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Dynamic response of piles under lateral loading: full scale field test and numerical analysis
No full textNegli ultimi anni il tema dell’interazione terreno-struttura ha suscitato un’attenzione
particolare nel campo dell’ingegneria geotecnica e strutturale. Le caratteristiche dinamiche del sistema terreno-fondazione possono condizionare fortemente il comportamento sismico della struttura di elevazione. Questo problema è particolarmente importante nel caso di
fondazioni profonde: le proprietà meccaniche e la geometria del terreno e delle fondazioni,così come la loro mutua interazione, possono influenzare le caratteristiche di rigidezza e
smorzamento del sistema suolo-palo durante il moto sismico.
Sia nella ricerca che nella progettazione avanzata questo problema può essere studiato in modo diretto, modellando l’intero sistema dinamico suolo-palo con un modello 3D agli
elementi finiti, o utilizzando metodi teorici. I risultati di questi approcci sono molto sensibili al grado di dettaglio del modello e a molti parametri che definiscono le caratteristiche dinamiche del sistema terreno-palo. In questo contesto, i risultati di test
sperimentali in situ, in vera grandezza o in scala ridotta, e i risultati di test di laboratorio rappresentano uno strumento essenziale per fornire i parametri necessari ai modelli numerici e analitici e per validarne i risultati. Tuttavia, in letteratura non sono molti i lavori
che riguardano prove in situ in vera grandezza su fondazioni profonde.
Questo lavoro presenta un esteso programma di prove sperimentali in situ in vera grandezza condotto su un gruppo di tre pali in acciaio al porto turistico di La Spezia, in Italia. Il programma comprende tre tipologie di prove dinamiche con differenti livelli di eccitazione:
prove di impatto, prove di vibrazione libera e di vibrazione forzata. I pali sono vibro-infissi in argilla marina soffice in una configurazione ad L e non sono vincolati in testa. Il programma di prove è stato eseguito in due tempi: la prima campagna una settimana dopo la vibro-infissione dei pali e la seconda dopo dieci settimane. I pali sono stati strumentati
con accelerometri disposti in testa a ciascun palo e con strain gages e trasduttori di pressione disposti lungo il palo d’angolo, utilizzando una tecnica non convenzionale per prove in sito in ambiente marino. Viene presentato e discusso il comportamento dinamico
del complesso sistema suolo-acqua-palo soggetto a diversi livelli di forza. Con riferimento al palo singolo, ne viene presentata la risposta in termini di frequenze naturali, smorzamenti
e forme modali dei primi due modi flessionali. Con riferimento al gruppo di pali sono valutati gli effetti dell’interazione palo-palo dovuti alla propagazione di diversi tipi di onde,
in relazione alla direzione dell’azione. Si discute la variazione del comportamento dinamico del sistema nel tempo (per le due campagne di prove) dovuta alla ri-consolidazione del terreno vicino ai pali susseguente la vibro-infissione. Inoltre, viene effettuata una stima
della velocità delle onde di taglio dello strato di terreno superficiale sulla base della misurazione dei ritardi temporali dei segnali accelerometrici alla testa dei pali. I risultati sperimentali sono inoltre confrontati con quelli ottenuti mediante differenti approcci numerici: in particolare sono stati sviluppati due differenti modelli 3D agli elementi finiti in ABAQUS, considerando elementi di tipo solido o di tipo shell per la modellazione dei pali,
i cui parametri sono calibrati sulla base dei risultati sperimentali. Infine si mostrano i confronti con i risultati ottenuti mediante il modello 3D per l’analisi cinematica di
fondazioni su pali formulato da Dezi et al. (2009), opportunamente modificato al fine di
simulare le prove della campagna sperimentale
NONLINEAR DYNAMIC RESPONSE OF NEAR-SHORE PILES SUBJECTED TO SNAP-BACK TESTING
No full textThis paper presents the results of free vibration tests carried out on a near-shore steel pipe pile vibrodriven into soft marine clay. The tests were carried out by quickly releasing a free head pile from a deformed configuration obtained by applying a quasi-static horizontal loading by means of a hydraulic jack. Different tests were performed at increasing loading levels with the aim of investigating the dynamic soil-water-pile system interaction and investigating how the nonlinear dynamic behaviour develops as the applied load increases. The results of experimental modal analyses, based on the signals recorded by strain gauges applied along the pile, are presented in terms of natural frequencies and damping ratios of the soil-water-pile system and the variations with loading level, due to the nonlinear behaviour of the system, are discussed. The dynamic effects induced on two pipe piles vibrodriven near the loaded pile to form an L-shaped in plan configuration are also discussed on the basis of signals measured by accelerometers applied at the head of the receiver piles; in order to investigate the pile-to-pile dynamic interaction
Experimental study of near-shore pile-to-pile interaction
Full text linkThis paper presents the results of lateral impact load field tests carried out on a system of three steel pipe piles vibro-driven into soft clay in a near-shore marine environment, with the aim of evaluating the pile-soil-pile dynamic interaction. Piles are arranged in an “L” shaped horizontal layout and are instrumented with accelerometers at their free heads. The obtained results show the complex dynamic behaviour at very small strain of the vibrating soil water piles system. The role of different type of waves in the pile to pile interaction is investigated by analyzing the results in the time and frequency domains and by means of a time-frequency analysis. The effects of the pile spacing and input direction on these interaction mechanisms are also presented. Finally, important dynamic parameters of the soil, such as the velocities of the shear waves and surface waves (Scholte waves) of the upper soil are directly estimated from the time delays between signals recorded at the pile heads
Lateral impact loading and snap-back testing to estimate linear and nonlinear dynamic response of near-shore piles
No full textThis paper presents some results of dynamic lateral loading tests on a near-shore steel pipe pile vibrodriven into soft marine clay. Two typologies of tests are carried out on a free head pile: impact load tests and free vibration tests at different load levels. The aim of this experimentation is to investigate the dynamic soil-water-pile interaction and determine the dynamic characteristics of the whole system by means of the two different typologies of test and for different levels of the dynamic input. The obtained results show the complex dynamic behaviour of the vibrating soil-water-pile system in terms of natural frequencies and damping ratios. The values obtained from the two test typologies are compared and the variations with the level of the input force, due to the nonlinear behaviour of the system, are discussed
Prove in vera grandezza e analisi numeriche di pali con forzante orizzontale impulsiva
No full textSommario
This paper presents the results of lateral impact load field tests carried out on three steel pipe piles at the tourist port of La Spezia, Italy. The piles are vibro-driven into marine soft clay, with a L-shaped plan layout, kept free at the head. Two test campaigns are carried out, the first 1 week and the second 10 weeks after vibrodriving of the piles. The dynamic behaviour of the complex soilwaterpile system at very small strain is discussed. In particular, the response of the single pile and the mutual interaction between loaded and receiver piles are presented. The variation in dynamic behaviour of the whole system in time (for the two campaigns), due to reconsolidation of the soil close to the pile subsequent to the vibrodriving, is observed. The experimental data are compared with the results obtained from a 3-D finite element models of the whole soilwaterpile system considering both a solid and a shell model for the piles and linear behaviour for the soil
Field tests of vibro-driven piles under lateral impact loading
No full textABSTRACT
This paper presents lateral impact load tests carried out on three steel pipe piles vibro-driven into marine soft clay at the tourist port “Mirabello” in La Spezia. Piles are displaced with a L-shape plane layout and kept free at the head. The instrumentation system of piles, unconventional for field test in marine environment, includes accelerometers, strain gauges and pore pressure gauges, with the relevant chemical and mechanical protections. The impact load tests were carried out in two different campaigns, the first 1 week and the second 10 weeks after the pile vibro-driving. Some of the results collected are presented, i.e. time histories of strains and accelerations. The response of the single pile and the dynamic behaviour of the pile group, at very small strain, are investigated: the dynamic behaviour of soil-water-pile system is discussed in terms of natural frequencies, damping and mode shapes; the pile-soil-pile interaction is studied by analyzing the different types of wave propagating from loaded pile to the receivers and estimating the shear wave velocity. Furthermore, the variation in time (for the two experimental campaigns) due to the re-consolidation of the soil close to the pile subsequent to the vibro-driving is observed
Full-scale pile subjected to different lateral dynamic loadings
No full textThis paper presents some results of dynamic lateral loading tests on a near-shore steel pipe pile vibrodriven into soft marine clay. Two typologies of tests are carried out on a free head pile: impact load tests and free vibration tests at different load levels. The aim of this experimentation is to investigate the dynamic soil-water-pile system interaction and determine the dynamic characteristics of the whole system by means of two different typologies of test and for different levels of the dynamic input. The obtained results show the complex dynamic behaviour of the vibrating soil water pile system in terms of natural frequencies and damping ratios. The values obtained from the two test typologies are compared and the variations with the level of the input force, due to the nonlinear behaviour of the system, are discussed
Linear and Nonlinear Dynamic Response of Piles in Soft Marine Clay
No full texthis paper presents the results of free vibration tests carried out at different load levels on a system of three near-shore steel pipe piles vibro-driven into soft marine clay. Piles are arranged in an L-shaped horizontal layout and are free at the head. The instrumentation consists of strain gauges placed at selected levels along the shaft of the loaded pile and accelerometers at the head of the receiver piles. The aim of this experiment is to analyze the dynamic soil–water–pile and pile-to-pile interaction and to investigate the development of nonlinearities at increasing load levels. The results of experimental modal analyses, in terms of natural frequencies and damping ratios of the system, are presented and the complex dynamic behavior of the vibrating soil–water–pile system and the pile-to-pile interaction are discussed. For a better reliability assessment of the system response in the range of linear behavior, the results of free vibration tests at the lowest level of the applied loads are compared with those obtained from impact load tests
Dynamic tests on an existing r.c. school building retrofitted with external steel "dissipative towers
No full textThis paper deals with the dynamic tests carried out on a school building (four story reinforced concrete frame) seismically retrofitted with an innovative system that uses external steel dissipative towers. Before the retrofitting, ambient vibration tests were carried out with the aim of evaluating the actual linear dynamic of the building including the contribution of non-structural components (e.g. external and internal walls). Modal parameters determined by means of experimental tests are crucial for the final design of the retrofitting system allowing the calibration of a predicting f.e. model. Three low noise servo-accelerometers per floor were opportunely positioned at each floor to monitor its rigid motion. The modal parameters are identified with the Enhanced Frequency Domain Decomposition technique obtaining the natural frequencies, mode shapes and damping ratios. After the building retrofitting, snap-back tests at different load levels were performed in order to evaluate the dynamic characteristics of the new structural system. The load was applied in a quasi-static manner by means of two Dywidag bars 47 anchored at the last floor and connected to a steel triangular truss pulled by two hydraulic jacks; the instantaneous release was obtained by cutting a dog-bone shaped steel plate with a blowtorch. In addition to the accelerometers, displacement transducers were positioned at the base of the steel towers, close to the viscous dampers, to measure the motion of the two towers. The natural frequencies and the damping ratios of the retrofitted building are estimated from the time histories of accelerations (free decay functions) by means of the crossing time and the logarithmic decrement techniques, respectively. The adopted experimental methodology adopted has revealed to be effective for the dynamic characterization, both at very low strain and at higher strain level, of a low-rise reinforced concrete frame building in service, which is characterized by a high overall stiffness due to the contribution of the non-structural components
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