bluuebunny/crossref_metadata_2025 huggingface dataset:
Drum centrifuge study of three-leg jackup models on clay
Authors: Dean E. T. R. , James R. G. , Schofield A. N. , Tsukamoto Y.
Published: October 2009

This paper reports and discusses centrifuge test data of model three-leg jackups on kaolin clay. The tests modelled one prototype jackup with 6·5 m dia. 13° conical spudcans, one with 6·5 m dia. flat-based spudcans, and one with 13·0 m dia. flat-based spudcans. The model jackups were subjected to preloading, slow cyclic loading, rapid cyclic loading and pullout. The data obtained in this study are the loadpaths consisting of the vertical load, horizontal load and moment associated with each leg, the vertical settlement, horizontal displacement and rotation of the model jackups as well as of the model spudcans.

Ce papier rend compte et discute les données d'essai de centrifugation de modèles de plates-formes é1évatrices à trois jambes sur du kaolin. Les essais ont modélisé une plate-forme prototype sur des caissons d'enfichage coniques de 6·5 m de diamètre et d'angle 13°, une autre sur des caissons de base plane de 6·5 m de diamètre, et une autre sur des caissons de base plane de 13 m de diamètre. Les modèles de plate-forme ont été soumis à des précharges, à des charges cycliques lentes, à des charges cycliques rapides et à des arrachements. Les données issues de cette étude sont des courbes de charge constituées de la charge verticale, de la horizontale et du moment associé à chaque jambe, du tassement vertical, du déplacement horizontal et de la rotation des plates-formes modélisées ainsi que des caissons d'enfichage modélisés.
Auscultation et modélisation numérique du processus de creusement à l'aide d'un tunnelier
Authors: Benmebarek S. , Kastner R. , Ollier C. , Charif A.
Published: October 2009

En vue de constituer une base de données expérimentales précise et liable pour la validation et la qualification des outils de simulation des mouvements induits par les opérations ´un tunnelier, deux plots ´essais de ĺextension de la ligne D du métro de Lyon ont été intrumentés intensivement. Dans cet article, on présente les résultats de mesures ´une auscultation de deux sections transversales de cette ligne. En faisant une comparaison avec les mesures ´autres pro-jets antérieurs, on a mis en évidence que ĺinstabilité du front de taille et le vide annulaire dégagé après ĺéchappement de la jupe constituent les sources principales de tassement à court terme. A partir de ces mesures, on propose, ensuite, une nouvelle procédure de simulation numérique en déformation plane qui dent compte des différentes étapes de construction. La comparaison des mouvements observés et simulés montre que le comportement des terrains meubles est mieux modélisé par un contrôle en déplacements que par un contrôle en contraintes à la périphérie de ĺexcavation.

In order to obtain a precise and reliable measurement database for the validation and qualification of simulation tools describing the deformations induced by tunnelling operations, two sites on the Lyon Metro D line extension have been intensively monitored. This paper presents the results of measurements from two experimental transverse tunnel sections of this line. A comparison with data from previous projects shows that the instability of the tunnel face and the annular void created by the passage of the shield constitute the principal sources of short-term settlement. Based on these measurements, a new procedure of numerical simulation in the plane strain conditions is proposed, taking into account various stages of construction. The comparison of the observed and simulated movements shows that the behaviour of soft ground is better simulated by displacement control than by stress control of the periphery of the excavation.
Spatial variation in liquefaction risk
Authors: Fenton G. A. , Vanmarcke E. H.
Published: October 2009

Basic concepts and methods of random field theory, useful for characterizing 'distributed disordered systems', are applied to a soil deposit to evaluate the spatial extent of liquefaction under typical conditions. The soil deposit, modelled after a site located at the Wildlife Management Area in California, is represented by a three-dimensional random properties field and is subjected to multiple realizations of a space-time random field of earthquake ground motion patterned after the Superstition Hills event of 1987. Based on Monte Carlo analysis using a deterministic one-dimensional nonlinear finite-element program, spatial variability of liquefaction risk is assessed and techniques for improving estimates of liquefaction risk, accounting for local spatial variation, are investigated.

Les concepts et m6thodes de base de la théorie de champ aléatoire, servant ´ caracteogonriser les 'systémes désordonnés répartis' sont appliqueogons ´ un d´pôt de sol pour évaluer 1'étendue spatiale de la liquéfaction dans des conditions types. Le dépôt de sol, reproduisant un site situogon dans la Wildlife Management Area en Californie, est représenteacute par un champ de proprieacuteteacutes aleacuteatoires en trois dimensions et soumis a´ de multiples reacuteacutealisations d'un champ aléatoire espacetemps pour les mouvements de terrain seacuteismiques, copieacutes sur Nveacutenement de Superstition Hill en 1987. En nous basant sur une analyse de Monte Carlo utilisant un programme d'éléments finis non linéaires déterministes, nous évaluons la variabiliteacute spatiale du risque de liquéfaction et nous étudions les techniques permettant d'améliorer les évaluations du risque de liqu faction, en tenant compte de la variation spatiale locale.
Centrifuge model tests on doubly propped embedded retaining walls in overconsolidated kaolin clay
Authors: Richards D. J. , Powrie W.
Published: October 2009

A series of tests on model diaphragm walls embedded in an overconsolidated clay was carried out at the London Geotechnical Centrifuge Centre. The walls were propped at the crest at the start of excavation and, following the simulation of excavation, were immediately propped at formation level. A retained height of 10 m was modelled in all tests, and the depth of embedment below dredge level varied between 5 m, 10 m and 15 m. A 'softer' construction sequence was also investigated, with excavation to 5 m below the retained surface taking place prior to the installation of the crest-level prop. The effect of pre-excavation lateral earth pressure was also investigated. The test results show that an in- crease in embedment depth will lead to an increase in wall bending moment and a reduction in bottom prop load. There is no real advantage in terms of limiting ground movements in increasing the embedment depth without also increasing the wall stiffness.

Nous avons effectué une série d'essais sur une maquette de murs souterrains dans une argile surconsolidée au London Geotechnical Centrifuge Centre. Les murs ont été soutenus à la crÊte au début de Pexcavation et, après ľexcavadon simulée, ont immédiatement éré soutenus au niveau de formation. Une hauteur retenue de 10 m a été mise en maquette dans tons les essais et la profondeur de ľenfouissement sous le niveau de drague variait entre 5, 10 et 15 m. Nous avons également étudié une séquence de construction 'plus tendre' avec une excavation de 5 m en dessous de la surface retenue, avant ľinstallation du souden de crÊte. Nous avons également étudié ľeffet de la pression terrestre latérale avant excavation. Les résultats des essais montrent qu'une augmentation de la profondeur d'enfouissement conduit à une augmentation du couple de flexion du mur et à une réduction de la charge de souden du bas. Il n'y a aucun avantage réel, si ľont vent limiter les mouvements du sol, à augmenter la profondeur d'enfouissement sans augmenter également la rigidité des murs.
Effects of sampler design on tube sampling disturbance—numerical and analytical investigations
Authors: Clayton C. R. I. , Siddique A. , Hopper R. J.
Published: October 2009

The detailed features of the design of a sampling tube, such as area ratio, inside clearance, cutting edge taper angle, etc., have an important influence on the disturbance caused when a tube sampler is pushed into cohesive soil. Currently, most tube sampler designs have evolved on the basis of empirical design rules, and local good practices, developed for particular soil conditions. The strain path method provides a basis upon which the influence and importance of different features of tube sampler design can be evaluated, and the design of future samplers optimized. This paper examines the use of Baligh's strain path method for the assessment of tube sampling disturbance, and implements the method via a finite-element approach to assess the influence of area ratio, cutting-edge angles and inside clearance, on sample disturbance evaluated on the basis of the strains imposed on the centre-line of a soil sample.

Les caractéristiques précises du modèle de tube d'échantillonnage, comme le rapport de section, le jeu interne, l'angle de cône du bord coupant, etc., ont une influence prépondérante sur la perturbation causée par Penfoncement d'un échantillonneur à tubes dans un sol cohérent. A heure actuelle, la plupart des modèles d'échantillonneur à tubes sont conçus a partir de r6gles de calcul empiriques et de pratiques locales adaptées au conditions de terrain particulières. La méthode des courbes de contrainte fournit un moyen permettant d'une part, d'évaluer l'influence et l'importance des diff&entes caractéristiques du modèle d'échantillonneur à tubes et d'autre part, d'optimiser le dimensionnement des futurs échantillonneurs. Ce papier expose utilisation de la méthode des courbes de contrainte de Baligh pour étudier la perturbation de l'échantillonnage tubulaire, et décrit la méthode à é1éments finis mise en oeuvre pour évaluer l'influence du rapport de section, des angles de bord coupant et du jeu interne, sur la perturbation de l'échandllon évaluée en s'appuyant sur les contraintes impos&aucte;es sur l'axe d'un &aucte;chantilIon de sol.
The effect of inÆltration on the stability of the slopes of a dry ash dump
Authors: Fourie A. B. , Rowe D. , Blight G. E.
Published: October 2009

The effects of prolonged periods of infiltration on the stability of the slopes of a dry ash dump are examined. The ash, which is placed by stackers, comes to rest at slope angles that are steeper than those predicted by conventional slope stability analyses using effective stress strength parameters. It is shown that the additional strength required to ensure stability of the ash dump slopes is provided by matric suction developed in the unsaturated ash. Infiltration of rainfall into the slopes can reduce the suction sufficiently to trigger shallow slope failures. Two methods for analysing this problem, an approximate method based on statistical rainfall records together with a simplified approach to simulating the rate of infiltration into a slope, and a more rigorous finite element approach, are discussed. Although the approximate method is more conservative than the finite element method, the results are similar and illustrate the importance of accurately quantifying the contribution of matric suction to the shear strength of an unsaturated soil when considering slope stability problems. The relevance of antecedent infiltration on the susceptibility of a slope to rainfallinduced instabilities is highlighted, and a strategy to take account of this factor is suggested.

Nous examinons l'effet de pluies prolongées sur la stabilité des pentes d'un terril de cendre sèche. La cendre, qui est amenée par les engins emplileurs, se dépose en formant des déclivités plus abruptes que celles prévues dans les analyses de stabilité de pente conventionnelles utilisant des paramètres effectifs de resistance à la contrainte. Nous montrons que la résistance additionnelle nécessaire pour assurer la stabilité des pentes d'un terril de cendre est donnée par la succion matrique développée dans la cendre partiellement saturée. L'infiltration des pluies dans les pentes peut suffisamment réduire la succion pour déclencher des ruptures dans les pentes peu profondes. Nous examinons deux méthodes pour analyser ce probl&graveme: une méthode approximative basée sur les statistiques de pluviosité, employée avec une simulation simple du taux d'infiltration dans une pente; et une méthode d'éléments finis, plus rigoureuse. Bien que la méthode approximative soit plus conservatrice que la méthode d'élément finis, les résultats sont similaires et illustrent l'importance de bien quantifier l'influence de la succion matrique sur la résistance au cisaillement d'un sol partiellement saturé en ce qui concerne les problèmes de stabilité de pente. Nous montrons que les taux de pluviosité passés ont une grande influence sur la propension des pentes à se déstabiliser et nous suggérons une stratégie permettant de prendre en compte ce facteur.
Virgin compression of structured soils
Authors: Liu M. D. , Carter J. P.
Published: October 2009

This paper describes a study of the virgin compression behaviour of both naturally and artificially structured soils under one-dimensional compression or isotropic compression. It is proposed that during virgin compression, the additional voids ratio sustained by soil structure is inversely proportional to the current mean effective stress. The proposed formula has been verified for 20 different naturally and artificially structured soils, and it is seen that the proposal describes well the behaviour of all these soils. Finally, a general discussion on soil structure and its features is presented, and the proposed compression equation is extended to describe the compression of structured soil along a general stress path.

Cet expos&acaute; est une étude du comportement d'une consolidation vierge sur des sols structur&acuate;s de mani&gravere naturelle et artificielle, avec une consolidation unidimensionnelle on isotrope. Nous avançons que pendant la consolidation vierge, le taux de pore additionnel subi par la structure du sol est inversement proportionnel &grave la contrainte effective moyenne actuelle. La for-mule propos&acuate;e a ´té v&acuute;rifi&acuate;e sur vingt sols structurés de manière artificielle et naturelle et l'on volt que la proposition décrit bien le comportement de tons ces sols. Enfin, nous présentons une &acuate;tude générale de la structure du sol et de ses caractéristiques et nous d&acuate;veloppons l'équadon de consolidation proposée pour d&acaute;crire la consolidation du sol structuré le long d'une trajectoire de contrainte g&acutge;nérale.
Undrained shear strength of a glacial clay overconsolidated by desiccation
Authors: Mesri G. , Ali S.
Published: October 2009

Block samples of a Boston Blue clay crust were used in a detailed study of the relationship between undrained shear strength and consolidation pressure in the compression and extension shear modes. The glacial silty clay sample that was mainly composed of quartz, feldspar, illite and chlorite has developed an overconsolidation ratio near to 3·0 by desiccation. The in situ coefficient of earth pressure at rest was near to 0·9 as compared to K0p = 0·56 of the normally consolidated young clay in the compression range beyond the preconsolidation pressure. In the compression range, for each consolidation-shear mode, undrained shear strength to vertical consolidation pressure ratio as well as shearing-induced pore water pressure to vertical consolidation pressure ratio, friction angle mobilized at yield, and the coefficient of earth pressure at rest, were constant and independent of consolidation pressure. The mobilized undrained shear strength of the normally consolidated young clay obtained by consolidating samples to the compression range was 0·22σ′vc. However, mobilized undrained shear strength from specimens that were subjected to the in situ effective stress condition before undrained shear was 0·17σ′p. This value of in situ strength from the laboratory tests as well as the increase in undrained shear strength in the recompression range from the effective overburden pressure to the preconsolidation pressure are probably related to the microfissures in the desiccated stiff clay. Undrained shear strength of the desiccated stiff clay tended towards zero as the vertical consolidation pressure decreased starting from the in situ condition.
Centrifuge modelling of models of seismic effects on saturated earth structures
Authors: Dewoolkar M. M. , Ko H.-Y. , Pak R. Y. S.
Published: October 2009

Centrifuge modelling of seismic effects on saturated, liquefiable, cohesionless soil structures is discussed in this paper. A methylcellulose solution (metolose) was used as a substitute pore fluid to alleviate problems arising from the conflict in time scaling in the modelling of dynamic and dissipative phenomena. A modelling of models type of study was performed on retaining wall models with saturated cohesionless backfill using both metolose and water as the pore fluid. These experiments were analysed using the finite element program DIANA-SWANDYNE II, and parametric studies were also performed. Based on the experimental and numerical results, it was confirmed that the conflict between dynamic and consolidation time scales exists and a substitute pore fluid is necessary for modelling prototype behaviours. It was shown that metolose is a suitable replacement pore fluid. It was also concluded that seismic modelling of models can be achieved only if a suitable substitute pore fluid is used. However, depending on the type of model, type of earthquake motion, and drainage boundary conditions, a false indication of achievement of seismic modelling of models can be obtained with water as the pore fluid which may not necessarily represent real prototype behaviour.
Plastic response of circular footings on sand under general planar loading
Authors: Gottardi G. , Houlsby G. T. , Butterfield R.
Published: October 2009

Data from a set of tests of model circular footings on dense sand are presented. The footings were subjected to a variety of combinations of vertical, horizontal and moment loading. The tests were designed to provide the information necessary to construct a complete model of the footing behaviour, based on the concepts of plasticity theory. In particular, the tests provide detailed information about the shape of the yield surface, and so allow generalization of bearing capacity calculations to cases other than purely vertical loading. Information is also obtained about the hardening law and flow rule appropriate for a plasticity model, and the elastic response within the yield surface.

Nous présentons ici les données provenant d'un ensemble d'essais sur des modèles d'assises circulaires sur un sable dense. Ces assises ont été soumises à une variété de combinations de charge verticale, horizontale et de moment. Les essais avaient été con&ecil;us pour donner l'information nécessaire à la construction d'un modéle complet du comportement de l'assise, en se basant sur les concepts de la théorie de plasticité. Notamment, les essais donnent une information détaillée sur la forme de la surface d'écoulement, ce qui permet de généraliser les calculs de résistance de portée et de les Réndre à des cas autres que les cas de charge purement verticale. Iis renseignent aussi sur la loi de durcissement et la règle d'écoulement convenant à un modée de plasticitéla réastique àieur de la surface d'écoulement.
Sedimentation and self-weight consolidation: constitutive equations and numerical modelling
Authors: Toorman E. A.
Published: October 2009

The solution of the mass balance equation for sedimentation and consolidation in sediment suspensions requires closure equations for the parameters diffusivity, permeability and effective stress. Experimental data for non-cohesive and cohesive materials are discussed. New semi-empirical relations, which are a better approximation of reality, are proposed. The non-uniqueness for cohesive sediments is explained in terms of differences in aggregation history. The numerical implementation of the unifying theory for sedimentation and consolidation faces some serious problems. Particularly, the discontinuities require the addition of artificial diffusivity to create the necessary damping to avoid overshoot-induced oscillations. Improved stability is obtained using a special finite-element technique. The model is validated with several data sets and proves to predict the right behaviour and density profile evolution for both non-cohesive and cohesive sediments. The remaining differences between measured and computed density profiles for cohesive materials are attributed to the inadequacy of the currently used constitutive equations.

La solution de ĺéquation ´équilibrage des masses pour la sédimentation et la consolidation dans les suspensions de sédiments nécessite des équations de clôture pour les paramètres suivants: diffusivité, perméabilité et contraintes effectives. Les données expérimentales relatives aux matières non cohésives et cohésives sont discutées et de nouveaux rapports semi-empiriques, représentant une meilleure approximation de la réalite, sont proposés. Le caractère non unique pour les sédiments cohésifs est expliqué sur le plan des différences dans ĺhistoire de ĺagrégation. Ĺexécudon numérique de la théorie de ĺunification pour la sédimentation et la consolidation se heurte à de graves difficultés. En particulier, les discondnuités nécessitent ĺadjonction ´une diffusivité ardficielle pour créer ĺamortissement nécessaire pour éviter le dépassement des oscillations induites. Une technique spéciale aux é1éments finis permet ´optimiser la stabilité. Le modèle est confirmé avec plusieurs ensembles de données et preuves afin de prévoir le comportement exact et ĺévolution du profil de densité pour les sédiments non cohésifs et cohésifs. Les différences restantes entre profils de densités mesurés et calculés pour le matériel cohésif sont attribuées à ĺinsuffisance des équations constitutives actuels.
Fall cone and compression tests on clay±gravel mixtures
Authors: Kumar G. V. , Wood D. Muir
Published: October 2009

Fall cone tests have been performed on mixtures of kaolin and fine gravel, from which both the variation of liquid limit with clay content and the link between undrained shear strength and water content for different clay contents have been determined. A sharp change in response is observed when the clay content falls below about 40%. One-dimensional compression tests have been performed in a hydraulic oedometer and in a consolidation tube: both devices gave similar results. Again a change in character of response is observed at clay contents below about 40%. For clay contents above about 35% it is the clay matrix alone which controls the mechanical behaviour of the mixture. The gravel only begins to influence the behaviour when the granular volume fraction reaches about 0·45.

Nous avons fait des essais á la chute avec un cône sur des mélanges de kaolin et de gravier fin, et de lá, nous avons déterminé la variation de la limite liquide en fonction du contenu argileux et le lien entre résistance au cisaillement non draé et contenu en eau pour différentes teneurs argileuses. Nous avons observé un changement marqué des réponses quand la teneur argileuse tombe en dessous de 40% environ. Des essais de compression en une seule dimension ont été faits dans un dométre hydraulique et dans un tube de consolidation: ces deux dispositifs ont donné des résultats similaires. Encore une fois, nous avons observé un changement dans le caractére de la réponse dans les teneurs argileuses inférieures á environ 40%. Pour les teneurs argileuses supérieures á environ 35%, c’est la matrice ďargile seule qui contrôle le comportement mécanique du mélange. Le gravier ne commence á influencer ce comportement que lorsque la fraction du volume granulaire atteint environ 0·45.
Numerical simulations of deviatoric shear deformation of granular media
Authors: Thornton C.
Published: October 2009

Quasi-static shear deformation of granular media is examined by performing numerical simulations on polydisperse systems of elastic spheres in a periodic cell. Results of axisymmetric compression test simulations are reported for both a dense and a loose system. At the macroscopic scale, the simulated stress–strain–dilation responses are in excellent qualitative agreement with the mechanical behaviour of sand observed in real experiments. Numerical simulation permits a detailed examination of the evolution of internal variables associated with the micromechanical processes occurring at the particle scale. In this context, the evolution of the induced structural anisotropy, the percentage of sliding contacts, the average coordination number and the normal and tangential contact force contributions to the stress tensor are presented and discussed. Further simulations are reported which illustrate the effect of interparticle friction on both the macrocopic and microscopic mechanical behaviour. Finally, the versatility of numerical simulation is demonstrated by presenting results obtained from simulations of general radial deviatoric loading, a constant deviatoric strain test and a multi-axial plane strain test.
Mechanical modelling of drained creep triaxial tests on loose sand
Authors: Di Prisco C. , Imposimato S. , Vardoulakis I.
Published: October 2009

The problem of mechanical instability of loose sand specimens during drained load controlled triaxial compression tests (creep tests) is discussed. In order to model mathematically such a phenomenon, a strain hardening elasto-visco-plastic constitutive model is extended so as to incorporate frictional strain-rate softening. The model is defined by a non-associated flow rule and by a Drucker–Prager-type yield surface which is equipped with two state variables, r and a. Variable r accounts for shear strain-induced anisotropy, while variable a accounts for volumetric strain hardening. The dynamic non-linearities of the strain response are discussed and theoretical predictions seem to reproduce experimental results quite well.
Distribution of earth pressure on a retaining wall
Authors: Wang Y.-Z.
Published: October 2009

On the basis of Coulomb's concept that the earth pressure against the back of a retaining wall is due to the thrust exerted by a sliding wedge of soil between the back of the wall and a plane which passes through the bottom edge of the wall and has an inclination of θ, a differential equation of first order is set up by considering the equilibrium of the forces on an element of the wedge. A theoretical result for the unit earth pressure on a retaining wall is obtained. A comparison is made between Coulomb's formula and the formula presented here, and the earth pressure calculated by the formula presented here is also compared with experimental observations.
Drained probing triaxial tests on a weakly bonded artificial soil
Authors: Malandraki V. , Toll D.
Published: October 2009

The results of drained probing triaxial tests are presented for a weakly bonded artificial soil in which the stress space is probed by following different stress path directions. The bounding surface (failure envelope) was found to vary with stress path direction. The influence of the stress path direction on the development of tangential stiffness, strain contours and the position of yield is also demonstrated. The yield behaviour is examined by identifying a reduction in tangential stiffness. Three different yield loci are presented in the stress space, each of which corresponds to the stress path direction followed. The changes in yield loci due to different stress path directions are explained in terms of anisotropic breakage of the bonded structure. The limits between different zones of behaviour were found to rotate in correspondence to the stress path direction. The work demonstrates that the yield loci and the zones of behaviour for bonded soils are not unique but depend on the stress path followed.
Fall-cone penetration and water content relationship of clays
Authors: Feng Tao-Wei
Published: October 2009

Results of previous studies on using the fall-cone test to determine the plastic limit of soils show that further research is warranted. The present investigation devotes itself to modification of the specimen preparation technique, examination of the relationship between depth of cone penetration and water content, and estimation of the plastic limit using the penetration depth against water content relationship. Specimen rings are made and used to replace the specimen cup so as to facilitate the preparation of specimens. Data of 26 soils, covering a wide range of liquid limits from 30% to 526%, show that the logarithm of depth of cone penetration against logarithm of water content relationship is linear. Therefore, a linear equation with two soil-dependent parameters is developed for estimation of the plastic limit. For a soil, the value of the soil-dependent parameters can be determined from as little as four fall-cone tests and substituted into the equation to estimate the plastic limit. The computed plastic limits of the soils analysed are within 0·8–1·2, with an average value of 1, times the measured plastic limits. The effects of strain rate and undrained strength ratio on the estimation of the plastic limit are evaluated and found to be limited.
Measurements and numerical simulations of surface waves using a plane model
Authors: Jonsson J. O.
Published: October 2009

A falling-weight impactor with a controlled force energy spectrum was used to generate surface waves in soft clay soil. Field measurements of both load and response in terms of vertical acceleration were made. The acceleration-type frequency response function (FRF) between the point of source and the point of response was established. The Rayleigh wave velocity of the soil was estimated from the measured acceleration time histories and from the established FRFs. Simulations of the field measurements were conducted and the dynamic response of the soil was studied in terms of the vertical acceleration and displacement at the soil surface. Two-dimensional numerical results were scaled to three dimensions. The effect of incorporating contributions from shear waves and compression waves at the surface was demonstrated. FRFs based on simulated data were established. The scaled numerical results and measured values are in good agreement.
Influence of compaction procedure on the mechanical behaviour of an unsaturated compacted clay. Part 2: shearing and constitutive modelling
Authors: Wheeler S. J. , Sivakumar V.
Published: October 2009

The influence of compaction pressure, compaction water content and type of compaction (static or dynamic) on subsequent soil behaviour was investigated by conducting controlled-suction triaxial tests on samples of unsaturated compacted speswhite kaolin. Compaction pressure influences initial state, by determining the initial position of the yield surface, thus affecting, among other things, the shape of stress–strain curves during shearing. Compaction pressure also influences, to a limited degree, the positions of the normal compression lines for different values of suction, but it has no effect on critical state relationships. The effect of compaction pressure can probably be modelled solely in terms of initial state if an anisotropic elastoplastic model incorporating rotational hardening is employed, whereas the parameters defining the slopes and intercepts of the normal compression lines for different values of suction require adjustment with variation of compaction pressure if a conventional isotropic hardening elastoplastic model is employed. Compaction water content influences the initial suction, but also has a substantial influence on normal compression lines and a noticeable effect on the volumetric behaviour at critical states. It is likely that soil samples compacted at different water contents will have to be modelled as different materials, irrespective of whether an isotropic or anisotropic hardening elastoplastic model is employed. A change from static to dynamic compaction has no significant effect on subsequent behaviour.
Numerical studies of the bearing capacity of shallow foundations on cohesive soil subjected to combined loading
Authors: Taiebat H. A. , Carter J. P.
Published: October 2009

This paper presents the results of three-dimensional finite-element analyses of circular foundations on the surface of homogeneous, purely cohesive soil. The foundations were assumed to adhere fully to the soil, and compressive, tensile and shear stresses may develop at the interface between the footing and the soil. The predicted ultimate response of the foundations to combined vertical, moment and horizontal loading was compared with other available theoretical predictions. A three-dimensional failure locus is presented for these foundations, based on the numerical predictions. An equation that approximates the shape of the failure locus is also suggested, and this provides a convenient means of calculating the bearing capacity of circular foundations on a uniform clay and subjected to combined loading.
Dilatancy for cohesionless soils
Authors: Li X. S. , Dafalias Y. F.
Published: October 2009

Dilatancy is often considered a unique function of the stress ratio η = q/p′, in terms of the triaxial stress variables q and p′. With this assumption, the direction of plastic flow is uniquely related to η, irrespective of the material internal state. This obviously contradicts the facts. Consider two specimens of the same sand, one is in a loose state and the other in a dense state. Subjected to a loading from the same η, the loose specimen contracts and the dense one dilates. These two distinctly different responses are associated with a single η but two different values of dilatancy, one positive and the other negative. Treating the dilatancy as a unique function of η has developed into a major obstacle to unified modelling of the response of a cohesionless material over a full range of densities and stress levels (before particle crushing). A theory is presented that treats the dilatancy as a state-dependent quantity within the framework of critical state soil mechanics. Micromechanical analysis is used to justify and motivate a simple macroscopic constitutive framework. A rudimentary model is presented, and its simulative capability shown by comparison with experimental data of the response of a sand under various initial state and loading conditions.
Limit equilibrium approach to study the evolution of shear band systems in soils
Authors: Leśniewska D. , Mróz Z.
Published: October 2009

The paper is concerned with the analysis of shear band systems appearing in dry soil behind flexible retaining walls under plane strain conditions. The experimental problem studied is that of an initially vertical, plane retaining wall, deforming because of the gradual excavation of sand from one side of the wall. The aim of the paper is to clarify some of the possible physical reasons for multiple shear bands developing behind such walls. The problem of shear band arrangement in a regular pattern, and their possible sequence, are also considered. The behaviour of soil adjacent to the wall is modelled by a set of rigid blocks, cut off from the soil body by lines parallel to the classical Coulomb wedge ‘slip line’ and sliding subsequently on each other. The distance between subsequent shear bands is calculated from the equilibrium of an actual soil wedge, which may contain previously activated wedges. An elastic behaviour of the wall is taken into account in a simplified way when equilibrium is verified. The plastic softening of soil is expressed in terms of the angle of internal friction or cohesion decreasing linearly with soil wedge displacement. The present simplified analysis is aimed as an explanation of shear band patterns, and constitutes a first step towards more refined treatments.
Review and interpretation of primary floor failure mechanism at a longwall coal mining face based on numerical analysis
Authors: Indraratna B. , Nemcik J. A. , Gale W. J.
Published: October 2009

This paper describes the conceptual model of primary rock failure mechanisms in the floor ahead of the longwall coal mining face based on underground observations and numerical modelling. During longwall mining, the stress fields and displacements of strata are constantly changing. High stress concentrations ahead of the longwall face can initiate fractures in the rock floor that can, under unfavourable stress conditions, develop into secondary floor failure, where large displacements of strata can occur. Severely broken floor strata or floor heave can seriously interfere with mining operations. Underground observations indicate that the sub-vertical shear fractures and the shear failure along the bedding planes in stratified rock are the most common fracture types observed in the rock floor below the longwall face. To explain how these fractures occur, basic principles of rock failure mechanism are applied to an overstressed rock subject to stress concentrations typically found at the longwall face. The computational simulation of the progressive longwall mining operations was undertaken in stratified rock to model complex stress distributions ahead of the longwall face and to simulate the rock failure in the mining floor. The model indicates that the shear fractures and the bedding plane failure dominate the floor failure ahead of the moving longwall face, as was observed underground.
Limiting resistance of a spherical penetrometer in cohesive material
Authors: Randolph M. F. , Martin C. M. , Hu Y.
Published: October 2009

The shear strength of soft sediments may be assessed by field tests, principally either vane tests or cone penetrometer tests. The continuous profile of strength obtained from the cone penetrometer is an advantage over the vane test, although the significant corrections for overburden stress, and pore pressure acting on the back face of the cone, limit its accuracy in soft soils. Alternative shapes for penetrometers have been suggested, for example a cylindrical T-bar and a spherical ball, both of which allow full flow of soil around the probe, thereby obviating the need for corrections due to the ambient stress level. Plasticity solutions have been available for some while relating the (average) shear strength of the soil to the net bearing pressure for plane strain flow around the cylindrical T-bar. This paper documents a corresponding solution for a spherical penetrometer, using upper and lower bound approaches supported by finite element analysis. Effects of strength anisotropy are discussed in the light of example results comparing the measured resistance of cone, T-bar and ball penetrometers.
CPT-based liquefaction analysis, Part 2: Reliability for design
Authors: Juang C. H. , Chen C. J. , Rosowsky D. V. , Tang W. H.
Published: October 2009

This paper is the second of a set of two papers that address the issue of earthquake-induced liquefaction potential. In the present study, a probabilistic approach is taken to analyse field liquefaction performance records. The CPT-based limit state function for liquefaction resistance assessment presented in the first paper forms the basis for this probabilistic analysis. At the limit state, the liquefaction resistance of a soil, expressed as cyclic resistance ratio (CRR), is equal to the critical cyclic stress ratio (CSR), known as the seismic load. A reliability technique is used to analyse the 225 case records using the established limit state function and considering uncertainties in the input parameters. The probabilities of liquefaction are inferred from the calculated reliability indices based on Bayes’ theorem. A mapping function between the factor of safety and the actual probability of liquefaction based on field case records is developed. The factor of safety, calibrated to the actual probability of liquefaction, can thus form the basis for rational design decisions.
Stiffness of geomaterials at very small strains
Authors: Clayton C. R. I. , Heymann G.
Published: October 2009

Experiments have been conducted in the triaxial apparatus to observe the behaviour at very small strains of three widely different natural materials: Bothkennar clay, London clay, and a high-porosity Chalk. Local strain measurements made using LVDTs were calibrated by a Fabry–Pérot laser interferometer. A displacement accuracy as high as ±0·027 μm was obtained, leading to an axial strain accuracy of the order of ±10−4%. Special care was taken to ensure that the rates of creep reduced to acceptably small levels before loading commenced. At very small strain levels the observed stress–strain behaviour appeared to be linear for all three materials. The extent of this linear behaviour was similar (0·002–0·003%) for all three materials, despite the maximum stiffness (Emax) ranging over two orders of magnitude. The initial influence of stress change and strain level on secant modulus was relatively small, with stiffnesses at 0·01% strain being of the order of 80–95% of Emax. Effects of loading path were observed, giving different rates of stiffness degradation as a function of strain, but recent stress history effects did not occur even though stress excursions went well beyond the range normally thought to induce plastic strains. Stiffnesses measured in the triaxial apparatus at very small strain levels were found to be similar to those obtained using field geophysical techniques, despite assumptions of isotropy, and the effects of sampling. The implications of these observations for practice are discussed.
Failure mechanism of the Aznalcóllar Dam, Seville, Spain
Authors: Olalla C. , Cuéllar V.
Published: October 2009

In this paper an explanation is given of the geotechnical causes that induced the failure of the Aznalcóllar Dam in Seville, Spain. On the day of the failure the dam was 28 m high with a crest length of approximately 3000 m. It stored nearly 20 million m3 of mining waste. The construction of the tailing pond started in 1979. On the night of 24 April 1998 a breach developed in the embankment of the dam. To analyse the problem an extensive investigation of in-situ and laboratory tests was carried out. The data obtained have confirmed that the rupture was caused by a failure of the dam foundation at a depth of approximately 14 m below original ground level. This gave rise to an almost flat slide with a horizontal movement of 60 m affecting a section of about 600 m along the dam. The foundation consists of carbonated, slightly expansive overconsolidated silty clays known locally as ‘Andalusian blue marls’. In spite of their low dry density, they are hard and exhibit strain-softening behaviour. These phenomena, together with the presence of bedding planes (not observed by the borehole investigation, but detected by special geophysical techniques) and particularly the pore pressure generated in the foundation by the weight of the tailings and by the weight of the dam itself, have had a considerable effect on the failure process.
Structure degradation and changes in the mechanical behaviour of a stiff clay due to weathering
Authors: Cafaro F. , Cotecchia F.
Published: October 2009

The paper discusses the results of research into the causes of weathering of stiff Pleistocene clays located in the Montemesola Basin (TA, Italy), and on the effects of weathering on the clay's mechanical behaviour. The weathered clays are yellow-brown and overlie the original grey clays in the whole basin. The liquidity indexes of both the yellow and the grey clays, their fabric, as observed by means of scanning electron microscopy, as well as the yellow clay oxidation features suggest that drying has been the main weathering process in the basin. This is confirmed by the results of drying–wetting cycle tests carried out in the laboratory on undisturbed clay samples, which show the drying–wetting cycles to produce changes in the grey clay similar to those present in situ due to weathering. Results of oedometer, stress path and triaxial tests show that weathering has caused a degradation of the clay bonding and an associated reduction in the size of the clay state boundary surface. Both one-dimensional and iso-tropic compression data and the shear data indicate that the effects of weathering on the mechanics of the clay may be established according to the decrease in the clay stress sensitivity. Weathering also causes a decrease in the normalised shear stiffness, as observed by means of bender element tests, which is also related to a decrease in the stress sensitivity.
Strip loads on a non-linear heterogeneous half-space
Authors: Holzlöhner U.
Published: October 2009

A non-linear heterogeneous half-space is considered as being defined by one modulus and two exponents to describe the increase of stiffness with depth and the degree of non-linearity. This ‘parabolic’ half-space contains the linear elastic models of Boussinesq, Gibson and Booker et al. as subsets. If the parameters are selected appropriately the material turns into a hardening plastic-like material. Differential equations are established for plane deformation of a half-space consisting of such material. An exact solution is presented for the half-surface load. This and the existing line-load solution are used to calculate approximate solutions for strip loads and the rigid strip footing. The differential equations established are used for accuracy assessment. The ‘parabolic’ half-space of this type is applicable to problems with predominantly plastic soil deformation if monotonically loaded rigid structures are considered. Although ultimate failure cannot be modelled, the influence of stiffness increase with depth on bearing capacity and on displacements of footings can be better described using this model than by other simple methods.
A laboratory study of post-rupture strength
Authors: Georgiannou V. N. , Burland J. B.
Published: October 2009

Stiff intact clays exhibit rapid loss of strength after failure, with the formation of slip surfaces and the development of a well-defined ‘post-rupture’ strength envelope. This paper describes an experimental study of the influence of a number of factors on the post-rupture strength of initially intact samples of Vallericca Clay. These include changes in confining pressure after the formation of a slip surface, the magnitude of the relative displacement across the surface, comparison between the post-rupture strength envelopes in triaxial compression and extension, and the influence of existing slip surfaces on the formation and strength of subsequent rupture planes formed by rotation of the principal stresses. The results show that the post-rupture strength envelope is well defined, repeatable, and not very sensitive to stress history or the rotation of the principal stresses. Little reduction in shearing resistance takes place for relative displacements up to about 10 mm. The results are believed to be of significance in understanding the formation of fissures in stiff clays, and in evaluating their operational strength.
Modelling the behaviour of circular footings under combined loading on loose carbonate sand
Authors: Cassidy M. J. , Byrne B. W. , Houlsby G. T.
Published: October 2009

A work-hardening plasticity model for the behaviour of rigid circular footings on loose carbonate sand is described. The model incorporates vertical, moment and horizontal force resultants and the corresponding displacements of the footing, and allows predictions of response to be made for any load or displacement combination. The model is a modification of an existing work-hardening model for circular footings on dense silica sand (published by Houlsby and Cassidy, 2002 ) and is based on a series of specifically designed tests on loose carbonate sand ( Byrne and Houlsby, 1998 , 2001 ). Retrospective simulation of the experimental data verifies the model's suitability for use in this soil type and conditions.