Particle scale analysis of soil stiffness and elastic wave propagation

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.Open Acces

Sensitivity of G0 and stress-strain relation of geomaterials to grain shape and surface roughness

It is empirically known that the packing property and mechanical responses of cohesionless granular materials are influenced by grain shape. For example, the attainable range of void ratio depends on grain shape; angular materials tend to exhibit greater friction angles. Besides, shear wave velocity (Vs ) and small-strain shear modulus (G0 ) of sphere assemblies are affected by surface roughness. However, consensus has yet to be reached on the combined effect of grain shape and surface roughness on G0 and stress-strain relation. This contribution aims to evaluate the shape-roughness combined effect on the strain-dependent mechanical responses of granular materials. Three groups of glass beads having different grain shapes and a silica sand were used, and their grain surfaces were roughened through a systematic procedure using a milling machine. In total, eight materials were subjected to triaxial compression after measurement of Vs. The experimental results reveal that the stress-strain relation of angular particles is remarkably affected by the surface roughness, whereas the roughness effect on the stress-dependent variation in G0 is limited for angular particles

Selecting an appropriate shear plate configuration to measure elastic wave velocities

The (small-strain) elastic moduli of soil can be determined from stress wave velocity measurements. Bender/extender elements are widely used in laboratory experiments; however, discussion on how to accurately determine wave velocities using this method continues. Planar piezoelectric transducers (sometimes called shear plates) are a relatively new technology, whose use is not yet widely established, that appear to offer some advantages in comparison with bender/extender elements for laboratory geophysics tests. This contribution critically assesses the use of planar piezoelectric elements experimentally and using discrete element method (DEM) simulations. Planar piezoelectric elements capable of generating and receiving either shear or compression waves were placed in the top and base caps of a triaxial apparatus. Samples of glass ballotini were used so that stress wave propagation simulations could be performed on equivalent virtual samples using DEM. The appropriate shear plate configuration to effectively measure the shear wave velocity is explored. Considering both time- and frequency-domain responses, it is revealed that shear plate signals are sensitive to the surface area and thickness of the piezoelectric elements and to the lateral boundary conditions. Using a shear plate with the widest possible surface area exposed to the soil specimen is recommended to increase the signal-to-noise ratio and to produce more planar shear waves, resulting in a more accurate measurement of shear wave velocity

Influence of packing density and stress on the dynamic response of granular materials

Laboratory geophysics tests including bender elements and acoustic emission measure the speed of propagation of stress or sound waves in granular materials to derive elastic stiffness parameters. This contribution builds on earlier studies to assess whether the received signal characteristics can provide additional information about either the material’s behaviour or the nature of the material itself. Specifically it considers the maximum frequency that the material can transmit; it also assesses whether there is a simple link between the spectrum of the received signal and the natural frequencies of the sample. Discrete element method (DEM) simulations of planar compression wave propagation were performed to generate the data for the study. Restricting consideration to uniform (monodisperse) spheres, the material fabric was varied by considering face-centred cubic lattice packings as well as random configurations with different packing densities. Supplemental analyses, in addition to the DEM simulations, were used to develop a more comprehensive understanding of the system dynamics. The assembly stiffness and mass matrices were extracted from the DEM model and these data were used in an eigenmode analysis that provided significant insight into the observed overall dynamic response. The close agreement of the wave velocities estimated using eigenmode analysis with the DEM results confirms that DEM wave propagation simulations can reliably be used to extract material stiffness data. The data show that increasing either stress or density allows higher frequencies to propagate through the media, but the low-pass wavelength is a function of packing density rather than stress level. Prior research which had hypothesised that there is a simple link between the spectrum of the received signal and the natural sample frequencies was not substantiated

Soil liquefaction in Tokyo Bay area due to the 2011 Tohoku (Japan) earthquake

A devastating earthquake hit the Tohoku and Kanto regions of Japan on 11 March 2011, causing extensive damage to life and property as a result of a large-scale tsunami and damage to nuclear power plants. Although located about 400 km away from the epicentre, many residential and commercial buildings and lifeline facilities in Tokyo Bay area suffered extensive damage due to soil liquefaction and associated ground deformation. This paper discusses the results of the damage investigation conducted in the area after the earthquake, with emphasis on liquefaction-induced damage to buildings, roads, lifelines and other infrastructure. In addition, the performance of ground improved by various remediation techniques is discussed. Finally, lessons learned from the event are summarised.</jats:p

Shaping force-transfer arch to retain subsurface cavity in coarse sandy ground

Ground cave-ins, which are the collapse and discontinuous subsidence of the ground surface, are thought to be caused by the expansion and upward movement of subsurface cavities due to fluctuations in the groundwater table or earthquakes. Compared to cohesive clays or plastic silts, cohesionless sands are more vulnerable to cavity formation and subsequent ground cave-ins. With recent technology, such as ground-penetrating radar, geometrical information on cavities, e.g., location and shape, can be detected. In practice, the soil cover thickness-to-cavity width ratio (H/B) is often used for risk assessments of cave-ins. However, it is questionable whether H/B alone is sufficient for these risk assessments since the mechanical responses, such as the resistance of the remaining soil above the cavity, are not considered. For this reason, the aim of the present contribution is to understand the mechanism underlying the subsurface cavity stability by considering the force transfer around the cavity. Suction measurement, cavity retention, and needle penetration model tests were conducted using various coarse granular materials. The results revealed that suction is essential to preventing cavities from collapsing, and that suction is higher for smaller particles, particles with lower degrees of saturation, and particles with angular shapes and smoother surfaces. In addition to H/B, the mechanical interlock from angularity or roughness contributes to cavity stability. Laboratory needle penetration tests revealed the existence of a force-transfer arch between the sound and weakened zones around a cavity, which is related to the cavity stability. Furthermore, the position of the arch is affected not only by H/B, but also by the particle characteristics (e.g., friction angle) and cavity roof shape. Therefore, considering the material type and the shape of the cavity roof, along with H/B, will lead to enhanced assessments of the cave-in potential of subsurface cavities

Particle crushing and critical state of volcanic pumice – 2018 Hokkaido Eastern Iburi Earthquake

Volcanic pumice, with special characteristics such as crushable particles and high water retention, is distributed throughout Japan and serves as the source layer for slope hazards characterised by post-failure gentle slope flows and long-distance flows. The aim of this study is to determine the relationship between the crushing characteristics and the mechanical properties of porous pumice, which often contributes to such disasters. As the porous pumice material, Ta-d pumice, which caused numerous slope disasters during the 2018 Hokkaido Eastern Iburi Earthquake in Japan, was collected and subjected to a series of triaxial compression tests. The grain size distribution of the pumice before all the tests was adjusted to be uniform, and the amount of crushing was quantified by measuring the grain size distribution after the tests. The results suggest that the critical state and isotropic consolidation of porous pumice can be systematically expressed in a three-dimensional space with the axes of the void ratio, mean effective stress, and degree of particle crushing. Furthermore, a gentle slope disaster with an inclination of less than 21°, that had occurred at the site from which the Ta-d pumice was collected, was discussed in terms of its flow potential, showing that the flow distance can be adequately explained

Contractancy and shear behavior of extremely loose structure soils with particle breakage in saturated and unsaturated conditions

Pumices with high pore voids of volcanic origin are distributed throughout Japan and are the causal layer of slope failures. In many cases of surface failures, it is difficult to assume that the resulting layers are fully saturated. The high waterholding capacity of the pumice suggests that they were deposited in an unsaturated state with a high degree of saturation. In this study, saturated triaxial compression tests and fully undrained unsaturated triaxial compression tests were conducted on artificially produced pumice and natural pumice while measuring the amount of crushing. This is to clarify the relationship between the crushing and mechanical properties of pumice with porous particles, which are often the cause of such disasters, and their behaviour under unsaturated conditions. The results showed that the pumice stones have an ultra-high pore structure. Moreover, pumice with porous particles reached a steady state under both saturated and highly saturated unsaturated conditions, and the amount of crushing increased under highly saturated unsaturated conditions

Effects of seepage flow on liquefaction resistance of uniform sand and gap-graded soil under undrained cyclic torsional shear

Internal erosion is the transportation of soil particles from within or beneath geotechnical structures, caused by seepage flow, that impacts the subsequent mechanical and hydraulic behaviour of the soil. However, it is difficult to predict the liquefaction resistance of eroded soil due to several factors related to the soil fabric. The present study investigates the impact of seepage flow on the undrained cyclic behaviour of two types of soil: uniform sand and gap-graded soil with a fines content of 20%, using a novel erosion hollow cylindrical torsion shear apparatus. From the results for the uniform sand, the soil fabric formed by moist tamping (MT) leads to higher liquefaction resistance than that formed by air-pluviation (AP). However, after applying seepage flow, the liquefaction resistance of the eroded MT specimens becomes even lower than that of the non-eroded AP specimen. Therefore, the liquefaction resistance of soil is expected to decrease due to the rearrangement of the initially stable coarse particles during seepage flow. On the other hand, the liquefaction resistance of the gap-graded soil tends to increase after the removal of fines as the number of stable contacts between the coarse particles is increased. Under these test conditions, the latter effect is found to be greater for the given gradation, leading to a slight increase in the liquefaction resistance of the tested gap-graded soil after internal erosion. Furthermore, the intergranular void ratio and small-strain shear modulus are seen to be well correlated with the liquefaction resistance of the tested soil