An experimental study of the behaviour of two rockfills accounting for the effects of degree of saturation

Rockfill dams have become more and more recognized for their safety, economy and adaptability to widely varying site conditions. As a contribution to the understanding of the main factors affecting the rockfill behaviour, the paper reports and discusses experimental data on several aspects relevant to the interpretation and analysis of their in-situ response. The experimental programme involved three series of oedometric tests on specimens of two different gravels having the same grading, reconstituted at the same initial relative density. Experimental observations on rockfills compressibility are presented and discussed: attention is paid to the role of degree of saturation (Sr) through the analysis of "driest", "fully saturated"conditions, and the transition from one to the other. Grain crushing tests on dry and saturated soil particles are also reported. Grain size distributions of the specimens, both after compaction and after the oedometer tests, are compared in the paper. The results show that the effect of Sr cannot be overlooked in the mechanical characterization of the material, especially in rockfill/stress conditions prone to crushin

Experimental and numerical investigations of a river embankment model under transient seepage conditions

The evaluation of riverbank stability often represents an underrated problem in engineering practice, but is also a topical geotechnical research issue. In fact, it is certainly true that soil water content and pore water pressure distributions in the riverbank materials vary with time, due to the changeable effects of hydrometric and climatic boundary conditions, strongly influencing the bank stability conditions. Nonetheless, the assessment of hydraulic and mechanical behavior of embankments are currently performed under the simplified hypothesis of steady\u2010state seepage, generally neglecting the unsaturated soil related issues. In this paper, a comprehensive procedure for properly defining the key aspects of the problem is presented and, in particular, the soil characterization in partially saturated conditions of a suitably compacted mixture of sand and finer material, typical of flood embankments of the main river Po tributaries (Italy), is reported. The laboratory results have then been considered for modelling the embankment performance under transient seepage and following a set of possible hydrometric peaks. The outcome of the present contribution may provide meaningful geotechnical insights, for practitioners and researchers, in the flood risk assessment of river embankments

Ccella triassiale con controllo dell’umidità relativa

Il comportamento meccanico dei rockfill dipende dal livello tensionale e dalla densità relativa, ma anche dalla rottura delle particelle, fenomeno caratteristico dei terreni a grana grossa. Come ampiamente riconosciuto in letteratura, la rottura dei grani è influenzata dalla composizione mineralogica, dall’assortimento granulometrico ed anche dall'umidità relativa (RH). Recentemente, presso il Laboratorio di Geotecnica del Dipartimento di Ingegneria Civile, Edile ed Ambientale dell’Università degli Studi di Napoli “Federico II”, è stata avviata una ricerca finalizzata proprio allo studio sperimentale del comportamento meccanico di rockfill in condizioni di parziale saturazione. Lo studio si avvale dell’impiego di una cella triassiale di grandi dimensioni modificata per imporre nel provino anche condizioni di parziale saturazione: il controllo dell’umidità relativa, e quindi della suzione totale, consiste nell’imporre il trasferimento di vapore acqueo mediante un sistema di convezione forzata, ed avviene per mezzo di soluzioni saline collocate all’interno di un recipiente esterno. La cella è stata anche modificata introducendo, per la misura delle deformazioni assiali e radiali, un sistema magnetico innovativo di rilevamento forma/volume denominato Magnetic Shape Detector (MSD), attualmente in fase di messa a punto

Effetto della suzione e tensione di confinamento sulla resistenza a taglio di una miscela sabbio-limosa costipata

La presente nota illustra sinteticamente le fasi di esecuzione e le modalità interpretative di prove triassiali consolidate drenate (TXCD) condotte su una miscela di sabbia e limo argilloso costipata in condizioni di parziale saturazione. I test a controllo di suzione sono stati eseguiti mediante un’apparecchiatura triassiale a doppia parete, il cui piedistallo è stato equipaggiato con una pietra porosa a elevato valore di ingresso d’aria, al fine di prevenire il fenomeno di cavitazione all’interno delle linee di drenaggio. Nel corso delle prove la suzione target è stata imposta attraverso la tecnica della traslazione degli assi e la variazione di volume dei provini stimata mediante un dispositivo cilindrico a doppia camera, in grado di correlare tali modificazioni alle letture di un sensore di spostamento induttivo (LVDT). I risultati della campagna di indagine, comparati ai corrispettivi ottenuti da prove effettuate in condizioni sature, evidenziano che la suzione contribuisca in maniera significativa alla resistenza a taglio dei terreni e che il suo effetto vari in maniera non lineare in funzione del grado di saturazione e della tensione di confinamento

Analysis of transient seepage through a river embankment by means of centrifuge modelling

Earthen river embankments are typically in unsaturated conditions during their lifetime and the degree of saturation within their bodies may vary significantly throughout the year, due to seasonalfluctuations of the river stage, as well as infiltrations of meteoric precipitation and evapotranspiration phenomena. Given the significant effects of partial saturation on the hydro-mechanical behaviour of soils, realistic assumptions on the actual water content distribution inside the embankments are essential forproperly modelling their response to hydraulic loadings. In this framework, centrifuge modelling is a useful tool to get insights into the evolution of saturation conditions of a water retaining structure during flood events. It allows for the direct observation of the groundwater flow process, which is hardly detectable at the prototype scale, enabling, at the same time, the validation and calibration of predictive numerical tools.In this paper, the results of a centrifuge test carried out on small-scale physical model of a compacted silty clayey sand embankment subjected to a simulated high-water event, at the enhanced gravity of 50-g, are presented and discussed. The physical model was carefully instrumented with potentiometers, miniaturized pore pressure transducers and tensiometers. Pore pressures and suctions measured during the experiment showed that the stationary flow conditions were reached only after an unrealistic hydrometric peak persistence. It therefore emerges that, for the design and/or the assessment of the safety conditions of a river embankment similar to the one tested, the simplified hypothesis of a steady-state seepage, in equilibrium with the maximum river stage expected could result, in many cases, an excessively conservative assumption

Magnetic measurement system of sandy gravel specimens shape during tests in a large triaxial apparatus

As well known, rockfill behaviour is strongly influenced by both intrinsic (mineralogy and size of particles, grain shapes, soil grading etc.) and state parameters (relative density, stress state, relative humidity). To investigate their mechanical response to stresses and relative humidity (RH) loadings, a large size triaxial device (H = 410 mm, D = 200 mm) has been developed at the University of Naples Federico II (Italy), including modifications required to impose partially saturated conditions in the specimen by means of the vapour equilibrium technique. In order to evaluate local axial and radial strains and global volumetric strains in partially saturated conditions, a magnetic shape detector device has been designed and installed. The accuracy of this system has been evaluated in some isotropic compression triaxial tests on compacted sandy-gravel specimens. The experimental data clearly show the effectiveness of the magnetic system in the measurement of axial displacements while the measurement in radial direction appear to be strongly affected by the non-linearity of the complex magnetic field generated during the test and requires further checking tests

On the Effect of Grading and Degree of Saturation on Rockfill Volumetric Deformation

The use of rockfill in civil engineering works such as earth and rockfill dams, road and railway embankments, increased along the last century especially for sustainability reasons. In order to ensure the stability of these structures under a set of conditions expected to occur during their life (grain crushing, changes in degree of saturation), the prediction of mechanical behavior of rockfill is fundamental for land management and protection. The mechanical behavior of rockfill mainly depends on stress level and relative density, as usual for granular soils. However, because of the large size of the particles, their mineralogical characteristics play a relevant role too, influencing grain crushing and therefore the overall behavior. Furthermore, it has been demonstrated in literature that for a given state (defined as the combination of stress state, void ratio and average number of grain to grain contacts) the effect of grains mineralogy on grain crushing (or microcrushing) largely depends on relative humidity (RH). The paper presents the results of triaxial tests performed on gravel specimens in which different levels of relative humidity are explored in order to analyze the “driest” and “fully saturated” conditions, and the transition from one to the other. Grain size distributions of the specimens, both after compaction and after the triaxial tests, are also compared in the paper

Sistema magnetico di rilevamento forma/volume in cella triassiche di grandi dimensioni

Il comportamento dei materiali granulari è fortemente influenzato sia da parametri intrinseci (mineralogia e dimensione delle particelle, forma dei grani, assortimento granulometrico, …) che da parametri di stato (densità relativa, stato tensionale, umidità relativa, …). Presso l’Università degli Studi di Napoli Federico II un’apparecchiatura triassiale di grandi dimensioni (D = 200 mm, H = 410 mm) è stata upgradata con l’obiettivo di studiare la risposta meccanica di terreni a grana grossa. Nell’ambito delle operazioni di potenziamento è stato progettato ed installato un dispositivo magnetico di rilevamento forma/volume, concepito per la misura delle deformazioni assiali e radiali locali nonché delle deformazioni volumetriche globali. L'accuratezza di questo sistema è stata valutata in alcune prove triassiali su campioni di ghiaia debolmente sabbiosa compattati. I dati sperimentali mostrano chiaramente l'efficacia del sistema magnetico nella misurazione degli spostamenti assiali mentre la misurazione in direzione radiale sembra essere fortemente influenzata dalla non linearità del campo magnetico generato durante il test e richiede ulteriori verifiche