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Numerical investigation on the modal properties of slopes
Full text linkEarthquake-induced displacements of slopes are often evaluated through decoupled approaches combined with
empirical relationships accounting for the characteristics of the input motion and the dynamic properties of the sliding
mass. As per the latter point, recent studies highlighted the key role played by the system modal properties on the slope
performance. Notwithstanding, the natural periods of the slope are commonly evaluated through over-simplified
expressions relating to homogenous soil deposit with horizontal ground level. With the aim to provide a more realistic
and comprehensive dynamic identification, this study presents a numerical investigation on the modal features of
slopes with planar slip surface, implemented in the analysis framework OpenSees. The consideration of several slope
geometries and mechanical properties point out i) the influence of the soil incompressibility, of particular interest under
saturated conditions, and ii) effective ranges for the multiaxial modal periods and participation masses as a function
of the soil stiffness. The results of the parametric study are finally used to derive a practice-oriented identification
procedure of the modal features of slopes, as starting information for calibrating simplified numerical tools devoted to
their large-scale assessment
Influence of the displacement predictive relationships on the probabilistic seismic analysis of slopes
Full text linkSeismically induced landslides can often cause severe human and economic losses. Therefore, it is worth assessing the seismic performance of slopes through a reliable quantification of the permanent displacements induced by seismic loading. This paper presents a new semiempirical relationship linking the permanent earthquake-induced displacements of slopes to one or two synthetic ground motion parameters developed considering the Italian seismicity, and a comparison with existing simplified displacement models is illustrated. Once combined with a fully probabilistic approach, these relationships provide a useful tool for practicing engineers and national agencies for a preliminary estimate of the seismic performance of a slope. In this perspective, the predictive capability of different semiempirical
relationships is analyzed with reference to the permanent displacements evaluated for the Italian seismicity assimilating the slope to a rigid body and adopting the Newmark integration approach. The consequences of the adoption of these relationships on the results of the probabilistic approach are illustrated in terms of displacement hazard curves and hazard maps for different slope scenarios
Regional scale assessment of the seismic performance of natural slopes
Full text linkThis paper analyses the seismic response of natural slopes within a probabilistic framework with reference to the Italian
seismicity. Once combined with a proper semi-empirical relationship linking the permanent earthquake-induced
displacements of slopes to one or two synthetic ground motion parameters, the probabilistic approach allows to
evaluate the permanent displacements for a given return period and a given yield seismic coefficient, the latter
accounting synthetically for the seismic resistance of the slope. The results are illustrated in terms of hazard maps
showing the distribution of the displacements at a regional scale and are therefore useful to identify slopes in areas that
are prone to earthquake instability. In detail, scalar and vector probabilistic approaches based on the ground motion
parameter PGA and the couple PGA, PGV, respectively, are employed to produce hazard maps for two Italian regions
characterised by severe seismicity. Finally, the results obtained through the fully probabilistic approach are critically
analysed and compared with those determined through a simplified decoupled method that combines a standard
probabilistic seismic hazard analysis (PSHA) with the displacement semi-empirical relationships. The maps presented
in this work represent a reliable and simple tool for preliminary, screening level analyses of natural slopes in terms of
seismic-induced displacements
Probabilistic assessment of seismic-induced slope displacements: an application in Italy
Full text linkEarthquake-induced slope instability is one of the most important hazards related to ground
shaking, causing damages to the environment and, often, casualties. Therefore, it is important
to assess the seismic performance of slopes, especially in the near fault regions, evaluating
the permanent displacements induced by seismic loading. This paper applies a probabilistic
approach to evaluate the seismic performance of slopes using an updated database
of ground motions recorded during the earthquakes occurred in Italy. The main advantage
of this approach is that of accounting for the aleatory variability of both ground motions
and prediction of seismic-induced displacements of slopes. The results are presented in
terms of hazard curves, showing the annual rate of exceedance of permanent slope displacement
evaluated using ground motion data provided by a standard probabilistic hazard
analysis and a series of semi-empirical relationships linking the permanent displacements
of slopes to one or more ground motion parameters. The procedure has been implemented
on a regional scale to produce seismic landslide hazard maps for the Irpinia district, in
Southern Italy, characterised by a severe seismic hazard. Seismic landslide hazard maps
represent a useful tool for practitioners and government agencies for a regional planning
to identify and monitor zones that are potentially susceptible to earthquake-induced slope
instability, thus requiring further detailed, site-specific studies
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
Evaluation of Seismic Landslide Hazard Based on a New Displacement Semi-empirical Relationship
Full text linkThis paper presents a new semi-empirical relationship that links the
permanent earthquake-induced displacements of slopes to the synthetic ground
motion parameter PGA or to the couple PGA and PGV. The displacements are
evaluated under the hypothesis of a rigid sliding block performing Newmark’s
integrations for all the acceleration time histories of the updated Italian seismic
database. The relationship reproduces well the displacements for any values of
yield seismic coefficient in the whole range of peak ground acceleration. The two
parameters expression is more reliable for the study of Italian slopes under seismic
loading than that based on the single PGA parameter as characterised by a lower
standard deviation. The proposed relationship is also combined with a fully probabilistic
approach to produce displacement hazard curves and hazard maps for
different sites and regions of Italy that represent a useful tool for practicing engineers
and national agencies for a preliminary estimate of the seismic performance
of a slope
Elastic anisotropy and elastoplastic coupling of soils: a thermodynamic approach
Full text linkL'attività di ricerca verte sulla modellazione costitutiva della risposta anisotropa dei terreni, che rappresenta un aspetto rilevante del comportamento meccanico degli stessi in numerose applicazioni geotecniche. È stato dapprima formulato un modello iperelastico non lineare anisotropo in deformazioni infinitesime, in funzione di una serie di invarianti misti dei tensori di deformazione elastica e di struttura (fabric tensor), che rappresenta al livello macroscopico le relative proprietà microstrutturali del materiale. La nuova formulazione proposta può essere efficacemente impiegata per riprodurre il carattere anisotropo della rigidezza a piccole deformazioni osservato sperimentalmente sia nelle sabbie che nelle argille. Il legame costitutivo è poi stato adottato all’interno del modello elastoplastico a singola superficie per argille proposto da Dafalias & Taiebat (2013), caratterizzato da incrudimento misto isotropo e rotazionale. Si è individuata quindi una relazione tra la variabile interna che governa l’anisotropia in regime plastico ed il tensore di struttura prima definito con riferimento alla risposta reversibile. In tal modo, quest’ultimo ingrediente risulta avere un carattere evolutivo, correlato all’evoluzione dell’incrudimento rotazionale del materiale. Ciò dà luogo ad una forma di accoppiamento elastoplastico i cui aspetti quantitativi sono stati valutati con riferimento ad una serie di osservazioni sperimentali effettuate su un’argilla ricostituita in laboratorio da Mitaritonna et al. (2014). Il modello di Dafalias & Taiebat è stato in seguito riformulato secondo la teoria dell’iperplasticità (Houlsby & Puzrin, 2000). La nuova formulazione, oltre a garantire il rispetto dei principi della termodinamica, consente di introdurre all’interno del modello forme di accoppiamento elastoplastico rigorose, in cui anche il regime di risposta plastico viene modificato in virtù di tale accoppiamento. Sono stati quindi introdotti due tipi di accoppiamento, il primo in forma scalare sulla pressione di preconsolidazione e il secondo, di natura più prettamente direzionale, sul tensore di struttura.The research activity focuses on the constitutive modelling of the anisotropy of soils, which represents a relevant aspect of the mechanical behaviour of the same in many geotechnical applications. A non-linear anisotropic hyperelastic model in infinitesimal strain was first formulated, as a function of a series of mixed invariants of elastic strain tensor and fabric tensor, which represents at the macroscopic level the relative microstructural properties of the material. The proposed new formulation can be used to reproduce the anisotropic character of the small strain stiffness observed experimentally both in sandy and in clayey soils. The constitutive relationship was then adopted within the single surface elastoplastic model for clays proposed by Dafalias & Taiebat (2013), characterised by isotropic and rotational hardening laws. Therefore, a relationship has been identified between the internal variable that governs the anisotropy in the plastic regime and the fabric tensor defined before with reference to the reversible response. In this way, the latter ingredient is no longer constant as related to the evolution of the rotational hardening of the model. This leads to a form of elastoplastic coupling whose aspects have been evaluated with reference to a series of experimental laboratory tests carried out by Mitaritonna et al. (2014) on a reconstituted clay. The model of Dafalias & Taiebat was then reformulated according to the theory of hyperplasticity (Houlsby & Puzrin, 2000). The new formulation, besides to guarantee the respect of the laws of thermodynamics, makes it possible to introduce rigorous forms of elastoplastic coupling within the model, in which also the plastic regime is modified by virtue of the coupling. Two types of coupling were introduced, the first one through the preconsolidation pressure and the second one, accounting for the directional properties of soils, through the fabric tensor
Elasto-Plastic Coupling in Soils: A Thermodynamic-Based Approach
No full textIn the present study the implications of a thermodynamic-based
constitutive framework on the mechanical behaviour of soils are critically
analysed. The primary advantage of this approach as compared to the traditional
hardening plasticity is that the models are guaranteed to obey the laws of
thermodynamics. Furthermore, the use of potential functions allows to introduce
some crucial ingredients of the mechanical behaviour of soils that directly affect
the shape of the yield surface and the flow rule of the model. To illustrate the
above features, different forms of elasto-plastic coupling are presented and their
implications on the response of the models are explored with reference to a
series of numerical simulations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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