K. Senetakis

The Small-Strain Shear Modulus and Damping Ratio of Quartz and Volcanic Sands

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.

Small-Strain Shear Modulus and Damping Ratio of Sand-Rubber and Gravel-Rubber Mixtures

This study examines the small-strain dynamic properties of mixtures composed of sandy and gravelly soils with granulated tire rubber in terms of shear modulus (GO), and damping ratio in shear (Dmin). Torsional resonant column tests are performed on dry, dense specimens of soil-rubber mixtures in a range of soil to rubber particles size 5:1–1:10 and rubber content from 0 to 35% by mixture weight. The experimental results indicate that the response of the mixtures is significantly affected by the content of rubber and the relative size of rubber to soil particles. Concering the small-strain shear modulus, an equivalent void ratio is introduced that considers the volume of rubber particles as part of the total volume of voids. Based on a comprehensive set of test results a series of equations were developed that can be used to evaluate the shear modulus and damping ratio at small shear strain levels if the confining pressure, the content of rubber by mixture weight, the grain size of soil and rubber particles, and the dynamic and physical properties of the intact soil are known.

The Development of a New Micro-Mechanical Inter-Particle Loading Apparatus

The inter-particle coefficient of friction comprises an essential input parameter in computer codes that utilize the discrete element method. This paper describes the main features of a custom-built apparatus of a new generation, capable of performing inter-particle shearing tests at very small displacements on the order of tens to hundreds of microns and measuring the frictional forces developed at the contacts of coarse-grained particles of sand to gravel size. Linear, micro-stepping motors are used for the inter-particle shearing tests of a displacement-controlled type and the application of the vertical confinement of a force-controlled type at the particle contacts. The apparatus is designed to work at very small confining forces, in general between 1 and 20 N, and utilizes a system of bearings of small friction, which can be calibrated following simple procedures. The experiments are controlled and monitored through a computer code developed for the apparatus. The signal conditioning and data-logging systems were optimized to give the minimum environmental and electrical noise in the experimental data. The particles tested must have a relatively convex shape and be fairly symmetrical about the axis of shearing to avoid significant lateral forces in the out-of-plane horizontal direction during sliding, and, in general, the size of particles is limited from about 0.50 to 5.0 mm. Tests on reference particles composed of chrome steel balls and quartz particles demonstrated high repeatability of the results and agreement with the literature data. The experimentally derived horizontal force–displacement data showed that the stiffness of the apparatus is sufficiently high to prevent significant stick-slip phenomena, allowing a stable sliding.

Wave Propagation Attenuation and Threshold Strains of Fully Saturated Soils with Intraparticle Voids

The prediction of ground response against wave propagation is essential for construction materials and the safe design of civil engineering infrastructures, such as, embankments, retaining walls, or foundations subjected to machine vibrations. For ground response analysis studies, the shear modulus and material damping, which are expressed as a function of shear strain, are the important properties of soils. The volumetric threshold strain is also a key property in order to evaluate possible permanent deformations or substantial increase in pore water pressure in saturated soils during dynamic loading. The paper presents dynamic test data derived from resonant column experiments on volcanic granular soils which are characterized by low unit weight and weak grains of intraparticle voids. These materials can be used as potential lightweight backfill in retaining walls or other applications with a demand in reduction of vertical or horizontal stresses to the ground and structural facilities. Additional experiments on quartz sands were conducted for comparison. The volcanic soils had much lower small-strain shear modulus than that of quartz sands and higher linearity in the range of medium strains, by means of normalized stiffness and material damping curves. The elastic and volumetric thresholds were shifted to larger strains for the volcanic soils in comparison to the quartz sands. Different prevailed micromechanisms possibly contributed to these observed trends.

Effect of Gradation and Particle Shape on Small-Strain Young’s Modulus and Poisson’s Ratio of Sands

AbstractThe influence of gradation and particle shape on the small-strain Young’s modulus of dry sands is investigated through a comprehensive set of resonant column tests in the flexural mode of vibration. Experiments are performed on an array of sands with different coefficients of uniformity and particle shapes. The effect of gradation is investigated using tests on sands with similar particle shapes. The effect of particle shape is then examined through the experimental results on sands with a range of particle shapes. A new model is developed to incorporate the effects of gradation and particle shape into the prediction of the small-strain Young’s modulus. The proposed model is verified and compared with the existing models in the literature using the state parameter in the critical state soil mechanics framework. It is shown that the proposed model outperforms the previous ones considering the significant effect of particle shape on the small-strain Young’s modulus. Using the theory of elasticity, a...

Effect of Young’s Modulus and Surface Roughness on the Inter-Particle Friction of Granular Materials

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 Tribological Behavior of Two Potential-Landslide Saprolitic Rocks

We investigate in this study the tribological behavior of two potential-landslide saprolites conducting grain-scale experiments with an advanced custom-built micro-mechanical loading apparatus. A lateritic rock from India and a completely decomposed granite from Hong Kong were used. These geological materials have been subjected to decomposition and disintegration due to weathering. The characterization of the materials at the grain scale was conducted based on energy-dispersive X-ray spectroscopy analysis, scanning electron microscope images, and interferometry analysis. Both materials showed high roughness values and their surface-morphological characteristics were relatively inconsistent. Repeated micro-mechanical shearing tests were conducted on pairs of grains, and their normal load—deflection, tangential load—deflection, tangential stiffness, and frictional responses were explored with a focus on the effect of repeating the shearing tests following the same shearing paths. It was shown that the behavior was markedly different, between shearing the grains at lower and greater normal loads; in the latter case the production of debris slightly increased the friction, but it substantially increased the tangential stiffness. These observations were also linked to the changes of the normal load—deflection behavior of surfaces subjected to pre-shearing. These findings are important in the fundamental understanding of the behavior of geological materials interfaces as well as the discrete modeling of natural and engineering problems. Digital microscopic images were also implemented to further support the observations from the study.

Effects of Particle Grading and Stress State on Strain-Nonlinearity of Shear Modulus and Damping Ratio of Sand Evaluated by Resonant-Column Testing

ABSTRACT The effects of particle grading characteristics, pressure level, and anisotropic stress state in triaxial compression on the decrease in the normalized shear modulus and the increase in the damping ratio of sand when the shear strain amplitude increases from 0.001% to 0.1% were evaluated conducting tests with a fixed-partly fixed resonant column. Specimens having different grading characteristics were reconstituted using crushed rock particles. The results show noticeable effects of the coefficient of uniformity, the mean confining pressure and the stress ratio on the shear strain-nonlinearity of shear modulus and damping ratio over the strain range examined.

Effect of Anisotropic Stress State on Elastic Shear Stiffness of Sand–Silt Mixture

Soil stiffness is a critical property in the evaluation of earth structure deformation and its accurate estimation is an essential part of any geotechnical investigation. In this study, the significant contribution of non-plastic silica fines content in the relationship between small-strain shear modulus and stress anisotropy of silty sands is investigated through a comprehensive set of bender element tests. All the samples of the study were created in the laboratory and tested in a stress path triaxial apparatus applying isotropic and anisotropic stress paths. The mixtures had variable contents of sand and a non-plastic silica silt. Data analysis demonstrated that the exclusive effect of fines content on the changing rate of elastic shear modulus due to anisotropic stress state turns out to be considerable and increasing for all the tested parent sands up to a specific silt portion, about 20%, after which this effect becomes negligible. Using the experimental results, the previously proposed expression for the prediction of the small-strain shear modulus under stress anisotropy for granular materials is extended to sand–silt mixtures. A comparison of the measured against the predicted elastic shear moduli under stress anisotropy, demonstrated that the concept of the skeleton (granular) void ratio could not be applied effectively in the evaluation of the dynamic behaviour of sand–silt mixtures.

Elastic Stiffness of Volcanic Sand through Resonant Column Tests

The resonant column method is established as a standard laboratory method for the study of the elastic properties of soils. The study presents low-amplitude resonant column test results on volcanic sands with intra-particle voids. The experiments were performed on dry samples prepared at variable relative densities and tested in torsional mode of vibration. In the first part of the article, the important factors that control the elastic stiffness of uncemented sands are described shortly and recent findings on granular soils dynamic properties are presented briefly. The second part describes the basic features of the resonant column used in the investigation and the materials of the study and in the third part representative results of an extensive experimental testing program on volcanic granular soils are presented and discussed with a focus on comparisons between the elastic stiffness of volcanic and quartz granular soils. The importance of the effect of the presence of intra-particle voids within the particle mass of the volcanic soils is emphasized, which in turn affects markedly the global void ratio of the samples.