1,721,227 research outputs found
Using Transfer Function for Estimating Dissipative Properties of Soils from Surface Wave Data
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Numerical and experimental comparison between 2-station and multistation methods for spectral analysis of surface waves
Full text linkCombined use of geophysical methods in site characterization
No full textGeophysical tests are widely used in site characterization for geotechnical and geoenvironmental applications. A wide range of methods is available to reconstruct geometrical features of the subsoil accounting for different responses of soils as a porous medium and for the characteristics of the pore fluid. Seismic tests provide the advantage of an evaluation of the mechanical response of the medium, although only at very small strain levels, with the possibility of testing geomaterials in their undisturbed state on site. Interpretation of wave propagation in different frameworks can provide information at several levels of details. One crucial aspect of non-invasive geophysical method is that they are based on the solution of an inverse problem aimed at estimating soil parameters. Combined use of different geophysical datasets can provide more robust approaches, increasing the reliability of the final reconstructions. Several applications are reported in this respect with particular focus on surface wave methods, which are getting more and more popular because of their cost and time effectiveness
Il modello geotecnico per le analisi sismiche: affidabilità delle tecniche sperimentali in sito
No full textL’attendibilità del modello geotecnico dipende dall’accuratezza e dall’affidabilità delle prove sperimentali utilizzate per valutarne i parametri di riferimento. La velocità di propagazione delle onde di taglio, direttamente legata al modulo di taglio a piccole deformazioni, rappresenta un parametro fondamentale per le analisi sismiche. La presente memoria discute l’affidabilità delle tecniche geofisiche in sito correntemente utilizzate per la sua valutazione, evidenziando le incertezze epistemiche e sperimentali associate a ciascun metodo. In particolare vengono commentate alcune recenti esperienze su siti campione in cui le misure sono state ripetute utilizzando diverse tecniche e/o multiple realizzazioni della stessa tecnica. Le incertezze nella determinazione dei parametri vanno debitamente tenute in conto valutando le implicazioni sulla specifica applicazione. A titolo di esempio, nel presente lavoro si fa riferimento allo studio della risposta sismica locale, quantificando le differenze riguardanti i modelli ottenuti con ripetute realizzazioni della stessa prova (confronto intra-metodo) e/o con l’utilizzo di diverse metodologie (confronto inter-metodo). I risultati mostrano che un’attenta e scrupolosa esecuzione delle prove porta a risultati consistenti, con margini d’incertezza che possono essere ritenuti accettabili
Multistation Methods for Geotechnical Characterization using Surface Waves
Full text linkThis dissertation deals with soil characterization methods based on surface wave propagation applied to geotechnical engineering purposes. This topic has gained much interest in the last decade because of the appealing possibilities given by non-invasive methods, which are at once very flexible and cost effective. An overview of the properties of Rayleigh waves in layered linear elastic and linear viscoelastic media is presented, together with their applications for site characterization, of whose the SASW (Spectral Analysis of Surface Waves) method is by far the most well-known in geotechnical engineering. The research has been mainly focused on the application of multistation methods, compared with the classical two-station approach typical of the SASW method. Results from both numerical simulations and experimental testing are reported to compare two-station and multistation methods and to clarify the advantages that can be obtained using the latter ones. In particular the research has been developed following two different directions: on the one hand the application of classical geophysical analysis tools (such as domain analysis and slant stack transform) to tests performed with impulsive sources. On the other one the possibility of obtaining from surface wave testing not only a stiffness profile, but also a damping ratio profile for the site. In this respect a new method for simultaneous measurements of Rayleigh dispersion and attenuation curves is proposed. Regarding the first topic, the necessity of a multistation approach to determine the experimental dispersion test is essentially related to the spatial variation of phase velocity. Analyses in the frequency-wavenumber domain and in the frequency-slowness domain are very powerful approaches, still there was a need of studying the effects of the change of scale from geophysical applications to geotechnical ones. Indeed because of the peculiar properties of Rayleigh waves, surface testing is strongly affected by the distance travelled by the analysed wave. The numerical simulations performed in the research show that the phase velocity obtained using multistation methods with a limited number of receivers close to the source is not a modal value as it is for geophysical applications, but an apparent phase velocity arising from modal superposition. The experimental tests showed the good performances of multistation methods when compared to the SASW method. In particular some drawbacks of the latter method, due essentially to its two-station nature, are avoided and the field-testing appears to be very promising for future applications. In particular the application of the frequency-wavenumber domain analysis can lead to much faster and more stable estimates of the experimental dispersion curve and the process is easily automated, with a great saving of time and less requirement for subjective decisions. Another important advantage is given by the stability with respect to a near field effects that lead to a better reconstruction of the dispersion curve for the low frequencies and hence to a deeper characterization. The necessity of a new method for the simultaneous determination of surface wave dispersion and attenuation curves is linked to the strong coupling existing between the two. Such coupling is extremely important for the subsequent inversion process, in a consistent method leading from the field measurements to the stiffness and damping profiles. The proposed method uses a new testing configuration, designed to measure the experimental transfer function. Successively a regression process of the complex quantity with the corresponding expression obtained modelling soil as a linear viscoelastic layered system leads to the experimental dispersion and attenuation curves. Some preliminary results are reported showing very encouraging results, also if a more extensively testing programme is required for the complete validation of the metho
Developments in surface wave testing for seismic site characterization
No full textThe paper provides an overview of surface wave methods for seismic site characterization. Advantages and disadvantages of the technique are highlighted with respect to other geophysical methods and specific aspects of different implementations are presented. The influence of higher propagation modes is discussed in the light of recently proposed inversion approaches. A crucial aspect is related to solution non-uniqueness: its consequences are considered together with possible mitigation strategies. With respect to the latter, as in any inverse method, the possibility of using a-priori information or additional experimental data increases the reliability of the solutio
Alcune considerazioni sulla considerazioni sulla classificazione sismica del sottosuolo
No full textMolte norme tecniche attualmente in vigore consentono di valutare in modo approssimato la risposta sismica di un terreno utilizzando un approccio deterministico semplificato, basato sulla definizione di categorie di sottosuolo. A ciascuna categoria vengono associati opportuni parametri che modificano gli spettri di risposta per tener conto dell’amplificazione stratigrafica del moto sismico. Cruciale importanza assume quindi la procedura di classificazione del sottosuolo adottata. Al fine di valutare le procedure correnti di classificazione, il presente studio riporta i risultati di un database di risposta sismica locale, realizzato tramite una procedura stocastica di scelta dei parametri di modello, implementando un’analisi elastica lineare equivalente 1D con il programma SHAKE91. È stato inoltre possibile un confronto tra sottosuoli che presentano inversione nel profilo di velocità e quelli con caratteristiche meccaniche sempre crescenti con la profondità. I risultati mostrano un’ampia dispersione dei fattori di amplificazione per ciascuna classe di sottosuolo con una ridotta rilevanza della presenza dell’inversione nei profili di velocità
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