Search

This study presents the use of controlled blasting as a source of seismic energy to obtain the coupled, dynamic, linear-elastic to nonlinear-inelastic response of a plastic silt deposit. Characterization of blast-induced ground motions indicates that the shear strain and corresponding residual excess pore pressures (EPPs) are associated with low-frequency near- and far-field shear waves that are within the range of earthquake frequencies, whereas the effect of high-frequency P-waves is negligible. Three blasting programs were used to develop the initial and pre-strained relationships between shear strain, EPP, and nonlinear shear modulus degradation. The initial threshold shear strain to initiate soil nonlinearity and to trigger generation of residual EPP ranged from 0.002% to 0.003% and 0.008% to 0.012%, respectively, where the latter corresponded to ~70% of the maximum shear modulus, [G.sub.max]. Following pre-straining and dissipation of EPPs within the silt deposit, the shear strain necessary to trigger residual excess pore pressure increased two-fold. Greater excess pore pressures were observed in situ compared to those of intact direct simple shear (DSS) test specimens at a given shear strain amplitude. The reduction of in situ undrained shear strength within the blast-induced EPP field measured using vane shear tests compared favorably with that of DSS test specimens. Key words: shear modulus degradation, excess pore pressure generation, in situ testing, controlled blasting, soil dynamics. Nous presentons ci-dessous l'utilisation d'un dynamitage controle comme source d'energie sismique pour obtenir la reponse couplee, dynamique, lineaire-elastique a non-lineaire-inelastique d'un depot de limon plastique. La determination des mouvements du sol induits par le dynamitage indique que la deformation de cisaillement et les surpressions interstitielles residuelles correspondantes (EPP) sont associees a des ondes de cisaillement de basse frequence dans le champ proche et lointain qui se situent dans la gamme des frequences sismiques, tandis que l'effet des ondes P de haute frequence est negligeable. Trois programmes de dynamitage ont ete utilises pour developper les relations initiales et pre-contraintes entre la deformation de cisaillement, l'EPP et la degradation du module de cisaillement non lineaire. Le seuil initial de deformation en cisaillement pour initier la non-linearite du sol et pour declencher la generation de l'EPP residuelle varie de 0,002 a 0,003% et de 0,008 a 0,012%, respectivement, ou cette derniere correspond a ~70% du module de cisaillement maximal, [G.sub.max]. Apres la pre-deformation et la dissipation des EPP dans le depot de limon, la deformation de cisaillement necessaire pour declencher la surpression interstitielle residuelle a ete multipliee par deux. Des surpressions interstitielles plus importantes ont ete observees in situ par rapport a celles d'eprouvettes intactes de cisaillement simple direct (DSS) pour une amplitude de deformation de cisaillement donnee. La reduction de la resistance au cisaillement non drainee in situ dans le champ d'EPP induit par l'explosion, mesuree a l'aide d'essais de cisaillement a l'aide d'ailettes, se compare favorablement a celle des eprouvettes DSS. [Traduit par la Redaction] Mots-cles : degradation du module de cisaillement, generation de surpression interstitielle, essais in situ, dynamitage controle, dynamique des sols., Introduction Geotechnical earthquake engineering practice regarding the seismic response of silt deposits generally focuses on assessments of their cyclic and post-cyclic responses. Such assessments may consider the plasticity index, PI; [...]

Changes in soil fabric following liquefaction have been studied using various in situ methods that often return inconclusive or conflicting observations. The time-rate variation of stiffness, when observed, is usually not evaluated over significant periods of time, limiting investigations about aging in post-liquefaction regain of stiffness. Even more uncommon is the application of geophysical techniques to evaluate changes in shear wave velocity ([V.sub.S]) as a proxy for small-strain stiffness. This study uses controlled blasting to examine long-term post-liquefaction regain of stiffness following multiple blast events. The multi-channel analysis of surface waves (MASW) technique was used to observe changes in [V.sub.S] of aged deposits at a test site in South Carolina. Blast-induced liquefaction of the target liquefiable layer resulted in significant reduction to its initial small-strain stiffness owing to the destruction of the aged soil fabric. The time-rate variation in [V.sub.S] indicated that the initial small-strain stiffness was not re-established over many months following liquefaction. Following a second blast event, the small-strain stiffness reduced again, but recovered more quickly, similar to previously reported observations of young sand deposits. This study provides a significant basis for interpreting in situ body and surface wave measurements of aged and young sand deposits densified using blast liquefaction. Key words: liquefaction, blasting, surface waves, shear wave velocity, aging effects. Les changements dans le tissu du sol suite a la liquefaction ont ete etudies a l'aide de diverses methodes in-situ, et donnent souvent lieu a des observations peu concluantes ou contradictoires. La variation dans le temps de la rigidite, lorsqu'elle est observee, n'est generalement pas evaluee sur des periodes significatives, ce qui limite les recherches sur le vieillissement dans la reprise de la rigidite apres la liquefaction. Plus rare encore est l'application de techniques geophysiques pour evaluer les changements de la vitesse des ondes de cisaillement ([V.sub.S]) en tant qu'indicateur de la rigidite a faible contrainte. Cette etude utilise le dynamitage controle pour examiner la reprise de rigidite a long terme apres la liquefaction a la suite de multiples explosions. La technique d'analyse multicanal des ondes de surface (MASW) a ete utilisee pour observer les changements de [V.sub.S] des depots ages sur un site test en Caroline du Sud. La liquefaction par explosion de la couche liquefiable cible a entraine une reduction significative de sa rigidite initiale a faible contrainte en raison de la destruction du tissu de sol vieilli. La variation dans le temps de la [V.sub.S] indique que la rigidite initiale a faible contrainte n'a pas ete retablie au cours des nombreux mois qui ont suivi la liquefaction. A la suite d'une deuxieme explosion, la rigidite des petites deformations a de nouveau diminue, mais elle s'est retablie plus rapidement, comme ce fut le cas lors d'observations anterieures de jeunes depots de sable. Cette etude fournit une base importante pour l'interpretation des mesures in-situ des corps et des ondes de surface des depots de sable jeunes et ages densifies par liquefaction a l'explosif. [Traduit par la Redaction] Mots-cles: liquefaction, explosion, ondes de surface, velocite des ondes de cisaillement, effets du vieillissement., Introduction Controlled blasting uses the detonation of explosive charges to simulate the multiple strain cycles necessary to induce pore pressure buildup and subsequent liquefaction in loose, cohesionless, saturated soils. Subsequently, [...]

This study compares the in-situ dynamic response of a low plasticity silt deposit subjected to multidirectional loading from vibroseis shaking and controlled blasting to a suite of element-scale, cyclic laboratory test specimens. The agreement between excess pore pressures and simple shear strain relationships over a wide range in strains is remarkable. Slightly larger excess pore pressures observed in-situ are attributed to three-dimensional loading and pore pressure migration/redistribution in the shallower portions of the deposit. Noted differences in shear modulus, G, are attributed to strain rate effects, spatial variability in the in-situ stiffness, and hydraulic boundary conditions. The variation in in-situ G/Gmax follows the trend from torsional shear specimens up to 0.4% shear strain; larger strains in the silt deposit imposed by controlled blasting yielded a stiffer response than that from cyclic torsional shear and direct simple shear specimens due in part to field drainage for deeper portions of the deposit. The in-situ cyclic resistance ratio for the deeper portion of the deposit in which plane body waves could be assumed and for the selected excess pore pressure ratio criterion was larger than that of stress-controlled cyclic direct simple shear (CDSS) test specimens, despite the detrimental effect of multidirectional shaking in the field. The effect of strain history, spatial variability, and drainage boundary conditions to drive differences between the in-situ and laboratory test specimens is identified. [ABSTRACT FROM AUTHOR]

Aggregate piers represent an economical ground improvement technique used to increase bearing capacity and reduce settlements of weak soils. Several approaches have been developed to estimate the bearing capacity of aggregate pier-reinforced clay, but these models exhibit large prediction bias and uncertainty. This study uses newly developed footing loading test data to investigate the relationship between the bearing capacity and the area replacement and slenderness ratios. The bearing capacity of a single aggregate pier, whether isolated or in groups, below a loaded footing increases as the area replacement ratio decreases due to increase in extent of confined soil surrounding the pier. The length and diameter of an aggregate pier is also shown to result in significantly increased bearing capacity, an effect that diminishes with increasing slenderness. New modifications are proposed to existing simplified and cavity expansion models to account for the effect of confinement, area replacement ratio, and slenderness ratio using a leave-one-out cross-validation technique. The cross-validation analysis resulted in robust bearing capacity models that are more accurate than existing analytical models. Additionally, the stress concentration ratio for shallow foundations supported by aggregate pier-reinforced plastic soils at failure was estimated and compared with the available data, indicating its sensitivity to design variables and showing that this critical design parameter may be predicted using the updated models. Key words: ground improvement, aggregate pier, load tests, bearing capacity, stress concentration ratio. Les piles de granulats representent une technique economique d'amelioration du sol utilisee pour augmenter la capacite portante et reduire les tassements de sols faibles. Plusieurs approches ont ete developpees pour estimer la capacite portante de l'argile renforcee en piles de granulats, mais ces modeles presentent un biais de prediction et une incertitude importants. Cette etude utilise des donnees d'essai de chargement de semelle recemment developpees pour etudier la relation entre la capacite portante et les ratios de remplacement de la surface et d'elancement. La capacite portante d'un seul et meme pilier d'agregats, qu'elle soit isolee ou en groupes, au-dessous d'un sol en charge augmente a mesure que le taux de remplacement de la surface diminue en raison de l'augmentation de l'etendue du sol confine entourant le pilier. Il a egalement ete demontre que la longueur et le diametre d'une pile de granulats entrainaient une augmentation significative de la capacite portante, un effet qui diminuait avec l'elasticite croissante. De nouvelles modifications sont proposees pour les modeles existants d'expansion simplifiee et d'expansion de cavite afin de prendre en compte l'effet du confinement, du taux de remplacement de la surface et du coefficient d'elancement a l'aide d'une technique de validation croisee << un a un >>. L'analyse de validation croisee a donne lieu a des modeles de capacite portante robustes, plus precis que les modeles analytiques existants. De plus, le taux de concentration des contraintes pour les fondations peu profondes soutenues par des sols plastiques renforces par des piles de granulats a ete estime et compare aux donnees disponibles, indiquant sa sensibilite aux variables de conception et montrant que ce parametre de conception critique peut etre predit a l'aide des modeles mis a jour. [Traduit par la Redaction] Mots-cles: amelioration du sol, jetee d'agregat, tests de charge, capacite portante, concentration de stress., Introduction Aggregate piers (or stone columns) have been widely used as a method of reinforcement of soft soils. The installation of aggregate piers increases bearing capacity and reduces total and [...]

Microbial-induced calcium carbonate precipitation (MICP) represents a promising approach to improve the geotechnical engineering properties of soils through the precipitation of calcium carbonate (CaC[O.sub.3]) at soil particle contacts and soil particle surfaces. An extensive experimental study was undertaken to investigate the influence of initial relative density on the efficiency of the biocementation process, the reduction of liquefaction susceptibility, and the cyclic response in biocemented calcareous soils. For this purpose, stress-controlled undrained cyclic triaxial shear (CTS) tests were carried out on untreated and MICP-treated calcareous sand specimens for different initial relative densities and magnitudes of biocementation. Improvement in the cyclic response was quantified and compared in terms of excess pore pressure generation, evolution of axial strains, and the number of cycles to liquefaction. The cyclic experiments show that MICP treatment can change the liquefaction failure mechanism from flow failure to cyclic mobility and can significantly change the excess pore pressure generation response of initially loose specimens. Scanning electron microscope (SEM) images indicate the CaC[O.sub.3] crystals alter the characteristics of the sand particles and confirm the physical change in soil fabric that impacts the dynamic behavior and liquefaction resistance of MICP-treated specimens. Furthermore, the effect of biocementation was contrasted against the effect of relative density alone, and MICP treatment was shown to exhibit greater efficiency in improving the cyclic resistance than densification. Key words: biocementation, cyclic response, relative density, improvement factor, calcareous sand. La precipitation de carbonate de calcium induite par des microbes (MICP) represente une approche prometteuse pour ameliorer les proprietes d'ingenierie geotechnique des sols grace a la precipitation de carbonate de calcium (CaC[O.sub.3]) au contact de particules de sol et a la surface de particules de sol. Une vaste etude experimentale a ete entreprise pour etudier l'influence de la densite relative initiale sur l'efficacite du processus de biocementation, la reduction de la susceptibilite a la liquefaction et la reponse cyclique dans les sols calcaires biocementes. A cette fin, des essais de cisaillement triaxial cyclique non draine, sous contrainte controlee, ont ete effectues sur des echantillons de sable calcaire non traites et traites au MICP pour differentes densites et amplitudes relatives initiales de biocementation. L'amelioration de la reponse cyclique a ete quantifiee et comparee en termes de generation de pression interstitielle excedentaire, d'evolution des deformations axiales et du nombre de cycles jusqu'a la liquefaction. Les experiences cycliques montrent que le traitement MICP peut modifier le mecanisme d'echec de la liquefaction, passant de l'echec de l'ecoulement a la mobilite cyclique, et peut modifier de maniere significative la reponse a la generation de pression interstitielle excedentaire des specimens initialement laches. Les images au MEB indiquent que les cristaux de CaC[O.sub.3] modifient les caracteristiques des particules de sable et confirment le changement physique dans la structure du sol qui influe sur le comportement dynamique et la resistance a la liquefaction des echantillons traites au MICP. En outre, l'effet de la biocementation a ete mis en contraste avec l'effet de la densite relative seule, et il a ete demontre que le traitement par MICP presentait une efficacite plus grande pour ameliorer la resistance cyclique que la densification. [Traduit par la Redaction] Mots-cles : biocementation, reponse cyclique, densite relative, facteur d'amelioration, sable calcaire., Introduction Microbially induced calcium carbonate precipitation (MICP) is a biologically induced cementation process that can be used to improve the geotechnical engineering properties of a granular soil through the precipitation [...]

Calcareous sands are known as problematic soils in nature and challenge geotechnical engineers in many practical projects. Microbially induced calcite precipitation (MICP) is an innovative soil improvement technique that uses biomineralisation processes to induce cementation in-situ. The work described in this paper investigates the strength, deformation, and microstructure characteristics of biocemented calcareous sand under different cementation solution to sample volume ratios. A series of laboratory experiments was conducted, including unconfined compressive strength tests, splitting, tensile (i.e., Brazilian) strength tests, and consolidated drained triaxial tests. The results indicate that an exponential function reasonably describes the unconfined compressive strength and splitting tensile strength with increasing cementation solution to sample volume ratios. The tangent modulus at 50% peak strength increases exponentially with an increase in cementation solution to sample volume ratio, whereas it increases linearly with an increase in strength. The strength parameters for this MICP-improved soil, including the peak cohesion and friction angle, are derived to facilitate engineering design. Microstructure analyses are used to illustrate the physical basis for the increase in strength and stiffness with increases in the calcite content, as demonstrated using the cementation solution to sample volume ratio. Key words: calcareous sand, microbially induced calcite precipitation (MICP), cementation solution to sample volume ratio, unconfined compressive strength, splitting tensile strength, microstructure. Les sables calcaires sont connus comme des sols problematiques dans la nature et defient les ingenieurs geotechniciens dans de nombreux projets pratiques. La precipitation de calcite induite par les micro-organismes (MICP) est une technique innovante d'amelioration des sols qui utilise des processus de biomineralisation pour induire la cementation in situ. Le travail decrit dans cet article etudie les caracteristiques de resistance, de deformation et de microstructure du sable calcaire biocemente sous differents rapports solution volume/volume de cementation. Une serie d'experiences en laboratoire a ete realisee, comprenant des essais de resistance a la compression non confinee, des essais de dedoublement, des essais de resistance a la traction (c'est-a-dire bresiliens) et des essais triaxiaux draines consolides. Les resultats indiquent qu'une fonction exponentielle decrit de maniere raisonnable la resistance a la compression non limitee et la resistance a la rupture par traction avec des rapports croissants solution de cementation/volume d'echantillon. Le module tangent a 50% de la resistance maximale augmente de fa on exponentielle avec l'augmentation du rapport solution de cementation/volume d'echantillon, alors qu'il augmente lineairement avec l'augmentation de la resistance. Les parametres de resistance de ce sol ameliore par MICP, y compris la cohesion maximale et l'angle de frottement, sont derives pour faciliter la conception technique. Les analyses de microstructure sont utilisees pour illustrer la base physique de l'augmentation de la resistance et de la rigidite avec l'augmentation de la teneur en calcite, comme demontre par le rapport solution de cementation/volume d'echantillon. [Traduit par la Redaction] Mots-cles : sable calcaire, precipitation de calcite d'origine microbienne (MICP), rapport solution de cementation/volume d'echantillon, resistance a la compression non confinee, fendre la resistance a la traction, microstructure., Introduction Calcareous sands, which mainly exist in tropical and subtropical continental shelves, are composed of debris and skeletal remains of marine organisms. Due to their carbonate mineralogy and unique particle [...]

This study proposes a reliability-based design procedure to evaluate the allowable load for augered cast-in-place (ACIP) piles installed in predominately granular soils based on a prescribed level of reliability at the serviceability limit state. The ultimate limit state (ULS) ACIP pile-specific design model proposed in the companion paper is incorporated into a bivariate hyperbolic load-displacement model capable of describing the variability in the load-displacement relationship for a wide range of pile displacements. Following the approach outlined in the companion paper, distributions with truncated lower-bound capacities are incorporated into the reliability analyses. A lumped load-and-resistance factor is calibrated using a suitable performance function and Monte Carlo simulations. The average and conservative 95% lower-bound prediction intervals for the calibrated load-and-resistance factor resulting from the simulations are provided. Although unaccounted for in past studies, the slenderness ratio is shown to have significant influence on foundation reliability. Because of the low uncertainty in the proposed ULS pile capacity prediction model, the use of a truncated distribution has moderate influence on foundation reliability. Key words: augered cast-in-place (ACIP) piles, reliability, serviceability limit state, statistics, design. Cette etude propose une procedure de conception basee sur la fiabilite (RBD, << reliability-based design >>) pour evaluer la charge admissible pour pieux coules par tariere (ACIP, << augered cast-in-place piles >>) installes principalement dans les sols granulaires bases sur un niveau de fiabilite a l'etat limite de service. L'etat limite ultime (ULS) du modele de conception specifique du pieu propose dans le document d'accompagnement est integre dans un modele de charge-deplacement hyperbolique a deux variables capables de decrire la variabilite de la relation charge-deplacement pour une large gamme de deplacements. Suivant l'approche decrite dans le document d'accompagnement, les distributions avec capacites de limite inferieure tronquee sont incorporees dans l'analyse de fiabilite. Une charge localisee et un facteur de resistance sont etalonnes a l'aide d'une fonction de performance appropriee et de simulations de Monte-Carlo. Les intervalles moyens et conservateurs de prevision de limite inferieure de 95 % pour la charge calibree et le facteur de resistance resultant de la simulation sont fournis. Bien que disparues dans le cours des etudes, le rapport d'elancement est montre avoir une influence significative sur la fiabilite de la fondation. En raison de l'incertitude faible de modele de prediction de la capacite du pieu d'ULS propose, l'utilisation d'une distribution tronquee a une influence moderee sur la fiabilite de la fondation. [Traduit par la Redaction] Mots-cles: pieux coules par tariere (ACIP), fiabilite, etat limite de conception, statistiques, conception., Introduction A suitable foundation design will satisfy the strength limit or ultimate limit state (ULS) as well as the serviceability limit state (SLS), which is often associated with the allowable [...]

The use of augered cast-in-place (ACIP) piles for transportation infrastructure requires an appropriate reliability-based design (RBD) procedure. In an effort to improve the accuracy of an existing design model and calibrate appropriate resistance factors, this study presents a significantly revised RBD methodology for estimating the shaft and toe bearing capacity of ACIP piles using a large database consisting of static loading tests in predominately granular soils. The proposed design models are unbiased, as opposed to those currently recommended. Based on the reasonable assumption that a finite lower-bound resistance limit exists, lower-bound design lines are developed for shaft and toe bearing resistance by applying a constant ratio to the proposed design models. Resistance factors are calibrated at the strength or ultimate limit state (ULS) for ACIP piles loaded in compression and tension for two commonly used target probabilities of failure with and without lower-bound limits. For piles loaded in compression, separate resistance factors are calibrated for the proposed shaft and toe bearing resistance models. The inclusion of a lower-bound limit for piles loaded in tension results in a 24%-50% increase in the calibrated resistance factor. For piles loaded in compression, the application of a lower-bound limit results in a 20%-150% increase in the calibrated resistance factor, and represents a significant increase in useable pile capacity. Although the impact of a lower-bound limit on resistance factor calibration is directly dependent on the degree of uncertainty in the distribution of resistance, this effect is outweighed by the type of distribution selected (i.e., normal, lognormal) at more stringent target probabilities of failure due to differences in distribution shape at the location of the lower-bound limit. A companion paper explores the use of the revised ULS model in a reliability-based serviceability limit state design framework. Key words: augered cast-in-place (ACIP) piles, ultimate limit state, reliability, statistics, resistance factors, lower bound limits. L'utilisation de pieux coules par tariere (ACIP, << augered cast-in-place piles >>) pour l'infrastructure de transport exige une conception basee sur la fiabilite (RBD, << reliability-based design >>). En vue d'ameliorer la precision d'un modele de conception et d'etalonner les facteurs de resistance, cette etude presente une methodologie RBD revisee pour l'estimation de la capacite portante de l'arbre et du bout des pieux ACIP en utilisant une base de donnees importante composee d'essais de chargement statique principalement dans les sols granulaires. Les modeles de conception proposes sont non biaises, par opposition a celles qui sont actuellement recommandees. Basees sur l'hypothese raisonnable qu'une limite de resistance inferieure limite existe, des limites inferieures de conception sont mises au point pour la resistance de roulement de l'arbre et du bout en appliquant un rapport constant aux modeles de conception proposes. Les facteurs de resistance sont etalonnes a la resistance ou a l'etat limite ultime (ULS) pour les pieux ACIP charges en compression et en traction pour deux probabilites de cible couramment utilisee en defaillance avec et sans limites les limites inferieures. Pour les pieux charges en compression, des facteurs de resistance separes sont etalonnes pour les modeles de resistance de roulement de l'arbre et du bout. L'inclusion d'une limite inferieure pour les pieux charges en traction donne lieu a une augmentation de 24-50 % dans le facteur de resistance etalonne. Pour les pieux charges en compression, l'application d'une limite inferieure entrame une augmentation de 20-150 % dans le facteur de resistance calibree, et represente une augmentation significative de la capacite du pieu utilisable. Bien que l'impact d'une limite inferieure sur l'etalonnage des facteurs de resistance depend directement du degre d'incertitude dans la repartition de la resistance, cet effet est compense par le type de distribution (c.-a-d., normale, lognormale) a des probabilites cibles plus rigoureuses de defaillance due a des differences dans la forme de distribution a l'emplacement de la limite inferieure. Un autre article se penche sur l'utilisation de modele de l'ULS revise base sur la fiabilite dans un cadre de conception d'etat limite de service. [Traduit par la Redaction] Mots-cles: pieux coules par tariere (ACIP), etat limite ultime, fiabilite, statistiques, facteurs de resistance, limites de la borne inferieure., Introduction Subjective global factors of safety, defined as the ratio of resistance and load, have traditionally been used to account for variations in the predicted resistance and loads to provide [...]