Ingegneria Civile e Ambientale

Nella presente nota si riporta lo studio dei fenomeni di liquefazione verificatisi nella fase cosismica del terremoto emiliano del 20 Maggio 2012. In particolare sono stati identificati fenomeni associabili a liquefazione tramite l'Interferometria Differenziale Radar ad Apertura Sintetica (DInSAR-Differential Interferometry Synthetic Aperture Radar) e si sono valutati i cedimenti indotti al piano campagna. A tale scopo, si sono utilizzate 4 immagini SAR COSMO SkyMed in banda X ad alta risoluzione nel periodo compreso tra il 1 Aprile ed il 6 Giugno 2012. All'interno del campo di deformazione cosismico di natura tettonica si sono individuate aree affette da subsidenza localizzata, probabilmente associata alla compattazione di strati sabbiosi per liquefazione. Lo studio geologicogeotecnico delle aree suddette ha confermato la presenza di uno strato sabbioso profondo potenzialmente liquefacibile. Il cedimento dovuto al liquefazione, calcolato mediante metodi empirici utilizzando i dati ottenuti da prove penetrometriche statiche, risulta in buon accordo con i valori osservati tramite telerilevamento, a meno di discrepanze legate alla variabilità locale dello strato sabbioso o alla sottostima dei cedimenti misurati a causa della decorrelazione del dato SAR.

The paper describes a numerical model implemented to simulate the construction of a shallow tunnels in a sandy soil with the technique of jet grouted canopies. For a number of reasons, including an uncommon difficulty in reproducing the whole geometrical arrangement of the tunnel, its construction sequence and the complex mechanical interaction among the different elements, the design of this constructive methodology is typically based on simplified mechanical schemes. On the contrary, a more complete study of the construction process, capable of reproducing with the highest possible accuracy the shape and position of the different elements, their installation and the characteristics of the different materials, is necessary to capture the nature of the complex phenomena taking place in the lining and in the surrounding soils and to predict the effects of tunneling operations. Bearing this goal in mind, a three dimensional model has been created: the construction sequence of the temporary and permanent lining systems and the excavation procedure have been derived from a case study reported in the literature; a hypoplastic model with intergranular strain concept has been adopted to describe the stress-strain response of soil from early stages of deformation; a linear elastic-perfectly plastic model with time dependent parameters has been adopted to simulate the aging of the cemented materials after installation. The results of calculation are reported in terms of settlements at the ground level, deformation of the soil and stresses in the lining.

L’articolo riporta l’analisi sismica prestazionale di una diga in rockfill con manto in conglomerato bituminoso situata nel sud Italia. Dopo una breve descrizione sui principi dell’analisi sismica prestazionale con riferimento alle odierne normative nazionali, sono stati definiti gli obiettivi prestazionali secondo quanto riportato nella letteratura. Per l’analisi sismica della diga è stato realizzato un modello numerico bidimensionale comprendente la roccia di fondazione ed il corpo diga, sollecitato con input sismici naturali spettrocompatibili, selezionati mediante un approccio deterministico. La risposta tenso-deformativa del materiale incoerente costituente il corpo diga è stata simulata con un modello isotropo viscoelastico perfettamente plastico, i cui parametri sono stati calibrati sulla base di prove triassiali monotoniche e cicliche. L’efficacia di tale modello è stata inizialmente testata simulando i risultati sperimentali ottenuti da prove in centrifuga realizzate su sez...

An overall analysis of single-fluid jet grouting is presented. Current concepts and design rules are first reviewed and compared. Alternative approaches to the analysis of single-fluid jet grouting are proposed and implemented with experimental data and observations taken from recent and well-documented case histories. In-situ observations and measurements as well as laboratory tests are presented, indicating the influence of both treatment parameters and original soil properties on treatment effects. On the basis of the experimental evidence, the mechanical phenomena induced by jet grouting are discussed.

The present work is focused on the investigation of physical and mechanical properties of fine grained soil treated with selected fly ash and lime in order to observe the effects that mixtures prepared with different component proportions induce on the observed behavior. The experimental investigation was carried out on Speswhite kaolin treated with fly ash coming from fluidized bed combustion and lime. pH measurements, sedimentation tests and oedometer tests were performed in order understand the mechanisms governing the physical and mechanical behavior of the treated soils.

Nella maggior parte dei problemi geotecnici si identificano i valori caratteristici delle diverse proprietà, cercando di tener conto della variabilità con criteri deterministici o probabilistici. In questa operazione si trascura spesso l’influenza della variabilità spaziale, ciò è vero anche per i terreni trattati con jet grouting. Per tale motivo è necessario definire un criterio che permetta di calcolare la resistenza caratteristica del terreno cementato tenendo conto dei diversi fattori di variabilità. A tal fine, sono stati analizzati, diversi dati sperimentali comprendenti un gran numero di prove di compressione monoassiale e la distribuzione della velocità delle onde di compressione, misurate tramite prove cross-hole e interpretate con la tecnica tomografica sonica, ottenuti sul campo prova di Polcevera. Questi dati sono stati poi elaborati con metodi di analisi geostatistica, per definire la variabilità della resistenza caratteristica e ricavare i valori di quest’ultima al va...

The paper focuses on the role of jet grouting on tunnel support, although the influence of supplementary techniques is also considered. For this purpose, experience gained from two well documented case histories is presented. Reported observations and experimental data have been selected in order to help in the identification of proper design limit states. Jet grouting has proven to be quite effective, allowing tunnelling in very difficult conditions, but some failures have also been recorded. Failure mechanisms can be classified according to three different modes: excavation face failure, canopy tip failure, provisional lining failure. With regard to the control of settlements, jet grouting has proven to be very effective but some ground uplift can be temporarily induced by soil treatment.

An experimental research has recently been performed for developing an original sediment management and reuse procedure, consisting in the following operations: dredging of the sediments from the seabed, pumping of the dredged soil to the shoreline, reduction of water content by desiccation, addition of lime for stabilisation, placement in the embankment. In the paper some results of the experimental laboratory work carried out to highlight the effectiveness of the procedure have been reported. The suitability of lime treatment has been investigated with particular reference to the high initial water content of the soil. The efficiency of the improvement has been investigated by mechanical testing after short term since lime addition.

Ground improvement has become one of the most effective tools of geotechnical engineering, being adopted for an always larger variety of civil engineering applications. To reduce the role of subjective choices of operators, the use of different techniques tends to be codified by specific guidelines. In the European Union there is an ongoing effort to standardize execution and design within codes continuously reviewed by designated committees. A widespread and systematic standardisation on the ground improvement as a mean to mitigate the effects of liquefaction on buildings and infrastructures is missing. The paper presents and overview of traditional and new ground improvement technologies suitable for this application. The methods are firstly classified by considering their effects on the ground (e.g. densification, stabilization, drainage, desaturation, etc.). Design principles are then outlined for new or pre-existing buildings and infrastructures, considering the ongoing review ...

In the attempt to codify a procedure exportable to other similar cases, a thorough investigation of the seismic performance of a bituminous concrete faced rockfill dam built in Italy in the early eighties is herein presented. The dam presents a 90 m tall embankment built in a narrow canyon and is situated in a highly seismic region. The implemented methodology encompasses the indications provided by the most recent literature to point out the problems potentially caused by earthquakes and to account for the paramount factors affecting the response of the dam. Particular attention has been paid to the concept of performance, defining its goals in accordance with the most recent standards and deriving the correspondent limit conditions from observations reported in the literature. In order to optimize the computational effort, dynamic analyses with two and three dimensional finite difference codes have been combined to study the coupled response of the embankment, rocky foundation and bituminous facing. After validating the numerical models with centrifuge tests performed on small scale models of the embankment, the performance of the dam has been investigated with reference to a number of possible scenarios focusing on the amplification spectra, the deformation of the embankment and the integrity of the bituminous lining.

The paper reports on the behaviour of a masonry dam built in the Italian Alps in the early twenties of the last century and still in operation for electric power supply. The dam is 31 m high and its waterproofing is ensured by a multi-layered impervious facing, a concrete cut-off and a grout curtain protruding into the foundation soils. The geological location of the dam is rather complex, because its left abutment is located on a rock formation while the right one rests on a thick moraine deposit. Since its first impounding, large downstream water flows and significant movements of the dam and of its moraine abutment were observed. A comprehensive investigation has thus been conducted in order to understand the overall behaviour of the reservoir, after eighty years of service. For this purpose, historical documents have been first reviewed in order to reconstruct the design assumptions, construction operations and early observations on site. Experimental investigations of the moraine deposit have then been conducted, in order to estimate the subsoil properties. Seepage flow rates and reservoir impoundment levels, recorded for more then twenty years, have also been analysed, showing the correlation existing between these two variables. More recently, a system for monitoring the displacements of the dam and the moraine has been implemented and the recorded data have been examined. All the available observations have been evaluated and a reasonable interpretation of the coupled hydraulicalmechanical behaviour of the dam and of its moraine abutment has been inferred. Numerical calculations have finally been conducted, in order to verify such explanation.

This paper illustrates the results of an experimental study on sand-bentonite mixtures for their use as confinement barriers for solid waste landfills. The mixtures have been prepared parametrically varying the percentage of bentonite. The sample preparation method was established willing to simulate the compaction processes on site. In fact, the compacted samples were tested following two different stress-wetting paths representative of the possible stress and imbibition sequences occurring on a landfill confinement barrier. In the first case, the barrier comes into contact with rainwater before being subjected to the overloading stress induced by waste disposal, while, in the second case, the barrier is overloaded by the waste before being wetted by the leachate. The compressibility and permeability of the sand-bentonite mixtures were determined, in both cases, by oedometric compression tests. The experimental results are analysed and compared in order to evaluate the influence of the bentonite content on the mechanical and hydraulic behaviour of the mixture. Interpretation of the results is also accomplished with a micro-mechanical investigation of the mixtures fabric. Suitable compositions of sand and bentonite are finally proposed for the design of effective confinement barriers. Keywords Bentonite compaction Á Bentonite swelling Á Bentonite hydraulic conductivity Á Compaction of sand-bentonite mixtures Á Hydraulic conductivity of sand-bentonite mixtures Á Oedometer test on bentonite Á Oedometer test on sandbentonite mixtures Á Sand-bentonite mixtures Á Swelling of sand-bentonite mixtures Á Waste landfills confinement & Erminio Salvatore