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Many steep soil slopes are apparently stable beyond what is indicated by slope stability analysis. The mechanism of slope stability in dilating soils is explained in detail, and the development of shear strength in such soils is demonstrated by drained and undrained tests on dense sand. It is argued that appropriate shear strength parameters for analysis of slope stability in dilating materials describe the residual strength. It is explained how reliance on peak shear strength parameters is unsafe, because the component of shear strength created by the additional effective confining pressure caused by development of suction due to inhibited dilation can be exhausted by either access to water or by drying the soil. The fleeting phenomenon of apparent additional shear strength causes super-stability of the slope. Exhaustion of the soil's capacity to dilate results in reduction of shear strength and instability of the steep slope. It is difficult to predict the time when the soil's capacity to dilate is exhausted and when the consequent decline in shear strength occurs. This is because this decline occurs with access to water. This is demonstrated by triaxial compression tests on saturated and partly saturated, dilating specimens. Key words: dilation, instability, noncemented soil, shear strength, slope stability, suction. Resume : Un grand nombre de talus de terre a forte pente sont apparemment stables au-dela de ce qu'indiquent les analyses de la stabilite des pentes. On explique en detail le mecanisme de la stabilite des pentes dans un sol en dilatation et on demontre le developpement de la resistance au cisaillement d'un tel sol par des essais draines et non draines menes sur du sable compact. On fait valoir que les parametres appropries de resistance au cisaillement aux fins de l'analyse de la stabilite des pentes dans des materiaux en dilatation decrivent la resistance residuelle. On explique comment il est risque de se fier aux parametres de resistance maximale au cisaillement, car l'element de la resistance au cisaillement cree par la pression de confinement effective additionnelle, causee par le developpement de la succion due a la dilatation inhibee peut s'attenuer par soit l'acces a l'eau ou le ressuyage du sol. Le phenomene passager de resistance au cisaillement supplementaire apparente cause la superstabilite de la pente. L'epuisement de la capacite du sol a dilater entraine une reduction de la resistance au cisaillement et l'instabilite de la pente raide. Il est difficile de prevoir le moment ou la capacite du sol a dilater est epuisee et celui ou la diminution consequente de la resistance au cisaillement se produit; la raison etant que cette diminution survient de pair avec l'acces a l'eau. On demontre ceci par des essais de compression triaxiale sur des specimens en dilatation satures et partiellement satures. [Traduit par la Redaction] Mots-cles : dilatation, instabilite, sol non cimente, resistance au cisaillement, stabilite des pentes, succion., Introduction In connection with studies of conditions for stability and instability of granular materials, it has become clear that dilating sands can be very strong and perfectly stable as long [...]

Effects of strain rate on the stress-strain and subsequent stress relaxation behaviors have been studied by performing triaxial compression tests on dense Virginia Beach sand specimens at three different strain rates (ratio of 256 between the slowest and the fastest) under low and high confining pressures. For the tests performed under low confining pressure, the specimens that were initially sheared at a faster rate showed a slightly higher amount of stress relaxation, but almost identical stress-strain behaviors were achieved. For tests performed under high confining pressure, the same amount of strength was achieved at high axial strains (10% to 20%), but specimens sheared at higher strain rates showed a slightly stiffer stress-strain response at low axial strains (up to 10%). Similar to the tests performed under low confining pressure, higher strain rates produced higher amounts of stress relaxation to some extent. Effects of correction of axial strain due to load cell expansion and drainage condition during stress relaxation have also been studied and the results indicated that correction of axial strain and undrained condition will both increase the observed amount of stress relaxation. Moreover, a 1 day stress relaxation curve was obtained by connecting the ending stress-strain points of six stress relaxation tests initiated at different deviator stress levels, and this curve was found to be different from the 1 day creep curve obtained from a previous study. A long-term stress relaxation test was also performed, and it showed linear reduction of deviator stress with the logarithm of time during stress relaxation. Observations made are all aligned with the phenomenon of static fatigue and the proposed mechanism for time effects in granular materials. Key words: time effects, stress relaxation, particle crushing, static fatigue, structuration. Les effets de la vitesse de deformation sur la relation entre contrainte et deformation et sur les phenomenes de relaxation de contrainte qui suivent ont ete etudies par l'intermediaire d'essais de compression triaxiale realises sur des echantillons de sable dense preleves a Virginia Beach, a trois vitesses de deformation differentes (coefficient de 256 entre les vitesses minimale et maximale) et sous des pressions de confinement faibles et elevees. Dans le cas des essais effectues sous des pressions de confinement elevees, les echantillons initialement soumis au cisaillement avec une vitesse de deformation elevee ont presente un niveau de relaxation de contrainte legerement plus eleve, mais des comportements de contrainte-deformation presque identiques ont ete observes. Dans le cas des essais realises sous des pressions de confinement elevees, le meme niveau de resistance a ete atteint pour des taux de deformations axiales importants (10 a 20 %) bien que les echantillons soumis au cisaillement avec des vitesses de deformation elevees aient presente une courbe contrainte-deformation legerement plus raide pour de faibles taux de deformation axiale (maximum de 10 %). Comme dans le cas des essais effectues sous des pressions de confinement faibles, des vitesses de deformation elevees ont donne lieu, dans une certaine mesure, a des niveaux de relaxation de contrainte plus eleves. Les effets de la correction de la deformation axiale due a la dilatation de la cellule manometrique et les conditions de drainage au cours de la relaxation de contrainte ont ete egalement etudies, les resultats montrant que la cette correction et des conditions non drainees font augmenter le niveau de relaxation de contrainte observe. En outre, on a obtenu une courbe de relaxation de contrainte sur une journee en connectant les valeurs de contrainte-deformation mesurees au cours de six essais de relaxation de contrainte realises a differents niveaux de contrainte deviatorique. On a alors constate que cette courbe etait differente de la courbe de fluage sur une journee obtenue lors d'une precedente etude. Un essai de relaxation de contrainte sur une longue duree a egalement ete realise et montre une reduction lineaire de la contrainte deviatorique en fonction du logarithme du temps au cours de la relaxation de contrainte. Les observations effectuees corroborent toutes le phenomene de fatigue statique et le mecanisme lie aux effets de temps dans les materiaux granulaires. [Traduit par la Redaction] Mots-cles: effets de temps, relaxation de contrainte, ecrasement de particules, fatigue statique, structuration., Introduction Aging effects and rate effects in granular material have been widely referred to in the literature. Aging effects are described as changes in the material stress-strain relation with time, [...]

Triaxial compression tests were performed on dense specimens of Virginia Beach sand at low and high confining pressures to study time effects that relate to grain crushing due to static fatigue or delayed fracture. Experiments to study effects of loading strain rate on subsequent creep showed negligible time effects and no grain crushing at low confining pressures, while tests at high confining pressures indicated increasing amounts of creep with increasing initial loading strain rates and with increasing deviator stress at creep. Investigation of effects of grain-size distribution indicated stiffer initial response and smaller amounts of creep for more uniformly graded soils at high confining pressures. The experimental results showed that structuration effects were not present in the dense Virginia Beach sand. A long-term creep test at high confining pressure indicated continuous creep with no indication of its termination. Sieve analyses following each triaxial test showed that grain crushing, as quantified by Hardin's relative breakage factor, was proportional to energy input and amount of creep observed for each soil specimen. The creep is due to the time-dependent static fatigue by which the grains crush and cause rearrangement of the grain structure, and this is the reason behind the time effects in granular materials. Key words: time effects, creep, grain crushing, static fatigue, structuration. Des essais de compression triaxiale ont ete realises sur des echantillons denses de sable de Virginia Beach, a des pressions de confinement faibles et elevees, afin d'etudier les effets du temps sur l'ecrasement des grains du a la fatigue statique ou a la rupture retardee. Les essais evaluant les effets du taux chargement en deformation sur le fluage subsequent ont demontre des effets du temps negligeables et aucun ecrasement des grains a des pressions de confinement faibles, tandis que les essais a pression de confinement elevee ont indique une augmentation du fluage lorsque les taux de chargement en deformation initiaux augmentent, de meme qu'avec l'augmentation de la contrainte deviatrice lors du fluage. Les investigations des effets de la distribution granulometrique ont demontre un comportement initial plus rigide et un plus faible fluage pour des sols plus uniformes a pressions de confinement elevees. Les resultats experimentaux demontrent que les effets structuraux ne sont pas presents dans le sable dense de Virginia Beach. Un essai de fluage a long terme, realise sous une pression de confinement elevee, a indique un fluage continu avec aucune indication qu'il va se terminer. Des analyses par tamisage faites apres chaque essai triaxial ont demontre que l'ecrasement des grains, tel que quantifie par le facteur de bris relatif de Hardin, est proportionnel a l'energie ajoutee et a la quantite de fluage observe pour chaque echantillon de sol. Le fluage est cause par la fatigue dependante du temps, par laquelle les grains sont ecrases et ainsi engendrent un rearrangement de la structure des grains, et cet effet explique l'influence du temps dans les materiaux granulaires. [Traduit par la Redaction] Mots-cles: effets du temps, fluage, ecrasement des grains, fatigue statique, structure., Introduction Time effects in granular materials have been widely reported in the literature and comprise rate effects and aging effects. Rate effects include rate dependency, creep (loading rate of zero) [...]

Triaxial tests have been performed to demonstrate the conditions for stability and instability in loose silty sand. Drucker (1951) and Hill (1958) stability conditions in terms of the sign of the second work increment were employed in the design of the stress paths used in the triaxial compression and extension tests performed with quasi-constant shear stress while the mean normal stress was reduced until failure occurred. It is shown that the sand is completely stable under drained conditions for any stress path and irrespective of the sign of the second work increment. This is demonstrated by completely stopping the change in stresses and observing the stable behavior in the range of stresses where the sand contracts and where it dilates. Once the effective stress failure surface is passed, the sand becomes unstable, and the sign of the second work increment is always negative. Run-away instability can occur inside the failure surface for loose silty sand under undrained conditions for which the sand tends to contract, pore pressures continue to develop, and the second work increment is negative. Liquefaction may follow if the loose silty sand is sufficiently loose. Key words: failure, instability, laboratory tests, liquefaction, shear strength, silty sand, triaxial test. Des essais triaxiaux ont ete realises afin de demontrer les conditions de stabilite et d'instabilite dans des sols silteux laches. Les conditions de stabilite de Drucker (1951) et Hill (1958), en termes du signe du deuxieme increment de travail, sont utilisees dans la conception des cheminements des contraintes pour les essais triaxiaux en compression et en extension realises avec une contrainte de cisaillement pratiquement constante, tandis que la contrainte normale moyenne etait reduite jusqu' a la rupture. Il a ete observe que le sable est completement stable dans des conditions drainees quelque soit le cheminement des contraintes, et quelque soit le signe du deuxieme increment de travail. Ceci est demontre en arretant completement le changement de contraintes et en observant le comportement stable dans la variation des contraintes parmi lesquelles le sable se contracte et se dilate. Une fois la surface de rupture de la contrainte effective passee, le sable devient instable et le signe du deuxieme increment de travail est toujours negatif. L' emballement de l' instabilite peut se produire a l' interieur de la surface de rupture pour du sable silteux lache dans des conditions non drainees, durant lesquelles le sable a tendance a se contracter, les pressions interstitielles continuent a se developper, et le deuxieme increment de travail est negatif. La liquefaction peut se produire par la suite, si le sable silteux lache est suffisamment lache. Mots-cles : rupture, instabilite, essais en laboratoire, liquefaction, resistance au cisaillement, sable silteux, essai triaxial. [Traduit par la Redaction], Introduction Failure and instability of loose silty sands, both of which may lead to catastrophic results, are phenomena in which the shear strength drops after reaching an initial peak, i.e., [...]

The mechanism of instability in granular soils is explained and its requirement as a forerunner to the liquefaction of level or sloping ground is described. Case histories support the observation that it is silty sands that liquefy under static and a majority of earthquake-induced conditions. Recent experiments show that clean sands do not behave similarly to silty sands. Tests on loose, silty sand indicate a 'reverse' behavior with respect to confining pressure and this violates the basic assumption that loose, silty sands behave similarly to loose, clean sands. Strong correlations between fines content, compressibility, and liquefaction potential are often found for these soils. A procedure for the analysis and evaluation of static liquefaction of slopes of fine sand and silt, such as submarine slopes, mine tailings, and spoil heaps, is presented. It involves determination of the region of instability in stress space in which potential liquefaction may be initiated and determination of the state of stress in the slope. A method of finding the state of stress is developed to predict the zone of potential liquefaction in simple slopes. Trigger mechanisms for initiation of instability followed by soil liquefaction are reviewed and mechanisms of soil strengthening are discussed. Key words: flow failure, in situ tests, instability, laboratory tests, liquefaction, silty sand. Le mecanisme d'instabilite dans les sols granulaires est explique;, et son implication en tant que precurseur dans la liquefaction des sols plats ou en pente est decrite. Des etudes de cas supportent l'observation que ce sont les sables silteux qui se liquefient sous chargements statiques et sous la majorite des conditions induites par les seismes. Des travaux recents ont demontre: que les sables propres ne se comportent pas de la meme facjon que les sables silteux. Des essais sur un sable silteux lache indiquent un comportement inverse par rapport a la pression de confinement, et ceci contredit la supposition de base disant que les sables silteux laches se comportent de facon similaire aux sables propres et laches. Des correlations elevees entre le contenu en particules fines, la compressibilite et le potentiel de liquefaction sont souvent observees pour ces sols. Une procedure pour l'analyse et revaluation de la liquefaction statique de pentes composees de sable fin et de silt, telles les pentes sous-marines, les residus miniers et les depots de deblais est presentee. Cette procedure implique la determination de la region d'instabilite dans l'espace de contrainte dans lequel la liquefaction statique peut etre initieie, ainsi que la determination de l'eetat des contraintes dans la pente. Une meethode pour trouver l'eetat des contraintes est developpeee afin de predire la zone de liquefaction potentielle dans des pentes simples. Les mecanismes deeclencheurs pour l'initiation de l'instabilites, suivie de la liquefaction, sont revises, et les meecanismes de renforcement des sols sont discutes. Mots-cles: rupture en ecoulement, essais in situ, instabilite, essais en laboratoire, liquefaction, sable silteux. [Traduit par la Redaction], Introduction Since the realization of what constitutes instability was made in the 1980s and procedures for analyzing instability conditions in fine sands and silts were proposed in the 1990s, some [...]

Based on previously obtained experimental results, a mechanistic picture of time effects in granular materials is presented. Accordingly, time effects are caused by grain crushing, which in turn is time dependent, as indicated by static fatigue of brittle materials. Triaxial compression tests have been performed on Virginia Beach sand at high pressures, where grain crushing is prevalent, to study effects of initial loading strain rates on subsequent amounts of creep and stress relaxation. Grain size distribution curves were determined after each test and the amount of crushing, as characterized by Hardin's breakage factor, is related to the energy input to the triaxial specimens. A pattern emerges that indicates the importance of crushing for the axial and volumetric strains, while rearrangement and frictional sliding between intact grains play much smaller roles in the stress-strain and volume change behaviors of granular materials at high stresses and shear strains. Because particle crushing is a time-dependent phenomenon described as static fatigue or delayed fracture, the close relation between time effects and crushing in granular materials is established. DOI: 10.1061/(ASCE)GT.1943-5606.0000335 CE Database subject headings: Creep; Granular media; Sand; Material; Soil properties; Strain rates; Time dependence; Triaxial tests. Author keywords: Creep; Crushing; Granular materials; Sand; Soil properties; Strain rate; Time dependence; Triaxial tests.

Stress relaxation and stress drop-relaxation tests have been performed to complement a test series performed to study strain rate, creep, and stress drop-creep effects on crushed coral sand. Drained experiments with constant effective confining pressure of 200 kPa were performed in which triaxial specimens of crushed coral sand were loaded to initial stress differences of 500, 700, and 900 kPa, followed by stress drops of 0, 100, 200, 300, and 400 kPa at which points the axial strains were kept constant while the axial stress relaxation and the volumetric strains were observed. The stress drops produced delays in initiation of stress relaxation that were proportional with the magnitudes of the stress drops. The experiments show that sands do not exhibit classic viscous effects, and their behavior is indicated as 'nonisotach,' while the typical viscous behavior of clay is termed 'isotach.' Thus, there are significant differences in the time-dependent behavior patterns of sands and clay. A mechanistic picture of time effects in sands is proposed. DOI: 10.1061/(ASCE)GT.1943-5606.0000212 CE Database subject headings: Creep; Granular media; Sand, Soil type; Soil properties; Stress relaxation; Time dependence; Triaxial tests. Author keywords: Creep; Granular materials; Sand; Soil properties; Stress relaxation; Time dependence; Triaxial tests.

The part of sand behavior that is affected by time, such as creep, relaxation, and loading rate effects are not similar to those observed for clay. To throw more light on the time effects in sand, many series of drained triaxial compression experiments have been performed on crushed coral sand. These tests were all performed with a constant effective confining pressure of 200 kPa. The test series included experiments with specimens loaded at five different strain rates with a 256-fold ratio between the extreme rates, tests with sudden changes in strain rate from slow to fast and vice versa, and tests in which axial and volumetric creep strains were observed at stress differences of 500, 700, and 900 kPa. Creep creates structuration and this has to be overcome to produce further plastic straining. Experiments were also performed in which the stress difference was dropped quickly from three different values of 500, 700, and 900 kPa followed by creep. In these stress drop-creep tests five different magnitudes of stress drops were employed: 0, 100, 200, 300, and 400 kPa. The results involving conventional creep effects and stress drop-creep effects are presented and analyzed. DOI: 10.1061/(ASCE)GT.1943-5606.0000067 CE Database subject headings: Creep; Granular materials; Sand; Soil properties; Strain rate; Time dependence; Triaxial tests.

Current laboratory testing often imposes or assumes uniform stress and strain distribution in a specimen for convenient data reduction to interpret soil behavior. This paper presents an inverse analysis framework, Self-learning Simulations (SelfSim), to interpret the drained behavior of sand from triaxial compression tests with fully frictional loading platens. The frictional platens result in significant bulging of and nonuniform stresses and strains within sand specimens. SelfSim treats the specimen as a boundary value problem (BVP) and extracts these nonuniform stresses and strains from within each specimen using external load and displacement measurements. The extracted behavior shows significant principal stress rotation, variation of intermediate principal stress, and nonuniform volume change throughout the specimen. Mobilized friction angles are interpreted on the two-dimensional slip surface associated with the Mohr-Coulomb failure criterion, on the octahedral plane associated with the Drucker-Prager failure criterion, and on the spatially mobilized plane (SMP) associated with the Matsuoka-Nakai failure criterion. The extracted stress-strain behavior is used to examine the sand's stress-dilatancy characteristics. Proposed integration of SelfSim inverse analysis with laboratory testing opens the way for new and efficient approaches to soil behavior characterization under general loading conditions, needed for the solution of general geotechnical boundary value problems, from readily available laboratory tests. Key words: sand behavior, laboratory testing, inverse analysis, friction angle, failure criterion, stress-dilatancy. Les essais en laboratoire utilises couramment imposent ou assument souvent une distribution uniforme des contraintes et deformations dans un specimen pour reduire les donnees servant a interpreter le comportement du sol. Cet article presente un cadre pour une analyse inverse, Self-learning Simulations (SelfSim), pour interpreter le comportement draine d'un sable lors d'essais en compression triaxiale avec des plateaux de chargement entierement frictionnels. Les plateaux frictionnels provoquent un gonflement significatif et des contraintes et deformations non uniformes a l'interieur du specimen de sable. SelfSim considere le specimen comme un probleme de valeur frontiere << BVP >> et extrait les contraintes et deformations non uniforme de chaque specimen en utilisant des mesures de chargement externe et de deplacement. Le comportement extrait montre une rotation importante des contraintes principales, des variations des contraintes principales intermediaires, et un changement de volume non uniforme a travers le specimen. Les angles de friction mobilises sont interpretes a l'aide des surfaces de glissement en deux dimensions associees au critere de rupture Mohr-Coulomb, sur le plan octaedral associees au critere de rupture Drucker-Prager, et sur le plan mobilise dans l'espace << SMP >> associees avec le critere de rupture Matsuoka-Nakai. Le comportement en contrainte-deformation extrait est utilise pour examiner les caracteristiques de contrainte-dilatation du sable. L'integration proposee de l'analyse inverse SelfSim avec les essais en laboratoire ouvre la voie pour des approches nouvelles et efficaces pour caracteriser les sols soumis a des conditions de chargement generales, qui sont necessaires pour solutionner des problemes geotechniques generaux de valeurs frontieres a partir d'essais en laboratoire deja disponibles. Mots-cles : comportement du sable, essais en laboratoire, analyse inverse, angle de friction, critere de rupture, contraintedilatation. [Traduit par la Redaction], Introduction Sand behavior has been widely investigated with the aid of a range of laboratory devices, including triaxial cell, plane strain cell, true triaxial cell (Lade and Duncan 1973), directional [...]

The results from a laboratory experimental study on silty sand are presented. A scanning electron microscope (SEM) was used to investigate the effect of depositional method and silt content on the grain contact structure of silty sand. Triaxial specimens containing Nevada sand with various quantities of nonplastic silt were isotropically consolidated to 25 kPa and then preserved through epoxy impregnation. Specimens were formed using both dry funnel deposition and water sedimentation. A procedure was developed that allowed the microstructure to be quantified in terms of potentially stable and unstable grain contacts. As silt content increased, the percentage of stable grain contacts decreased. Specimens formed by dry funnel deposition contained a higher percentage of potentially unstable grain contacts than those reconstituted by water sedimentation, possibly reflecting a more compressible particle structure. This effect became more pronounced as silt content increased. A description of the specimen preservation technique as well as the methodology used to both prepare and analyze SEM specimens for grain contact analysis are presented., Key words: depositional method, epoxy intrusion, fabric, fines, liquefaction, sand, silt, specimen preservation, specimen reconstitution. Resume : On presente les resultats d'une etude experimentale en laboratoire sur le sable silteux. [...]

The results from a laboratory experimental study on silty sand are presented. Undrained triaxial compression tests were performed on Nevada sand containing nonplastic silt. All specimens underwent monotonic loading after isotropic consolidation to 25 kPa. Various depositional techniques were used to create specimens to compare their stress-strain behavior within three separate density ranges and three different silt contents. As density increased, the effect of the depositional method on the undrained behavior decreased. The influence of the depositional method on specimen behavior also was found to increase with silt content, particularly at lower densities. These findings therefore are of special interest with regard to laboratory testing for the purpose of characterizing liquefaction potential, where loose, silty sands may be encountered. A description and analysis of five different depositional techniques is provided. These methods include: dry funnel deposition, water sedimentation, slurry deposition, mixed dry deposition, and air pluviation. The results of the undrained testing are analyzed using void ratio after consolidation and silt content as the bases of comparison. Key words: density, deposition, depositional method, fines, liquefaction, sand, silt, specimen reconstitution, stress-strain, undrained testing. On presente les resultats d'une etude experimentale en laboratoire sur un sable silteux. Des essais de compression triaxiale non drainee ont ete realises sur le sable du Nevada contenant un silt non plastique. Tous les specimens ont ete soumis a un chargement monotonique apres une consolidation isotrope sous 25 kPa. Diverses techniques de deposition ont ete utilisees pour creer des specimens dans le but de comparer leur comportement contrainte-deformation a l'interieur de trois plages separees de densite et de trois teneurs en silt. Lorsque la densite croit, l'effet de la methode de deposition sur le comportement non draine decroit. On a aussi trouve que l'influence de la methode de deposition sur le comportement du specimen croit avec la teneur en silt, particulierement aux faibles densites. Par consequent, ces resultats presentent un interet special pour les essais en laboratoire dans le but de caracteriser le potentiel de liquefaction, dans le cas ou des sables silteux meubles peuvent etre rencontres. On fournit une description et une analyse de cinq differentes techniques de deposition. Ces methodes comprennent: une deposition a sec avec entonnoir, une sedimentation dans l'eau, une deposition en boue, une deposition seche melangee, et une deposition a l'air en pluie artificielle. Les resultats des essais non draines sont analyses au moyen de l'indice des vides apres consolidation et la teneur en silt comme bases de comparaison. Mots-cles: densite, deposition, methode de deposition, fines, liquefaction, sable, silt, specimen, reconstitution, contraintedeformation, essais non draines., [Traduit par la Redaction] Introduction Past research has debated the effect of nonplastic silt content on the liquefaction behavior of sand. Many studies suggest that liquefaction resistance increases with increasing [...]

Experimental evidence and analyses of results of three-dimensional (3D) tests show that the shape of the failure surface for soils is influenced by the intermediate principal stress, shear banding, and cross anisotropy. Presented here is a formulation of a general 3D failure criterion for cross-anisotropic soils for both nonrotating and rotating stresses. The formulation relates the loading direction to the principal directions of the cross-anisotropic microstructure of the soil. The criterion is based on a function of stress, previously used as the 3D failure criterion for isotropic frictional materials, which is set equal to a scalar that varies over a sphere. The formulation is specialized for true triaxial tests and torsion shear tests and determination of material parameters is demonstrated. The failure criterion for cross-anisotropic soils is compared with experimental results from the literature to show that it is able to capture the conditions obtained in true triaxial tests without stress rotations as well as the conditions in torsion shear tests performed to study effects of stress rotation. Sets of data from some classic true triaxial tests are reinterpreted to show their true cross-anisotropic behavior. DOI: 10.1061/(ASCE)1090-0241(2008)134:1(117) CE Database subject headings: Anisotropy; Failures; Shear strength; Shear tests; Triaxial tests.

Abstract: Shear banding in Santa Monica beach sand deposited by dry pluviation in hollow cylinder specimens is studied in 34 drained torsion shear tests with rotation of principal stress directions. [...]

A constitutive model has been developed to capture the behavior of cross-anisotropic frictional materials. The elastoplastic, single hardening model for isotropic materials serves as the basic framework. Based on the experimental results of cross-anisotropic sands in isotropic compression tests, the principal stress coordinate system is rotated such that the model operates isotropically within the rotated framework. Experimental plastic work contours on the octahedral plane are plotted for a series of true triaxial tests on dense Santa Monica Beach sand to study the effects of cross anisotropy on the evolution of yield surfaces. The amount of rotation of the yield and plastic potential surfaces decreases to zero (isotropic state) with loading. The model is constructed for cases where the principal stress and material symmetry axes are collinear and no significant rotation of principal stresses occur. The model incorporates fourteen parameters that can be determined from simple experiments, such as isotropic compression, drained triaxial compression, and triaxial extension tests. A series of true triaxial and isotropic compression tests on dense Santa Monica Beach sand are used as a basis for verification of the capabilities of the proposed model. CE Database subject headings: Granular materials; Anisotropy; Isotropic materials; Compression tests; Triaxial tests; Models.

Drained true triaxial tests on dense Santa Monica Beach sand deposited with a cross-anisotropic fabric have been performed to study the failure condition in the principal stress space. The failure surface was assumed to be symmetric around the vertical axis (on the octahedral plane of the principal stress space), but varying as a function of the Lode angle. Data from previously performed consolidated-undrained true triaxial tests on San Francisco Bay Mud and data from triaxial compression, triaxial extension, and plane strain tests on Toyoura sand showed similar behavior in terms of effective stresses. A three-dimensional failure criterion is proposed for characterization of failure in cross-anisotropic soils, under commonly occurring conditions when loading and depositional directions coincide and no significant rotation of principal stresses occur. This cross-anisotropic criterion is developed using a coordinate rotation of the principal stress space and utilization of an existing isotropic failure formulation. Derivation of the three required parameters is explained and illustrated. The proposed criterion is compared with various experimental results; and it is demonstrated that the failure criterion for cross-anisotropic soils captures the experimental behavior with good accuracy. CE Database subject headings: Anisotropy; Clays; Sand; Failures; Shear strength; Triaxial tests.

The volume change behavior of cross-anisotropic sand is studied using results of a series of cubical triaxial tests. The relationships between the volumetric response, failure, and shear localization are addressed, Rates of dilation under various three-dimensional stress conditions are evaluated in conjunction with the peak shear resistance and initiation of shear banding in specimens of dense Santa Monica beach sand. The location of the line in principal stress space along which the tendency to deform changes from compressive to dilative (the characteristic line) is determined using two different methods. The uniqueness of this characteristic line for cross-anisotropic materials is analyzed. DOI: 10.1061/(ASCE)0733-9399(2003)129:2(167) CE Database keywords: Failures; Anisotropy; Sand; Shear; Stress-strain relations.

The significance of material cross anisotropy in sands is underscored and experimentally evaluated in a series of true triaxial tests on Santa Monica beach sand in a cubical device. Failure patterns, initiation and development of shear banding, and complete stress-strain behavior are described for the entire range of the Lode angle under general three-dimensional loading conditions. Localized failure was found to govern the ultimate resistance of the sand for intermediate values of parameter b = ([[sigma].sub.2]-[[sigma].sub.3])/([[sigma].sub.1]-[[sigma].sub.3]) in each of the three sectors of the octahedral plane. Variations of the friction angle are fully described and show its significant dependence on the inherent cross-anisotropic material structure. DOI: 10.1061/(ASCE)0733-9399(2003)129:2(160) CE Database keywords: Failures; Anisotropy; Sand; Shear; Stress-strain relations.

The results from an experimental study on silty sands are presented and evaluated in view of the framework of critical-state or steady-state soil mechanics. Drained and undrained compression tests were performed on Nevada sand containing nonplastic silt. The drained and undrained steady-state lines diverged. This divergence was caused by the tendency for static liquefaction at low pressures. Results from undrained tests with different initial void ratios produced different steady-state lines. Unique steady-state lines may therefore not always exist for silty sands. Denser specimens liquefied at low confining pressures, while looser specimens at higher confining pressures showed stable behavior. Thus, a unique relation between void ratio and confining pressure, namely the steady-state line, may not represent the behavior of loose silty sands. This occurs because silty sands exhibit 'reverse' behavior characteristics; more dilative behavior is observed with increasing confining pressure under undrained shearing. Load and deformation control testing was performed to evaluate the effect of strain rate. Compressibility was examined as an alternative measure of liquefaction potential.

The experimental techniques pertaining to accurate measurement of creep in sand are explained in detail. Triaxial compression and proportional loading tests were performed in a triaxial setup for which special procedures were developed to maintain constant temperature, constant confining pressure, and constant axial load, whereas the axial and volumetric deformations were measured using two types of mechanical measurement systems, both free of zero drift. Special attention was paid to the avoidance of air and water leakage through the membrane in the long-term tests. The experiments show that the nature of creep strains is similar to that of plastic strains. They may be predicted from the framework provided by the hardening plasticity theory. In particular, the potential surface determined for the prediction of plastic strains may also be used for the prediction of time-dependent creep strains. From the experiments it also appears that the yield surface and the plastic potential surface move out together, and the point at which to evaluate inelastic strain increment directions is the current location of the yield surface and the accompanying plastic potential surface.

A study was conducted on the confining pressure-caused crushing of granular particles. Results show that particle crushing is reduced by enhancing mineral hardness and that smaller amounts of breakage for a given stress is achieved with stronger or harder particles. Results are consistent with those obtained from the crushing of sedimentary rock-derived oversized particles.

A high-pressure triaxial compression and extension testing apparatus was utilized to investigate the effect of initial relative density on soil instability. Stress paths from a series of undrained tests, at confining pressures from 8 to 60 MPa, were evaluated to determine the location of the potential instability region. Experiments were performed on specimens with relative densities of 30% and 60%, and compared with previously published results for 90%. The results indicate that initial relative density has a small effect on the slope of the instability line, with the slope of the line decreasing as initial density is reduced. In addition, the conditions under which a specimen exhibits unstable behavior were confirmed by performing series of specialized instability tests. For tests with undrained conditions imposed inside the region of potential instability, instability was observed immediately. For tests with undrained conditions imposed outside, but relatively close to this region, creep effects would cause pore pressures to increase resulting eventually in instability. In general, it was found that initiation of unstable behavior depends on the relationship between the shape of the yield surface and the effective stress path followed.

Particle breakage occurs when the stresses imposed on soil particles exceed their strength. To quantify the amount of breakage, many particle breakage factors have been proposed. Correlations of particle breakage parameters with standard soil parameters such as effective mean normal stress at failure and void ratio at failure do not provide a fully unified correlation with many different types of tests. However, particle breakage factors appear to correlate very well with the total energy input into the test specimen, thus providing a single unifying parameter for all types of soil tests. Therefore, the amount of particle crushing may be predicted if the stresses and strains in the soil can be estimated, such as in a finite-element analysis. The principal significant use of these particle breakage factors may be related to permeability estimates when there are changes in gradation due to particle breakage. A new particle breakage factor, [B.sub.10], is proposed that will allow easy permeability computations when related to the total input energy.

The results from an experimental study of undrained granular material behavior at high pressures are presented. Axisymmetric specimens of dense Cambria sand were tested in undrained triaxial compression tests between initial effective confining pressures of 6.4 and 68.9 MPa. Undrained triaxial extension tests between initial effective confining pressures of 12.0 and 52.0 MPa were also performed. Uniform strains in extension specimens were enforced by creating a rigid jacket around the specimen, which prevented necking, but allowed unimpeded deformations in the specimen. It consisted of two layers of spaced steel plates, which were bent to fit the contours of the specimen. The layers of plates are separated by greased rubber membranes. The high confining pressures cause large amounts of particle crushing to occur, which in turn cause the development of large positive pore pressures. This caused the effective stress path to move rapidly toward the effective stress failure envelope. The Mohr-Coulomb, effective stress, secant friction angles in both compression and extension appear to be approximately the same. The applicability of critical state soil mechanics at high pressures is explored.

A one-dimensional testing apparatus was developed to test soils to axial stresses up to 850 MPa. The apparatus was instrumented with strain gauges such that lateral soil stresses, and therefore [K.sub.0], could be inferred from measured circumferential strains. Three different initial densities of quartz, Cambria, and gypsum sands were tested and it was found that the effect of initial density was eliminated at high stress magnitudes. This stress magnitude was higher for mineralogically harder grains than for softer grains. The inferred values of [K.sub.0] for the mineralogically harder Cambria sand was found to be constant at high pressures, but slightly below that indicated by Jaky's equation. However, the softer gypsum sand indicated increasing values of [K.sub.0] as the stress magnitude increased. This apparently was caused by inelastic, viscous flow during shearing.

The results of an experimental study of granular materials at high pressures are presented. Axisymmetric specimens of dense Cambria sand were tested in drained triaxial compression tests between confining pressures of 0.05 and 52 MPa. Drained triaxial extension tests between confining pressures of 0.25 and 52 MPa were also performed. As the confining pressure is increased, it was found that the stress-strain curve, and volumetric and axial strains to failure rapidly increased at a certain stress magnitude. This was shown to be directly related to a rapid increase in particle crushing. Beyond a certain higher value of stress magnitude, the stress-strain curves steepen, and the volumetric and axial strains to failure decrease. This was also shown to be directly related to the cessation of particle crushing and is typified by a zero rate of volume change at failure. The Mohr-Coulomb secant friction angle was found to be related to the rate of volume change at failure, whether the soil was dilatant or highly contractive and subject to large amounts of particle crushing.

An experimental study on granular materials in triaxial extension at confining pressures from 0.25 to 68 MPa is presented. Drained and undrained tests were performed utilizing cylindrical specimens. Strain localization was repeatedly encountered in the form of specimen necking. The strain localization was determined to initiate very early in the test. The cause of the strain localization was determined to be the inherent instability in the axisymmetric extension test, which allows stresses, and therefore deformations, to concentrate at the weakest part of the specimen. This instability is a result of the inward radial strains experienced during the extension test. The effect is to lower the measured failure stresses and strains resulting in the conclusion that the conventional extension test is unreliable for determining soil strength in extension. A method was developed to enforce uniform strains in triaxial extension tests on cylindrical specimens by the use of small metal plates separated by lubricated latex membranes. Using this method a series of uniform-strain tests were performed. Detailed comparisons between strain-localized and uniform-strain tests are presented.