Dissipative behaviour of reinforced-earth retaining structures under severe ground motion

This paper focuses on the seismic performance of geosynthetic-reinforced retaining walls (GRWs) that several evidences have shown to be generally adequate. This can be attributed to the dissipation of energy produced by the internal plastic mechanisms activated during the seismic shaking, and to an overall ductile behaviour related to the large deformation that can be accommodated by the soil-reinforcement system. Using a number of numerical computations, this work compares the behaviour of three idealized structures that were conceived in order to have a similar seismic resistance, that however is activated through different plastic mechanisms. The analyses include numerical pseudo-static computations, carried out iteratively to failure, and time-domain nonlinear dynamic analyses, in which acceleration time-histories were applied to the bottom boundary of the same numerical models used for the pseudo-static analyses. The results of the dynamic analyses were interpreted in the light of the plastic mechanisms obtained with the pseudo-static procedure, confirming that GRWs develop local plastic mechanisms during strong motion resulting in a significant improvement of their seismic performance

Probabilistic Seismic Hazard Curves and Maps for Italian Slopes

The seismic performance of an earth slope is commonly evaluated through the permanent displacements developed at the end of an earthquake. In this paper a probabilistic approach is adopted to assess the displacement of the slope for a given hazard level using an updated database of ground motions rec-orded during the earthquakes occurred in Italy. The results are presented in terms of hazard curves, showing the annual rate of exceedance of permanent slope displacement evaluated using ground motion data provided by a standard probabilistic hazard analysis and a series of semi-empirical relationships linking the permanent displacements of slopes to one or more ground motion parame-ters. The probabilistic approach permits to take into account synthetically the characteristics of the slope through the yield seismic coefficient, the aleatory variability of the ground motions and the different subsoil classes of the record-ing stations. Finally, the procedure has been extended on a regional scale to produce seismic landslide hazard maps for Irpinia, one of the most seismically active regions in Italy. Seismic landslide hazard maps are very attractive for practitioners and government agencies for a screening level analysis to identify, monitor and minimise damages in zones that are potentially susceptible to earthquake-induced slope instability

Influence of the displacement predictive relationships on the probabilistic seismic analysis of slopes

Seismically induced landslides can often cause severe human and economic losses. Therefore, it is worth assessing the seismic performance of slopes through a reliable quantification of the permanent displacements induced by seismic loading. This paper presents a new semiempirical relationship linking the permanent earthquake-induced displacements of slopes to one or two synthetic ground motion parameters developed considering the Italian seismicity, and a comparison with existing simplified displacement models is illustrated. Once combined with a fully probabilistic approach, these relationships provide a useful tool for practicing engineers and national agencies for a preliminary estimate of the seismic performance of a slope. In this perspective, the predictive capability of different semiempirical relationships is analyzed with reference to the permanent displacements evaluated for the Italian seismicity assimilating the slope to a rigid body and adopting the Newmark integration approach. The consequences of the adoption of these relationships on the results of the probabilistic approach are illustrated in terms of displacement hazard curves and hazard maps for different slope scenarios

Evaluation of Seismic Landslide Hazard Based on a New Displacement Semi-empirical Relationship

This paper presents a new semi-empirical relationship that links the permanent earthquake-induced displacements of slopes to the synthetic ground motion parameter PGA or to the couple PGA and PGV. The displacements are evaluated under the hypothesis of a rigid sliding block performing Newmark’s integrations for all the acceleration time histories of the updated Italian seismic database. The relationship reproduces well the displacements for any values of yield seismic coefficient in the whole range of peak ground acceleration. The two parameters expression is more reliable for the study of Italian slopes under seismic loading than that based on the single PGA parameter as characterised by a lower standard deviation. The proposed relationship is also combined with a fully probabilistic approach to produce displacement hazard curves and hazard maps for different sites and regions of Italy that represent a useful tool for practicing engineers and national agencies for a preliminary estimate of the seismic performance of a slope

Una procedura per il dimensionamento di muri in terra rinforzata soggetti ad eventi sismici intensi

In questa nota si presenta una procedura per il dimensionamento di muri in terra rinforzata mediante il metodo pseudo-statico. In essa, il coefficiente sismico k è calibrato su prefissati livelli di prestazione del muro, espressi in termini di valori limite degli spostamenti permanenti accumulati durante l’evento sismico. Un’equivalenza di upper bound tra gli spostamenti indotti da sisma e i valori di k è stata ottenuta applicando il metodo di Newmark al database di accelerogrammi italiani, aggiornato alle registrazioni del 2017, e correggendo le relazioni empiriche ottenute per tenere conto della forma dei meccanismi plastici interni alla zona rinforzata, ai quali corrisponde la massima domanda di resistenza dei rinforzi. Fissata la prestazione sismica, il muro viene quindi dimensionato per ottenere un coefficiente sismico critico associato ai meccanismi interni (kcint) inferiore a quello associato ai meccanismi esterni (kcext), così da promuovere l’attivazione di meccanismi plastici che prevedano la mobilitazione della resistenza del sistema terreno-rinforzo, dotato di rinforzi caratterizzati da adeguati livelli di duttilità

Seismic design of geosynthetic-reinforced earth retaining walls following a pseudo-static approach

Geosynthetic-reinforced earth (GRE) retaining walls show a better performance than conventionally-designed walls during destructive earthquakes, due to their capability of redistributing seismic-induced deformations within the reinforced zone. In this paper, a recently-proposed method to design GRE walls is first recalled, where the wall is designed to trigger an internal plastic mechanism in the presence of strong earthquakes. Following a pseudo-static approach, the seismic coefficient k is therefore assumed equal to the internal seismic resistance of the wall kcint. The seismic coefficient is then calibrated against given seismic wall performance, expressed in terms of limit values of earthquake-induced displacements. Permanent displacements are evaluated through empirical relationships that were previously developed on the basis of a parametric integration of an updated Italian seismic database. Effectiveness of the proposed procedure is then demonstrated by assessing, through Finite Difference nonlinear dynamic analyses, the seismic performance of two walls, namely a GRE and a conventional gravity wall, characterised by the same seismic resistance but triggering an internal and external plastic mechanism, respectively. They are both subjected to a real strong motion, capable of activating a plastic mechanism. Results showed that lower permanent displacements are accumulated in the GRE wall where internal mechanisms are triggered

Screening-level analyses for the evaluation of the seismic performance of a zoned earth dam

Many existing earth dams have been designed and built worldwide before the establishment of a seismic code, so that it is of relevant interest to evaluate their seismic performance and post-seismic operational conditions. This requires an accurate geotechnical characterisation of the dam and foundation soils, a proper definition of the seismic scenarios at the site of the dam, the use of simplified procedures for screening-level seismic analyses and advanced nonlinear dynamic analyses to study the most critical seismic scenarios. This process has been used for the evaluation of the seismic performance of a zoned earth dam located in a high seismic hazard area of Southern Italy. In this paper the available data of historical seismicity at the site of the dam and the results of a probabilistic seismic hazard analysis are first discussed and input ground motions are selected using compatibility criteria with the energy and frequency content of the expected target motion in a range of vibration period relevant for the non-linear response of the dam. Seismic performance of the dam is then evaluated through procedures based on Newmarktype computations, in which permanent displacements are related to ground motion characteristics and to the seismic resistance of the dam, the latter evaluated detecting the earthquake-induced plastic mechanisms and the corresponding critical accelerations. Also, an original improvement of the well-known Makdisi & Seed procedure, was proposed to better capture the actual influence of non-linear soil behaviour in the evaluation of horizontal acceleration and permanent displacements of the crest of the dam. The analysis results pointed out the relevant role of the earthquake-induced shear strength reduction on the dam permanent displacements

The role of seismic intensity on the performance of caisson foundations supporting bridge piers: preliminary results from dynamic centrifuge

Seismic performance of caisson foundations supporting bridge piers may take advantage of soil inelastic response when subjected to strong seismic events, thanks to the soil nonlinear and hysteretic behaviour. This can bring to a substantial optimisation in caisson design and major cost savings. In the framework of Capacity Design extended to geotechnical systems, temporary attainment of plastic mechanisms may be permitted provided that the resulting permanent displacements are lower than given threshold values, which in turn depend on the considered limit state and performance level required to the structure. Clearly, this new design approach needs to be validated against physically-sound numerical and experimental simulations. A campaign of dynamic centrifuge tests was therefore recently carried out at the Schofield Centre, University of Cambridge, where the seismic performance of caisson foundations was assessed. In this paper, a preliminary interpretation of the experimental results is given, shedding some light on the interplay between seismic intensity and mechanical soil properties. Specifically, the results obtained in two tests are discussed, where a caisson-pierdeck system was subjected to earthquakes of increasing intensity. In the two tests, a soft and very soft clay layer was reproduced, to either avoid or promote the plastic soil behaviour. It is shown that the highly nonlinear and hysteretic response of the very soft clay limits the inertial forces transmitted to the superstructure, thus validating the above-mentioned design approach. The beneficial effect of inelastic soil behaviour entailed permanent displacements increasing with earthquake intensity, which should be checked against limit state prescriptions

Scavi profondi nei centri urbani. Una convivenza possibile

Il costante aumento della popolazione nelle grandi città richiede una sempre maggiore capacità di trasporto che, in aree densamente costruite, è spesso possibile solo grazie alla costruzione di infrastrutture in sotterraneo. È dunque cruciale ridurre al minimo gli spostamenti indotti dall’esecuzione di queste opere per proteggere le strutture e le infrastrutture preesistenti, specialmente in presenza di monumenti ed edifici storici di inestimabile valore. Ciò ha portato negli anni, da un lato, allo sviluppo di procedure di calcolo in grado di fornire previsioni attendibili degli effetti indotti dall’esecuzione di scavi profondi e gallerie e ha stimolato, dall’altro, la ricerca di soluzioni costruttive in grado di limitare tali spostamenti. Nella prima parte della relazione si richiamano i metodi semplificati, disponibili in letteratura per una previsione preliminare degli spostamenti indotti dalle opere in sotterraneo, e si discutono alcuni aspetti delle analisi numeriche di interazione terreno-struttura che, se portati in conto, permettono di migliorare la qualità delle previsioni; tra questi, l’effetto del peso e della rigidezza delle costruzioni esistenti, gli effetti di installazione di alcune lavorazioni, l’influenza dei tempi di costruzione e delle condizioni di drenaggio. La seconda parte della relazione è invece dedicata ad un esame delle soluzioni progettuali e degli interventi di mitigazione che è possibile utilizzare per una protezione efficace delle strutture e delle infrastrutture esistenti in prossimità delle aree di scavo