The nature of soil stiffness at small strains remains poorly understood. The relationship between soil stiffness (e.g. shear stiffness, G0) and isotropic confining pressure (p′) can be described using a power function with exponent (b), that is, G0 = A (p′/pr)b, where A is a constant and pr is an arbitrary reference pressure. Experimentally determined values of b are usually around 0·5 and these are higher than the value of 0·33 that can be analytically determined using Hertzian theory. Hertzian theory considers contact between two smooth, elastic spheres; however, in reality, inter-particle contacts in soil are complex with particle shape and surface roughness affecting the interaction. Thus Hertzian theory is not directly applicable to predict real soil stiffness. It has, however, provided a useful basis to develop an analytical framework to consider the influence of particle surface roughness on small-strain soil stiffness. Here, earlier contributions using this framework are extended and improved by p...

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Quantitative assessment of the influence of surface roughness on soil stiffness

In this study experimental data on a model soil in a cubical cell are compared with both discrete element (DEM) simulations and continuum analyses. The experiments and simulations used point source transmitters and receivers to evaluate the shear and compression wave velocities of the samples, from which some of the elastic moduli can be deduced. Complex responses to perturbations generated by the bender/extender piezoceramic elements in the experiments were compared to those found by the controlled movement of the particles in the DEM simulations. The generally satisfactory agreement between experimental observations and DEM simulations can be seen as a validation and support the use of DEM to investigate the influence of grain interaction on wave propagation. Frequency domain analyses that considered filtering of the higher frequency components of the inserted signal, the ratio of the input and received signals in the frequency domain and sample resonance provided useful insight into the system response. Frequency domain analysis and analytical continuum solutions for cube vibration show that the testing configuration excited some, but not all, of the system’s resonant frequencies. The particle scale data available from DEM enabled analysis of the energy dissipation during propagation of the wave. Frequency domain analysis at the particle scale revealed that the higher frequency content reduces with increasing distance from the point of excitation.

https://link.springer.com/content/pdf/10.1007%2Fs10035-015-0599-4.pdf

Micromechanics of seismic wave propagation in granular materials

Resource utilization of iron tailings powder is an effective measure to reduce the dam-break risk of an iron tailings reservoir. Adding polypropylene fiber to iron tailings powder can improve its shear performance. Direct shear tests were carried out on reinforced iron tailings powder with polypropylene fiber with dosages of 0%, 0.25%, 0.5%, 0.75%, and 1%, respectively. The normal stresses during the tests were 100, 200, 300, and 400 kPa, respectively. The test results show that with the increase of polypropylene fiber dosage, the cohesive force of iron tailings powder firstly increases and then decreases gradually, and the internal friction angle firstly decreases and then increases. The back propagation (BP)neural network was used to fit the shear force (F) and shear displacement (s) of the test to obtain the F-s function relationship that satisfies the accuracy. Based on the energy dissipation theory, the direct shear energy dissipation of polypropylene-fiber-reinforced iron tailings powder was calculated. The mathematical model of energy dissipation of fiber interfacial failure was derived by the fiber distribution model. The interfacial strength parameters of polypropylene fiber were calculated based on the direct shear test data and the mathematical model of fiber interfacial energy dissipation. The test results show that the addition of polypropylene fiber from the perspective of energy dissipation can improve the shear properties of iron tailings powder.

Investigation on Direct Shear and Energy Dissipation Characteristics of Iron Tailings Powder Reinforced by Polypropylene Fiber

The fabrication of soil samples of uniform density is a fundamental problem in laboratory and physical experiments. A novel technique that relies on a systematic increase of density induced by thermal cycling is presented here. The principle is simple; when the sample is heated the grains and their container undergo thermal expansion and this leads to the settling of the material due to the differential thermal expansion between the grains and the container and the metastable nature of the granular assembly. This change in fabric is not reversible upon cooling down of the sample, so the newly formed fabric can be used for experimental testing. Moreover, the soil fabric can be incrementally enhanced using successive thermal cycles of heating–cooling. In this paper, the change of void ratio is investigated in two carbonate sands, a silica sand and glass ballotini. The effect of particle morphology and initial density on the thermal-induced mechanisms of grain rearrangement is discussed. On the basis of expe...

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Enhancing soil sample preparation by thermal cycling

The behavior of a sandy soil in laboratory tests is highly influenced by the sedimentation technique. In this study, a calcareous sand from the reclamation site in Persian Gulf is used as the mater...

X-ray analysis on the effect of sample preparation on the microstructure of calcareous sands

https://research-information.bris.ac.uk/files/155636261/Energy_efficiency_of_fibre_reinforced_soil_formation_at_s_2018_Geotextiles_a.pdf

Energy efficiency of fibre reinforced soil formation at small element scale: Laboratory and numerical investigation

A pluviation device for the fabrication of cubical samples of granular materials has been recently developed. The granular material is pluviated under gravitational loading through a rectangular Pe...

Experimental and numerical assessment of a cubical sample produced by pluviation

Understanding wave propagation through soils is essential for site response analysis in earthquake engineering, interpretation of geophysical surveys and SASW (Spectral Analysis of Surface Waves), interpretation of laboratory bender element tests, etc. Analysis of wave propagation has largely been based on continuum descriptions and two dimensional analyses. This study presents recent developments in multiaxial testing that permit the combination of laboratory seismic testing with exploration of three-dimensional principal stress space. A Cubical Cell Apparatus with bender-extender piezoceramic elements fitted in all six faces are used so that wave propagation velocities of an analogue granular material can be determined. The results of a first series of wave propagation tests for a sample under isotropic confinement are presented.

Jean-Francois Camenen

Papers

Energy efficiency of fibre reinforced soil formation at small element scale: Laboratory and numerical investigation

Experimental and numerical assessment of a cubical sample produced by pluviation

Experimental investigation of wave propagation in three dimensions in unbounded particulate assemblies