Effect of initial packing density, stress level and particle size ratio on the behavior of binary granular material: a m
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ORIGINAL PAPER
Effect of initial packing density, stress level and particle size ratio on the behavior of binary granular material: a micromechanical approach Fatemeh Vahidi‑Nia1 · Hamed Bayesteh1 · Mahdi Khodaparast1 Received: 16 November 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The geo-mechanical behavior of granular materials rigorously depends on their initial packing density, stress level and particle size distribution. Moreover, the behavior of binary granular soil is affected by the quality and quantity of their small particles named the fines content (FC). The contribution mechanism of the FC to the load-bearing structure of the soil at various particle sizes, stress levels and densities is still pending. The present study aimed to use a micromechanical approach to simulate the behavior of binary mixtures with particle size ratios (α) of 2.5 to 7.1 and study the effect of stress level and initial packing density on the stress–strain behavior using discrete element method (DEM). Accordingly, the effect of FC on the internal friction angle, anisotropy parameters, coordination number and coarse–fine (C–F) contacts have been studied. The results indicate that the variation in peak shear strength versus FC is bell-shaped with a minimum for FC of 30% to 40% and this is independent of initial packing density and stress level, but depending on the value of α. The greater particle size ratios (α = 7.1) increase the tendency of fines particles to rotate during biaxial testing, reduced the shear strength in comparison with samples with small particle size ratios (α = 2.5). Exceeding the threshold FC (30–40%) led to formation of stronger force chains among the fines and increased the role of FC on the peak shear strength. Keywords DEM · Micromechanics · Binary · Granular material · Fines content List of symbols ac Anisotropy coefficient of contact normal an Anisotropy coefficient of normal contact force at Anisotropy coefficient of tangent contact force ε1 Axial strain C–C Coarse–coarse C–F Coarse–fine P Confining pressure CN Coordination number D Packing density FC Fines content F–F Fine–Fine μ Friction coefficient between particles φ Internal friction angle Dc Maximum diameter of coarse grains Df Minimum diameter of fine grains * Hamed Bayesteh [email protected] 1
Department of Civil Engineering, University of Qom, Qom, Iran
Kn Normal stiffness Fn Normal force Nt Number of particles PSD Particle size distribution α Particle size ratio PFA Peak friction angle δ Relative tangential surface displacement at the contact q Shear stress q/p Stress ratio KS Tangent stiffness εv Volumetric strain ζ Relative displacement of particle in the normal direction
1 Introduction Granular mixtures are naturally created by landslides, debris flow and alluvial deposits and comprise coarse particles (sand-gravel) and fines (sand-silt or clay) that form binary-graded mixtures (e.g. [51, 55].). Understanding the
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