Analisi predittive del comportamento idro-meccanico di un modello di argine fluviale in condizioni di flusso transitorio

Il frequente manifestarsi di eventi alluvionali per collasso di opere in terra costituisce un problema di particolare rilevanza nella valutazione del rischio idrogeologico e della pianificazione territoriale. In questo contesto risulta cruciale un’accurata stima delle condizioni di stabilità che tenga conto dello stato di parziale saturazione dei terreni costituenti il rilevato. Altrettanto importante è tenere conto delle condizioni di flusso transitorio che si instaurano negli argini e nei terreni di fondazione a seguito delle continue variazioni del livello idrometrico dei corsi d’acqua. Tutto ciò influenza notevolmente la risposta idro-meccanica delle opere in parola. In tale prospettiva la nota illustra uno studio numerico, basato sull’approccio agli elementi finiti e sul metodo dell’Equilibrio Limite, per l’analisi del comportamento di un argine fluviale rappresentativo delle opere di difesa idraulica degli affluenti alpini e appenninici del fiume Po. Lo studio proposto tiene debito conto delle condizioni di parziale saturazione del corpo arginale interessato da un moto di filtrazione in regime transitorio. Le analisi numeriche proposte costituiscono parte integrante della progettazione di una prova in centrifuga geotecnica su un modello fisico in scala ridotta, finalizzata a investigare la vulnerabilità delle infrastrutture arginali nei confronti di eventi di piena

Analisi sperimentale per lo studio del comportamento idromeccanico di un terreno costipato

La valutazione delle condizioni di stabilità delle opere in terra rappresenta una difficoltà spesso sottovalutata nella pratica ingegneristica e, viceversa, di attualità della ricerca scientifica in geotecnica. Infatti, è certamente vero che il contenuto d’acqua dei terreni costituenti tali opere e la distribuzione della pressione interstiziale nelle strutture in terra variano nel tempo a causa dei mutevoli effetti delle condizioni idrometriche e climatiche al contorno, cosa che influenza significativamente le condizioni di stabilità dell’opera. In questo lavoro è presentata una procedura per la caratterizzazione del comportamento idromeccanico di una miscela di sabbia e limo al variare dell’energia di costipamento. I dati sperimentali vengono discussi al fine di contribuire allo studio del comportamento in esercizio delle opere in terra

Modellazione fisica in centrifuga di un argine fluviale soggetto a forzanti idrauliche

In questo lavoro sono riportati i risultati sperimentali di una prova in centrifuga su un modello di argine fluviale soggetto ad una progressiva sollecitazione idraulica. Le misure di pressioni neutre e di suzione nell’argine mostrano che il fronte di saturazione raggiunge il lato campagna attingendo la condizione stazionaria solo dopo una persistenza irrealistica dell'evento di piena simulato. Pertanto svolgere la progettazione o la valutazione delle condizioni di sicurezza di un argine fluviale con proprietà simili a quello testato nel presente lavoro, assumendo l'ipotesi semplificata di un regime stazionario di filtrazione potrebbe risultare in molti casi un approccio eccessivamente conservativo e, comunque, poco significativo

Mechanical behaviour of rockfill with different degrees of saturation

Coarse-grained soils are used in many civil engineering structures, such as land reclaiming, rockfills embankments and dams. In order to ensure an appropriate performance of these structures in working conditions, it is required to understand the main factors affecting the mechanical behaviour of granular materials. Looking for answers to these problems, the thesis, of a distinctly experimental nature, is aimed at the analysis of the main state variables influencing the stress-strain response of coarse-grained soils. After an in-depth examination of the scientific literature, the thesis describes the equipment and experimental procedures used, with emphasis on a "large" triaxial cell for specimens of an unconventional size (diameter equal to 200 mm and height of 410 mm). This prototype is capable to investigate on the behaviour of saturated and partially saturated granular materials, as well as their mechanical response on transition from unsaturated to the saturated state. The contribution of unsaturated soil mechanics is required for a proper understanding of the basic mechanisms of soil deformation that occur in a wide variety of engineering situations. In fact, the environmentally induced change in water content is generally the triggering mechanism of soil deformations in many situations involving unsaturated soils. This is a well-known phenomenon which has received a continuous interest ever since the widespread use of rockfill in dams for which rapid “collapse” settlements have been associated with water impoundment. The extensive experimental program presented starts from the description of the procedures used for the preparation of compacted specimens of two gravels with different mineralogy. The aim is to highlight how the characteristics of individual particles and the packing conditions affect the mechanical behaviour. For both materials, the results of mechanical tests obtained using grain size distributions different by uniformity coefficient and maximum particle size are presented and critically discussed. For each of the materials resulting from the different mineralogy and the grain size distribution, the influence of several stress and hydro-volumetric state variables on their mechanical response is analysed. This analysis leads to the following two main topics: i) the change of the grain size distribution due to particle breakage, and ii) the relationship between the stress-strain response and particle breakage. Concerning this second topic, a micromechanical analysis has been performed to justify the observed changes of the grain-size distribution due to variations in the stress paths followed during the tests. Some of the tests performed are aimed at assessing the effect of the degree of saturation and the transition from partial to total saturation on the stress-strain behaviour of the tested rockfills. Once the factors affecting rockfill behaviour have been identified, a model able to predict the influence of the relevant factors on the performance of rockfill materials has been introduced, with specific insights into some aspects such as the effects of particle breakage and degree of saturation on their behaviour

Soil water retention curves of a silty clayey sand compacted at different dry density

The study of the hydraulic behaviour of compacted soils in partially saturated conditions is essential for understanding the engineering performance of geotechnical structures such as river embankments. The paper presents the results of an experimental study focusing on the effects of the initial water content and dry density on soil water retention curve (SWRC) and volume change of a compacted soil mixture under hydraulic loading. For this purpose, soil specimens compacted at different water contents(dry, wet and optimum water content), chosen on the basis of standard Proctor compaction test, were used. Specimens of a mixture composed of 70% Ticino sand (TS) and 30% Pontida silt (PON) have been tested. This is a heterogeneous mixture of sand and silt, that usually constitute the embankments of tributaries of river Po (Italy). The SWRCs along drying paths were performed by means of evaporation tests starting from the saturated conditions gained in permeameter tests. For the investigation of the change in the void index during the evaporation test, the volume variation of the tested specimens was estimated by means of a calliper and photographic comparison. Changes in the SWRCs are consistent with changes in specimen initial condition while the soil volume change exhibited at the end of evaporation test is always negligible

Sviluppo di una cella triassiale per la caratterizzazione meccanica di un rockfill in condizioni di parziale saturazione

L'articolo descrive lo sviluppo di una cella triassiale per provini di diametro 100 mm o alternativamente 200 mm inizialmente concepita per lo studio del comportamento meccanico dei terreni a grana grossa in condizione di totale saturazione e poi potenziata per rendere possibile l’analisi del comportamento meccanico di rockfill parzialmente saturi. Il controllo dello stato di parziale saturazione avviene imponendo valori prescritti della suzione totale, come ben noto direttamente correlata all’umidità relativa dell’aria nei pori. La tecnica di controllo dell’umidità relativa consiste nell’imporre il trasferimento di vapore acqueo mediante un sistema di convezione forzata che, guidato da una pompa d’aria, trasporta l’umidità dell’aria contenuta in un recipiente al campione. Il controllo dell’umidità relativa avviene per mezzo di soluzioni saline collocate all’interno del recipiente stesso

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

Simplified Procedure to Identify the Critical State Line of Crushable Rockfills

Particle breakage plays a relevant role on the stress-strain behavior of rockfills. As is widely recognized, the amount of particle breakage of coarse-grained soils depends on a combination of several factors, namely grain size, mineralogy, the shape of the particles, stress level, and degree of saturation and its variation. The results of monotonic triaxial tests carried out on a large apparatus on specimens of two types of rockfills and three different gradings highlighted the role of the coefficient of uniformity, grain tensile strength, and loading path on particle breakage and overall soil behavior. Starting from experimental evidence, a simplified conceptual model to predict the level of break- age during a generic stress path was developed. Experimental data supported the hypothesis of the influence of particle breakage on the position of the critical state lines (CSLs) in the compression plane. This implies that CSL parameters are state dependent, changing with current grading modifications, and are not intrinsic. A simplified procedure for predicting the position of the CSLs as a function of current particle breakage was, therefore, developed and calibrated against laboratory test results

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-state 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

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