1,721,013 research outputs found
Predicted and observed behaviour of pre-installed barriers for the mitigation of tunnelling effects
No full textTunnelling activities in densely urbanised areas are likely to interfere with existing structures, often requiring the adoption of techniques to mitigate the effects of the induced ground movements. If the tunnel passes to the side of an existing structure, embedded barriers pre-installed between the tunnel and the buildings can act as a passive intervention reducing the displacements that would occur in greenfield conditions. A reliable prediction of the displacement field induced in the presence and the absence of the barrier is, therefore, of primary importance for the design of such a mitigation measure. In this respect, this paper discusses the efficiency of a barrier made of a line of bored piles through the comparison of field monitoring data from a test site in Rome with the results of soil-structure interaction (SSI) FE analyses and simple empirical relationships. Three numerical procedures to simulate plane-strain tunnel excavation were adopted to back-analyse the settlement
trough induced at the ground surface by the first passing tunnel. The comparison between the observed and computed settlement profiles permitted to identify the most reliable procedure for 2D simulations of tunnelling
excavation which are usually adopted in routine analyses, thus providing some useful guidance for a rational
design of the intervention
Alcuni aspetti del comportamento di grandi dighe in terra omogenee
No full textIn questa nota si mostrano alcuni aspetti del comportamento sismico di dighe in terra omogenee. A partire da una diga esistente ben documentata è stato definito un modello di sbarramento opportunamente semplificato con il quale sono state eseguite analisi numeriche pseudo-statiche condotte a rottura e analisi dinamiche in tensioni efficaci. I risultati mostrano che la deformabilità del bedrock permette una parziale attenuazione dell’energia del sisma e quindi una riduzione degli spostamenti della diga rispetto al caso di bedrock infinitamente rigido. Al contrario, la componente verticale dell’azione sismica induce una sensibile amplificazione delle accelerazioni e un incremento degli spostamenti dello sbarramento, specialmente in condizioni non drenate
La risposta di grandi dighe in terra durante eventi sismici intensi
No full textIn questa nota vengono discussi alcuni aspetti del comportamento sismico di grandi dighe di terra omogenee. Nello specifico, vengono mostrati i risultati di analisi dinamiche eseguite su un modello numerico alle differenze finite, in condizioni di deformazione piana, di una diga esistente e ben documentata in letteratura: la diga sulla Marana Capacciotti. Le analisi dinamiche non lineari nel dominio del tempo sono state condotte applicando alla base del reticolo di calcolo le componenti orizzontale e verticale di quattro registrazioni reali. Gli input sismici selezionati sono caratterizzati dallo stesso contenuto energetico e da un periodo di ritorno molto elevato. Nelle analisi si è tenuto conto del possibile sviluppo di sovrappressioni interstiziali positive mediante il modello di Finn-Byrne.
Nella nota viene discusso l’effetto della presenza di pressioni interstiziali negative nella zona satura del corpo diga sui meccanismi deformativi e sui cedimenti permanenti della cresta, che possono essere considerati un indice di prestazione sismica della diga.Instability phenomena of large existing earth dams are one of the major sources of seismic vulnerability in Italy, as most of them were constructed in the absence of specific seismic regulations. Therefore, it is necessary to investigate the response of such earth structures when subjected to severe earthquake loading, estimating the safety conditions with respect to deformation patterns which can impair the water retention capability. However, earth dams are complex structures and their seismic behavior is strongly affected by the inertial forces that develop during the ground motion and change with time and space. Moreover, positive excess pore water pressure may build up in the saturated portion of the dam during cyclic loading, leading to a reduction of the shear strength of the earth dam and the foundation soil.
This paper focuses on the seismic performance of large earth dams. Specifically, a plane strain numerical model of an existing homogeneous earth dam was calibrated using the monitoring data taken during the construction and the impoundment phases. The seismic performance of the dam was evaluated through a series of nonlinear dynamic analyses carried out in the time-domain using four real seismic records applied at the base of the model. The selected input motions are characterised by the same energy content and by a very high return period. Both the horizontal and the vertical component of the seismic action were applied at the base of the grid. In calculations, cyclic excess pore water pressure build-up was accounted for using the Finn-Byrne model. The seismic response of the dam is discussed showing the effects of the pore water pressure distribution within the embankment on the deformation patterns of the dam. Two hypothesis were adopted for the pore pressure distribution in the portion of the dam located above the “free surface”, defined by the condition u = 0: in the first, negative values of pore water pressure were allowed to develop, while in the second, the condition u = 0 was maintained above the “free surface”. The results show that the differences in the deformation patters observed for the two schemes can be ascribed mainly to the different distribution of the available shear strength of the soil within the embankment. Anyway, despite the high return period of input motions considered in the analyses, this corresponding to a very severe seismic scenario, the maximum permanent settlement computed at the crest of the dam was always lower than the available freeboard, this indicating a satisfactory performance of the dam to intense earthquake loading
Comportamento sismico di opere in terra rinforzata
No full textNei metodi di progettazione basati sulla valutazione della prestazione sismica di opere in terra rinforzata si ammette che la resistenza del sistema possa essere localmente raggiunta e che, a seguito di ciò, si attivino meccanismi plastici con sviluppo di spostamenti permanenti. Assume particolare importanza, quindi, la ricerca di meccanismi intrinsecamente duttili, in cui l’opera accumuli spostamenti irreversibili senza perdere i requisiti di sicurezza o funzionalità richiesti. In questa nota vengono presentati alcuni risultati preliminari dello studio del comportamento sismico di opere in terra rinforzata. Nello specifico, lo studio è stato articolato in tre fasi. Nella prima fase sono state eseguite analisi con meccanismi plastici locali e globali predeterminati, seguendo il classico approccio pseudo-statico. Sono stati quindi individuati due schemi di riferimento che, a parità di coefficiente sismico critico, sono caratterizzati da meccanismi plastici diversi. Tali schemi sono stati studiati nella seconda fase mediante analisi numeriche pseudo-statiche, in cui i meccanismi plastici sono conseguenza del calcolo, e nella terza fase mediante analisi numeriche dinamiche, in cui i meccanismi plastici risultano dall’interazione dinamica tra il terreno e i livelli di rinforzo
Effect of soil improvement on ground movements induced by conventional tunnelling
Full text linkLine C of Rome underground will cross the city from southeast to northwest, with a total length of about 25 km, passing through the historical city centre. This is a difficult environment due to many archaeological finds and pre-existing buildings of great historical value. Along stretch T3 of the line two conventional tunnels connect the TBMs launching pit to the new San Giovanni station. They run for a length of 140 m at a depth of about 25 m and reach the station passing at a short distance from the ancient Aurelian Walls (3rd century CE). Excavation of these tunnels was carried out following a three-stage procedure: (i) excavation of two small diameter tunnels (D = 3 m) using a mini slurry shield machine; (ii) soil improvement via cement grouting using tubes à manchettes installed radially from the mini-tunnels; and (iii) conventional excavation of the two running tunnels in the improved soil. An extensive monitoring system was set to control ground movements induced throughout the excavation process. This paper presents the displacement measured at the ground surface during the construction activities, highlighting the effects induced by grouting. The effectiveness of a protective barrier, made by a line of piles, in reducing the movements induced by tunnelling in the Aurelian Walls is also assessed. A 2D FE back-analysis is finally presented, showing that a satisfactory description of the observed behaviour can only be obtained by simulating the volume changes induced by the cement grouting
An approach to evaluate the efficiency of compensation grouting
No full textCompensation grouting is often employed as a mitigation technique to reduce the settlements induced by tunnelling. The efficiency of compensation grouting, defined as the ratio of the volume of heave induced at ground surface to the volume of injected grout, is strongly dependent on grout properties, injection characteristics and soil properties. In clayey soils, site observations and laboratory tests show high values of compensation efficiency immediately after injection. However the efficiency can decrease with time if positive excess pore water pressures develop in the soil during the injection process. Conversely, in sandy soils, a loss of volume occurs due to pressure filtration of the grout during and soon after the injection process, which reduces the compensation efficiency. This paper describes an analytical model that can be used to evaluate the volume loss produced by pressure filtration of cement-bentonite grouts as a function of soil, grout and injection parameters. The magnitude of pressure filtration is evaluated as the grout efficiency, which is given by the ratio of the volumes of the final grout body to the injected grout. Results show that the grout efficiency increases with decreasing soil permeability and with increasing grout bentonite content. The result of a grout injection test on a silty soil is back-analysed using this approach in which the grout efficiency is first evaluated through the proposed filtration model and then used to calibrate a numerical simulation of the test. The good agreement between experimental data and computed results shows that prediction of the efficiency of compensation grouting can be improved using the proposed approach
Seismic behaviour of large earth dams: from site investigations to numerical modelling
No full textThis paper presents selected results from the study on the seismic behaviour of a large earth dam. The study relies on the availability of field and laboratory tests, as well as on monitoring data collected during the construction and impounding phases. Specifically, a two-dimensional numerical model was calibrated by comparing the settlements of the dam measured during the construction with the computed values. Iterative pseudo-static numerical analyses were carried out to investigate the plastic mechanisms forming under critical conditions. The seismic performance of the dam was then evaluated through a series of dynamic analyses in terms of effective stress. The results evidenced that a significant reduction of the seismic energy is obtained if the deformability of the bedrock is accounted for in the analyses, resulting in lower permanent displacements of the dam body, and, conversely, the vertical component of the seismic action induces a sensible increase of the seismic displacements
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