The debris flow hazard in the Lagarelle Creek in the eastern Umbria region, central Italy

International audienceThis paper analyzes the Lagarelle Creek watershed, situated in the Municipality of Vallo di Nera, in the eastern Umbria region, central Italy. In this part of the Region, narrow valleys and very steep slopes characterize the morphology of the Appennine ridge. The presence of strongly-tectonized rocky masses, subdivided by several joint systems, is the main cause for the formation of sorted debris deposits, which accumulate mainly along the topographic convergences. This determines the conditions for possible events of debris flows. According to previous studies, the basin of the Lagarelle Creek, has been classified as an area prone to a high hazard of debris flows (Regione dell'Umbria ? C.N.R. I.R.P.I., 1996). For this reason, systematic studies have been carried out on the whole watershed which, in this first phase, have examined the definition of the geological and morphological features of the zone, by means of the acquisition of cartographies and of field surveys, and the elaboration of the topographical data of the basin, by means of a digital model of the terrain. Once the potential triggering areas of debris flows were identified, an assessment of the mobilizable volumes possibly involved in a debris flow event was carried out. To perform such an assessment both the geomorphologic method proposed by Hungr (Hungr et al., 1984) and the empirical relations calibrated on debris flow events of the alpine arc were applied. The results obtained were compared with those derived from information supplied by the inhabitants of the small mountain village, who have described in detail the most important events of the last century

A comparison of different approaches for the modelling of shallow foundations in seismic soil-structure interaction problems

In this work, the performance of two different macroelement models for shallow foundations on sands is assessed by considering the dynamic response of a RC bridge subject to earthquake loading. The first macroelement model is formulated within the framework of kinematic hardening elastoplasticity with prescribed bounding surface (Grange et al. 2009). The second macroelement model has been recently developed within the framework of the theory of hypoplasticity (Salciarini and Tamagnini 2009). The results of a series of FE simulations show that a significant reduction of the computed structural loads can be obtained by taking properly into account the foundation-soil behavior, rather than assuming zero displacements and rotations at the pier bases. The two macroelements considered provide quite similar results, in spite of the large differences existing in their mathematical formulation

La modellazione dei fenomeni di interazione terreno-struttura mediante macroelementi: elastoplasticità vs. ipoplasticità

International audienceIn this work two different macroelements for shallow foundations, recently developed in the framework of the theories of hardening plasticity and hypoplasticity, have been considered to evaluate the effects of the soil-structure interaction for a reinforced concrete viaduct subject to earthquake loading. To this aim, the predictions of the dynamic behaviour of the viaduct obtained with the two macroelements – in terms of forces and displacements – have been compared with those obtained for the case of rigid soil. Despite the differences existing in the mathematical structure of the constitutive equations of the two macroelements, the analyses have shown very similar results, both in terms of structural loads and in terms of computed displacements and rotations of the foundations. In the ideal case study considered in this work, the soil-structure interaction reduces significantly the structural load and the ductility demand of the piers. The example presented shows that the two macroelements adopted may represent a significant step forward in the implementation of simple, robust and accurate computational tools for the soil-structure interaction analysis, both for research and geotechnical design.Nel presente lavoro due diversi macroelementi per fondazioni superficiali, recentemente sviluppati nell’ambito delle teorie della plasticità incrudente e dell’ipoplasticità, sono impiegati per valutare gli effetti dell’interazione terreno-struttura per un viadotto in c.a. precompresso soggetto ad un evento sismico. A tale fine, le previsioni del comportamento dinamico del viadotto ottenute con i macroelementi – in termini di sollecitazioni e spostamenti – sono state confrontate con quelle ottenute nel caso di terreno di fondazione infinitamente rigido. Nonostante la struttura matematica delle equazioni costitutive implementate nei due macroelementi sia molto diversa, le simulazioni eseguite mostrano risultati molto simili, sia in termini di sollecitazioni negli elementi strutturali, che in termini di spostamenti orizzontali e rotazioni delle fondazioni. Nel caso esaminato, l’interazione terreno-struttura riduce in misura significativa le sollecitazioni nella struttura e la richiesta di duttilità nelle pile. L’esempio analizzato mostra che i due macroelementi presi in esame possono costituire un signi-ficativo passo in avanti nella messa a punto di strumenti di calcolo semplici, robusti ed accurati per l’analisi dei fenomenidi interazione terreno-struttura, non solo per scopi di ricerca ma anche a livello di progettazione ordinaria

Regional prediction of landslide hazard using probability analysis of intense rainfall in the Hoa Binh province, Vietnam.

The main objective of this study is to assess regional landslide hazards in the Hoa Binh province of Vietnam. A landslide inventory map was constructed from various sources with data mainly for a period of 21 years from 1990 to 2010. The historic inventory of these failures shows that rainfall is the main triggering factor in this region. The probability of the occurrence of episodes of rainfall and the rainfall threshold were deduced from records of rainfall for the aforementioned period. The rainfall threshold model was generated based on daily and cumulative values of antecedent rainfall of the landslide events. The result shows that 15-day antecedent rainfall gives the best fit for the existing landslides in the inventory. The rainfall threshold model was validated using the rainfall and landslide events that occurred in 2010 that were not considered in building the threshold model. The result was used for estimating temporal probability of a landslide to occur using a Poisson probability model. Prior to this work, five landslide susceptibility maps were constructed for the study area using support vector machines, logistic regression, evidential belief functions, Bayesian-regularized neural networks, and neuro-fuzzy models. These susceptibility maps provide information on the spatial prediction probability of landslide occurrence in the area. Finally, landslide hazard maps were generated by integrating the spatial and the temporal probability of landslide. A total of 15 specific landslide hazard maps were generated considering three time periods of 1, 3, and 5 years

Probabilistic vs. Deterministic Approach in Landslide Triggering Prediction at Large–scale

Reliability in the prediction of rainfall-induced shallow landslides at large scale has constituted a great challenge in the last decades. Different approaches have been adopted to include in the forecasts both the geometric, mechanical and climatic factors that affect the triggering phase of the process. A quite promising one is based on the probabilistic physically–based model implemented in the code PG TRIGRS, which takes into account the uncertainty in soil spatial variability and characterization. The model uses the Kriging technique to assess the spatial distribution of soil properties for the study areas, starting from available georeferenced measurements, alongwith their probability distribution functions. The Point Estimate Method (PEM) is then used to evaluate the Probability of Failure (PoF) within the study area, where PoF is defined as the probability that the Factor of Safety is less or equal than 1. This version is an extension of the original TRIGRS code, which combines a 1D hydrologic model with a stability analysis to assess the safety level of a given slope in a deterministic manner. In this work we compare the results provided by PG TRIGRS versus the original version of the code, by applying both of them to the same study area in Central Italy

3D Finite Element Modelling of Hydraulic Efficiency of Drainage Trenches

In this work the hydraulic efficiency of a parallel draining trenches system is evaluated by performing a series of pseudo-3D consolidation analyses using the finite element method. A parametric study has been conducted to investigate the influence of relevant aspects of the problem such as the slope inclination and trench length, which are not considered in currently available 2D solutions. Starting from a case study as a reference, an ideal slope has been considered, in silty, saturated soil. The evolution in space and time of the pore-water pressure and hydraulic head has been evaluated and, consequently, the evolution of the average hydraulic efficiency of the drainage system has been assessed, starting from the construction phase to the final steady-state conditions. The temporal evolution of the hydraulic efficiency has been assessed as a function of the trench inclination, depth, length and interspacing. Results have showed that the trench inclination and depth have a limited effect on the average value of the hydraulic efficiency, whereas the trench interspacing and length (the latter accountable only with a 3D model) have a remarkable effect on it