Numerical and Experimental Investigation into the Effect of Retention Properties of Partially Saturated Soil for the Stability Analysis of River Embankments

Stability assessment for river embankment represents a fundamental issue in safety assessment towards geo-hazard; several topical issues are involved with this study. Primary, unsaturated soil conditions are experienced during the entire lifetime of these structure; transient analysis are often required in regards to time-variability of external conditions; uncertainties related to soil hydraulic, mechanical and retention parameters are intrinsic for this problem, as well as those related to the knowledge of initial conditions. To overcome these problems, several simplified hypothesis are usually adopted both for the design and assessment of embankments. With the purpose to comprehensively deal with main part of the involved geotechnical issues, numerical and experimental investigations has been performed for the stability analysis of river embankments. The effect of retention properties of partially saturated soils on the hydraulic and stability conditions have been investigated, accounting for hydrometric water-level time-variability; numerical studies have been performed with increasing level of sophistication, using different mathematical methodologies for probabilistic numerical analysis. Various experimental studies and site monitoring data are presented, sharing the purpose of investigating retention properties for partially saturated soils. In its entirety, the final purpose of this study is to analyse the assessment process of riverbanks stability towards global instability mechanism triggered by variable hydraulic boundary conditions. Indications and suggestions, which can eventually be referenced for similar problems or implemented in the standard engineering practice, are provided; the use of site monitoring is justified to improve present flood early-warning system able to define in continuous progressive alert level combining measured and forecasted data with numerical probabilistic analysis results

Assessing River Embankment Stability Under Transient Seepage Conditions

Abstract The evaluation of riverbank stability is a fundamental problem in flood risk management, representing a critical task for engineering practice. Soil heterogeneity together with initial and boundary conditions are among the crucial issues that should be considered to obtain an accurate solution of the problem. Generally, attention and efforts are mostly devoted to the soil characterization, the hydrometric level forecasts and the estimation of the rainfall intensity, while in situ measurements usually receive less attention. Nevertheless, suction and soil water content have a strong influence on the reliability of seepage and stability analyses. A preliminary study aiming at the design of a monitoring system for the measurement of soil moisture and suction in the unsaturated silty soils of a river embankment has been carried out, with the purpose of linking the collected data to the boundary conditions and hence obtaining a more accurate estimate of the riverbank probability of failure. Furthermore, a general outline of the research project, its methodology and application are presented in the paper

A New Technique for Deep in situ Measurements of the Soil Water Retention Behaviour

In situ measurements of soil suction and water content in deep soil layers still represent an experimental challenge. Mostly developed within agriculture-related disciplines, field techniques for the identification of soil retention behaviour have been so far employed in the geotechnical context to monitor shallow landslides and seasonal volume changes beneath shallow foundations, within the most superficial ground strata. In this paper, a novel installation technique is presented, discussed and assessed, which allows extension of the use of commercially available low-cost and low-maintenance instruments to characterise deep soil layers. Multi-depth installations have been successfully carried out using two different sensor types to measure the soil suction and water content up to 7\u2009m from the soil surface. Preliminary laboratory investigations were also shown to provide a reasonable benchmark to the field data. The results of this study offer a convenient starting point to accommodate important geotechnical works such as river and road embankments in the traditional monitoring of unsaturated soil variables

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

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

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

On the role of partially saturated soil strength in the stability analysis of a river embankment under steady-state and transient seepage conditions

River flood risk is considered being one of the most costly hazards in Europe and under a further major potential impact of climate change, in combination with land-use changes and water management practices, flood risk is expected to increase for many river basins. In engineering practice, the design of river embankments is usually performed using simplified approaches, considering steady-state flow conditions induced by the retained water and almost neglecting issues related to partially saturated soils, thus leading to potentially heavily over conservative results of stability analyses. To provide a realistic assessment of river bank stability conditions and to get a more accurate prediction of flood risk it is necessary to consider river bank soil behaviour at different saturation degrees in connection with transient seepage flow. A numerical study on stability conditions of a specific river embankment focusing on the partially saturated soil strength contribution is presented herein. Seepage and stability analyses have been carried out using the information collected on river Secchia flooding case study, occurred north of the city of Modena (Italy) in January 2014. Limit equilibrium method has been adopted for assessing the overall stability in steady-state and transient flow conditions. Useful indications for accounting unsaturated soil strength in similar circumstances are finally provided in the paper

On the role of partially saturated soil strength in the stability analysis of a river embankment under steady-state and transient seepage conditions

River flood risk is considered being one of the most costly hazards in Europe and under a further major potential impact of climate change, in combination with land-use changes and water management practices, flood risk is expected to increase for many river basins. In engineering practice, the design of river embankments is usually performed using simplified approaches, considering steady-state flow conditions induced by the retained water and almost neglecting issues related to partially saturated soils, thus leading to potentially heavily over conservative results of stability analyses. To provide a realistic assessment of river bank stability conditions and to get a more accurate prediction of flood risk it is necessary to consider river bank soil behaviour at different saturation degrees in connection with transient seepage flow. A numerical study on stability conditions of a specific river embankment focusing on the partially saturated soil strength contribution is presented herein. Seepage and stability analyses have been carried out using the information collected on river Secchia flooding case study, occurred north of the city of Modena (Italy) in January 2014. Limit equilibrium method has been adopted for assessing the overall stability in steady-state and transient flow conditions. Useful indications for accounting unsaturated soil strength in similar circumstances are finally provided in the paper

Monitoring soil retention properties in a riverbank susceptible to fluvial erosion

One of the principal source of vulnerability for riverbanks is given by slopes instabilities, which is triggered on the riverside by fluvial erosion. In order to mitigate such erosion, the establishment of a dense herbaceous cover aims at promoting the slope protection and reducing the likelihood of embankment failure. In fact, the aerial parts of vegetation reduce the mechanical impact of river level fluctuations and rainfall on the embankment and retain sediment transported, while the belowground parts reinforce mechanically the materials forming the top of the embankment, facilitating drainage in the topmost layers and promoting plant water uptake, thus contributing to the regulation of the drying/wetting cycle. Plating deep-rooting perennial, herbaceous species on earth embankments therefore represent a sustainable, green intervention for the protection of a riverbank susceptible to fluvial erosion, contributing to the preservation of the fluvial ecosystem environment and avoiding a wide use of grey solutions. The European research project OPERANDUM is testing also this typology of NBS, with an experimental site selected on the river Panaro, one of the main tributary of the main Po River, Italy. To investigate the effect of vegetation on the riverbank soil, a monitoring system has been installed at shallow depths. The system estimates soil water content, matric suction and pore water pressure, in order to quantify the effects of the growth of different vegetation species, which have been recently seeded on site, for analyzing the plant-soil-atmosphere interaction. The work will present the site preparation and the system implementation. The analysis of the first collected data and the outcomes of the preliminary investigations, including site and laboratory experiments, will then be discussed. Monitoring data collected along the entire vegetation growth cycle, that is expected to take around two years, will allow to quantify the influence of vegetation in the soil-atmosphere interaction processes and, on the long-term, verify its effective contribution in riverbank protection

Metodologie di analisi per la valutazione della stabilità di argini fluviali in condizioni di flusso transitorio

Un’accurata stima delle condizioni di stabilità di argini fluviali rappresenta un problema di notevole interesse nell’ambito della gestione e pianificazione territoriale, con un diretto e significativo impatto nei processi di valutazione del rischio idrogeologico. Pur limitandosi ai meccanismi di collasso connessi all’insorgenza di fenomeni di instabilità globale, numerose sono le tematiche di interesse geotecnico coinvolte nella risoluzione di tale problema; fra queste, le condizioni di parziale saturazione dei terreni costituenti il rilevato arginale, aventi significativo impatto nella risposta idro-meccanica dell’opera in terra e presenti durante l’intera vita utile. Inoltre, le condizioni di flusso transitorio associate ai moti di filtrazione che si sviluppano nel terreno arginale e di fondazione risultano di non semplice descrizione, anche in relazione alla variabilità temporale delle condizioni al contorno (di tipo atmosferico ed idraulico) ed alle incertezze associate alla conoscenza sia dello stato di saturazione che delle proprietà idrauliche e di ritenzione del terreno. Queste, unite all’eterogeneità intrinseca dei suoli, implica necessariamente l’adozione di approcci probabilistici, complicando certamente la definizione di criteri sicurezza nei confronti dell’insorgenza di possibili meccanismi di collasso. Uno studio comprensivo delle varie problematiche associate alla valutazione della stabilità di argini fluviali in condizioni di flusso transitorio è stato condotto al fine di esplorare ed approfondire varie metodologie di analisi. I differenti approcci, nel seguito brevemente descritti, si distinguono in base al grado di approfondimento e di dettaglio dei modelli, all’onere computazionale e di calcolo richiesto ed alla tipologia di informazioni e dati di input a disposizione per la risoluzione del problema in esame. Nella presente nota, infine, varie indicazioni son fornite sulla base delle evidenze e dei risultati raccolti, con lo scopo di fornire un utile riferimento all’attuale pratica ingegneristica e di investigare nel dettaglio le varie problematiche geotecniche coinvolte