1,721,030 research outputs found
Constitutive models with kinematic hardening: Effects on anisotropy and non-coaxiality
No full textThe aim of the paper is to show the capability of constitutive models based on mixed isotropic-kinematic hardening and characterized by an associated flow rule to simulate the evolution of anisotropy and non-coaxiality of soil elements subjected to unconventional loading conditions
An experimental investigation into the mechanical behaviour of a structured stiff clay
No full textIn recent years, fundamental research has been carried out into the properties of some natural stiff clays and the corresponding reconstituted materials, highlighting the role of microstructural features in the observed differences. In this paper the results of an experimental investigation into the mechanical behaviour of an Italian stiff clay of marine origin are presented. Medium-pressure and high-pressure stress-controlled triaxial cells were used in which natural samples underwent isotropic and anisotropic compression and swelling before drained or undrained shearing. Comparison of soil behaviour observed after different compression histories up to different values of maximum effective stress allowed the following aspects to be discussed: the effects of the initially structured state on the medium to large strain response and shear strength characteristics of the soil; the relevance of volumetric and deviatoric plastic strain to the structure degradation; the role and implications of the imposed non-isotropic stress histories; the permanent differences between reconstituted samples and fully destructured natural samples; and the uniqueness of the critical state condition
Prospettive nell'analisi del comportamento delle dighe di terra
No full textRelazione General al XXVI Convegno Nazionale di Geotecnica - Nella prima parte della relazione si discute degli effetti della parziale saturazione sul comportamento dei terreni compattati, evidenziando come l’ampia sperimentazione e l’applicazione dei principi di meccanica dei terreni non saturi in codici calcolo oggi disponibili proiettino nuova luce sulla progettazione e sull’osservazione del comportamento delle opere di terra. La seconda parte è dedicata al comportamento delle dighe di terra in presenza di sisma. Vengono esaminati criticamente gli approcci di analisi proposti in letteratura, a partire dai classici metodi pseudo-statici sino a discutere quelli numerici avanzati di recente sviluppati in ambito accademico. Il complesso comportamento meccanico dei terreni quando soggetti ad azioni cicliche richiede l’introduzione di ipotesi semplificative nei suddetti approcci, cui sono correlati i loro vantaggi e limiti. Nella pratica tecnica, la consapevolezza di questi ultimi può consentire di selezionare con maggiore efficacia lo strumento di analisi appropriato allo specifico problema da esaminare
Relating elastic and plastic fabric anisotropy of clays
No full textA relationship between elastic anisotropy, as typically observed in clayey soils subjected to shear wave propagation tests, and plastic anisotropy, detected at yielding and leading to rotated yield loci, is proposed. Such a relationship is expected because both elastic and plastic anisotropies can be ascribed to the same directional ingredients that characterise the fabric of the soil at the microscale. The relationship takes the form of an analytical relation between an elastic and a plastic fabric tensor, the former entering a hyperelasticity theory while the latter is the rotational hardening variable of a clay plasticity theory. The elastic anisotropy can be measured experimentally by wave propagation along orthogonal planes, identifying the ratio of the corresponding elastic shear moduli while a sample is compressed at fixed stress ratio, and paired with plastic anisotropy obtained by the integration of its plastic fabric tensor evolution equation during the foregoing compression. Such experiments were available and used to calibrate and validate the proposed elastic–plastic anisotropy relationship. The findings have a two-way beneficial effect for the solution of a geotechnical boundary value problem, where one can easily measure initial elastic clay anisotropy in the field, which can be used to initialise the plastic anisotropy for the subsequent analysis of the problem, while the evolving plastic anisotropy can be used to update the elastic fabric tensor during deformation
Dynamic soil-structure interaction. A three-dimensional numerical approach and its application to the Lotung case study
No full textThis paper presents a three-dimensional non-linear finite element (FE) approach to analyse the dynamic soil-structure interaction (SSI) phenomena observed at the Lotung Large-Scale Seismic Test (LSST) site. The numerical study is carried out in the time domain by a commercial FE code, taking into account the non-linear behaviour of soil and the multi-directional nature of real seismic events. The soil response is simulated by an isotropic hardening elasto-plastic hysteretic model (HSsmall) implemented in the material model library of the code. This model allows to describe the non-linear cyclic response ranging from small to large strain amplitudes and to account for the variation of the initial stiffness with depth. In the paper, the FE numerical approach is first validated through a series of parametric analyses simulating simplified cases (i.e. linear visco-elastic structures founded on a homogeneous linear visco-elastic soil deposit) for which analytical solutions exist. Then, it is adopted to back-analyse the behaviour of the 1/4-scale nuclear power plant containment structure constructed at the Lotung LSST site which was shook by several earthquakes of different intensities and frequency contents. The FE results are thus compared to the recorded in-situ free-field and structural motions, highlighting the satisfactory performance of the numerical model in replicating the observed response. The overall outcome of this research proves that nowadays complex dynamic SSI phenomena can be tackled by direct approach, overpassing the strong simplifications of the well-established substructure approaches
Coupling plasticity and energy-conserving elasticity models for clays
No full textA class of two-invariant stored energy functions describing the hyperelastic characteristics of soils is coupled with a critical-state plasticity model. The functions include constant as well as pressure-dependent elastic shear modulus models, and automatically satisfy the requirement that the elastic response for any loading path be energy conserving. The elastic responses predicted by the hyperelastic model are compared with measured undrained elastic responses of an overconsolidated clay in order to assess, both qualitatively and quantitatively, the predictive capability of the hyperelastic model. The importance of the pressure-dependent nature of the elastic shear modulus is assessed within the context of elastic and plastic responses. An energy-conserving model provides a fundamentally correct description of elastic material behavior even in the regime of plastic responses
Seismic ground response at Lotung. Hysteretic elasto-plastic-based 3D analyses
No full textThis paper presents a non-linear finite element study to back-interpret the free field seismic response recorded at the Lotung Large-Scale Seismic Test site. The study is carried out in the time domain by the Finite Element (FE) code PLAXIS 3D, considering the vertical wave propagation of both the horizontal components of motion. The non-linear soil behaviour is simulated through a constitutive model, the Hardening Soil model with Small-Strain Stiffness (HSsmall), capable of describing the cyclic response of the material at different strain levels. In the paper, the constitutive response of the HSsmall model is firstly investigated through numerical simulations of strain-controlled cyclic shear tests under single and multi-directional conditions at low strain levels. Then, it is adopted to back-analyse the recorded free field seismic response, comparing the FE numerical results to the in-situ down-hole and surface signals recorded during two earthquakes occurred on May 20th and July 17th 1986, characterized by different peak ground accelerations
The influence of pre-failure soil modelling on the behaviour of open excavations
No full textBalkema,RotterdamPrediction of displacements occurring around an excavation is influenced by the constitutive model used to describe the pre-failure behaviour of the soil. An experimental program has been carried out, in which samples of a reconstituted clay were subjected to stress paths similar to those occurring around excavations. A simple linear elastic-perfectly plastic soil model, and an elastic-plastic soil model with isotropic deviatoric hardening, have been calibrated on the experimental results, and then used in a finite element analysis to simulate the behaviour of an open excavation supported by an embedded retaining wall propped at the top. Comparison among the results of the analyses permitted to assess the influence of pre-failure non linearity on the ground movements predicted around the excavation. Also, the influence of the initial in situ stress state, the wall stiffness and the drainage conditions were studied using the elastic-plastic soil model with isotropic deviatoric hardening
On the use of artificial neural networks as generic descriptors of geomaterial mechanical behaviour
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