The Cimaganda rockslide (2012) : recent geomorphological evolution of the paleo-event

The San Giacomo Valley (Sondrio, Italy), as many alpine areas, is quite frequently affected by rock slope landslides at different scales, due to its geological and morphological features. Their interaction with anthropic activities represents one of the main natural risk along the Valley. The analysis and understanding of slope instability processes are thus so crucial to forecast landslide events, and to plan risk mitigation and civil protection actions. This work deals with the study of the Cimaganda rockslide, occurred in September 2012 after some days of persistent rainfall. It involved a rock volume of about 20.000 m3, blocking the main road (SS36) and isolating the municipality of Madesimo and Campodolcino in the upper Valley. The rockslide developed in an active geomorphological context, along the right flank of the historical Cimaganda landslide, dated at least to the 17th century with a volume involved valued in 7.5 million of m3. Following a procedural scheme including field surveys, remote sensing and geomechanical laboratory tests, this work develops an accurate characterization of the slope, that lead to a solid geological and geomechanical conceptual model extended to the slope directed involved in the 2012 event and to the surrounding area of the historical large landslide. Geological, geomorphological and geomechanical surveys allowed to recognize typical features of deep-seated gravitational deformations and largescale stress release: trenches and counter-slopes at the crown of the ancient landslide, sub-vertical tensile fracturing along the slope and shear planes mainly along the right flank. Using Finite Element Method (FEM) the conceptual model was built specifying the joint network orientation and the elasto-plastic properties of rock and joints. Hydraulics properties and anisotropy conductivities of rock mass, necessary to simulate hydrogeological flow, were calculated based on joint features of each discontinuity set. First, the numerical modelling, developed to simulate the slope scenarios before the landslide event, was able to reproduce a deformation pattern coherent with filed observations. Then, the introduction of a rainfall infiltration process, as triggering factor, with a semi-coupled hydro-mechanical analysis, allowed to simulate the evolution of the 2012 rockslide. This work represents a solid base to improve the analysis of the Cimaganda paleo-landslide and explore instability-forecasting scenarios in order to enhance rockslide risk management

Relazioni tra precipitazioni e sviluppo di falde sospese in versanti terrazzati

Il lavoro vuole contribuire alla comprensione delle dinamiche di infiltrazione e circolazione idrica, in un versante terrazzato con muretti a secco, per capire i meccanismi di formazione di falde sospese temporanee,i il loro rapporto con gli eventi di precipitazione e con la stabilit\ue0 dei muri. Sulla base di dati di precipitazione e misure piezometriche recenti \ue8 stata proposta una soglia pluviometrica di sviluppo di falde sospese e un\u2019analisi degli eventi meteorologici dal 1980 ad oggi. Si \ue8 poi costruito un modello numerico che riesce a riprodurre la formazione e lo sviluppo di falde sospese sul versante

Relazioni tra precipitazioni e sviluppo di falde sospese in versanti terrazzati

Il lavoro vuole contribuire alla comprensione delle dinamiche di infiltrazione e circolazione idrica, in un versante terrazzato con muretti a secco, per capire i meccanismi di formazione di falde sospese temporanee, il loro rapporto con gli eventi di precipitazione e con la stabilit\ue0 dei muri. Sulla base di dati di precipitazione e misure piezometriche recenti \ue8 stata proposta una soglia pluviometrica di sviluppo di falde sospese e un\u2019analisi degli eventi meteorologici dal 1980 ad oggi. Si \ue8 poi costruito un modello numerico che riesce a riprodurre la formazione e lo sviluppo di falde sospese sul versante

Rock Mass Rating spatial estimation by geostatistical analysis

This work aims to estimate the Rock Mass Rating of 200km2 area of the Italian Central Alps, along San Giacomo Valley (province of Sondrio). The regional geological setting is related to the Pennidic Nappe arrangement, which is characterized by the emplacement of sub-horizontal gneissic bodies, separated by meta-sedimentary cover units. The resulting RMR map can be a useful tool to forecast the quality of outcropping rock masses as well as to derive their geomechanical behaviour. Almost 100 geomechanical field surveys have been carried out in the research area, in order to characterize the outcropping rock masses; afterwards rock mass quality indexes have been evaluated in each surveyed site. In order to estimate the Rock Mass Rating values in un-sampled locations, different geostatistical techniques (kriging and simulations) have been applied, using both bi-dimensional and almost three-dimensional approaches. The validation process shows that kriging tends to produce smoothened distributions, while conditional simulations allow respecting local extreme values. Although geostatistical analysis reveals that geomechanical properties show spatial correlations, it is to remind that rock mass quality is strongly related to its geological and structural history

TOWARDS THE COMPREHENSION OF ROCKFALL MOTION, WITH THE AID OF IN SITU TESTS

Rockfalls frequently occur in Alpine areas, creating serious risks to population and buildings; the protection measures against rockfalls cannot be adequately designed unless the comprehensive understanding of rockfall phenomenon. Some experimental rockfall tests have been performed on a talus slope in Grosina Valley (northern Italy), with the aim to check the reliability of common simulation methods and to analyse the motion of falling blocks. First, a-priori kinematic simulations have been performed, and, after the rockfall tests, the results have been compared with the real stopping positions of blocks. Afterwards, the recorded trajectories of falling blocks have been analysed, allowing the calculation of the motion parameters of falling blocks. The motion of blocks was mainly characterized by rebounds, therefore particular attention has been paid to restitution coefficients, which describe the loss of energy during the impact and greatly affect the results of rockfall simulations. Although the talus slope features are quite constant, an unexpected wide range of restitution coefficients results from the movies: the variability is greater than that one of bibliography, moreover normal restitution coefficients are extremely high (they often overpass the unit). The qualitative relationships between restitution coefficients and slope features, falling block characteristics and pre-impact motion conditions have been searched and described

ROCKFALLS: IN SITU TEST, KINEMATIC SIMULATION AND MITIGATION MEASURES – SASSO FARINACCIO CASE STUDY

This research deals with the analysis of rockfall motion and the importance of kinematic simulations as well as in situ tests in designing of protection measures. The study area is a talus slope located in Northern Italy (Provence of Sondrio), where rockfalls frequently occur. The positions and dimensions of fallen blocks were measured and used in the calibration process, performed through the back-analysis approach. The calibrated values of restitution coefficients were compared with those obtained from in situ tests, which were carried out on the examined slope. Looking at the resulting rock fall hazard, the bidimensional kinematic simulations were performed in order to individuate the most suitable location and dimensions of mitigation measures

A fracturing state map for Chiavenna Valley: preliminary geostatistical analysis and optimal spatial sampling design

The interplay between tectonic processes and post-glacial debuttressing reflects in the actual fracturing state of the Chiavenna Valley (SO), which is the focus of this study. Fractures in rock masses condition water flow and stability, therefore knowledge of their pattern and properties is crucial to understand and predict the evolution of these processes. The overarching goal of the study is the creation of a fracturing map, at the slope scale, of the study area. Specific objectives are: i) to explore the interaction between meso-scale rock mass properties and understand the geomechanical and hydrogeological relationships between them; ii) to evaluate the available rock mass properties database and highlight its weakness according to the goal of the study; iii) to define a strategy to improve the database; and quantify its effectiveness. For each specific objective, the methods applied and the main results obtained are presented below. I) The geomechanical database available for the study area includes 132 survey points, with different levels of information regarding both primary variables (e.g. Jv, JRC, Aperture, Joint Compressive Strength, Weathering grade) and inferred variables (e.g. RMR, GSI and permeability). Correlation matrices between primary properties were calculated and a Principal Component Analysis performed. The Joint Volumetric Count (Jv), the index that summarizes better the general fracturing state of the rock mass, did not show any strong correlation with other primary variables. II) A tentative map of Jv was derived by means of Ordinary Kriging (following the approach of Ferrari et. al, 2014) following standard verifications of data non-clustering, normality, and casualty. The best result was obtained with a Gaussian model with a Lag of 1000 m and an anisotropy in the direction 35\ub0-215\ub0. However, the cross validation showed a high prediction standard error. This is mainly attributable to the distribution of the geomechanical field surveys locations, which are highly concentrated along roads (i.e. accessible areas). The result suggested the necessity of additional, spatially distributed field surveys. III) Thirty new sampling locations were selected with the Spatial Simulated Annealing model-based sampling algorithm (SSA) implemented in the R platform (Brus, 2018). The algorithm was specifically modified and adapted for the present study. The additional 30 geomechanical field surveys (both classical and with UAV) in the selected locations are now ongoing. The evaluation of the resulting variogram and Jv map improvement will follow, eventually exploring different geostatistical techniques

Geomechanical surveys and geostatistical analyses in Valchiavenna (Italian Central Alps)

The present study concerns the forecast of geomechanical features in rock masses and how mechanical properties can be regarded as regionalized variables. It considers an area of about 200 km2, extended in the Italian Central Alps along the Valchiavenna Valley (SO) - San Giacomo and Bregaglia valleys - where different civil and mining works are present. The regional geological setting is related to the Pennidic Nappe arrangement, characterized by the emplacement of sub-horizontal gneissic bodies resulting from the Mesoalpine isoclinalic folding of crystalline basements (\u201cTamb\uf2\u201d and \u201cSuretta\u201d Units) emplaced throw East and separated by metasedimentary cover units. More than one hundred detailed structural and geomechanical field surveys, mainly located in San Giacomo Valley, were carried out in order to characterize the rock masses, in accordance with the I.S.R.M. suggested methods. The procedure allowed to identify the number of joint sets and their average orientations, supplying a quantitative description of the discontinuities in terms of spacing, persistence, roughness, aperture, filling, wall strength and weathering. From the collected data the rock mass quality indexes have been evaluated in each surveyed site. Geostatistical methods have been applied to study the spatial distribution of the main rock masses characteristics, such as joint set orientation, fracture intensity and joint spacing, being the direct survey data local. Where no data are available the rock mass features have been estimated, the results obtained by kriging and conditional simulation techniques are presented

Hydro-geotechnical modelling of Como subsidence

The urban area of Como (Italy), due to its subsoil structure and historical evolution, is particularly susceptible to subsidence. Moreover, this phenomenon exposes the lakefront areas to an increasing risk of flooding. Following a procedural scheme including dataset creation and management, 2D and 3D conceptual model and coupled hydrogeological and geotechnical numerical simulations, the most critical zones and scenarios have been analyzed, assessing natural and anthropic groundwater perturbations as the major cause in triggering subsidence phenomenon. The main purpose of this work is to establish an effective tool in supporting groundwater management, forecasting subsidence phenomena and planning mitigation works