The size and shape of soil particles reflect the formation history of the grains. In turn, the macroscale behavior of the soil mass results from particle level interactions which are affected by particle shape. Sphericity, roundness, and smoothness characterize different scales associated with particle shape. New experimental data and results from published studies are gathered into two databases to explore the effects of particle shape on packing density and on the small-to-large strain mechanical properties of sandy soils. In agreement with previous studies, these data confirm that increased angularity or eccentricity produces an increase in emax and emin. Furthermore, the data show that increasing particle irregularity causes a decrease in stiffness yet heightened sensitivity to the state of stress; an increase in compressibility under zero-lateral strain loading; an increase in the critical state friction angle cs; and an increase in the intercept of the critical state line there is a weak effect on the slope . Therefore, particle shape emerges as a significant soil index property that needs to be properly characterized and documented, particularly in clean sands and gravels. The systematic assessment of particle shape will lead to a better understanding of sand behavior.

Particle Shape Effects on Packing Density, Stiffness, and Strength: Natural and Crushed Sands

The shear strength of liquefied soil, su(LIQ), mobilized during a liquefaction flow failure is normalized with respect to the vertical effective stress (σ 'vo) prior to failure to evaluate the liquefied strength ratio, su(LIQ)/σ 'vo. Liquefied strength ratios mobilized during 33 cases of liquefaction flow failure are estimated using a procedure developed to directly back-analyze the liquefied strength ratio. In ten cases, sufficient data regarding the flow slide are available to incorporate the kinetics, i.e., momentum, of failure in the back-analysis. Using liquefied strength ratios back-calculated from case histories, relationships between liquefied strength ratio and normalized standard penetration test blowcount and cone penetration test tip resistance are proposed. These relationships indicate approximately linear correlations between liquefied strength ratio and penetration resistance up to values of qc1 and (N1)60 of 6.5 MPa and 12 blows/ft (i.e., blows/0.3 m), respectively.Key words: liquefaction,...

http://tstark.net/wp-content/uploads/2012/11/38-LiquefiedStrengthRatio-CGJ.pdf

Liquefied strength ratio from liquefaction flow failure case histories

SUMMARY

Fabric and its evolution need to be fully considered for effective modeling of the anisotropic behavior of cohesionless granular sand. In this study, a three-dimensional anisotropic model for granular material is proposed based on the anisotropic critical state theory recently proposed by Li & Dafalias [2012], in which the role of fabric evolution is highlighted. An explicit expression for the yield function is proposed in terms of the invariants and joint invariants of the normalized deviatoric stress ratio tensor and the deviatoric fabric tensor. A void-based fabric tensor that characterizes the average void size and its orientation of a granular assembly is employed in the model. Upon plastic loading, the material fabric is assumed to evolve continuously with its principal direction tending steadily towards the loading direction. A fabric evolution law is proposed to describe this behavior. With these considerations, a non-coaxial flow rule is naturally obtained. The model is shown to be capable of characterizing the complex anisotropic behavior of granular materials under monotonic loading conditions and meanwhile retains a relatively simple formulation for numerical implementation. The model predictions of typical behavior of both Toyoura sand and Fraser River sand compare well with experimental data. Copyright © 2013 John Wiley & Sons, Ltd.

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A critical state sand plasticity model accounting for fabric evolution

A database of the mechanical behavior of 25 natural sands was compiled from the existing literature. Particle shape and size analysis, obtained by dynamic imaging analysis, for each material in the database has subsequently been linked to its mechanical properties; selected sands were also subject to interferometry study for particle surface roughness measurements. This paper reviews the effect of the particle shape properties of these sands on their Critical State and stiffness parameters, introducing a new parameter to optimize the correlations.

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Effect of Particle Shape on the Mechanical Behavior of Natural Sands

This paper attempts to evaluate the undrained shear strength of sand during flow failures, based on both laboratory testing and field observations. In the laboratory, the minimum shear resistance d...

Undrained shear strength of clean sands to trigger flow liquefaction

Les ruptures par liquefaction de depots de sables lâches peuvent representer une menace importante pour les structures cotieres. On a evalue, a partir des concepts d'etats stationnaires, les contributions possibles de divers mecanismes de declenchement lors d'une rupture par liquefaction qui a eu lieu en 1985 a l'embouchure du chenal principal de la riviere Fraser. Parmi les mecanismes qui ont pu declencher le glissement par liquefaction en 1985, on a etudie la sedimentation, les vagues de surface et les marees basses. L'analyse montre qu'une sedimentation rapide genere des contraintes de cisaillement dans la pente mais ne pourrait pas faire demarrer des ruptures par liquefaction. Lorsqu'on evalue l'effet des vagues de surface et leur capacite a causer des glissements de type coulee profonde, on trouve qu'il ne s'accumule pas de pressions interstitielles significatives dues aux vagues de surface pouvant exister dans la region. Les marees basses ne peuvent pas non plus declencher de rupture dans des sables submerges et completement satures. L'apparition de gaz provoque cependant la desaturation des sediments ce qui peut induire des pressions interstitielles residuelles pendant les marees basses. On emet donc l'hypothese que c'est la combinaison de sediments lâches, de faibles quantites de gaz et de marees basses qui favorise les ruptures par coulees liquefiees dans le delta. Ces ruptures conduisent a des glissements regressifs.

/pdf/seabed-instability-due-to-flow-liquefaction-in-the-fraser-357mqw6acp.pdf

Seabed instability due to flow liquefaction in the Fraser River delta

Les glissements de type coulee par liquefaction sont des phenomenes qui arrivent souvent dans le delta de la riviere Fraser du cote de Sand Heads. Ces ruptures n'ont pas trouve d'explication et leurs causes restent du domaine de la speculation. L'etude d'un tel glissement ayant eu lieu en 1985 dans la riviere Fraser a proximite de Sand Heads a ete entreprise en utilisant les concepts d'etats stationnaires. Pour qu'un glissement par coulee impliquant la liquefaction de depots de sables puisse prendre place, le sable doit presenter un ramollissement lors du cisaillement non draine et les contraintes de cisaillement en place doivent etre superieures a la resistance non drainee mobilisable. On a effectue des essais de laboratoire sur des echantillons reconstitues de sable de la riviere Fraser de facon a etablir les parametres d'etat stationnaire. Les specimens de sol ont ete prepares par compaction humide et pluviation dans l'eau. Des mesures de vitesse de propagation des ondes de cisaillement ont ete utilisees pour evaluer l'etat en place des depots de sable recents et non cimentes. On a trouve que l'etat en place des depots recents au voisinage de Sand Heads se situait du cote lâche de l'etat stationnaire, ce qui correspond aux instabilites observees sur le site.

/pdf/evaluation-of-the-in-situ-state-of-fraser-river-sand-45enpjox1x.pdf

P K Robertson

Papers

Seabed instability due to flow liquefaction in the Fraser River delta

Evaluation of the in situ state of Fraser River sand