1,721,239 research outputs found
Application of a finite deformation multiplicative plasticity model with non-local hardening to the simulation of CPTu tests in a structured soil
Full text linkIn this paper an isotropic hardening elastoplastic constitutive model for structured soils is applied to the simulation of a standard CPTu test in a saturated soft structured clay. To allow for the extreme deformations experienced by the soil during the penetration process, the model is formulated in a fully geometric non-linear setting, based on: i) the multiplicative decomposition of the deformation gradient into an elastic and a plastic part; and, ii) on the existence of a free energy function to define the elastic behaviour of the soil. The model is equipped with two bonding-related internal variables which provide a macroscopic description of the effects of clay structure. Suitable hardening laws are employed to describe the structure degradation associated to plastic deformations. The strain-softening associated to bond degradation usually leads to strain localization and consequent formation of shear bands, whose thickness is dependent on the characteristics of the microstructure (e.g, the average grain size). Standard local constitutive models are incapable of correctly capturing this phenomenon due to the lack of an internal length scale. To overcome this limitation, the model is framed using a non-local approach by adopting volume averaged values for the internal state variables. The size of the neighbourhood over which the averaging is performed (characteristic length) is a material constant related to the microstructure which controls the shear band thickness. This extension of the model has proven effective in regularizing the pathological mesh dependence of classical finite element solutions in the post-localization regime. The results of numerical simulations, conducted for different soil permeabilities and bond strengths, show that the model captures the development of plastic deformations induced by the advancement of the cone tip; the destructuration of the clay associated with such plastic deformations; the space and time evolution of pore water pressure as the cone tip advances. The possibility of modelling the CPTu tests in a rational and computationally efficient way opens a promising new perspective for their interpretation in geotechnical site investigations
Extension of plasticity theory to debonding, grain dissolution, and chemical damage of calcarenites
Full text linkThe mechanical properties of calcarenites are known to be significantly affected by water saturation: both stiffness and strength decrease for wetting in the short term and for chemical dissolution in the long term. Both processes mainly affect bonds among grains: immediately after inundation depositional bonds fall in suspension, whereas diagenetic bonds dissolve more slowly. In this paper, the authors started from the micro-structural analysis of the weathering processes to conceive a strain hardening hydro-chemo-mechanical coupled elastoplastic constitutive model. The concept of extended hardening rules is here enriched: weathering functions have been determined by employing a micro to macro simplified upscaling procedure. Chemical damage is incorporated into the formulation by means of a scalar damage function. Its evolution is also described by using a multiscale approach. A new term is added to the strain rate tensor in order to incorporate the dissolution induced chemical deformations developing once the soft rock is turned into a granular material. A calibration procedure for the constitutive parameters is suggested, and the model is validated by using both coupled and uncoupled chemo-mechanical experimental test results.</p
A finite deformation multiplicative plasticity model with non–local hardening for bonded geomaterials
No full textThe paper presents a finite deformation, isotropic hardening, non-associative elastic-plastic constitutive model (FD_MILAN model) for describing the mechanical behavior of a wide range of bonded natural geomaterials such as stiff overconsonsolidated clays, porous soft rocks or bio-improved soils. The formulation of the model is based on the multiplicative split of the deformation gradient and on the assumption of hyperelastic behavior. To deal with the occurrence of strain localization, typically observed in this class of geomaterials, the model has been equipped with a non-local version of the hardening laws. This approach is capable of regularizing the pathological mesh dependence occurring in the post-localization regime when adopting classical plasticity models. In view of its application to practical geotechnical problems characterized by large displacements and deformations within a hydro-mechanical coupled environment, the model has been implemented in the recently developed Particle Finite Element code G-PFEM for geomechanics applications. To demonstrate the effectiveness of the numerical implementation, a series of numerical simulations has been performed considering two representative boundary value problems: the modeling of shear localization in plane strain biaxial tests and the simulation of CPTu tests in a saturated porous soil. The results of biaxial test simulations have highlighted the role of the characteristic length in controlling the thickness of the localized zone and the effect of the confining pressure in determining the pattern of shear band formation. An interesting feature emerging from the partially drained CPTu simulation results is the progressive formation of persistent shear bands, which originate from the cone tip and propagate outwards along the entire penetration depth
Calibrazione di un modello costitutivo elastoplastico incrudente per terreni fini cementati mediante una metodologia innovativa
No full textTra le tecniche più diffuse di miglioramento meccanico di terreni rientrano quelle del deep soil mixing e il jet grouting; tali tecniche, miscelando il terreno in sito con cemento, creano dei legami intergranulari tra le particelle. Il comportamento meccanico del terreno cementato è solitamente descritto utilizzando modelli costitutivi semplici, quali ad esempio il modello elasto-perfettamente plastico di Mohr-Coulomb. Modelli costitutivi più avanzati, basati principalmente sull’elastoplasticità incrudente con opportune estensioni delle leggi di incrudimento, sono in grado di descrivere in modo decisamente più accurato il comportamento meccanico del terreno cementato. Il loro utilizzo è tuttavia ostacolato dalla difficoltà di calibrazione del maggior numero di parametri e di variabili di stato che utilizzano e pertanto il loro impiego nella pratica ingegneristica è limitato. Il presente lavoro intende mostrare una procedura di calibrazione innovativa di un modello costitutivo avanzato utilizzando semplici grandezze fisiche e meccaniche della miscela terreno-cemento: i) il rapporto tra indice dei vuoti e contenuto di cemento, ii) la resistenza a compressione semplice e iii) le proprietà meccaniche del materiale non cementat
Direct observation of particle kinematics in biaxial shearing test
Full text linkBiaxial shearing tests on dual-sized, 2d particle assemblies are conducted at several confining pressures. The effect of particle angularity, an important mesoscale shape descriptor, is investigated at the macro and micro levels. Macroscopically, it is observed that assemblies composed of angular particles exhibit higher strengths and dilations. The difference observed in bulk behavior due to particle angularity can be explained reasonably by considering particle-level mechanisms. A novel 2D image analysis technique is employed to estimate particle kinematics. Particle rotation results to be a key mechanism strongly influenced by particle shape determining the overall granular behavior. Unlike circular particles, angular ones are more resistant to rotations due to stronger interlocking and consequently exhibit higher strengths
Micromechanical study of potential scale effects in small-scale modelling of sinker tree roots
Full text linkWhen testing an 1:N geotechnical structure in the centrifuge, it is desirable to choose a large scale factor (N) that can fit the small-scale model in a model container and avoid unwanted boundary effects, however, this in turn may cause scale effects when the structure is overscaled. This is more significant when it comes to small-scale modelling of sinker root-soil interaction, where root-particle size ratio is much lower. In this study the Distinct Element Method (DEM) is used to investigate this problem. The sinker root of a model root system under axial loading was analysed, with both upward and downward behaviour compared with the Finite Element Method (FEM), where the soil is modelled as a continuum in which case particle-size effects are not taken into consideration. Based on the scaling law, with the same prototype scale and particle size distribution, different scale factors/g-levels were applied to quantify effects of the ratio of root diameter (𝑑𝑟) to mean particle size (𝐷50) on the root rootsoil interaction
Experimental Study on the Water-Induced Weakening of Calcarenites
No full textCarbonatic rocks, such as calcarenites, are very often subject to damage processes, causing a progressive degradation of their mechanical properties. In nature, in some cases, this phenomenon can cause the collapse of cliffs and underground cavities, with dangerous consequences for the anthropic environment. In this paper, the results of an experimental campaign, intended to both clarify and quantify the mechanical consequences of this process, are illustrated. To achieve such a goal, suitable physical and geotechnical indices are introduced and different time scales to describe the physical/chemical reactions induced by the water saturation of the material are taken into consideration. In particular, the authors have observed: (1) a short-term marked and instantaneous reduction in strength when water fills the pores of the rock; (2) a long-term dissolution; and (3) a progressive chemically induced reduction in the grain size. To describe the degradation processes induced by the material water saturation, owing to the complexity of the hydro-chemo-mechanical phenomena taking place within the material, suitably designed tests under controlled “weathering” conditions were also performed and discussed.</p
2D Image-based calibration of rolling resistance in 3D discrete element models of sand
Full text linkContact rolling resistance is the most widely used method to incorporate particle shape effects in the discrete element method (DEM). The main reason for this is that such approach allows for using spherical particles hence offering substantial computational benefits compared to non-spherical DEM models. This paper shows how rolling resistance parameters for 3D DEM models can be easily calibrated with 2D sand grain images
On the efficiency of coupled discrete-continuum modelling analyses of cemented materials
Full text linkComputational load of discrete element modelling (DEM) simulations is known to increase with the number of particles. To improve the computational efficiency hybrid methods using continuous elements in the far-field, have been developed to decrease the number of discrete particles required for the model. In the present work, the performance of using such coupling methods is investigated. In particular, the coupled wall method, known as the “wall-zone” method when coupling DEM and the continuum Finite Differences Method (FDM) using the Itasca commercial codes PFC and FLAC respectively, is here analysed. To determine the accuracy and the efficiency of such a coupling approach, 3-point bending tests of cemented materials are simulated numerically. To validate the coupling accuracy first the elastic response of the beam is considered. The advantage of employing such a coupling method is then investigated by loading the beam until failure. Finally, comparing the results between DEM, DEM-FDM coupled and FDM models, the advantages and disadvantages of each method are outlined
DEM element modelling of silent piling group installation for offshore wind turbine foundations
Full text linkOffshore wind farms are now built in deeper water and bigger foundations are required to stabilise wind turbines of increasing sizes. Pile driving is the most widespread foundation installation method, but more stringent environmental regulations necessitate costly mitigation methods to reduce underwater noise emissions. The silent piling (push-in) concept presented in this work is composed of a cluster of four piles, progressively installed by successive jacking sequences. During one sequence, each pile is moved downward by 0.5m stroke, while the other piles are used as reaction. This paper presents the results of Discrete Element Method (DEM) of the installation process. This work identifies the main features of the push-in installation method, such as pile interaction, progressive plugging and loss of efficiency as a function of depth. It is shown that the cluster capacity can reach six times the weight of the tool necessary to silently install the piles
- …
