Specimen Size Effect on Shear Behavior of Loose Sand in Triaxial Testing

Triaxial tests are widely used to determine the behavior and strength characteristics of soils without due attention to the differences in specimen size. Several drained and undrained monotonic triaxial compression shear tests are performed on three different specimen sizes of the same sand to investigate the influence of specimen size and scale effect on the shear behavior. The test results indicate that the behavior of loose sand is strongly influenced by the specimen size, with larger specimens exhibiting a stiffer behavior during isotropic compression, and mobilizing smaller shear strengths and effective friction angles. Triaxial testing also involves many sources of errors that could significantly affect shear strength parameters if not corrected. Extensive errors are investigated and it is found that negligence in making corrections accounting for these errors will result in an overestimation as much as 42% and 15 degrees in the critical shear strength and critical state friction angle, respectively. Furthermore, the measured critical state parameters and shear strengths are employed to compare the static and seismic slope stability of an earth embankment dam, calibrate a critical state soil constitutive model, study the soil behavior under shallow foundations, and evaluate liquefaction triggering and failure of retaining structures. The results show that all of these analyses are significantly affected by the strength parameters of the same soil determined from different specimen sizes. While using small size samples for determining shear strength parameters might result in un-conservative design, the choice of a large sample size is consequently a more accurate representation of soil strength conditions and field deformations

Experimental Study of the Displacements Caused by Cone Penetration in Sand

As more advanced theoretical methods become available for solution of the complex cone penetration (CPT) boundary-value problem, it is essential to develop methods to validate those solutions experimentally. A large scale model penetrometer testing facility, consisting of a half-circular calibration chamber with digital image correlation (DIC) capability, was developed for experimental simulation of cone penetration. The flat side of the chamber is transparent, made of Plexiglass. In individual tests, sand samples are prepared inside the half-circular chamber by pluviation; the half-circular cone is then pushed into the sand model flush against the Plexiglas, with the penetration process digitally imaged for DIC analysis. The displacement data obtained from DIC analysis provided valuable insights into the cone penetration boundary-value problem. Experiments with three different types of silica sands show that sand crushability influences the slip pattern around the advancing cone and also the cone penetration resistance. The derived strain paths show that very complex modes of deformation are experienced by the soil elements in close proximity of the penetrometer tip. The close-up imagery of the interface zone provided useful information for understanding the model penetrometer-sand interface behavior. The series of cone penetration tests conducted in layered sand profiles show that the sensing and development depths are dependent upon the position of weak and strong layers with respect to the direction of the advancing cone. The observed influence zone as interpreted from DIC analysis is smaller than stated in the literature

Discrete Element Modeling of the Mechanical Response of Cemented Granular Materials

abstract: With the growth of global population, the demand for sustainable infrastructure is significantly increasing. Substructures with appropriate materials are required to be built in or above soil that can support the massive volume of construction demand. However, increased structural requirements often require ground improvement to increase the soil capacity. Moreover, certain soils are prone to liquefaction during an earthquake, which results in significant structural damage and loss of lives. While various soil treatment methods have been developed in the past to improve the soil’s load carrying ability, most of these traditional treatment methods have been found either hazardous and may cause irreversible damage to natural environment, or too disruptive to use beneath or adjacent to existing structures. Thus, alternative techniques are required to provide a more natural and sustainable solution. Biomediated methods of strengthening soil through mineral precipitation, in particular through microbially induced carbonate precipitation (MICP), have recently emerged as a promising means of soil improvement. In MICP, the precipitation of carbonate (usually in the form of calcium carbonate) is mediated by microorganisms and the process is referred to as biomineralization. The precipitated carbonate coats soil particles, precipitates in the voids, and bridges between soil particles, thereby improving the mechanical properties (e.g., strength, stiffness, and dilatancy). Although it has been reported that the soil’s mechanical properties can be extensively enhanced through MICP, the micro-scale mechanisms that influence the macro-scale constitutive response remain to be clearly explained. The utilization of alternative techniques such as MICP requires an in-depth understanding of the particle-scale contact mechanisms and the ability to predict the improvement in soil properties resulting from calcite precipitation. For this purpose, the discrete element method (DEM), which is extensively used to investigate granular materials, is adopted in this dissertation. Three-dimensional discrete element method (DEM) based numerical models are developed to simulate the response of bio-cemented sand under static and dynamic loading conditions and the micro-scale mechanisms of MICP are numerically investigated. Special focus is paid to the understanding of the particle scale mechanisms that are dominant in the common laboratory scale experiments including undrained and drained triaxial compression when calcite bridges are present in the soil, that enhances its load capacity. The mechanisms behind improvement of liquefaction resistance in cemented sands are also elucidated through the use of DEM. The thesis thus aims to provide the fundamental link that is important in ensuring proper material design for granular materials to enhance their mechanical performance.Dissertation/Thesisundrained simulation with flexible membranecyclic direct simple shear simulationDoctoral Dissertation Civil, Environmental and Sustainable Engineering 201

The bearing capacity of footings on sand with a weak layer

Minor details of the ground, such as thin weak layers, shear bands and slickensided surfaces, can substantially affect the behaviour of soil–footing and other geotechnical systems, despite their seeming insignificance. In this paper, the influence of the presence of a thin horizontal weak layer on the ultimate bearing capacity of a strip footing on dense sand is investigated by single-gravity tests on small-scale physical models of the soil–footing system. The test results show that the weak layer strongly influences both the failure mechanism and the ultimate bearing capacity if its depth is lower than about four times the footing width. It is found that the presence of a thin weak layer can cause decreases of the ultimate bearing capacity of up to 80%. Numerical simulations, by finite-element analysis, of the behaviour of the reduced-scale models are able to capture the failure mechanism and the ultimate bearing capacity correctly, only if the mean equivalent constant value of the secant angle of shearing resistance used in calculations is selected, taking into account the curvature of the shear strength envelope of the sand within the very low normal stress range existing in the tested models

Évaluation du potentiel de liquéfaction à l’aide du nouvel essai triaxial en cisaillement simple (TxSS)

Abstract: In recent decades, several total and effective stress constitutive models have been developed in geotechnical engineering practice to perform one-dimensional site response analysis. These constitutive models have been incorporated in either finite difference or finite element dynamic analysis programs. Also, numerous research work has been done to predict more accurately earthquake-induced pore water pressure more accurately. Hitherto, the developed pore pressure models can be classified into stress, strain, and energy-based models. In the current study, strain-based and energy-based pore pressure models were proposed based on a series of strain-controlled cyclic TxSS tests performed on reconstituted specimens of Baie Saint Paul, Carignan, Ottawa C-109, Ottawa F-65, and Quebec CF6B sands. The proposed strain-based model was used to investigate the equivalent number of cycles concept and to assess the pore pressure as a damage metric. The energy-based model was combined with the Sigmoidal-model in FLAC 2D to introduce a simplified coupled energy-based pore pressure model. The proposed model was calibrated and verified, in terms of shear stress-strain response and excess pore pressure development, based on a series of strain-controlled cyclic TxSS test results. On an elemental-level, the model results were validated under cyclic strain-controlled and stress-controlled tests and a fair agreement was observed between the energy-based model and DSS results in terms of liquefaction resistance. In addition, the proposed model was validated by incorporating the energy-model in FLAC3D platform to study the cyclic behavior under triaxial and simple shear conditions. The numerical simulation clarifies the difference between cyclic triaxial and simple shear conditions as well as the load conditions (i.e. stress or strain-controlled conditions). Further validation was performed by numerical simulation of a centrifuge model conducted by Ramirez et al. (2017) at the University of Colorado Boulder by using the well-established Finn model and by the proposed energy-based model. The comparison shows the capability of the proposed energy-based model in conjunction with the Sigmoidal-model to very well simulate the seismic response in liquefaction analysis. The proposed simplified coupled energy-based pore pressure model was implemented to assess the compatibility of the liquefaction charts in the eastern and western North America as a part of this study. Different hypothetical sand deposits having different fundamental periods were subjected to two scaled-up earthquakes to perform 1-D site response analysis. One is compatible with the National Building Code of Canada 2005 (synthetic earthquake) and another incompatible real earthquake from the western region (Northridge earthquake). The comparison in terms of the generated pore pressure, the equivalent number of cycles and incorporated liquefaction charts (CRR-(N1)60CS) highlights the inaccuracy of using current liquefaction charts in Eastern regions.Au cours des dernières décennies, plusieurs modèles constitutifs des contraintes totales et effectives ont été devellope dans la pratique de geotechnique pour effectuer une analyse unidimensionnelle de la réponse du site. Ces modèles constitutifs ont en fait été incorporés dans l'analyse dynamique par différence finie ou par éléments finis. De nombreux travaux de recherche ont été effectués pour prédire avec plus de précision la pression d'eau interstitielle induite par le séisme. Jusqu'à présent, les modèles de pression interstitielle développés peuvent être classés en modèles basés sur la contrainte, la déformation et l'énergie-dissipe. Dans le cadre de la présente étude, un modèle de pression interstitielle basé sur la déformation et un autre modèle de pression interstitielle basé sur l'énergie sont proposés à partir d'une série d'essais TxSS cycliques contrôlés par déformation effectués sur des échantillons de sol reconstitué à Baie Saint-Paul, Ottawa C-109, Ottawa F-65 et dans les sables de Québéc. Le modèle basé sur la déformation proposé a été utilisé pour étudier le concept du nombre équivalent de cycles et pour évaluer la pression interstitielle comme mesure des dommages. Cependant, le modèle basé sur l'énergie est combiné avec le modèle Sigmoidal dans le logiciel FLAC pour introduire un modèle couplé simplifié de pression interstitielle basé sur l'énergie. Le modèle proposé est calibré et vérifié, en termes de réponse contrainte-déformation et de pression interstitielles, sur la base d'une série de résultats d'essais TxSS cycliques à contrainte contrôlée. Au niveau des éléments, les résultats du modèle ont été validés dans le cadre d'essais cycliques à contrainte contrôlée et d'essais alternatifs à contrainte contrôlée et une concordance a été observé entre les résultats du modèle énergétique et ceux du DSS en termes de potentielle de liquéfaction. De plus, le modèle proposé a été utilisé en incorporant le modèle d'énergie dans la plate-forme FLAC3D pour étudier le comportement cyclique dans des conditions de cisaillement simple et triaxial. La simulation numérique clarifie la différence entre les conditions de cisaillement cycliques triaxiales et les conditions de cisaillement simples ainsi que les conditions de charge (c'est-à-dire les conditions de contrainte ou de déformation contrôlées). D'autres validations ont été effectuées par simulation numérique d'un modèle expérimental de centrifugeuse mené par Ramirez et al (2017) à l'Université du Colorado Boulder par le modèle Finn bien établi et par le modèle énergétique proposé. La comparaison obtenue montre la capacité du modèle énergétique proposé conjointement avec le modèle Sigmoidal à capturer la réponse sismique dans l'analyse de liquéfaction. Dans le cadre de cette étude, le modèle simplifié de pression interstitielle couplée basée sur l'énergie a été mis en oeuvre pour évaluer la capacité des chartes de liquéfaction dans l'Est et l'Ouest de l'Amérique du Nord. Différents dépôts de sable de niveau hypothétique ayant des périodes fondamentales différentes ont été soumis à deux séismes de grande échelle pour effectuer une analyse 1-D de la réponse du site. L'un est compatible avec le Code Nationale du Batiment 2005 (tremblement de terre synthétique) et un autre tremblement de terre réel incompatible de la région ouest (séisme de Northridge). La comparaison en termes de pression interstitielle générée, de nombre équivalent de cycles et de charte de liquéfaction incorporés (CRR-(N1)60CS) souligne une certaines imprecision de l'utilisation des chartes de liquéfaction actuels dans les régions de l'Est

A numerical study of the suitability of rigid inclusion ground reinforcement beneath caisson quay walls

The objective of this study was to determine whether rigid inclusions are suitable for reinforcement of the foundation of a caisson quay wall functioning as a container terminal. Apart from their brittle behaviour under lateral loading, rigid inclusions are well suited to the large uniform loads and stringent post-construction deflection tolerances associated with container terminal structures. Their inherent strength and stiffness means they have certain advantages over other stiffening columns commonly used for ground reinforcement in port expansion projects. Their mechanical properties allow construction to unrestricted heights at any construction rate and, in theory, RIs can be applied to all soil types. Additionally the locations of many ports coincide with rivers, deltas and estuaries which are associated with poor soil conditions often requiring ground improvement. Their suitability is of practical significance to port planners and engineers who are faced with the challenge of providing satisfactory foundation performance that is cost effective. The addition of RI ground reinforcement for this structural application would allow for greater flexibility in meeting these challenges. The literature review for this study was broad in its scope with emphasis placed on describing the mechanics of the problem, analysis methods and suitable installation methods for execution in the marine environment. One of the key outcomes of the literature review was identifying the problem of lateral loading due to "free-field" lateral ground movements. In light of this, suitable strategies for limiting and accommodating lateral loading of the RIs were proposed. A numerical study of the proposed ground improvement scheme was undertaken using the 3D finite element method. The key model outputs were caisson deflections and RI forces, moments and stresses, for the various simulated construction phases up to operational conditions. The model results were assessed in terms of the key foundation performance criteria which were related to STS crane rail tolerances and limiting tensile stresses in the RIs. This study found that for a firm clay subsoil condition the proposed RI ground reinforcement scheme met the foundation performance criteria for this structural application provided (i) strategies to limit lateral loading were implemented and (ii) the RIs were reinforced over the length where they were not fully compressed. While this study provided insights into the behaviour of RIs for this structural application, ultimately suitability is a function of range of factors, in addition to the limited technical performance criteria derived for this study

Modelling of excess pore pressure accumulation in sand around cyclically loaded foundations

In particular during storm events an accumulation of excess pore pressures may occur in the soil around cyclically loaded offshore foundations. The excess pore pressure build-up reduces the effective stresses in the soil and, hence, may negatively affect the structural integrity by influencing the soil-structure interaction. Besides a loss in bearing capacity, large plastic deformations may occur to the structure. Especially for offshore wind turbines an accurate estimation of such deformations is of great importance. Even though the consideration of this degradation effect on the bearing capacity is commonly demanded by the involved certification or approval bodies, no general applicable and accepted method for the calculative verification currently exists. Over the past decades several researchers investigated the excess pore pressure build-up around offshore foundations due to environmental cyclic loads. They tried to capture the loss of bearing capacity, the accumulation of plastic rotation and the essential influence on the serviceability limit state and fatigue design. However, even if there are some sophisticated concepts, none of them is seen as the simple general applicable choice. Within this thesis a new numerical method – termed Excess Pore Pressure Estimation method (EPPE) – is presented in great detail. This method allows for the transfer of the soil behaviour obtained in cyclic simple shear tests to the bearing behaviour of the entire foundation. Herein, the numerical model accounts for the cyclic excess pore pressure accumulation by respecting the element-based mean stress and stress amplitude as well as an equivalent number of load cycles. The simulation of the excess pore pressure build-up due to certain cyclic loading is based on undrained conditions, i.e. the excess pore pressure build-up due to cyclic loading is derived by disregarding the simultaneous consolidation process. The respected transfer method, in the form of contour plots, enables the consideration of site-specific cyclic direct simple shear and triaxial test results from laboratory devices to elements within the finite element model. Each integration point is evaluated individually. Based on the derived excess pore pressure field, a consolidation analysis takes place in the second step. The actual accumulated excess pore pressure in each element at the end of the storm (or cyclic loading event) is then found by analytically superposing the excess pore pressure decay curves from the consolidation analysis. For a deeper understanding of cyclic soil behaviour, the cyclic response in different laboratory devices with different densities and under varying stress states was investigated by the author. A contour approach based on cyclic load- and displacement-controlled test results is derived to study the element response from the numerical point of view and use these for the calibration of an implicit model. Moreover, different explicit approaches are presented and compared in terms of their estimation behaviour of cyclic excess pore pressure generation, their predicted foundation capacity and their model assumptions. The intention is hence to examine existing approaches and their applicability by means of an elaborate comprehensive study. A simple modular explicit model is presented which can be easily assessed with engineering judgment. If needed, the different individual calculation steps can be exchanged with more sophisticated ones. For a reference sandy soil, results of cyclic laboratory tests are presented and used on a reference monopile foundation for a predefined storm event. The EPPE approach helps to quantify the risk of capacity degradation as well as to evaluate an appropriate safety margin. It is possible with the current methodology to evaluate the degradation potential for different sites quite easily and fast.Insbesondere bei Sturmereignissen kann es im Boden an zyklisch belasteten Offshore-Fundamenten zu einer Akkumulation von Porenwasserüberdrücken kommen. Der Porenwasserüberdruck reduziert die effektiven Spannungen im Boden und kann daher die strukturelle Integrität negativ beeinflussen, indem dieser die Boden-Bauwerk-Interaktion zusätzlich beeinträchtigt. Insbesondere für Offshore-Windenergieanlagen ist eine genaue Abschätzung von Verformungen von großer Bedeutung. Obwohl die Berücksichtigung dieses Degradationseffekts auf die Tragfähigkeit von den beteiligten Zertifizierungs- oder Genehmigungsstellen gefordert wird, existiert derzeit keine allgemein anwendbare und akzeptierte Methode für den rechnerischen Nachweis. In den vergangenen Jahrzehnten untersuchten mehrere Forschende die zyklische Porenwasserüberdruckakkumulation, die sich um Offshore-Windenergieanlagen aufgrund von zyklischen Belastungen aufbaut. Sie versuchten, den Verlust der Tragfähigkeit und die Akkumulation der plastischen Rotation zu quantifizieren. Auch wenn einige Konzepte existieren, so wird keines als allgemeingültige Methodik angesehen. In dieser Arbeit wird eine neue numerische Methode – die sogenannte Excess Pore Pressure Estimation Methode (EPPE) – vorgestellt, die es erlaubt, das in zyklischen Einfachscherversuchen ermittelte Bodenverhalten auf das Tragverhalten des gesamten Fundaments zu übertragen. Dabei berücksichtigt das numerische Modell die zyklische Porenwasserüberdruckakkumulation unter Verwendung der element-spezifischen mittleren Spannung und Spannungsamplitude sowie der äquivalenten Zyklenzahl. Die Simulation des Porenwasserüberdruckaufbaus infolge bestimmter zyklischer Beanspruchungen basiert auf undrainierten Bedingungen, d.h. der Porenwasserüberdruckaufbau infolge bestimmter zyklischer Beanspruchungen wird unter Vernachlässigung des gleichzeitigen Konsolidierungsprozesses abgeleitet. Die Übertragung von Laborergebnissen auf Elemente innerhalb des Finite-Elemente-Modells in Form von Konturdiagrammen ermöglicht die Berücksichtigung von standortspezifischen zyklischen Einfachscher- und Triaxialversuchsergebnissen. Jeder Integrationspunkt wird individuell auf der Grundlage von last- oder weggesteuerten zyklischen Laborversuchsergebnissen ausgewertet. Die gesamte Porenwasserüberdruckakkumulation während eines Sturmereignisses, wird dann für einen bestimmten Bemessungssturm ermittelt. Auf Grundlage des abgeleiteten Porenwasserüberdruckfeldes wird im zweiten Schritt eine Konsolidierungsanalyse durchgeführt. Als Ergebnis der Analyse werden elementbasierte Porenwasserdruckabbaukurven abgeleitet. Der Verlauf des akkumulierten Porenwasserüberdrucks bis hin zum Ende des Sturms (oder des zyklischen Belastungsereignisses) wird durch analytische Superposition ermittelt. Für ein tiefgehendes Verständnis des zyklischen Bodenverhaltens wird das zyklische Antwortverhalten in verschiedenen Laborgeräten bei unterschiedlichen Lagerungsdichten und unter verschiedenen Spannungszuständen untersucht. Ein Konturansatz, der auf last- und verschiebungsgesteuerten Versuchsergebnissen basiert, wird abgeleitet. Um die Elementantwort aus numerischer Sicht zu untersuchen, wurde auch ein implizites Modell kalibriert. Die Ergebnisse werden im Detail erläutert. Anschließend werden verschiedene explizite Ansätze vorgestellt und hinsichtlich ihres Abschätzungsverhaltens der zyklischen Porenwasserüberdruckerzeugung, ihrer prognostizierten Gründungskapazität und ihrer Modellannahmen verglichen. Damit ist beabsichtigt, bestehende Ansätze und deren Anwendbarkeit in einer umfassenden Gesamtstudie zu untersuchen. Es wird ein generisches und modulares, explizites Modell vorgestellt, das leicht mit fachspezifischem Sachverstand bewertet werden kann. Die verschiedenen Berechnungsschritte können nach Bedarf durch weitere Schritte ergänzt werden. Im Rahmen dieser Arbeit werden Ergebnisse aus zyklischen Laborversuchen für einen beispielhaften Nordseesand vorgestellt und auf eine Referenz-Monopile-Gründung innerhalb eines vordefinierten Sturmereignisses angewendet. Der EPPE-Ansatz hilft bei der Quantifizierung des Verflüssigungsrisikos und der Ermittlung eines angemessenen Sicherheitsniveaus. Mit der aktuellen Methodik ist es möglich, das Degradationspotenzial für verschiedene Standorte einfach und schnell zu bewerten.Deutsche Forschungsgemeinschaft/Sonderforschungsbereich 1463/434502799/E