In the study we experimentally examine the influence of elastic properties and surface morphology on the inter-particle friction of natural soil grains. The experiments are conducted with a custom-built micromechanical apparatus and the database is enhanced by testing engineered-reference grains. Naturally-occurring geological materials are characterized by a wide spectrum of mechanical properties (e.g., Young’s modulus) and surface morphology (e.g., roughness), whereas engineered grains have much more consistent characteristics. Comparing to engineered materials, geological materials are found to display more pronounced initial plastic behavior during compression. Under the low normal load range applied in the study, between 1 and 5 N, we found that the frictional force is linearly correlated with the applied normal load, but we acknowledge that the data are found more scattered for natural soil grains, especially for rough and weathered materials which have inconsistent characteristics. The inter-particle coefficient of friction is found to be inversely correlated with the Young’s modulus and the surface roughness. These findings are important in geophysical and petroleum engineering contents, since a number of applications, such as landslides and granular flows, hydraulic fracturing using proppants, and weathering process of cliffs, among others, can be simulated using discrete numerical methods. These methods employ contact mechanics properties at the grain scale and the inter-particle friction is one of these critical components. It is stressed in our study that friction is well correlated with the elastic and morphological characteristics of the grains.

The dynamic properties of soils in the region of very small strains are essential for any seismic design. This paper aims to investigate the dynamic small-strain shear modulus (GO) and damping ratio (DO) of reconstituted dry sands of variable mineralogy, shape, and grain-size distribution. In particular, the low-amplitude torsional resonant column test results of 31 specimens are synthesized, 19 specimens of natural and quarry sands predominately composed of quartz particles, and 12 specimens of volcanic sands composed of rhyolitic glassy rock of porous particles. It is concluded that the volcanic sands exhibit significantly lower GO values and slightly lower DO values in comparison to the quartz ones whilst the response of the quartz sands is significantly affected by the shape of the particles. The differences in the observed responses between quartz and volcanic sands are partially attributed to the variability in particles density, morphology, and mineralogy, as well as the higher void ratio and the lower dry density that the volcanic sands exhibit in comparison to the quartz ones. Overall, the effects of the mean effective confining pressure (σm′), the void ratio (e), and the grain-size distribution on the dynamic response of the volcanic soils follow a similar trend as in the quartz sands. Using the general form of available relationships presented in the literature, and after modifying the “constant” parameters, appropriate equations, stemming from the low-amplitude resonant column data test, are proposed that may be used for the estimation of the small-strain shear modulus and damping ratio separately for natural quartz sands, quarry quartz sands, and volcanic granular soils.