1,721,067 research outputs found
Seismic microzonation by means of finite element approaches
No full textWhen an earthquake occurs, seismic waves radiate away from the source and travel rapidly through the earth's crust. The motion recorded at the ground surface of an area could be really different, in terms of duration and frequency content, from the reference outcrop motion due to the following site conditions: sequence of soil layers, velocity contrast between soil layers, thickness of each layer, dynamic behaviour of the soils, topography and geometry of the sub-interface.
Concerning the site-effects, it is possible to discuss about one-dimensional (1D) effects, two-dimensional (2D) and three-dimensional (3D) ones. One-dimensional ef-fects are induced in case of horizontally layered deposits with a horizontal ground surface (vertically heterogeneous media). Two-dimensional site-effects are generated in case of a complex stratigraphic sequence (vertically and laterally heterogeneous media) and/or in case of an uneven ground surface. In presence of stratigraphic and topographical surface varying in any direction (vertically, laterally and transversally heterogeneous media), it is necessary to refer to three-dimensional site-effects.
Seismic microzonation (SM) studies are used to assess local geological and ge-otechnical site conditions and to identify earthquake characteristics. A SM study can be undertaken according to three different levels of details, as reported in the Italian guidelines. In particular, numerical analyses are requested for a level III SM to quantify the reference motion modification.
This work aims to evaluate complex site-effects for a real case study, i.e. the Bo-vino village, located in South of Italy. This case study has been chosen due to the presence of a soft soil valley surrounded by rock outcrop hills. As a consequence complex site-effects are expected.
Essential ingredients for predicting site effects are: topography, stratigraphy, interface between soil layers and dynamic soil behaviour. Moreover it is necessary to select the reference seismic event and to define properly the input motion used in the numerical analyses.
The present thesis addresses how to perform a site response analysis using the finite element (FE) method. Two different FE codes, in the time domain, have been used: QUAKE/W, based on the equivalent-linear approach, and PLAXIS 3D, which adopts a non-linear Hardening Soil model with small strain stiffness (HSs).
At first, some numerical approaches to simulate 1D site response are defined with reference to ideal case studies. These approaches have been validated by compari-son with results obtained with the code EERA, which is based on an equivalent-linear approach in the frequency domain. 1D schemes have also been used to clarify the definition of reference motion and numerical input motion.
The numerical simulations of seismic site response for the Bovino case study are then presented. Before discussing the results of these analyses, local geology and topography are described. The geotechnical model is subsequently defined, based on field investigations and laboratory data. The reference outcrop motion is then selected, according to the Italian probabilistic seismic hazard maps. Seven real accelerograms have been selected as reference outcrop motions.
The results of the 2D analyses performed using QUAKE/W, with reference to 22 sections, are presented first. The results of the 2D analyses are compared with those of 1D analyses performed with reference to 42 soil columns, extracted along two sections.
Finally, results of 1D, 2D and 3D analysis performed with PLAXIS 3D, assuming two reference motions, are discussed. The analyses allowed to identify the effects of dimensional scheme to seismic site response, the dependency of each amplification pattern to the selected reference motion, the ground motion modification due to dif-ferent topography and soil layers interface, the comparison between different consti-tutive approaches to the same problem (i.e. linear equivalent and non-linear)
Influenza dell'anisotropia indotta dallo stato di sforzo sulla previsione dei cedimenti indotti dallo scavo di gallerie superficiali
No full textIn questa nota vengono presentati i risultati di analisi agli elementi finiti volte a prevedere i cedimenti indotti dallo scavo di gallerie superficiali in terreni a grana fine. In particolare, l’attenzione è stata rivolta a stabilire l’importanza dell’anisotropia indotta dallo stato di sforzo, adottando un modello costitutivo recentemente implementato dagli autori nel codice di calcolo Plaxis mediante un algoritmo implicito. Le analisi numeriche, contrariamente a quanto pubblicato recentemente da alcuni ricercatori (Einav e Puzrin, 2004), sembrano indicare un’influenza trascurabile dell’aspetto costitutivo considerato per cedimenti prodotti in corrispondenza di volumi persi che sono tipici dello scavo mediante macchine EPB o Slurry Shield
Effetto temporaneo e permanente di un sisma sul rivestimento di una galleria
No full textIn questa nota viene analizzato il comportamento trasversale di gallerie superficiali in terreni argillosi sotto azioni sismiche. A questo scopo sono messi a confronto due diversi approcci, uno basato su analisi della risposta sismica locale 1D e su soluzioni analitiche disponibili in letteratura (approccio disaccoppiato), il secondo su analisi dinamiche complete effettuate con un codice di calcolo non lineare agli Elementi Finiti (approccio accoppiato). Le differenti ipotesi costitutive prese in considerazione nell’approccio accoppiato hanno dimostrato di avere un ruolo rilevante nel calcolo delle sollecitazioni temporanee indotte dal sisma e hanno permesso di mettere in luce come nel caso di comportamento plastico del terreno il rivestimento della galleria risenta dopo la fine del sisma di azioni permanenti di non trascurabile entità
Modellazione numerica del comportamento sismico trasversale di gallerie superficiali in terreni argillosi
No full textIn questa nota viene analizzato il comportamento trasversale di gallerie superficiali in terreni argillosi sotto azioni sismiche. A questo scopo sono messi a confronto due diversi approcci. Nel primo, il problema in esame è affrontato in maniera disaccoppiata, effettuando inizialmente analisi mono-dimensionali di risposta sismica locale e utilizzando successivamente le caratteristiche del sisma calcolate in corrispondenza della profondità della galleria per valutare le sollecitazioni indotte sulla struttura attraverso soluzioni di tipo analitico. Nel secondo approccio lo studio del comportamento sismico della galleria viene condotto effettuando delle analisi dinamiche complete (approccio accoppiato) utilizzando, come ad esempio nel caso della presente nota, un codice di calcolo non lineare agli Elementi Finiti
Key Features of the Novel Geothermal Heat Exchanger Prototype Installed at the Brenner Base Tunnel
No full textThe design, installation, and testing of an innovative geothermal heat exchanger, tailored for tunnels
excavated by Tunnel Boring Machines, will be presented. The prototype was developed by the joint
efforts of BBT SE, involved in the construction of a new railway base tunnel system connecting Italy and
Austria, and the University of Bologna, engaged in applied research over various aspects of the BBT
system. The geothermal heat exchanger consists in a modular horizontal closed-loop system located in the
exploratory tunnel of the BBT system, specifically in the space dedicated to collect the drained water at the
lining invert. Due to the type of the heat exchange process, working with the drainage water, and for its
compact design and simple installation procedure, the prototype was called “Smart Flowing”. Modules were
built outside and later moved inside the tunnel, and eventually placed and assembled concurrently to the
advancement of the Tunnel Boring Machine. Specific tests were performed to prove the reliability and the
efficiency of the system, by simulating the work of a heat pump conditioning system in both heating and
cooling modes. Finally, a preliminary assessment of the economic and environmental potential of this
innovative prototype was carried out. First results showed the performance of the system for both heat
dissipation and extraction. The drainage water flow guarantees a continuous recovery to the natural state,
thus improving efficiency compared to classic geothermal heat exchangers. Economic savings and
reduction of pollutants and greenhouse gases, as compared to burning fossil fuels, can reach up to 70%
Assessment of the drill-and-blast related overbreak in the BBT system
No full textDrill-and-blast is the common conventional tunnelling method in hard rock conditions. This paper analyses the characteristics of the blasting-induced overbreak in some of the tunnels of the Brenner Base Tunnel (BBT) system, in order to evaluate its correlation with the main geometrical and technical parameters of the excavation as well as the geomechanical properties of the rock mass. In particular, starting from Terrestrial Laser Scanning measurements and direct overbreak observations, it was possible to distinguish so-called geological overbreaks (due to the pre-existing discontinuities in the rock mass) from technical ones (related to the appropriate performance of the excavation technique) on the basis of the after-blasting presence of half-casts. Data were used to develop a multiparametric regression model based on the RMR index and length of the blast holes, recognized respectively as the main factors influencing the two mentioned overbreak types. Model parameters were initially calibrated over a portion of the Mules access tunnel excavated in micaschists and subsequently validated against experimental evidence collected along tunnels of different geometries driven through granite and paragneiss rock-masses. The overbreak model proved to successfully predict the correlation between overbreaks (especially that fraction related to the geomechanical characteristics of the rock-mass), RMR and length of the blast holes. As such, its preliminary application can provide a reliable first-assessment of extra costs and time during the design stage
Integrazione implicita di un modello elasto-plastico con incrudimento misto isotropo-cinematico
No full textIn questa nota si propone l’integrazione implicita di un modello costitutivo a singola superficie dotato di incrudimento misto isotropo-cinematico per argille strutturate. Alla descrizione sintetica delle caratteristiche principali del modello segue la descrizione della strategia numerica di tipo implicito utilizzata per la sua integrazione. Il modello costitutivo è stato implementato in un codice FEM (ABAQUS) con cui sono stati realizzati alcuni esempi numerici che attestano l’accuratezza e la stabilità del metodo di integrazione usato
Interpretazione dei cedimenti indotti dallo scavo della nuova linea 5 della metropolitana di Milano
No full textLa nota fa riferimento alla costruzione della nuova Linea 5 della Metropolitana di Milano, attualmente in fase di completamento. Le due gallerie della linea, caratterizzate da un diametro di circa 6.7 m e da una profondità media dell’asse di 15 m dal piano campagna, sono state realizzate con macchine TBM-EPB in terreni alluvionali sabbioso-ghiaiosi parzialmente sotto falda. Nello studio sono presi in esame i cedimenti associati allo scavo della prima galleria della linea (galleria dispari o di destra) nella porzione iniziale della tratta in condizioni di campo libero. Tali misure sono state analizzate a posteriori, in direzione trasversale e longitudinale allo scavo, mediante classiche espressioni semi-empiriche note dalla letteratura. È stata esaminata, inoltre, l’influenza di differenti parametri di scavo, quali, ad esempio, la pressione di supporto al fronte e la pressione delle iniezioni di intasamento a tergo del rivestimento definitivo sull’entità della perdita di volume e sul profilo di subsidenza nelle sezioni monitorate. L’analisi condotta ha consentito di interpretare le misure di sito e di fornire una descrizione di dettaglio dello scavo con macchine EPB nelle specifiche condizioni geotecniche esaminate
Evapotranspiration estimate in a clayey slope affected by landslide phenomena
No full textThe evapotranspiration process is a key ingredient in the overall soil surface hydrological balance and therefore should be carefully evaluated in any geotechnical problem that is markedly affected by weather conditions, such as the slope stability. This issue was investigated with reference to the clayey slope underlying the town of Orvieto in Central Italy. The site has been monitored since more than 30 years with piezometers and inclinometers and a clear correspondence between movement characteristics and pore pressure variations, and indeed rainfall events, was observed at different locations and depths. In order to better estimate the rainfall quantity that effectively infiltrates in the slope, thus affecting the pore pressure regime, a study was conducted to evaluate the reference evapotranspiration ET0 at the investigated site. In order to assess the reliability of the temperature-based estimate of ET0 through the Penman-Monteith equation suggested by FAO, at a nearby meteorological station values estimated through the FAO procedure were compared to those obtained by a more refined estimate that utilizes measured meteorological data. In addition, the calculation of solar radiation was refined by introducing the effects of slope gradient and orientation as well as those of the shadowing produced by the rock cliff overlooking the clay slope. It was found that the temperature-based FAO estimate of ET0 is significantly larger than that calculated with a wide set of measured parameters. Finally, a repercussion of the different ET0 estimates on the infiltration process was investigated with reference to the water content available in the shallower part of the slope. Calculations, conducted through a simplified approach, indicated that in this specific application the difference in ET0 calculation plays a negligible role
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