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Dynamic properties of earth-core Italian dams from field and laboratory tests
Full text linkThe seismic design of new earth-core dams as well as the seismic re-assessment of existing ones with advanced numerical simulations require the knowledge of dynamic properties of core materials in a wide range of shear strains. The key parameters to be determined are the shear wave velocity Vs (or the maximum shear modulus G0) and the normalized modulus reduction and damping ratio curves (G/G0-c and D-c). Indeed, limited data do exist in the scientific literature on the in-field measured Vs profiles and on the laboratory cyclic/dynamic tests on undisturbed core samples. In this paper the dynamic properties of core materials of six zoned Italian dams are described and the main peculiarities are discussed, also in the light of the dynamic behaviour of natural soils
Considerazioni sulla scelta della magnitudo nelle analisi di liquefazione con metodi semplificati
No full textCome è noto, il fenomeno della liquefazione è governato dalla generazione e
dall’accumulo di eccessi di pressione interstiziale, in occasione di un terremoto, in sabbie sature
sciolte. I dati sperimentali mettono in evidenza che l’incremento della pressione interstiziale
avviene in risposta alle sollecitazioni cicliche tangenziali a cui un elemento di volume di terreno
è sottoposto in condizioni non drenate durante lo scuotimento sismico. Per tale elemento di
volume, il valore finale della pressione interstiziale è funzione delle caratteristiche del moto
sismico, cioè ampiezza, contenuto in frequenza e numero di cicli, ovvero durata.
All’inizio degli anni ’70, quando sono state sviluppate le prime procedure semplificate per la
verifica a liquefazione di un sito (Seed e Idriss, 1971), la durata del moto sismico è stata correlata
alla magnitudo. Tale approccio all’epoca rappresentava la scelta più razionale in quanto i casi di
studio in cui era avvenuta liquefazione non erano generalmente corredati di informazioni sulla
durata del moto sismico; inoltre il moto sismico era comunemente caratterizzato da un singolo
scenario, ovvero da un’unica coppia magnitudo-distanza. Con questo approccio, quindi, la
magnitudo rappresentava in maniera univoca un proxy della durata del moto sismico.
Attualmente, a seguito della disposizioni normative in materia antisismica (Ministero delle
Infrastrutture e Trasporti, 2008) che hanno assimilato i più recenti studi per la redazione della
mappa di pericolosità del territorio nazionale secondo un approccio probabilistico (http://www.
mi.ingv.it/pericolosita-sismica), l’uso della magnitudo come proxy della durata è certamente
più problematico. Infatti, un’analisi probabilistica tiene conto di tutti i possibili contributi che
derivano dalle diverse sorgenti sismogenetiche che possono influenzare il moto del suolo ad
un dato sito. Il valore dell’accelerazione di picco amax (accelerazione massima in superficie),
relativo ad un certo tempo di ritorno TR, non è quindi associato a nessuna specifica magnitudo,
ma riflette i contributi di tutte le possibili magnitudo legate alle diverse sorgenti sismogenetiche
considerate nell’analisi di pericolosità. In altre parole, con un approccio probabilistico si perde
il link diretto esistente tra magnitudo e amax.
Tuttavia, per un’analisi a liquefazione convenzionale con i metodi semplificati, il progettista
deve selezionare una singolo valore di magnitudo. L’approccio prevalente nella pratica
ingegneristica è quello di scegliere la magnitudo che si ritiene rappresentativa dello scenario
sismico considerato. Allo stato attuale, comunque, non esistono criteri condivisi per la scelta di
tale valore e generalmente si opta per quello più cautelativo. Nel seguito, dopo aver brevemente richiamato i principali aspetti della verifica a liquefazione con metodi semplificati, si descrivono alcuni metodi utilizzati in letteratura, per quanto
estremamente limitata sull’argomento (p. es. Finn e Wightman, 2007, Lanzo et al., 2014), per la definizione della magnitudo nelle analisi suddette. Infine si illustra un esempio applicativo relativo ad un sito ideale potenzialmente liquefacibile ubicato in Italia Centrale
Effects of ground motion characteristics on seismic response of earth dams: some remarks on duration and vertical shaking
No full textAcceleration records used as input motion for nonlinear dynamic analyses of earth
dams can significantly affect the outcome of the analyses. The selection of an adequate set
of records is therefore an essential step of the study. Customary approaches rely on
matching the target spectrum and the average response spectrum of the selected records.
Furthermore, vertical motion is often considered to have a modest influence on dam
response. In this paper, FLAC was used to conduct dynamic analyses of an earth dam
located in Central Italy. The response was assessed in terms of permanent crest settlements
and correlation were attempted with several IMs. The analyses were conducted with and
without vertical component of motions. It was found that Arias Intensity may be considered
an additional parameter to guide selection of input motions. The inclusion of the vertical
components lead to a general increase, on average 75%, of the crest settlement
Laboratory investigation on nonlinear dynamic properties of core materials of Italian dams
Full text linkThis paper presents the results of a series of laboratory investigations into the
dynamic properties of core materials of Italian earth-core rockfill dams. All tests have been
performed on specimens of undisturbed samples reconsolidated in a wide range of effective
confining stresses. Most of the data were obtained from cyclic simple shear tests and few data
from resonant column tests. It is shown that the shifting of the G/G0- c and D- c curves with
plastic index and effective confining stress is not as significant as it is well established for
natural fine-grained soils. Generic literature curves do not predict properly the dynamic
behaviour of the core materials, especially in the small-to-medium strain range. The
importance of conducting site-specific measurements in order to accurately model the
behaviour of core materials for dynamic analyses of embankment dams is therefore
highlighted
Studi di geotecnica sismica relativi alla progettazione della SS 79 direttrice Terni-Rieti
No full textIn questa nota sono illustrati gli studi di geotecnica sismica effettuati nell’ambito della progettazione in fase definitiva
della S.S. 79 Direttrice Roma – Terni. Questa infrastruttura, pur avendo una lunghezza limitata, ricade in un’area
caratterizzata da significativa pericolosità sismica locale. Le specifiche condizioni geomorfologiche, e in particolare la
presenza di una valle riempita di terreni lacustri-palustri di scadenti proprietà meccaniche, rappresentano condizioni
favorevoli allo sviluppo di fenomeni di amplificazione locale e di liquefazione. Le indagini eseguite hanno consentito la
definizione di un attendibile modello geologico e di un adeguato modello geotecnico di sottosuolo. La valutazione dei
fenomeni di amplificazione sismica è stata effettuata mediante analisi bidimensionali di risposta sismica locale condotte
con il codice di calcolo FLAC mentre le verifiche alla liquefazione sono state condotte con metodi dinamici semplificati
e con metodi dinamici completi. I risultati di questi studi condizionano sia l’azione sismica da assumere per la
progettazione delle opere sia l’adozione di eventuali provvedimenti per contrastare l’occorrenza della liquefazione
Effects of uncertainties and soil heterogeneity on the seismic response of archaeological areas. A case study
No full textThe paper deals with a numerical study aimed at the evaluation of the seismic response of the Central Archaeological Area of Rome including Palatine hill, Roman Forum, and Coliseum. A relevant role is played by buried morphology and mechanical properties of the anthropogenic cover which reaches a thickness of 20 m. This layer is constituted by "dominant masonry" and "dominant infill" zones strongly variable in the space thus exhibiting wide spatial heterogeneity in dynamic properties whose definition is affected by high degree of uncertainty. In order to investigate the effects of the uncertainty in dynamic soil properties of the anthropogenic unit on ground motion at surface, 1D and 2D stochastic analyses were carried out. The numerical results were then compared with those obtained from a standard deterministic model defined by using "average" values of dynamic properties. The influence of uncertainty in shear wave velocity distribution as well as in nonlinear properties (i.e. normalized shear modulus and damping curves) of the anthropogenic cover is highlighted in terms of Housner Intensity amplification factors profiles and acceleration response spectra in representative nodes at surface
Laboratory investigation on nonlinear dynamic properties of core materials of Italian dams
No full textThis paper presents the results of a series of laboratory investigations into the dynamic properties of core materials of Italian earth-core rockfill dams. All tests have been performed on specimens of undisturbed samples reconsolidated in a wide range of effective confining stresses. Most of the data were obtained from cyclic simple shear tests and few data from resonant column tests. It is shown that the shifting of the G/G0-c and D-c curves with plastic index and effective confining stress is not as significant as it is well established for natural fine-grained soils. Generic literature curves do not predict properly the dynamic behaviour of the core materials, especially in the small-to-medium strain range. The importance of conducting site-specific measurements in order to accurately model the behaviour of core materials for dynamic analyses of embankment dams is therefore highlighted
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Strong ground motion characteristics of 2016 central italy earthquakes and implications for ground motion modeling
No full textThe 2016 Central Italy earthquake sequence produced three mainshocks: (1) M6.1 24 August, (2) M5.9 26 October, and (3) M6.5 30 October. Each mainshock was followed by many aftershocks, some of which with M > 5.0. All earthquake events occurred on southeast-northwest trending normal faults. As part of reconnaissance activities of these events performed by the Geotechnical Extreme Events Reconnaissance Association (GEER), ground motion data was processed and analyzed. After processing all data using procedures developed during the latest Next Generation Attenuation (NGA-West2) project, we analyze strong motion characteristics of all three mainshocks, two selected large aftershocks (M5.3 24 August and M4.8 26 August) and a foreshock (M5.4 26 October). Our analysis shows that stations near the hanging wall, exhibit fling-step in some cases but no obvious rupture directivity effects. We compare ground motion intensity measures (including peak ground acceleration and velocity, PGA and PGV, respectively) to Italy-specific and global ground motion models. Overall, the data exhibit fast attenuation at large distance (>100 km), which is captured by Italy-adjusted global models, but not by Italy-specific models. We also found that global models tend to over-predict ground motions at short periods. Both features were also observed from the 2009 L’Aquila earthquake data and may represent regional features. We estimate the spatial distribution of PGA for the three mainshocks by means of a Kriging analysis performed on within-event residuals using a global semi-variogram model. We found that the ground motion is most intense south-west of the Mt.Vettore-Mt.Bove normal fault. Given the importance of Italian normal fault earthquakes in worldwide ground motion databases, this data set is of global significance for studies of normal fault ground motions
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