Critical infrastructures, such as bridges and viaducts, are essential to societal and economic development. However, many existing structures have exceeded their intended lifespan and must now be reassessed according to modern design codes, which incorporate seismic actions not considered during their original design. In this context, the foundations of existing bridges, particularly piled raft foundations, are of critical importance. Conventional assessment methods often rely on oversimplified assumptions, such as neglecting piles-raft-soil interaction, which can lead to overly conservative and costly retrofitting solutions. This is especially true for large piled rafts where the pile length is shorter than the raft major dimension, and the raft contribution to the system capacity becomes more significant. This research addresses these limitations by advancing the understanding of the mechanical response of large piled raft foundations under combined loading conditions. The study focuses on the interaction between piles, raft, and surrounding soil for piled rafts embedded in homogeneous, overconsolidated clay behaving under undrained conditions. Non-linear three-dimensional finite element simulations were conducted on a representative real viaduct pier foundation to investigate both local and global responses. The results highlight the crucial role of piles-raft-soil interaction in governing the system stiffness, load distribution, and failure mechanisms under combined loading, in contrast to pile group systems with no raft-soil contact. Distinct failure modes were observed depending on the imposed load conditions. Under centred vertical loads, the raft and piles mobilize their capacities independently. In contrast, horizontal loads and overturning moments induce rotational failure mechanisms involving the raft, piles, and the intervening soil. Structural analyses further revealed that horizontal load induces a pile head rotation, typically neglected in simplified models, significantly affecting the distribution of internal forces. To address the high computational cost of advanced simulations, a novel, rapid, and rational verification framework was developed for geotechnical ULS and SLS verifications of large piled rafts. This framework explicitly accounts for piles-raft-soil interaction without requiring numerical analysis. A theoretical approach was proposed to define geotechnical interaction domains using classical bearing capacity formulas and the limit equilibrium method. Additionally, a macroelement model was developed to reproduce the non-linear response of piled rafts under combined loading. This model enables efficient prediction of displacements based on a limited set of physically meaningful parameters. It was further used to generate deck drift-based design charts that account for foundation deformability. The practical implications of incorporating pile-raft-soil interaction into verification procedures were critically examined. The study demonstrated that only approaches explicitly considering this interaction produce accurate and non-conservative results, thereby avoiding unnecessary retrofitting interventions. Finally, simplified dynamic verifications of the pier under real accelerograms showed that accounting for soil-structure interaction substantially reduces seismic demands on both the pier and its foundation. These findings support the adoption of the proposed verification framework in practical engineering applications and highlight the need to extend this approach by developing a macroelement model for piled rafts under cyclic loading. Furthermore, the method should be adapted for piled rafts in sandy soils, where the role of the raft is expected to be even more significant.
Le infrastrutture critiche, come ponti e viadotti, sono essenziali per lo sviluppo sociale ed economico. Tuttavia, molte delle strutture esistenti hanno superato la loro vita utile prevista e devono ora essere rivalutate secondo le normative progettuali moderne, che includono azioni sismiche non considerate durante la progettazione originaria. In questo contesto, le fondazioni dei ponti esistenti, in particolare le fondazioni miste plinto-pali (piled raft), rivestono un'importanza cruciale. I metodi di valutazione convenzionali si basano spesso su ipotesi semplificate, come la trascuratezza dell’interazione tra pali, plinto e terreno, il che può portare a soluzioni di adeguamento eccessivamente conservative e costose. Questo è particolarmente vero per le fondazioni miste di grandi dimensioni in cui la lunghezza dei pali è inferiore alla dimensione maggiore del plinto, e il contributo del plinto alla capacità del sistema diventa più rilevante. Questa ricerca affronta tali limitazioni approfondendo la comprensione del comportamento meccanico delle fondazioni miste di grandi dimensioni soggette a condizioni di carico combinate. Lo studio si concentra sull’interazione tra pali, plinto e terreno circostante per fondazioni miste inserite in argille omogenee sovraconsolidate, in condizioni non drenate. Sono state condotte simulazioni non lineari tridimensionali agli elementi finiti su una fondazione reale rappresentativa di una pila di viadotto, per indagare sia la risposta locale sia quella globale. I risultati evidenziano il ruolo cruciale dell’interazione pali-plinto-terreno nel determinare la rigidezza del sistema, la distribuzione dei carichi e i meccanismi di collasso sotto carichi combinati, in contrasto con i sistemi di pali senza contatto tra plinto e terreno. Sono stati osservati meccanismi di collasso distinti a seconda delle condizioni di carico imposte. Sotto carichi verticali centrati, il plinto e i pali mobilitano le loro capacità in modo indipendente. Al contrario, i carichi orizzontali e i momenti ribaltanti inducono meccanismi di collasso rotazionali che coinvolgono plinto, pali e terreno interposto. Le analisi strutturali hanno inoltre rivelato che i carichi orizzontali inducono una rotazione della testa del palo, tipicamente trascurata nei modelli semplificati, influenzando in modo significativo la distribuzione degli sforzi interni. Per affrontare l’elevato costo computazionale delle simulazioni avanzate, è stato sviluppato un nuovo metodo di verifica rapido e razionale per le verifiche geotecniche agli SLU e SLE delle fondazioni miste di grandi dimensioni. Questo metodo tiene esplicitamente conto dell’interazione pali-plinto-terreno senza necessità di analisi numeriche. È stato proposto un approccio teorico per definire i domini di interazione geotecnica utilizzando le formule classiche di capacità portante e il metodo dell’equilibrio limite. Inoltre, è stato sviluppato un modello macroelemento per riprodurre la risposta non lineare delle fondazioni miste sotto carichi combinati. Questo modello consente una previsione efficiente degli spostamenti basata su un numero limitato di parametri fisicamente significativi. È stato inoltre utilizzato per generare grafici di progetto basati sul drift dell’impalcato, che tengono conto della deformabilità della fondazione. Le implicazioni pratiche dell’inclusione dell’interazione pali-plinto-terreno nelle procedure di verifica sono state esaminate criticamente. Lo studio ha dimostrato che solo gli approcci che considerano esplicitamente tale interazione producono risultati accurati e non conservativi, evitando così interventi di adeguamento non necessari. Infine, verifiche dinamiche semplificate della pila sotto accelerogrammi reali hanno mostrato che tenere conto dell’interazione terreno-struttura riduce significativamente le sollecitazioni sismiche sia sulla pila sia sulla sua fondazione. Questi risultati supportano l’adozione del quadro di verifica proposto nelle applicazioni ingegneristiche pratiche e sottolineano la necessità di estendere tale approccio sviluppando un modello macroelemento per fondazioni miste soggette a carichi ciclici. Inoltre, il metodo dovrebbe essere adattato per fondazioni miste in terreni sabbiosi, dove ci si aspetta che il ruolo del plinto sia ancora più rilevante.
Seismic verifications of large piled rafts: from numerical analyses to upscaled modelling
Corigliano, Matteo
2024/2025
Abstract
Critical infrastructures, such as bridges and viaducts, are essential to societal and economic development. However, many existing structures have exceeded their intended lifespan and must now be reassessed according to modern design codes, which incorporate seismic actions not considered during their original design. In this context, the foundations of existing bridges, particularly piled raft foundations, are of critical importance. Conventional assessment methods often rely on oversimplified assumptions, such as neglecting piles-raft-soil interaction, which can lead to overly conservative and costly retrofitting solutions. This is especially true for large piled rafts where the pile length is shorter than the raft major dimension, and the raft contribution to the system capacity becomes more significant. This research addresses these limitations by advancing the understanding of the mechanical response of large piled raft foundations under combined loading conditions. The study focuses on the interaction between piles, raft, and surrounding soil for piled rafts embedded in homogeneous, overconsolidated clay behaving under undrained conditions. Non-linear three-dimensional finite element simulations were conducted on a representative real viaduct pier foundation to investigate both local and global responses. The results highlight the crucial role of piles-raft-soil interaction in governing the system stiffness, load distribution, and failure mechanisms under combined loading, in contrast to pile group systems with no raft-soil contact. Distinct failure modes were observed depending on the imposed load conditions. Under centred vertical loads, the raft and piles mobilize their capacities independently. In contrast, horizontal loads and overturning moments induce rotational failure mechanisms involving the raft, piles, and the intervening soil. Structural analyses further revealed that horizontal load induces a pile head rotation, typically neglected in simplified models, significantly affecting the distribution of internal forces. To address the high computational cost of advanced simulations, a novel, rapid, and rational verification framework was developed for geotechnical ULS and SLS verifications of large piled rafts. This framework explicitly accounts for piles-raft-soil interaction without requiring numerical analysis. A theoretical approach was proposed to define geotechnical interaction domains using classical bearing capacity formulas and the limit equilibrium method. Additionally, a macroelement model was developed to reproduce the non-linear response of piled rafts under combined loading. This model enables efficient prediction of displacements based on a limited set of physically meaningful parameters. It was further used to generate deck drift-based design charts that account for foundation deformability. The practical implications of incorporating pile-raft-soil interaction into verification procedures were critically examined. The study demonstrated that only approaches explicitly considering this interaction produce accurate and non-conservative results, thereby avoiding unnecessary retrofitting interventions. Finally, simplified dynamic verifications of the pier under real accelerograms showed that accounting for soil-structure interaction substantially reduces seismic demands on both the pier and its foundation. These findings support the adoption of the proposed verification framework in practical engineering applications and highlight the need to extend this approach by developing a macroelement model for piled rafts under cyclic loading. Furthermore, the method should be adapted for piled rafts in sandy soils, where the role of the raft is expected to be even more significant.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/241517