Applying GSH to a Wide Range of Experiments in Granular Media

Granular solid hydrodynamics (GSH) is a continuum-mechanical theory for granular media, the range of which is shown in this paper. Simple, frequently analytic solutions are related to classic observations at different shear rates, including: (i)~static stress distribution, clogging; (ii)~elasto-plastic motion: loading and unloading, approach to the critical state, angle of stability and repose; (iii)~rapid dense flow: the $\mu$-rheology, Bagnold scaling and the stress minimum; (iv)~elastic waves, compaction, wide and narrow shear band. Less conventional experiments have also been considered: shear jamming, creep flow, visco-elastic behavior and nonlocal fluidization. With all these phenomena ordered, related, explained and accounted for, though frequently qualitatively, we believe that GSH may be taken as a unifying framework, providing the appropriate macroscopic vocabulary and mindset that help one coming to terms with the breadth of granular physics.Comment: arXiv admin note: substantial text overlap with arXiv:1207.128

Numerical and Experimental Investigation of Soil Behaviour Under Stationary Excitation

The effect of earthquakes on the behavior of soil and/or structures is usually investigated using earthquake-like signals as input for loading. This holds true for most of numerical and experimental simulations. In contrast to those approaches sinusoidal excitation was used here. The benefit of this type of excitation is an increased observability of the system, which is a precondition for a systematic investigation. Since sinusoidal excitation allows a gradually, step by step increase of the applied loading, the state of the soil (density and effective stress) as well as pore water pressure are transient. Therefore they are varying slowly and the modes of vibration are changing with time accordingly. The modes can be identified by regarding the contour of a soil column for instance. The evolution and distribution of pore water pressure (up to liquefaction) has to be captured simultaneously. A further advantage of this method is that asymptotic behavior can be investigated for the evolutions of pore water pressure and settlement of the surface with an increasing number of cycles. On the base of a numerical study some test in a shake-box will be shown using the described concept. Numerical modeling of soil behavior under cyclic respectively dynamic loading requires the application of nonlinear constitutive laws. With the used FE-model it is possible to observe the dependence of excitation amplitude, frequency and initial state on the transient amplification of the sinusoidal input signal up to an onset of liquefaction. The appropriate experimental investigations confirm the numerically observed behavior. Therefore a shake-box under 1-g conditions with smooth boundaries is used for the tests. They were performed with a homogeneous soil column (about 0.8m length, 0.6m width, 2.1m height) from medium-grained quartz sand under saturated conditions

Granular Solid Hydrodynamics

Granular elasticity, an elasticity theory useful for calculating static stress distribution in granular media, is generalized to the dynamic case by including the plastic contribution of the strain. A complete hydrodynamic theory is derived based on the hypothesis that granular medium turns transiently elastic when deformed. This theory includes both the true and the granular temperatures, and employs a free energy expression that encapsulates a full jamming phase diagram, in the space spanned by pressure, shear stress, density and granular temperature. For the special case of stationary granular temperatures, the derived hydrodynamic theory reduces to {\em hypoplasticity}, a state-of-the-art engineering model.Comment: 42 pages 3 fi

Development of Stresses in Cohesionless Poured Sand

The pressure distribution beneath a conical sandpile, created by pouring sand from a point source onto a rough rigid support, shows a pronounced minimum below the apex (`the dip'). Recent work of the authors has attempted to explain this phenomenon by invoking local rules for stress propagation that depend on the local geometry, and hence on the construction history, of the medium. We discuss the fundamental difference between such approaches, which lead to hyperbolic differential equations, and elastoplastic models, for which the equations are elliptic within any elastic zones present .... This displacement field appears to be either ill-defined, or defined relative to a reference state whose physical existence is in doubt. Insofar as their predictions depend on physical factors unknown and outside experimental control, such elastoplastic models predict that the observations should be intrinsically irreproducible .... Our hyperbolic models are based instead on a physical picture of the material, in which (a) the load is supported by a skeletal network of force chains ("stress paths") whose geometry depends on construction history; (b) this network is `fragile' or marginally stable, in a sense that we define. .... We point out that our hyperbolic models can nonetheless be reconciled with elastoplastic ideas by taking the limit of an extremely anisotropic yield condition.Comment: 25 pages, latex RS.tex with rspublic.sty, 7 figures in Rsfig.ps. Philosophical Transactions A, Royal Society, submitted 02/9

LOQUAT: an open-source GPU-accelerated SPH solver for geotechnical modeling

Smoothed particle hydrodynamics (SPH) is a meshless method gaining popularity recently in geotechnical modeling. It is suitable to solve problems involving large deformation, free-surface, cracking and fragmentation. To promote the research and application of SPH in geotechnical engineering, we present LOQUAT, an open-source three-dimensional GPU accelerated SPH solver. LOQUAT employs the standard SPH formulations for solids with two geomechnical constitutive models which are the Drucker–Prager model and a hypoplastic model. Three stabilization techniques, namely, artificial viscosity, artificial pressure and stress regularization are included. A generalized boundary particle method is presented to model static and moving boundaries with arbitrary geometry. LOQUAT employs GPU acceleration technique to greatly increase the computational efficiency. Numerical examples show that the solver is convergent, stable and highly efficient. With a mainstream GPU, it can simulate large scale problems with tens of millions of particles, and easily performs more than one thousand times faster than serial CPU code

Pulse tests in soil samples.

Available from British Library Document Supply Centre-DSC:DXN049283 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Dynamic large deformation modelling of soils including static liquefaction

Aplicat embargament des de la data de defensa fins el dia 30/9/2022Modelling geotechnical problems involving large deformation is a research field that arouses growing interest since it is fundamental for evaluating the risk and quantifying the magnitude of consequences. However, it is still challenging and requires to develop solution schemes able to reproduce failure initiation as well as post-failure dynamics. This thesis focuses on studying large deformations geotechnical problems subjected to static and dynamic loadings including static liquefaction in saturated and partially saturated soils that induces flow-like landslides. Accounting for the difficulties to treat large deformation with the traditional lagrangian approaches, due to the excessive distortion of the mesh elements, the Material Point Method (MPM) is adopted. MPM discretises the continuum media into a set of material points that can move attached to the material and carry all the information. The governing equations are solved at the nodes of the computational mesh that remains fixed in space. This double discretization provides to MPM the capabilities of handling problems involving large displacements and deformations. The numerical developments and simulations are carried out in the open-source MPM Anura3D code. The method is applied for the simulation of geotechnical cases dealing with soil excavation. First, the stability of strutted excavations in clay is studied and MPM results, which provides a realistic interpretation of failure, are compared with analytical solutions based on simplified assumption. An interesting discussion of the discrepancies found between the methods is also included. In the second example, the well-documented Cortes de Pallás landslide due to toe excavation is simulated. Consistently with field observation, the excavation produces significant displacements after which a new stable configuration is reached. Later on, the thesis presents the study of a complex real landslide occurred during a dam construction project, including its seismic assessment. This is a case of creeping slide motion and it is characterized by two superimposed sliding surfaces. Different scenarios of geometry and groundwater conditions, representing the landslide history and futures stabilizing measures and rainfall regimes, are considered for the stability analysis with FEM. MPM is used instead to carry-out a motion back-analysis on an unstable scenario. The creeping behaviour observed is also discussed through an analytical calculation in which the case is simplified to a planar landslide. Finally, the seismic assessment is performed with a focus on the effect of the superimposed slip surfaces. Results of MPM are compared with Newmark’s approach in its classical form and by introducing the strain-rate dependence on the residual friction angle. The last part of the thesis focusses on modelling a liquefaction-induced flow landslide from the failure triggering to the subsequent slide-to-flow transition. With this aim, an advanced constitutive model able to simulate static and cyclic soil response such as accumulation of permanent strains, excess pore pressure and degradation of soil stiffness modulus, is implemented in the open-source MPM Anura3D code. For the validation and calibration of the code and the constitutive model, laboratory tests published in the literature are reproduced. After validation, the model is applied to a real case of flow landslide recently occurred in Catalonia (Spain) Its parameters are calibrated based on data of undrained triaxial tests that show the liquefaction potential of the material involved. MPM simulation is able to reproduce the failure initiation caused by a water inflow and the large deformations in the post-failure stage. The final run-out calculated is found to be significantly affected by the model parameter accounting for liquefaction susceptibility.La modelización de problemas geotécnicos que involucran grandes deformaciones es un campo de investigación que despierta un interés creciente ya que es fundamental para evaluar el riesgo y cuantificar la magnitud de las consecuencias. Sin embargo, sigue siendo un desafío y requiere desarrollar soluciones capaces de reproducir el inicio de la rotura, así como la propagación del movimiento. Esta tesis se centra en el estudio de problemas geotécnicos de grandes deformaciones, incluida la licuefacción estática en suelos saturados y parcialmente saturados que inducen flujos de tierra. Teniendo en cuenta las dificultades en tratar grandes deformaciones con los enfoques tradicionales lagrangianos, debido a la excesiva distorsión de los elementos de la malla, se ha adoptado el Método del Punto Material (MPM). Los desarrollos numéricos y las simulaciones se llevan a cabo en el código abierto Anura3D. El método se ha aplicado para la simulación de casos geotécnicos relacionados con excavación de suelo. Primero, se ha estudiado la estabilidad de excavaciones sostenidas en arcilla y se han comparado los resultados del MPM, que proporcionan una interpretación realista de la rotura, con soluciones analíticas basadas en hipótesis simplificadas. También se ha incluido una discusión interesante de las discrepancias encontradas entre los métodos. En el segundo ejemplo, se ha simulado el bien documentado deslizamiento de tierra de Cortes de Pallás debido a la excavación del pie. Consistentemente con las observaciones de campo, la excavación produce desplazamientos significativos después de los cuales se alcanza una nueva configuración estable. Luego, la tesis presenta el estudio de un complejo deslizamiento real ocurrido durante el proyecto de construcción de una presa, incluyendo su evaluación sísmica. Este es un caso de movimiento deslizante lento y se caracteriza por dos superficies de deslizamiento superpuestas. Para el análisis de estabilidad con FEM se han considerado diferentes escenarios de geometría y condiciones de aguas subterráneas, que representan el historial de deslizamientos y futuras medidas de estabilización y regímenes de lluvia. En cambio, MPM se ha utilizado para llevar a cabo un análisis inverso de movimiento en un escenario inestable. El comportamiento observado de movimiento lento también se ha analizado a través de un cálculo analítico en el cual el caso se ha simplificado a un deslizamiento de tierra plano. Finalmente, se ha realizado la evaluación sísmica con un enfoque en el efecto de las superficies de deslizamiento superpuestas. Los resultados del MPM se han comparado con el método de Newmark en su forma clásica e introduciendo la dependencia de la velocidad de deformación en el ángulo de fricción residual. La última parte de la tesis se centra en la modelización de un flujo de tierra inducido por licuefacción, desde el desencadenamiento de la rotura hasta la posterior transición de deslizamiento a flujo. Con este objetivo, se ha implementado en el código abierto Anura3D un modelo constitutivo avanzado capaz de simular la respuesta estática y cíclica del suelo, como la acumulación de deformaciones permanentes, el exceso de presión de poros y la degradación del módulo de rigidez del suelo. Para la validación y calibración del código y del modelo constitutivo se han reproducidos ensayos de laboratorio descritos en la literatura. Después de la validación, el modelo se ha aplicado a un caso real de flujo de tierra ocurrido recientemente en Cataluña (España). Sus parámetros se han calibrado basándose en datos de ensayos triaxiales no drenados que muestran el potencial de licuefacción del material involucrado. La simulación MPM es capaz de reproducir la iniciación de la rotura causada por afluencia de agua y las grandes deformaciones en la etapa post-rotura. El alcance final del deslizamiento calculado se ve significativamente afectado por el parámetro del modelo que representa la susceptibilidad a la licuefacción.Postprint (published version

Modellierung und Analyse von Wellen-Bauwerk-Boden Interaktion für monolithische Wellenbrecher

Monolithic breakwaters are preferred to other types of structures in terms of economical and environmental aspects. Nevertheless, they are more vulnerable to foundation failures, especially to stepwise failures. Due to the highly complex processes involved in wave-structure-foundation interaction, no reliable model yet exists for this failure mechanism. Therefore, a semi-coupled CFD-CSD model system and a simplified model are developed in OpenFOAM to describe wave-structure-foundation interaction for monolithic breakwaters, and particularly stepwise failures. The CFD model is an extension of the incompressible multiphase Eulerian solver of OpenFOAM by introducing different seepage laws and a simplified fluid compressibility model. The CFD model is successful in reproducing breaking wave impact including effect of entrapped air. A new CSD model is developed to solve the fully dynamic, coupled Biot equations with a new approach taking advantage of the PISO algorithm to resolve pore fluid velocity-pressure coupling. Soil-structure interaction is introduced via a frictional contact model and for soil behaviour, a multi-surface plasticity model is implemented. The model is validated against analytical models and physical tests. The model succeeds to reproduce wave-induced residual pore pressure buildup and soil densification followed by pore pressure dissipation. A one-way coupling of both models is achieved by transforming the CFD model output into input for the CSD model. The semi-coupled model system is applied successfully to reproduce selected results of a caisson breakwater subject to breaking wave impact in the Large Wave Flume (GWK). The model system is applied to expand the range of conditions tested in GWK for response of the soil foundation. A new load eccentricity concept, is proposed to classify response of the foundation in four load eccentricity regimes. Load eccentricity carries all significant information related to wave loads (horizontal and uplift) and to properties of the structure (mass and geometry). Using this concept, recommendations are drawn for design of monolithic breakwaters, and a new simplified nonlinear 3-DOF model is developed with elastoplastic springs. Model parameters are calibrated using results from the CFD-CSD model for different sand relative densities and different load eccentricities. The simplified model can simulate the stepwise failure (sliding, settlement and tilt) as well as the overall failure (overturning).Caisson-Wellenbrecher werden aufgrund ökonomischer und Umweltaspekte bevorzugt. Jedoch sind sie empfindlicher gegen das Versagen des Baugrundes insbesondere gegen schrittweises Versagen. Aufgrund der Komplexität der Wellen-Bauwerk-Boden Interaktion liegt noch kein verlässliches Modell für diesen Versagensmechanismus vor. Deswegen werden ein semi-gekoppeltes CFD-CSD Modellsystem und ein vereinfachtes Modell in OpenFOAM entwickelt. Das CFD-Modell stellt eine durch Sickerströmungsgesetze und ein vereinfachtes Modell der Fluidkompressibilität erweiterte Version des mehrphasigen Strömingslösers von OpenFOAM dar. Das CFD-Modell wurde erfolgreich eingesetzt, um Druckschlagbelastungen durch brechende Wellen mit Lufteinschlüssen zu reproduzieren. Ein neues CSD-Modell wurde für die Lösung der voll dynamischen, gekoppelten Biot-Gleichungen mit einem neuen Ansatz entwickelt. Dabei wird der PISO-Algorithmus genutzt, um die Kopplung von Geschwindigkeit und Druck des Porenfluids zu lösen. Die Bauwerk-Boden Interaktion wird über ein Reibungs-Kontaktmodell eingeführt und für die Plastizität des Bodens ein Mehrflächenmodell implementiert. Die Validierung des CSD-Modells erfolgte durch analytische Modelle und Laborversuche. Mit dem Modell ist es gelungen, den Porenwasserdruckaufbau, die Bodenverdichtung und die Dissipation des Porenwasserdruckes zu reproduzieren. Es wurde eine Einweg-Kopplung der Modelle implementiert, in dem der Output des CFD-Modells als Input für das CSD-Modell aufbereitet wird. Mit dem validierten semi-gekoppelten Modellsystem ist es gelungen die Experimente im Großen Wellenkanal (GWK) zu reproduzieren. Darüber hinaus wurde das Modellsystem eingesetzt, um die getesteten Bedingungen zu erweitern. Ein neues Lastexzentrizitätskonzept wurde eingeführt, um die Gründungsverhaltens in vier Regime zu klassifizieren. Die Lastexzentrizität fasst alle relevanten Informationen der Wellenbelastung (Horizontal und Auftrieb) und der Bauwerkseigenschaften (Masse und Geometrie) zusammen. Unter Anwendung dieses Konzepts werden Empfehlungen für die Bemessung monolithisches Wellenbrechers ausgesprochen. Darüber hinaus wurde ein vereinfachtes nichtlineares 3-DOF Modell mit elasto-plastischen Federn entwickelt. Die Modellparameter wurden für unterschiedliche relative Dichte des Bodens und Lastexzentrizität kalibriert. Das vereinfachte Modell kann das schrittweise Versagen (Gleiten, Setzung und Kippen) sowie das Gesamtversagen (Umkippen) simulieren

Stabilisation behaviour of cyclically loaded shallow foundations for offshore wind turbines

This work presents the results of model tests and numerical simulations of shallow foundations subjected to cyclic loads typical of offshore loadings. Main focus is spent on the accumulated rotational and settlement behaviour relevant in particular for fixed installed offshore wind turbines