Effect of consolidation on the behaviour of excavations in fine-grained soils

This study deals with the detrimental effects of consolidation on the behaviour of excavations carried out in fine-grained soils. With the aim of reproducing the main aspects of the mechanical behaviour of medium-soft clays, an advanced constitutive model was adopted, which is based on bounding surface plasticity and can take into account the damage to the soil microstructure induced by plastic strains. In a first stage of the work, this constitutive model was implemented into a finite element program, and its response was studied through a series of single-element tests, evidencing the effect of the different soil parameters and initial conditions. In the present paper, the constitutive model was used to simulate the behaviour of an idealised excavation, studying the effect of the progressive dissipation of the excess pore water pressures generated during the excavation stages. It was found that, under certain conditions, the initial degree of structure can be progressively lost during consolidation. This detrimental effect produces significant deformation increments and in some cases can drive the system to collapse

Dissipative behaviour of reinforced-earth retaining structures under severe ground motion

This paper focuses on the seismic performance of geosynthetic-reinforced retaining walls (GRWs) that several evidences have shown to be generally adequate. This can be attributed to the dissipation of energy produced by the internal plastic mechanisms activated during the seismic shaking, and to an overall ductile behaviour related to the large deformation that can be accommodated by the soil-reinforcement system. Using a number of numerical computations, this work compares the behaviour of three idealized structures that were conceived in order to have a similar seismic resistance, that however is activated through different plastic mechanisms. The analyses include numerical pseudo-static computations, carried out iteratively to failure, and time-domain nonlinear dynamic analyses, in which acceleration time-histories were applied to the bottom boundary of the same numerical models used for the pseudo-static analyses. The results of the dynamic analyses were interpreted in the light of the plastic mechanisms obtained with the pseudo-static procedure, confirming that GRWs develop local plastic mechanisms during strong motion resulting in a significant improvement of their seismic performance

Studio dell'interazione dinamica terreno-spalla-sovrastruttura per una spalla da ponte

La spalla è la parte di un ponte che maggiormente interagisce con il terreno, inoltre il suo comportamento dinamico è influenzato anche dall'interazione con la sovrastruttura, producendo un reciproco scambio di forze di inerzia durante il sisma. In questa nota si illustra una metodologia per l’analisi della risposta dinamica di spalle da ponte attraverso l’applicazione a un particolare caso di studio, per il quale sono disponibili informazioni sia sui terreni di fondazione, sia sulla struttura in elevazione. Lo studio si avvale di modelli numerici di interazione accoppiati sviluppati con il codice di calcolo OpenSees, descrivendo il comportamento meccanico dei terreni mediante il modello costitutivo a plasticità diffusa di Manzari e Dafalias (2004), calibrato sui dati sperimentali. L’interazione dinamica tra la spalla e la struttura del ponte è stata studiata sia mediante la semplice applicazione di forze statiche equivalenti, sia attraverso lo sviluppo di un metodo di validità generale per l’identificazione di un modello strutturale equivalente. Il presente studio si colloca all'interno di un più ampio progetto di ricerca sulla dinamica delle spalle da ponte: in questa nota si mostrano alcuni risultati intermedi riguardanti la risposta sismica locale del sottosuolo Pantano, evidenziando gli effetti di multi-direzionalità del moto sismico, le azioni sismiche trasmesse in fondazione e alcune peculiarità relative alla propagazione del moto verticale

Stati limite ultimi per spalle da ponte

Nonostante la risposta dinamica delle spalle possa influenzare la prestazione di un ponte, la valutazione di questo effetto è complicata per l’elevatissima domanda computazionale dei modelli numerici completi, che includano esplicitamente nel modello strutturale la spalla e il terreno che interagisce con essa. In questa nota si fornisce un contributo alla definizione di un approccio semplificato di analisi basato sulla simulazione del comportamento delle spalle tramite macro-elementi, focalizzando in particolare l’attenzione sulle condizioni ultime dell’insieme spalla-terreno. Tramite l’applicazione dei teoremi di estremo dell’analisi limite, attraverso il metodo degli elementi finiti con mesh adattiva, si individuano i potenziali meccanismi plastici del sistema spalla-terreno, considerando anche meccanismi combinati in cui i pali di fondazione della spalla possano raggiungere la propria resistenza insieme al terreno. Si propone quindi un modello per la previsione della resistenza ultima delle spalle in condizioni di carico multi-assiali, identificato tramite la calibrazione di un numero limitato di parametri costitutivi. È mostrato come gli effetti inerziali associati all’azione sismica possano essere incorporati nel modello tramite una contrazione e rotazione della superficie che rappresenta le condizioni ultime della spalla, definita nello spazio delle forze. La superficie così definita si presta a essere inclusa in una rappresentazione generale della risposta sismica di una spalla attraverso la definizione di un macro-elemento per le spalle, ma è anche utilizzabile più immediatamente come uno strumento di verifica della resistenza delle spalle in un approccio alla progettazione sismica in termini di forze statiche equivalenti

Numerical evaluation of the modal characteristics of a bridge abutment

The seismic analysis of bridges needs to account for the effects of soil-structure interaction with methods that strike a reasonable balance between completeness and reliability of the numerical soil-structure interaction models. In view of this, focusing on the marked influence that the behaviour of the abutments can have on the seismic performance of the whole bridge, this study presents an identification procedure of the dynamic response bridge abutments. Based on the results of dynamic simulations on a reference local model of a bridge abutment carried out in the analysis framework OpenSees, the modal characteristics of the soil-abutment system are computed and are used to evidence the role played by the soil interacting with the abutment in controlling the overall dynamic response of the system. This role is quantified through the definition of the mass participation factors for different directions of motion

Validazione e utilizzo di un macro-elemento termodinamico multi-assiale per spalle da ponte

Si propone un macro-elemento multi-assiale per descrivere l’interazione spalla-terreno nel modello strutturale di ponte mediante analisi dinamiche non lineari a basso onere di calcolo. La relazione forza-spostamento del macro-elemento è ricavata mediante un approccio termodinamico e include degli aspetti caratterizzanti la risposta delle spalle, quali la non-linearità costitutiva e l’accoppiamento direzionale. In un'analisi dinamica, la dipendenza dalla frequenza del comportamento d’insieme viene simulata combinando il macro-elemento con le masse partecipanti del duo spalla-terreno. La calibrazione del modello richiede la sola valutazione della capacità ultima e della risposta a piccoli spostamenti del sistema geotecnico. Il macro-elemento, implementato in OpenSees, è validato con riferimento ai risultati di una modellazione accoppiata avanzata

A multiaxial inertial macroelement for bridge abutments

This paper proposes a multiaxial macroelement for bridge abutments that can be included in the global structuralmodel of a bridge to carry out nonlinear dynamic analyses with very much smaller computational effort than can be achieved using continuum representations of embankment and foundation soil behaviour. The proposed macroelement derives a constitutive force–displacement relationship within a rigorous thermodynamic framework and includes important features of non-linearity and directional coupling in characterizing the interactions of the abutment with the soil. In a dynamic analysis, the frequency-dependent response of the system is simulated through the combination of the macroelement with appropriate participating masses. The calibration procedure of the macroelement is based on the assessment of its ultimate capacity and of its response at small displacements, and it is shown that these ingredients can be derived through standardised procedures. In the paper, the macroelement response is validated against the results of fully coupled continuum numerical analyses for a reference soil–abutment system, under both static and seismic loading conditions. We show that the two models achieve similar predictions of maximum and permanent abutment deformations (less than 10–14% difference, respectively) for a suite of three-axis seismic loading events

On the seismic performance of straight integral abutment bridges: From advanced numerical modelling to a practice‐oriented analysis method

The seismic performance of integral abutment bridges (IABs) is affected by the interaction with the surrounding soil, and specifically by the development of interaction forces in the embankment-abutment and soil-piles systems. In principle, these effects could be evaluated by means of highly demanding numerical computations that, however, can be carried out only for detailed studies of specific cases. By contrast, a low-demanding analysis method is needed for a design-oriented assessment of the longitudinal seismic performance of IABs. To this purpose, the present paper describes a design technique in which the frequency- and amplitude-dependency of the soil-structure interaction is modelled in a simplified manner. Specifically, the method consists of a time-domain analysis of a simplified soil-bridge model, in which soil-structure interaction is simulated by means of distributed nonlinear springs connecting a free-field ground response analysis model to the structural system. The results of this simplified method are validated against the results of advanced numerical analyses, considering different seismic scenarios. In its present state of development, the proposed simplified nonlinear model can be used for an efficient evaluation of the longitudinal response of straight IABs and can constitute a starting point for a prospective generalisation to three-dimensional response

Design methods and predictive tools for deep excavations and bored tunnels

Papers in the present session deal with the design of geotechnical systems. Among the Authors' struggles to perform reliable predictions of real structures, it is instructive to discern two distinct strategies: (a) gaining on previous experience, and (b) decoupling. This general report on TS5 briefly illustrates the implementation of these strategies, making specific reference to some of the papers included this session