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This paper investigates the effects of indigenous vegetation on the shear strength of loose pyroclastic soils of the Campania region (southern Italy); these soils are frequently affected by shallow landslides 1-2 m deep that experience static liquefaction during the post-failure stage. Perennial graminae grasses were seeded in a one-dimensional column 2 m high and filled by pyroclastic soils, allowing the root to grow under atmospheric conditions. A noninvasive sampling procedure was adopted to take the vegetated soil samples, in which the roots were in their natural geometrical distribution. For each rooted sample, the root biomass, RM, was measured and the root volume density, RVD, was calculated. Isotropic consolidated triaxial tests in both drained and undrained conditions were performed on the rooted specimens, as well as on bare specimens as a control. The obtained results showed that the roots generally provided an increment to the soil strength. In drained conditions a reduction in the volumetric deformation was observed, which, under undrained conditions, was reflected in a general reduction of the excess pore-water pressures with a possible inhibition of the static liquefaction occurrence. This study highlights the potential role of grass roots as bio-engineering practice for stabilizing shallow covers of pyroclastic soils. Key words: failure, vegetation, triaxial tests, shallow landslides, liquefaction. Cet article etudie les effets de la vegetation indigene sur la resistance au cisaillement des sols pyroclastiques meubles de la region de Campanie (Italie du sud); ces sols sont souvent affectes par des glissements de terrain peu profonds de 1 a 2 m de profondeur qui subissent une liquefaction statique au stade post-rupture. Des graminees vivaces ont ete semees dans une colonne unidimensionnelle de 2 m de haut et remplies par des sols pyroclastiques, permettant a la racine de pousser dans des conditions atmospheriques. Une procedure d'echantillonnage non invasive a ete adoptee pour prelever les echantillons de sol vegetalises, dans lesquels les racines etaient dans leur distribution geometrique naturelle. Pour chaque echantillon racinaire, on a mesure la biomasse racinaire, RM, et calcule la densite volumique racinaire, RVD. Des essais triaxiaux consolides isotropes dans des conditions drainees et non drainees ont ete effectues sur les echantillons racines, ainsi que sur des echantillons nus comme temoins. Les resultats obtenus ont montre que les racines augmentaient generalement la resistance du sol. En conditions drainees, on a observe une reduction de la deformation volumetrique qui, dans des conditions non drainees, s'est traduite par une reduction generale de la surpression de l'eau interstitielle et une inhibition possible de la liquefaction statique. Cette etude met en evidence le role potentiel de la base comme pratique de bio-ingenierie pour stabiliser les couvertures peu profondes des sols pyroclastiques. [Traduit par la Redaction] Mots-cles: defaillance, vegetation, essais triaxiaux, glissements de terrain peu profonds, liquefaction., Introduction Shallow flow-type landslides (Hungr et al. 2001) are widely recognized as catastrophic events. They generally occur on natural hillslopes covered by residual (Lim et al. 1996; Toll et al. [...]

Flow-type landslides are a major global hazard. They occur worldwide, and are responsible for a large number of casualties, significant structural damage to property and infrastructure, and economic losses. The features of debris avalanches are particularly important, as they involve open slopes and affect triangular source areas when initial slides turn into avalanches through further failures or eventual soil entrainment. In this paper, the propagation stage of debris avalanches is numerically modelled to provide information such as the propagation pattern of the mobilized material and its velocity, thickness, and run-out distance. The use of a 'depth-integrated' model has the following advantages: (i) it adequately accommodates the irregular topography of real slopes, which greatly affects the evolution of the propagation stage; and (ii) it is less time consuming than full three-dimensional approaches. The model is named 'GeoFlow_SPH' and has previously been applied to theoretical, experimental, and real case histories. The behaviour of debris avalanches is analysed with particular attention to the apical angle, one of the main features of this type of landslide, in relation to soil rheology, hillslope geometry, and the geometric aspect ratio of the triggering area. The role of bed entrainment is also investigated with reference to differences in steepness of the uppermost parts of open slopes. First, simplified benchmark slopes are analysed using both water-like materials (with negligible shear strength) and debris-type materials (saturated frictional soil). Next, the paper addresses three important case studies from the Campania region of southern Italy (Cervinara, Nocera Inferiore, and Sarno), where debris avalanches occur in pyroclastic soils that originated from the eruptive products of the Mount Vesuvius volcano. In all of the cases analysed, the effects of erosion rate are compared with those of simulated soil propagation height, run-out distance, and velocity. In a novel contribution to the existing research, the results obtained from analysis of both the benchmark slopes and the real case histories indicate that landslide propagation depends on the interplay of rheology and bed entrainment. In particular, increased erosion growth rates correspond to shorter run-out distances, lower velocities, and larger propagation depths. It is further shown that erosion depth increases with either friction angle or the consolidation coefficient of pore-water pressure; the latter reduces bed entrainment but does not significantly affect the apical angle of debris avalanches. Globally, the results are particularly satisfactory because they indicate that the GeoFlow_SPH model is a suitable tool for the analysis and forecasting of debris avalanches. Key words: landslide, modelling, numerical, smoothed particle hydrodynamics (SPH), entrainment. Les glissements de terrain de type ecoulement sont un danger global majeur. Ils se produisent a la grandeur de la planete, et sont responsables d'un grand nombre de pertes de vies humaines, de dommages structuraux significatifs a la propriete et aux infrastructures, et de pertes economiques. Les caracteristiques des avalanches de debris sont particulierement importantes, puisqu'elles impliquent des pentes ouvertes et affectent des zones de sources triangulaires lorsque les glissements initiaux deviennent des avalanches via d'autres ruptures ou l'entrainement du sol. Dans cet article, l'etape de propagation des avalanches de debris est modelisee numeriquement afin de fournir de l'information comme le patron de propagation du materiel mobilise et sa vitesse, epaisseur et distance d'ecoulement. L'utilisation d'un modele << profondeur integree ' offre les avantages suivants : (i) il represente adequatement la topographie irreguliere des pentes reelles, ce qui affecte grandement l'evolution de l'etape de propagation; et (ii) il est plus rapide que les approches tridimensionnelles completes. Le modele est nomme << GeoFlow_SPH >> et a ete utilise precedemment pour des cas theoriques, experimentaux et reels. Le comportement des avalanches de debris est analyse avec une attention particuliere a l'angle au sommet, une des principales caracteristique de ce type de glissement de terrain, en lien avec la rheologie du sol, la geometrie des pentes, et le rapport de longueur geometrique de la zone de declenchement. Le role de l'entrainement du lit est aussi etudie en reference aux differences d'inclinaison dans les parties superieures des pentes ouvertes. Premierement, des pentes temoins simplifiees sont analysees avec des materiaux semblables a l'eau (avec une resistance au cisaillement negligeable) et des materiaux de type debris (sol frictionnel sature). Ensuite, l'article discute trois etudes de cas importantes de la region de Campania au sud de l'Italie (Cervinara, Nocera Inferiore et Sarno), ou des avalanches de debris se produisent dans des sols pyroclastiques qui proviennent des produits d'eruption du volcan du Mont Vesuve. Dans tous les cas analyses, les effets du taux d'erosion sont compares aux taux simules de hauteur de propagation du sol, de distance d'ecoulement et de vitesse. En tant que nouvelle contribution a la recherche existante, les resultats obtenus de l'analyse des pentes temoins et des etudes de cas reels indiquent que la propagation des glissements de terrain depend de l'interaction de la rheologie et de l'entrainement du lit. Plus particulierement, l'augmentation des taux d'erosion correspond a des distances d'ecoulement plus courtes, des vitesses plus faibles et des profondeurs de propagation plus grandes. Il est aussi demontre que la profondeur d'erosion augmente avec soit l'angle de friction ou le coefficient de consolidation de la pression interstitielle; ce dernier reduit l'entrainement du lit mais n'affecte pas significativement l'angle au sommet des avalanches de debris. Globalement, les resultats sont particulierement satisfaisants puisqu'ils indiquent que le modele GeoFlow_SPH est un outil adequat pour l'analyse et la prediction des avalanches de debris. [Traduit par la Redaction] Mots-cles: glissement de terrain, modelisation, numerique, hydrodynamique des particules lissee (SPH), entrainement., Introduction Flow-type landslides (Hungr et al. 2001,2014) are a global hazard responsible for large numbers of casualties and widespread damage, as they travel long distances at extremely rapid velocities, and [...]

Active slow-moving landslides in clayey soils exhibit continuous movements generally controlled both in the accelerating and decelerating phases by the pore-water pressure regime that, in turn, is strictly correlated to the net rainfall regime. The paper stresses the importance of a reliable groundwater model to predict these types of movements. To this aim a procedure is proposed to define the transient groundwater regime in the slope on the basis of recorded rainfall and monitoring data; the model is then used to derive the time-dependent shear strength along the main slip surfaces. The displacements at selected points along the slip surface are computed using a phenomenological (i.e., empirical) relationship between the local factor of safety and the displacement rate at those points. The procedure is employed for the analysis of a well-documented case history: the Porta Cassia landslide (central Italy). DOI: 10.1061/(ASCE)GT.1943-5606.0000323 CE Database subject headings: Landslides; Groundwater models; Displacement. Author keywords: Active slow-moving landslides; Groundwater modeling; Landslides displacement scenarios.

The paper deals with the modeling of failure and postfailure stage of shallow landslides of the flow-type that often affect natural shallow deposits of colluvial, weathered, and pyroclastic origin. The failure stage is frequently associated to rainfall that directly infiltrates the slope surface and to spring from the underlying bedrock. The postfailure stage is characterized by the sudden acceleration of the failed mass. The geomechanical modeling of both stages, based on site conditions and soil mechanical behavior, represents a fundamental issue to properly assess the failure conditions and recognize the potential for long travel distances of the failed soil masses. To this aim, in this paper, the current literature on the failure and postfailure stages of the shallow landslides of the flow-type is first reviewed. Then, an approach for their geomechanical modeling is proposed and three different modeling alternatives are presented. These models are then used to analyze, at different scales, a relevant case study of Southern Italy (Sarno-Quindici event, May 4-5, 1998). Numerical analyses outline that both site conditions and hydraulic boundary conditions are among the key factors to evaluate the reliability of landslides of the flow-type. The potentialities and limitations of the available models are also evidenced as well as the perspectives related to the use of more advanced numerical models. DOI: 10.1061/(ASCE)GT.1943-5606.0000182 CE Database subject headings: Landslides; Rainfall; Infiltration; Bedrock; Numerical models. Author keywords: Shallow landslides; Flow-type; Rainfall infiltration; Spring; Bedrock; Modeling.

Abstract: Knowledge of the groundwater regime, which is generally necessary in studying the stability of slopes, is of fundamental importance in the analysis of landslides involving intensely weathered rocks, as [...]