334 research outputs found
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
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
Numerical techniques for fast generation of large discrete-element models
Full text linkIn recent years, civil engineers have started to use discrete-element modelling to simulate large-scale soil volumes thanks to technological improvements in both hardware and software. However, existing procedures to prepare ‘representative elementary volumes’ are unsatisfactory in terms of computational cost and sample homogeneity. In this work, a simple but efficient procedure to initialise large-scale discrete-element models is presented. Periodic cells are first generated with a sufficient number of particles (enough to consider the cell a representative elementary volume) matching the desired particle size distribution and equilibrated at the desired stress state, porosity and coordination number. When the cell is in equilibrium, it is replicated in space to fill the problem domain. And when the model is filled, only a small number of mechanical cycles is needed to equilibrate a large domain. The result is an equilibrated homogeneous sample at the desired initial state in a large volume
Vertical loading tests on a simplified tree root prototype
No full textThe increasing number of extreme weather events, often accompanied by very intense wind gusts, can cause diffuse damages to arboreal heritage, which hence represents a severe hazard in urban areas for buildings, cars, structures, infrastructures and even human lives. From a geotechnical perspective, assessing the stability of a tree against uprooting represents a problem of interaction between the soil and the root system, subject to complex loading conditions. The experimental study presented in this paper approaches such a problem by considering a 1D vertical loading condition, both under compressive and tensile loads, for a simplified small-scale tree prototype with a flat root system, resting in a dry mid-loose Ticino sand deposit. The root system is conceptually assimilated to a direct foundation, and the role of the bending and the tensile behaviour of the different root components is highlighted, by considering both monotonic and non-monotonic quasi-static loading paths. The influence of several geometrical parameters is investigated, and the results highlight the need of a large displacement approach, also considering second order geometrical effects, to correctly interpret the results. These latter can be considered preparative to the development of more complex 3D experimental tests and theoretical interpretative frameworks
Micromechanical investigation of grouting in soils
Full text linkGrouting and jet grouting are geotechnical consolidation techniques commonly employed to improve the mechanical behaviour of soils. Although these techniques are common, the micromechanical processes taking place at the local level are not yet totally understood and modelled. In this work, such a problem has been approached from a micromechanical perspective via the discrete element method by considering the local interaction among soil grains and pseudo-fluid particles. Homogeneous representative elementary volumes of a virtual analogue of silica sand have been first generated and tiny rigid frictionless particles have been subsequently injected through them, to simulate the grouting in granular materials. Various injection pressures, initial soil pressures and initial soil densities have been considered. The different diffusion patterns, the flow rate, the consequent increase in local stresses and the consequent reduction in local porosity have been discussed. To overcome the DEM computational restrictions and to speed up the injection simulations, a novel procedure based on the replication of pre-equilibrated cells has been adapted for both the initialization and injection phase. Finally, a qualitative laboratory-scale pressure grouting test has been reproduced to validate the results
DEM study of particle scale effect on plain and rotary jacked pile behaviour in granular materials
Full text linkThe capacity of open-ended piles strongly depends on the potential plugging occurring during installation. The Discrete Element Method (DEM) is particularly suitable to the study of pile plugging, as it can model large soil deformation. However, DEM simulation of the 3D boundary value problems is computationally expensive, and particle upscaling is usually used to reduce the number of particles modelled. In this paper, two granular beds were created with the same average porosity, initial stress field and contact parameters, but different particle scales. Two pile geometries were installed by plain and rotary jacking. Normalised results show that the pile penetration mechanism is strongly affected by the particle scale. Larger particles lead to earlier pile plugging, higher shaft resistance and require a greater force to penetrate the ground. This effect can be linked to a modified penetration mechanism, with larger shear zones and less well defined “nose cone” under the pile tip for larger particles. Changing particle scaling has a neutral effect on the total penetration resistance of a rotary jacked pile, but reduces the base penetration and increases the plug resistance. Maximising the ratio of particle scale to wall thickness is key to adequately capturing the pile coring mechanism
Experimental Methodology for Chemo-mechanical Weathering of Calcarenites
No full textCalcarenite is a soft rock strongly affected by weathering processes that markedly reduce the mechanical rock properties with time. As a conse-quence, cliffs and underground cavities formed in calcaernites are frequently affected by intense erosion and instabilities. The field and laboratory experi-mental results mainly show three peculiarities of calcarenite mechanical be-havior: a) a marked and instantaneous reduction in strength, up to 60% of the dry initial value, when water fills the pores of the rock; b) a slow reduc-tion in strength after saturation; c) progressive weakening of the material during wetting and drying cycles. In the present work we concentrate on the long term effect of water on calcarenite. In this context, an experimental pro-cedure necessary for the calibration of a strain hardening-chemical softening elasto-plastic constitutive model is presented. Suitably designed tests under controlled “weathering” conditions were performed in order to define the critical variables that can physically explain the variety of phenomena occur-ring in the material
Interacción de terraplenes con la atmósfera y su protección mediante el uso de geomembranas
Full text linkPérez-Romero J, Ciantia M O, Arroyo M & Vaunat J (2016). Interacción de terraplenes con la atmósfera y su protección mediante el uso de geomembranas. 10º Simposio Nacional de Ingeniería Geotécnica; Reconocimiento, Tratamiento y Mejora del Terreno. Sociedad Española de Mecánica del Suelo, La Coruña, octubre de 2016, pp. 707-715Una vez ejecutado el terraplén de una obra lineal se inicia el proceso de interacción del mismo con la atmósfera, el cual coincide en el tiempo con el periodo de explotación de la infraestructura. El proceso más relevante de dicha interacción consiste en los cambios de humedad que se producen en su interior y una de las consecuencias es la generación de asientos postconstructivos. En este artículo se presentan los resultados de la simulación numérica de la exposición de un terraplén, durante un periodo de diez años, a dos climas de diferentes características. Se describen los cambios de humedad que predice el modelo numérico, así como los asientos asociados. Por otra parte se plantea el uso de geomembranas como herramienta para limitar el acceso de humedad al interior del relleno, lo cual a su vez supone una mitigación de los asientos postconstructivos. Para ello se simulan geomembranas dispuestas en diferentes configuraciones geométricas, analizándose las ventajas que ofrecen cada una de ellas de cara a la protección del relleno a largo plazo.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech
Centrifuge dataset for screw pile cyclic performance
No full textThe dataset presents the installation and (monotonic and cyclic) uplift loading of a screw piles in a sand bed. The full details of the procedure can be found in the related paper. Centrifuge Modeling of the Installation Advancement Ratio Effect on the Cyclic Response of a Single-Helix Screw Pile for Floating Offshore Wind (2025). Wang, W., Brown, M., Sharif, Y. U., Davidson, C. & Ciantia, M. O.,. Journal of Geotechnical and Geoenvironmental Engineering. 151, 1
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