have previously been considered in the experimental work of Cavarretta (2009) and in the dynamic analyses using the discrete element method (DEM) described by O’Donovan (2013). This research aims to develop understanding of the extent of the sensitivity of soil stiffness to the contact rheology by adopting theoretical, numerical (DEM) and experimental approaches. The theoretical approach follows Yimsiri & Soga (2000) who combined micromechanical effective medium theory and a rough surface contact model; their approach is revisited here considering more recent UK-based tribology studies. The contact laws considered in the DEM analyses presented here include particle surface roughness, partial slip at tangential contacts, and spin resistance based on these developments by the work of O’Donovan (2013). The experimental approach used two types of dynamic tests: bender element tests in a cubical cell apparatus, and shear plate tests in a triaxial apparatus. For both test types, smooth and rough surface spherical ballotini are used to study the surface roughness effects on the sample shear modulus. Shear plates are not commonly used in soil mechanics dynamic testing and so the study also included an assessment of this technology. The data generated show that the small-strain stiffness of granular materials is measurably reduced sensitively with the surface roughness especially at a low stress level. This explains partially a higher exponent n value in the relationship between the shear modulus and the confining stress (n > 0.5). As the stress level increases the shear modulus of the assembly of rough particles approach the smooth equivalent.