The Rock Mass Quality Index (RQI): a quantitative tool for the quality evaluation of near-surface rock masses

The knowledge of rock masses behaviour is an important information in various fields such as civil engineering, land use planning and hazard/risk zoning. Different rock mass classification methods, initially aimed at assisting underground excavations (Hoek, 2007), are widely used nowadays for preliminary design procedures (Bieniawski, 1989; Hoek, 2007), like the RMR (Bieniawski, 1976) and the Q (Barton et al., 1974) and their modifications. These methods incorporate geological, geomechanical and geometric parameters in order to obtain a quantitative estimation of the rock mass quality, but, on the other hand, their implementation is time-consuming. Despite the dominance of these two methods, further rock mass classifications systems have been proposed in the last decades and, among these, the Geological Strength Index (GSI) classification system is currently widely used as it allows to estimate the strength of rock mass through empirical semiquantitative evaluation (Hoek, 1994; Cai et al., 2004), based on both rock mass structure and condition of the joints (Hoek et al., 1995). Estimating the GSI is straightforward and fast, but it comes at the cost of a certain degree of subjectivity. Moreover, the index does not adequately account for the lithology of the rock mass matrix. Hence, for the above reasons, these classification methods are not fully suitable to collect rock mass data over wide scale areas for engineering geological mapping. The Rock mass Quality Index (RQI, Disperati et al., 2016; Mammoliti et al., 2018) is a rock mass classification system developed for cartographic purposes and it is based on the systematic fieldwork measurement and processing of sets of the Schmidt hammer rebound values (R). Each representative rock mass outcrop is analysed by collecting ca. 20 R values at the 15-25 nodes of a regular grid conceived to investigate the typical features of the rock mass. This allows to perform statistical analyses and to calculate the RQI, a quantitative indicator of the global strength and quality of the rock mass. In the last decade, a dataset of ca 1100 outcrops sites spreading over a large area (ca. 12000 km2) were acquired in Tuscany (Italy), according to different lithology, weathering, jointing conditions. The dataset consists of both RQI measurements and GSI estimations for the main different lithological groups (flysch, limestones, marls, magmatic rocks and schists) of the Northern Apennines (Italy), as well as the laboratory determinations of the Slake Durability Index (Id2; Franklin & Chandra, 1974) obtained by testing representative outcrop rock samples. The large dataset has allowed to analyse the correlation among RQI, GSI and Id2 and to perform an in-depth critical analysis of the relationships among RQI, lithology, rock mass structure, as well as the suitability of the RQI as reference index for engineering geological mapping of near-surface rock mass quality

Dismantling a volcanic edifice by deep-seated landslides: the case of the eastern Monte Amiata (Italy)

The Monte Amiata is a volcano located in central Italy composed by trachytic to olivine latitic lava flows and domes emplaced between 305 and 231 ka (Pleistocene). These volcanic products, affected by saprolite alteration processes of spatially variable intensity, unconformably overlie Pliocene marine clayey sediments, as well as the Ligurian units stacked during the Northern Apennines orogeny. The Monte Amiata area has been attracting much attention from research and industry because of its economic importance in the field of geothermal energy, ore deposits and groundwater supply, hence a quite detailed geologic framework is available for this area. Instead, less efforts were made toward the understanding of the widespread gravitational processes affecting the eastern side of the volcanic edifice, often involving the transition between the volcanic rocks and the underlying sedimentary units, where many natural springs arise. The main urban agglomerations developed in this geologic setting, so buildings and infrastructures have been suffering damages caused by landslide processes over large areas. In this context, remote sensing imagery analysis, geomorphological surveys, engineering geology sub-surface investigations and ground displacement monitoring by integrating GNSS, robotic total station and geometric levelling allow us to map the main geomorphological features and infer the geometry and displacement rates of landslides occurring in the eastern side of the Monte Amiata volcano. The results suggest the occurrence of complex gravitational processes with different kinematic characteristics, state of activity and depth of the rupture surfaces. By cross-referencing the new quantitative data collected with the geomorphological evidences and the existing literature, we propose a model for the progressive dismantling of the eastern slopes of the Monte Amiata volcano caused by the interaction among complex gravitational movements affecting, at different structural levels, both the sedimentary units and the volcanic rocks. Moreover, detailed mapping of the saprolite derived by weathering of lava flows is provided and contextualised in the post-volcanic evolution of the Monte Amiata volcano

Investigating the relationships among vegetation characters, saturated hydraulic conductivity and surface morphology at catchment scale by integrating new field data and morphometric analysis

Shallow landslides susceptibility assessment by physically based methods relies on the parametrization of both hydraulic and geotechnical properties of soils, which in turn depend on the conditions of root structures and vegetation cover. Vegetation roots contribute to the shear strength of soils, but their quantitative contribution is currently uncertain. Saturated hydraulic conductivity (Ks) is also relevant for slope stability as it influences infiltration rates and runoff. While the literature clearly shows the dependence of Ks on soil texture, there is a general understatement of the role of root structures on this parameter. Moreover, the distribution patterns of vegetation follow relations with surface morphologies which are not fully understood and therefore, are worthy of further investigations. For these reasons, this work focuses on the quantitative assessment of the influence of vegetation on shear strength for shallow landsliding and the investigation of the relationships between vegetation characters, saturated hydraulic conductivity and topographic parameters. Study areas affected by shallow landslides are chosen in the Garfagnana and Alpi Apuane regions (Northern Apennines, Italy), as well as in the Mt. Amiata volcano area (Southern Tuscany, Italy), where field measurements of below-ground vegetation (Root Area Ratio - RAR), above-ground vegetation (Leaf Area Index - LAI and vegetation load) and Ks are acquired inside, in the neighbour and far from shallow landslide sites. To this aim, a multitemporal landslide inventory is already available for the study area. Below-ground data are collected in trench profiles, while above-ground data are acquired by using a digital relascope as well as implementing vegetation cover photography methods. Measurements of Ks are carried out by means of both constant and falling head approaches. The morphometric analysis is performed by using some morphometric variables (eg. slope and hillslope curvatures) derived from a digital elevation model with cell size of 10 m. Morphometric clustering of these variables allows us to extract a set of land units where the distribution of vegetation characters and Ks are assessed. First results show that: a) root reinforcement to soil in terms of root-related cohesion plays a relevant role within the soil depths involved in shallow landslides; b) the weight of above-ground vegetation plays a “mild” negative role on slope stability; c) Ks is correlated with both RAR and soil depth, suggesting possible criteria for the straightforward parametrization of input parameters

DNA methylation of the 5'-UTR DAT 1 gene in Parkinson's disease patients

The involvement of epigenetics mechanisms in the transcriptional regulation of key genes has been investigated in the initiation and progression of neurodegenerative disorders, including Parkinson's disease (PD). Among others, we, here, focused the attention on the dopamine transporter (DAT) gene playing a critical role in maintaining the integrity of dopaminergic neurons

A NEW APPROACH TO ASSESS THE SUSCEPTIBILITY TO SHALLOW LANDSLIDES AT REGIONAL SCALE AS INFLUENCED BY BEDROCK GEO-MECHANICAL PROPERTIES

Due to high velocity, high frequency and the lack of warning signs, shallow landslides represent a major hazardous factor in mountain regions. Moreover, increasing urbanisation and climate changes triggering intense rainfall events make shallow landslides a source of widespread risk. The interest of the scientific community in this process has grown in the last three decades with the aim to perform robust shallow landslide hazard assessment at regional scale. Generally, these slope failures involve relatively small volumes of material sliding along with a planar shallow rupture surface. In the literature it is widely accepted that shallow landslides involve only slope deposit (or colluvium) and the sliding surface correspond to the discontinuity between bedrock and the overlying loose material. The fieldwork conducted in this thesis highlighted that often shallow landslides involve also the weathered and fractured portion of bedrock. In this framework, the implementation of shallow landslides susceptibility modelling should take into account the engineering geological properties of slope deposits, as well as of the underlying bedrock. In this thesis a fieldwork-based method is proposed to acquire, process and spatialize engineering geological properties of slope deposits and bedrock. The aims of this thesis were to compile a new multi-temporal shallow landslide inventory, characterize the engineering geological properties of slope deposits and bedrock, implement and compare shallow landslide susceptibility modelling by means a physically-based and a data-driven methods and explore the role of bedrock in shallow slope failures. The study area corresponds to a 242 km2 portion of the Garfagnana basin (Northern Apennines), a mountainous region where the elevation ranges between 150 and 2000 m a.s.l. characterized by an incised and rugged morphology with steep slopes (average 28° degrees) and a mean annual rainfall between 1500 and 2500 mm/year. From a geological point of view, the Garfagnana basin is a narrow intra-mountainous valley, interposed betweeen the Alpi Apuane metamorphic complex to the east and the sedimentary northern Apennine’s ridge to the west. The fieldwork and laboratory tasks carried out to map engineering geology characters of slope deposits consisted on a set of hundreds of field sampling points, with the acquisition of depth to the bedrock, geotechnical horizons, unit weight, as well as soil samples for lab analysis. The distribution of points was chosen by observing that engineering geology properties of slope deposits depend on both bedrock lithology and morphometric conditions. In order to obtain the map distribution of engineering geology parameters, we implemented a spatial analysis by clustering morphometric variables stratified as a function of bedrock lithological units. In order to investigate the engineering geology characteristics of the bedrock, a field survey aimed to classify rock masses was conducted. For each survey site, 200-400 Schmidt hammer rebound measures, bedding and joint data, GSI (Geological Strenght Index) and samples for laboratory analyses (unit weight and slake durability test) were collected. The field data were processed and spatially analyzed by means uni-variate and multi-variate cluster analysis in order to delineate domains with different bedrock geo-mechanical properties. The shallow landslide susceptibility analysis was performed using both data-driven, Information Value, and physically-based, a modified version of SHALSTAB model (PROBSS), methods. The numerical modelling faced three issues: a) the comparison of PROBSS and Information value (IV) in the prediction of shallow landslides involving SD; b) the training and cross-validation of IV models using shallow landslides involving bedrock or not; c) implementation of a physically-based model to predict involving bedrock shallow landslides. First of all, the results highlight that the field-based methods proposed here to evaluate engineering geological properties of slope deposits and bedrock are adequate for the implementation of regionalised physically-based susceptibility models. The comparison between PROBSS and IV highlights that the simplification of shallow landslides adopted by the infinite slope model which do not take into account the occurrence of a sliding surface located below the slope deposits / bedrock discontinuity, may affect the performance of physically-based susceptibility models. The accuracy of IV model is slightly better that PROBSS model. Having implemented two data-driven susceptibility models using two different training datasets highlighted the different characteristics that slope deposits and bedrock involving shallow landslides have, suggesting and demonstrating that the latter occur in conditions that the physically based model cannot predict. By placing the slip surface below the discontinuity between slope deposits and bedrock and providing shear strength parameters compatible with a weathered and fractured rock material, satisfactory accuracy result was obtained with PROBSS model

Cartografia geologica, analisi strutturale e vorticita cinematica di una zona di taglio regionale: la "Ferriere-Mollieres Shear Zone" tra i valloni Forneris e di Pontebernardo (Massiccio dell'Argentera, Alpi Occidentali)

Riassunto Lo scopo di questa tesi è stato lo studio meso e micro strutturale e l’analisi della cinematica del flusso di una zona di taglio regionale che affiora nel massiccio dell’Argentera (Alpi Occidentali). Tale zona di taglio (Ferriere-Mollieres Shear Zone, FMSZ) è una zona di taglio destra che attraversa quasi interamente il Massiccio Cristallino Esterno dell’Argentera da NW a SE, separando due complessi migmatitici di alto grado di età varisica: il complesso della Tinèe a SW ed il complesso Gesso-Stura-Vesubiè (GSV) a NE. Sulle miloniti della FMSZ e sulle rocce del basamento varisico giacciono in discordanza stratigrafica i depositi continentali Permo-triassici (quarziti), al di sopra dei quali tramite un contatto tettonico definito dalla presenza di brecce tettoniche poligeniche (Carniole Auctt.) è giustapposta la Successione Sedimentaria Elvetico-Delfinese. Il rilevamento geologico-strutturale condotto alla scala 1:5000 ha focalizzato l’attenzione sulla caratterizzazione della deformazione milonitica nella porzione settentrionale della FMSZ (dove raggiunge il suo massimo spessore di circa 2 km), più precisamente fra l’abitato di Ferriere (comune di Argentera) ed il vallone di Pontebernardo (comune di Pietraporzio), estendendosi per un’area di circa 25 km2. Il rilevamento geologico-strutturale ha permesso di cartografare i litotipi coinvolti nella deformazione milonitica e il loro grado di deformazione in accordo con la suddivisione proposta da Passchier e Trouw (2005). Sono stati riconosciuti: micascisti a biotite e sillimanite (MBS), micascisti a clorite e muscovite (MCM), leucograniti (LEU) e migmatiti nei due complessi adiacenti. La foliazione milonitica della FMSZ ha un’orientazione media N110-N130 ed immersioni prevalentemente verso NE con un’inclinazione di 70-90°, mentre la lineazione mineralogica ha un’orientazione media N120-130 inclina di 20-30° verso NW. E’ stato osservato un gradiente di deformazione verso il nucleo della zona di taglio in cui sono concentrate le fasce di ultramiloniti e più rare filloniti, mentre le zone periferiche passano da protomiloniti a migmatiti non deformate da taglio. Nell’area di studio, le quarziti Permo-triassiche sono state osservate sia in discordanza angolare sul basamento varisico sia inglobate come lenti decametriche nei micascisti a clorite e muscovite. Ciò ha permesso di ipotizzare età varisiche per l’attivazione della FMSZ, poi parzialmente ripresa durante l’orogenesi alpina coinvolgendo anche le quarziti Permo-triassiche. Sui campioni raccolti, da cui sono state realizzate le sezioni sottili, è stata svolta un’analisi microstrutturale per caratterizzare i meccanismi deformativi e le relazioni metamorfismo-deformazione. L’analisi ha mostrato che i MBS hanno sperimentato temperature di deformazione più elevate (fra 500-700°C) rispetto ai MCM (fra 300-400°). Per caratterizzare la cinematica del flusso durante l’attività della zona di taglio sono state condotte analisi di vorticità cinematica su campioni opportunamente selezionati, che hanno fornito percentuali di taglio puro comprese fra il 58-66% con il metodo PAR (Passchier, 1987a) e comprese fra il 68-74% con il metodo dei piani S-C’ (Kurz e Northrup, 2008). Riportando i dati sul grafico proposto da Fossen et al. (1994) che mette in relazione la vorticità cinematica e l’angolo ξ fra l’ISA (Istantaneous Stretching Axes) massimo orizzontale e i limiti della zona di taglio, risulta che la FMSZ può essere interpretata come una zona di taglio transpressiva dominata da taglio puro. Su alcuni campioni utilizzati per le analisi di vorticità cinematica è stata eseguita l’analisi della deformazione finita con il metodo centro a centro di Fry (1979). I dati ottenuti indicano una forma dell’ellissoide di tipo oblato. Sono stati calcolati successivamente i valori di raccorciamento perpendicolare e allungamento parallelo al piano di flusso (limite della zona di taglio), ottenendo rispettivamente valori di 22-33% e di 28-39%. I dati di vorticità cinematica combinati con i dati ottenuti dall’analisi della deformazione finita, considerando che la foliazione milonitica è molto inclinata e che la lineazione mineralogica ha inclinazioni verso NW di 20-30°, indicano che la Ferriere-Mollieres Shear Zone è una zona di taglio transpressiva dominata da taglio puro, con cinematica destra top-to-the-SW attivatasi durante l’orogenesi varisica, sicuramente dopo l’evento di migmatizzazione sin-collisionale (datato a 323±12 Ma, Compagnoni et al., 2010) e durante o dopo la messa in posto dei leucograniti datati a 327±3 Ma (Musumeci e Colombo, 2002). Nel quadro più generale dell’orogenesi varisica, viste le caratteristiche strutturali, microstrutturali e di cinematica del flusso, la FMSZ potrebbe essere interpretata come una porzione della East Variscan Shear Zone, una zona di taglio regionale destra top-to-the-SW lunga circa 1500 km, attivatasi dal Carbonifero medio al Permiano, e osservata anche negli altri Massicci Cristallini Esterni, nel massiccio dei Mauri, in Corsica e in Sardegna. Abstract The purpose of this thesis was the study of the meso-structural and micro-structural features of a regional shear zone cropping out in the Argentera Massif (Western Alps) and to characterize the kinematic of its flow. This shear zone (Ferriere-Mollières Shear Zone, FMSZ) shows dextral sense of shear and crosses almost the External Crystalline Massif of Argentera from NW to SE, separating two Variscan migmatitic complexes: the Tinèe complex to SW and the Gesso-Stura Vesubiè complex to the NE. On the mylonitic rocks of FMSZ and of the Variscan basements, Permo-Triassic continental deposits (quarzites) lie unconformably, and above these the Helvetic-Dauphinoise Sedimentary Succession through a tectonic contact marked by tectonic polygenic breccias (Carniole Auctt.) crops out. Geological mapping performed at the scale 1:5000, focused on the attention on the characterization of the mylonitic deformation in the northern portion of FMSZ (where it reaches two km of width), from Ferriere village to Pontebernardo valley, extending for an area of 25 km2. Geological mapping allowed to characterize rock types involved in the mylonitic deformation and their deformation degree, according to Passchier & Trouw (2005). There were recognized: Bt-Sill-bearing micaschists, Chl-white mica micaschists, leucogranites and migmatites of the two adiacent complexes. Mylonitic foliation strikes N110-130 and dips toward NE of 70-90°, while mineral lineation is oriented N120-130 plunging towards NW of 20-30°. Towards the core of the shear zone a deformation gradient has been observed showing by the presence of ultramylonite and rare phyllonitic layers, whereas the outer zones are characterized by the development of protomylonite passing to unsheared migmatites. In the study area an angular unconformity of Permo-Triassic quarzites on Variscan basement has been recognized, even if in one outcrop the presence of a deformed quarzite decametric lens, incorporated in the Chl-white mica-bearing mylonitic micaschist has been recognised. These allow us to attribute a Variscan age for the activation of FMSZ, partially reactivated during Alpine Orogenesis involving Permo-Triassic quarzites. Microstructural analysis performed in samples collected in the study area showed that Bt-Sill-bearing micaschists experienced higher deformation temperatures (>500°) compared to Chl-white mica-bearing micaschists. To characterize the flow kinematic during the shear zone activity vorticity analyses has been performed in samples properly selected, and it provides a pure shear component of 58-66% with PAR method analyses and of 68-74% with SC’ method analyses during non-coaxial deformation. Plotting the data on the diagram proposed by Fossen et alii (1994), which related kinematic vorticity and the angle ξ, angle between the maximum horizontal ISA and shear zone boundaries, resulted that FMSZ could be interpreted as a pure shear dominated transpressive shear zone. Some of these samples have been used also to finite strain analyses applying Fry method. Data obtained gives an oblate ellipsoid shape. Shortening orthogonal to flow plane and stretching parallel to flow plane have been calculated following Wallis et al. (1993), obtaining values of 22-33% and 28-39%, respectively. Kinematic vorticity data coupled with data obtained from strain analysis, considering steeply dipping mylonitic foliation and shallowly plunging mineral lineation point out that FMSZ is a dextral top-to-the-SW pure shear dominated transpressional shear zone active during Variscan Orogenesis after the syn-collisional migmatization event (occurred at 323 ± 12 Ma, Compagnoni et alii. 2010) and during or after the emplacement of leucogranites occurred a 327 ± 3 Ma according to Musumeci e Colombo (2002). In the regional framework of Variscan Orogenesis, the FMSZ could represent a portion of the East Varisan Shear Zone, a 1500 km long regional dextral top to SW shear zone, acting from Middle-Carboniferous to Permian, recognized in others External Massifs, Mauri Massif, Corsica and Sardinia

Comparison of different infiltration methods for flood numerical analysis: modelling the 19 June 1996 extreme event of Cardoso (Alpi Apuane, Italy)

As the climate changes are expected to generally increase the hydrogeological hazard, a better knowledge of the catchment-scale response to intense rainfalls is a relevant issue. Hence, in this work, different approaches of infiltration processes estimation are analysed within hydrological modelling of the extreme rainfall event which involved in 1996 the Cardoso area (southern Alpi Apuane, Tuscany, Italy). On June 19, 1996, a convective supercell storm produced extreme rainfall rates (478 mm/12 h with maximum intensity of 158 mm/h) within a restricted area of the southern Alpi Apuane. The heavy rainstorm principally affected the Cardoso river watershed, where shallow landslides and debris flows were triggered in the steep slopes of the low-order hydrographic network covered by thick unconsolidated materials. Consequently, severe hyperconcentrated flows destroyed the Cardoso village, with 13 deaths and hundreds million Euros damages. The FLO-2D flood routing model was used for the numerical modelling of the infiltration processes occurred during the event, by implementing both the SCS-CN method and the Green-Ampt (GA) equation. FLO-2D is a combined hydrologic-hydraulic model, in which there is no need to separate rainfall/runoff and flood routing and spatially varying rainfall/infiltration may be simulated. The main advantage of the GA model arises in the temporal variation of the rainfall intensity, which is not considered in the CN model. In north-western Tuscany and especially in the Cardoso basin, the Authors have available a large set of engineering geological data obtained by field surveys performed under the coordination of the Geomatics lab of the University of Siena. Namely, the field saturated hydraulic conductivity (Ks) and soil depth measurements were used to implement both the GA equation (I) and the CN method (II, III). For each lithological unit, the continuous maps of the soil depth and the Ks were obtained by integrating the field data with the landforms extracted by processing a set of morphometric DTM derivatives. Regarding the CN method, the Hydrologic Soil Groups (HSG) were determined following the procedure proposed by the USDA-NRSC (Hydrology National Engineering Handbook, version 2009) (II), and by applying the subjective interpretation criteria (III) which does not consider the values of Ks and soil depth. Methods (I) and (II) show similar results and are consistent with the historical literature data and information, considering three relevant transects close to the village, as well as with post-event field evidences. The results obtained by applying the method (III) are strongly conditioned by the subjective assignment of the HSG and they can be different in terms of peak discharge, flood wave arrival time and maximum water level, if compared to (I) and (II). Moreover, the spatial distribution of soil depth and Ks allows a comprehensive representation of the hydrological-morphological framework of the Cardoso catchment

Regional-scale susceptibility modelling of shallow landslides involving the weathered and fractured sub-surface bedrock

Landsliding is a complex phenomenon and its modelling aimed at predicting where the processes are most likely to occur is a tricky issue to be performed. Apart the chosen modelling approach, for both data-driven and physically-based models, paying adequate attention to the predisposing and triggering factors, as well as the input parameters is no less important. Generally, shallow landslides mobilize relatively small volumes of material sliding along a nearly planar rupture surface which is assumed to be roughly parallel to the ground surface. In the literature it is also widely accepted that shallow landslides involve only unconsolidated slope deposits (i.e., the colluvium), then the rupture surface corresponds to the discontinuity between the bedrock and the overlying loose soil. In this work, based on systematic field observations, we highlight that shallow landslides often involve also portions of the sub-surface bedrock showing different levels of weathering and fracturing. Then, we show that the engineering geological properties of slope deposits, as well as those related to the underlying bedrock, must be considered to obtain more reliable shallow landslides susceptibility assessment. As a first task, a multi-temporal shallow landslide inventory was built by photointerpretation of aerial orthoimages. Then, a new fieldwork-based method is proposed and implemented to acquire, process and spatialize the engineering geological properties of both slope deposits and bedrock. To support the regional scale approach, field observations were collected within, in the neighbour and far from the shallow landslide areas. Finally, both physically-based and data-driven methods were implemented to assess and compare shallow landslide susceptibility at regional scale, as well as to analyse the role of spatial distribution of rock mass quality for shallow slope failure development. The results highlight that, according to geology, structural setting and morphometric conditions, bedrock properties spatially change, defining clusters influencing both the distribution and characters of shallow landslides. As a consequence, the physically-based modelling provides better prediction accuracy when two possible rupture surfaces are analysed, the shallower one located at the slope deposit / bedrock discontinuity, and the deeper one located at the bottom of the fractured and weathered bedrock horizon. Even though the physically-based and data-driven models provide similar results in terms of ROC curves, the resulting susceptibility maps highlight quite substantial differences

Shallow landslides involving weathered and fractured bedrock: a comparative susceptibility analysis between deterministic and statistical models

Shallow landslide susceptibility modelling at regional scale may be performed using both a physically based and statistical approach. For the same area, these two approaches can have inconsistent results, mainly because the two methods are conceptually different. Physically based models are based on the infinite slope model and consists on the computation cell by cell of a safety factor comparing between driving and resisting forces. The assumption that landslides occur in slopes that are characterized by predisposing factors similar to those in which landslides have occurred in the past, is the concept behind the statistical models. The aim of this work is to compare the two approach and investigate the differences between the two models. The study area is located in northern Tuscany, central Italy, in which an extensive field survey highlighted that about 60% of landslides involve bedrock. For this reason, we developed a physically based susceptibility analysis taking into account both the surficial layer (slope deposit, SD) and the underlying layer (BR), characterized by weathered and fractured bedrock. This model is compared to the statistically based one, which take into account topographic and geologic predisposing factor as well as bedrock geo-mechanical properties, such Geological Strength Index (GSI), Schmidt hammer rebound values (Rv) and Joint density (Jv). The accuracy of the models is evaluated using a multi-temporal landslide inventory, in which involving bedrock landslides are distinct from slope deposits landslides. Within this general framework results are discussed regarding the model’s predictive capacity and spatial agreement

Engineering geology characterization of slope deposits and physically-based assessment of shallow landslide susceptibility (Alpi Apuane, Italy)

In this work we present the results of engineering geology characterization of slope deposits and assessment of shallow landslide susceptibility by means of a probabilistic physically-based model for the Western sector of the Alpi Apuane (Northern Apennines, Italy). The Alpi Apuane are a Tertiary metamorphic complex which is undergoing fast tectonic uplift, exhumation and erosion in respect to neighboring regions (the coastal Versilia Plain and Garfagnana Valley). For these reasons, the morphology of the Alpi Apuane is characterized by high relief energy, as highlighted by elevation differences up to around 2,000 m and deep river valleys with steep slopes. Moreover, the study area records annual precipitations among the highest in Italy (up to around 2,500 mm/y) and, especially in the last decades, frequent intense rainfall events (i.e.: 1996, 1998, 2000, 2011, 2013, 2014). In this framework landslides are widespread, especially shallow landslides involving unconsolidated slope deposits overlying bedrock. In order to assess shallow landslide susceptibility, we used a hydrological model coupled to a limit-equilibrium infinite-slope stability model. Reliability of results by physically-based models depends on accuracy of map distribution of input data which, however, is usually almost unknown. Hence, fieldwork and laboratory tasks were carried out to map engineering geology characters of slope deposits. For a set of hundreds of field sampling points, we acquired: depth to the bedrock, geotechnical horizons, unit weight, as well as soil samples for lab analysis. The distribution of points were chosen by observing that engineering geology properties of slope deposits depend on both bedrock lithology and morphometric conditions. Then, for a subset of the sampling points, we performed hydraulic conductivity measurements. Geotechnical determinations allowed us to estimate the friction angle ranges for different slope deposit types. In order to obtain the map distribution of engineering geology parameters, we implemented a spatial analysis by clustering morphometric variables stratified as a function of bedrock lithological units. Multitemporal visual interpretation of orthophotos (2003-2016) allowed us to obtain the database for a new shallow landslide inventory, which later underwent field accuracy assessment. By integrating the inventory to geology, we identified those bedrock lithological units where the infinite-slope assumption for shallow landslide modeling could be reasonably applied. In order to take into account and evaluate the effects of input parameters uncertainty, we implemented the slope stability-hydrological model by means of a Monte Carlo simulation. Assuming that the cohesion of slope deposits changes in space and time depending upon seasonal variation of land cover and precipitations, we calibrated the model by means of a back-analysis aimed at estimating the cohesion intervals which allow for optimization of the final predictive performance within the shallow landslide regions. This task was performed by using both prediction-rate curve and ROC diagrams. Finally, the results of susceptibility assessment, as well as maps/diagrams useful to describe the variability/uncertainty of results are critically discussed