Elastic anisotropy and elastoplastic coupling of soils: a thermodynamic approach

L'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

Three-dimensional advanced numerical approaches to the seismic soil and structural response analyses

A 3D non-linear finite element approach is developed to study the free-field seismic ground response and the soil-structure interaction (SSI) phenomena at the Lotung site (Taiwan) during the earthquake event occurred on May 20 1986. The site was extensively instrumented with down-hole and surface ac- celerometers, these latter located also on a 1/4–scale nuclear power plant containment structure. An advanced constitutive model is adopt- ed for simulating the soil behaviour, while a linear visco-elastic be- haviour is assumed for the structural model. The free-field and SSI analyses are carried out applying both the NS and EW horizontal components of the acceleration time history as recorded at the depth of 47 m b.g.l. The predicted ground response re- sults are in fair agreement with the recorded motion at depth and at the surface. The dynamic response of structure is well captured for this specific seismic event, thus confirming the validity of the numerical approach

Evaluation of Seismic Landslide Hazard Based on a New Displacement Semi-empirical Relationship

This 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

Influence of the displacement predictive relationships on the probabilistic seismic analysis of slopes

Seismically 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

Probabilistic Seismic Hazard Curves and Maps for Italian Slopes

The seismic performance of an earth slope is commonly evaluated through the permanent displacements developed at the end of an earthquake. In this paper a probabilistic approach is adopted to assess the displacement of the slope for a given hazard level using an updated database of ground motions rec-orded during the earthquakes occurred in Italy. 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 parame-ters. The probabilistic approach permits to take into account synthetically the characteristics of the slope through the yield seismic coefficient, the aleatory variability of the ground motions and the different subsoil classes of the record-ing stations. Finally, the procedure has been extended on a regional scale to produce seismic landslide hazard maps for Irpinia, one of the most seismically active regions in Italy. Seismic landslide hazard maps are very attractive for practitioners and government agencies for a screening level analysis to identify, monitor and minimise damages in zones that are potentially susceptible to earthquake-induced slope instability

FollowMe: a Robust Person Following Framework Based on Re-Identification and Gestures

Human-robot interaction (HRI) has become a crucial enabler in houses and industries for facilitating operational flexibility. When it comes to mobile collaborative robots, this flexibility can be further increased due to the autonomous mobility and navigation capacity of the robotic agents, expanding their workspace and consequently, the personalizable assistance they can provide to the human operators. This however requires that the robot is capable of detecting and identifying the human counterpart in all stages of the collaborative task, and in particular while following a human in crowded workplaces. To respond to this need, we developed a unified perception and navigation framework, which enables the robot to identify and follow a target person using a combination of visual Re-Identification (Re-ID), hand gestures detection, and collision-free navigation. The Re-ID module can autonomously learn the features of a target person and use the acquired knowledge to visually re-identify the target. The navigation stack is used to follow the target avoiding obstacles and other individuals in the environment. Experiments are conducted with few subjects in a laboratory setting where some unknown dynamic obstacles are introduced.Comment: published in "2023 IEEE International Conference on Advanced Robotics and Its Social Impacts (ARSO)

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Un modello iperelastico non lineare anisotropo per lo studio della risposta reversibile dei terreni

In questa nota viene presentato un modello costitutivo iperelastico non lineare anisotropo concepito per descrivere la risposta reversibile dei terreni. Aspetti quali la dipendenza non lineare della rigidezza elastica dallo stato di sforzo e il carattere anisotropo della rigidezza assumono spesso un ruolo non trascurabile nella modellazione del comportamento meccanico dei terreni e nell’analisi di numerosi problemi geotecnici al finito. Il modello in oggetto è in grado di riprodurre in maniera termodinamicamente corretta sia l’effetto dell’anisotropia indotta dallo stato di sforzo/deformazione, sia gli effetti dell’anisotropia di struttura. Quest’ultima è descritta da un tensore di struttura del secondo ordine che è la rappresentazione macroscopica delle proprietà microstrutturali del materiale. La procedura di calibrazione dei parametri e le capacità previsionali del modello sono illustrate con riferimento a risultati di prove di laboratorio eseguite su provini di sabbia

Analisi della risposta sismica dei pendii mediante un approccio probabilistico

Questo lavoro è finalizzato alla valutazione della prestazione sismica dei pendii mediante un approccio probabilistico a partire dal database sismico italiano aggiornato. Il principale vantaggio di questo approccio è quello di fornire un utile strumento di analisi dei pendii in condizioni sismiche in grado di portare in conto la variabilità dell’azione sismica sul territorio nazionale, nonché le diverse caratteristiche dei pendii. I risultati sono presentati in termini di curve di pericolosità, che mostrano la frequenza annua di superamento associata agli spostamenti permanenti indotti dal sisma nel pendio, a partire da un’analisi di pericolosità sismica di base e mediante una serie di relazioni semi-empiriche che legano gli spostamenti permanenti dei pendii a uno o più parametri sintetici del moto sismico. La procedura è stata implementata su scala regionale per produrre mappe di pericolosità per la regione dell’Irpinia, nel Sud Italia, caratterizzata da una severa sismicità. Le mappe di pericolosità ottenute rappresentano uno strumento utile per una valutazione speditiva della pericolosità sismica associata ai pendii e consentono una pianificazione su base regionale atta a identificare e monitorare le zone potenzialmente suscettibili di instabilità dei versanti a seguito di eventi sismici