Inferences on modeling rainfall-induced shallow landslides from experimental observations on stratified soils

Le frane superficiali indotte da pioggia (quali soil slips o debris-flows) sono una tipologia di movimento franoso che può coinvolgere i primi 2-3 metri di terreno, in genere rappresentato da coltri di alterazione eluvio-colluviali. Tali fenomeni costituiscono un serio rischio per le attività antropiche se si considerano sia le elevate velocità che si possono raggiungere durante la fase di trasporto che gli ingenti volumi di terreno che possono essere mobilizzati per effetto dell'erosione sul fondo del canale. Per questo motivo, negli ultimi anni sono stati dedicati molti sforzi all'elaborazione di tecniche e metodologie funzionali alla predizione spazio-temporale di questi eventi. Tra le nuove metodologie in fase di sviluppo, rivestono particolare importanza i cosiddetti modelli numerici fisicamente basati. Tali modelli tentano di riprodurre i processi fisici che conducono all'instabilità mettendo in relazione pioggia, pressione interstiziale e condizioni di resistenza del terreno. In particolare, molti di questi modelli adottano uno schema di pendio infinito per bilanciare le forze agenti e resistenti sul volume di terreno, usando un modello di infiltrazione per determinare gli effetti della pioggia sulle variazioni di pressione interstiziale. Oltretutto, questo tipo di modelli, tenendo conto della variabilità spaziale dei parametri coinvolti (es: caratteristiche fisico-meccaniche del terreno, intensità di pioggia), possono risultare particolarmente utili per predire l'occorrenza di frane superficiali alla scala di bacino. Tuttavia, l'utilizzo di questi strumenti non sempre consente di risalire alle reali condizioni di innesco, perlopiù a causa della complessità del fenomeno simulato e dell'ingente numero di parametri in esso coinvolto. Tra i vari aspetti che necessitano di essere approfonditi, c'è anche quello del contributo alla stabilità del terreno per effetto della coesione apparente indotta dalla matrice di suzione presente in condizioni non sature. Tale effetto non può non essere preso in considerazione, soprattutto nel caso di terreni caratterizzati da una granulometria limoso-argillosa. Sebbene in letteratura esistano alcuni metodi e formule empiriche per caratterizzare la resistenza di un terreno in condizioni non sature, allo stato attuale sono ben pochi gli studi inerenti l'analisi delle condizioni idraulico-meccaniche basati su osservazioni reali. Da questo punto di vista, alcuni autori hanno evidenziato come la modellazione fisica di laboratorio su modelli di pendio in scala possa rappresentare uno strumento estremamente utile per questa tematica. Tuttavia, solo in pochissimi casi si è tentato di utilizzare i risultati sperimentali per validare e/o migliorare modelli numerici fisicamente basati dedicati alla predizione dell'innesco di frane superficiali alla scala di bacino. Pertanto, l'obiettivo di questo lavoro è quello di verificare, attraverso prove sperimentali di laboratorio, alcune assunzioni di SLIP (Shallow Landslides Instability Prediction), un modello numerico fisicamente basato finalizzato alla predizione di frane superficiali indotte da pioggia. Nello specifico il modello calcola le condizioni di stabilità, espresse in termini di Fattore di Sicurezza (FS), simulando il processo di saturazione del suolo per effetto di uno specifico input di pioggia e tenendo specificatamente conto del contributo alla resistenza indotto dalla parziale saturazione del terreno per effetto delle piogge antecedenti. Sono stati quindi analizzati i risultati di differenti prove effettuate su un profilo di terreno ricostituito all'interno di una canaletta sperimentale, con l'obiettivo di descrivere e quantificare alcuni aspetti particolari concernenti la modellazione del processo di innesco. Nello specifico, è stata analizzata l'influenza sull'insorgere dell'instabilità dello spessore di due differenti strati presenti all'interno del profilo di terreno, di cui uno dei due caratterizzato da un comportamento coesivo. Per simulare l'effetto della coesione, è stato infatti utilizzato uno strato di sabbia parzialmente saturo, mentre la stessa sabbia (ma in condizioni asciutte) è stata utilizzata per realizzare il secondo strato. Il modello di pendio così costituito è stato sottoposto a differenti tilt tests, e in ciascuna prova è stato variato lo spessore degli strati in modo tale da verificare l'influenza di questo parametro sulle condizioni di stabilità. I risultati ottenuti sono stati quindi utilizzati non solo per corroborare alcune assunzioni del modello, ma anche per verificare la relazione matematica proposta dal modello stesso, e che lega resistenza del terreno e spessore degli strati attraverso il parametro della coesione apparente.In this work, we analyzed the results of different soil laboratory tests performed in a flume test apparatus with the aim to describe and quantify some particular aspects of the modelling of soil slip phenomena. In particular, we analyzed the influence, in terms of slope stability, of the thickness of two strata (a cohesive one and a not cohesive one) composing the slope model. To simulate the presence of cohesion, a partially saturated sand was employed, while the same sand but in dry conditions was used to reproduce the not cohesive stratum. The so-constituted slope laboratory model was then submitted to tilting tests, and in each test the thickness of these layers has been varied in order to investigate the influence of this parameter on slope stability. The obtained results have been used to calibrate several parameters and verify specific assumptions of SLIP, a simplified physically-based and well-tested model for the prediction of shallow landslides occurrence

Structure and dynamics of deep-seated slope failures in the Magura Flysch Nappe, outer Western Carpathians (Czech Republic)

International audienceDeep-seated mass movements currently comprise one of the main morphogenetic processes in the Flysch Belt of the Western Carpathians of Central Europe. These mass movements result in a large spectrum of slope failures, depending on the type of movement and the nature of the bedrock. This paper presents the results of a detailed survey and reconstruction of three distinct deep-seated slope failures in the Raca Unit of the Magura Nappe, Flysch Belt of the Western Carpathians in the Czech Republic. An interdisciplinary approach has enabled a global view of the dynamics and development of these deep-seated slope failures. The three cases considered here have revealed a complex, poly-phase development of slope failure. They are deep-seated ones with depths to the failure surface ranging from 50 to 110m. They differ in mechanism of movement, failure structure, current activity, and total displacement. The main factors influencing their development have been flysch-bedrock structure, lithology, faulting by bedrock separation (which enabled further weakening through deep weathering), geomorphic setting, swelling of smectite-rich clays, and finally heavy rainfall. All of the slope failures considered here seem to have originated during humid phases of the Holocene or during the Late Glacial

Shaking table tests on deformation and failure mechanisms of seismic slope

The 2008 Wenchuan earthquake in China induced many landslides. Gigantic slope failures have attracted serious concerns in engineering practice; however, small slope failures should also be investigated as they are more common. In particular, the detailed characteristics of slope failures during earthquakes remain unknown. Therefore, the present study carried out 1-G shaking table tests on a straight shape slope model with different shaking intensities and frequencies. The test results showed the amplification of motion, the initiation of failure, and final failure mode of the straight shape slope. Also, the experimental results can be used to investigate the response and amplification behavior of some prototype slopes. The results are helpful to demonstrate the detailed collapsing behavior of the slope under earthquake excitation, and provide useful data to analyze the failure mechanism of landslides and valuable references for seismic design of landslide engineering

Post-failure evolution analysis of a rainfall-triggered landslide by multi-temporal interferometry SAR approaches integrated with geotechnical analysis

Persistent Scatterers Interferometry (PSI) represents one of the most powerful techniques for Earth's surface deformation processes' monitoring, especially for long-term evolution phenomena. In this work, a dataset of 34 TerraSAR-X StripMap images (October 2013–October 2014) has been processed by two PSI techniques - Coherent Pixel Technique-Temporal Sublook Coherence (CPT-TSC) and Small Baseline Subset (SBAS) - in order to study the evolution of a slow-moving landslide which occurred on February 23, 2012 in the Papanice hamlet (Crotone municipality, southern Italy) and induced by a significant rainfall event (185 mm in three days). The mass movement caused structural damage (buildings' collapse), and destruction of utility lines (gas, water and electricity) and roads. The results showed analogous displacement rates (30–40 mm/yr along the Line of Sight – LOS-of the satellite) with respect to the pre-failure phase (2008–2010) analyzed in previous works. Both approaches allowed detect the landslide-affected area, however the higher density of targets identified by means of CPT-TSC enabled to analyze in detail the slope behavior in order to design possible mitigation interventions. For this aim, a slope stability analysis has been carried out, considering the comparison between groundwater oscillations and time-series of displacement. Hence, the crucial role of the interaction between rainfall and groundwater level has been inferred for the landslide triggering. In conclusion, we showed that the integration of geotechnical and remote sensing approaches can be seen as the best practice to support stakeholders to design remedial works.Peer ReviewedPostprint (author's final draft

Landsliding near Enguri dam (Caucasus, Georgia) and possible seismoectonic effects

The Enguri dam and water reservoir, nested in southwestern Caucasus (Republic of Georgia), are surrounded by steep mountain slopes. At a distance of 2.5 km from the dam, a mountain ridge along the reservoir is affected by active deformations with a double vergence. The western slope, directly facing the reservoir, has deformations that involve a subaerial area of 1.2 km2. The head scarp interests the main Jvari-Khaishi-Mestia road with offset of man-made features that indicate slip rates of 2-9 cm/y. Static, pseudostatic and Newmark numerical analyses, based on field and seismological data, suggest different unstable rock volumes basing on the environment conditions. An important effect of variation of water table is showed, as well as the possible destabilization of the landslide following seismic shaking compatible with the expected local Peak Ground Acceleration. This worst scenario corresponds to an unstable volume in the order of up to 48 ± 12*106 m3. The opposite, eastern slope of the same mountain ridge is also affected by wide deformation involving an area of 0.37 km2. Here, field data indicate 2-5 cm/y of short-term and long-term slip rates. Ground Penetrating Radar surveys of the head scarps confirm that these slip planes are steep and extend downward. All these evidences are interpreted as resulting from two similar landslides, whose possible causes are discussed, comprising seismic triggering, mountain rapid uplift, river erosion and lake variations

Observations on soil-atmosphere interactions after long-term monitoring at two sample sites subjected to shallow landslides

Soil-atmosphere interaction has implications in different scientific research contexts and is increasingly investigated through field measurements. This paper reports a detailed description of interaction between shallow soil and atmosphere at two test sites in Oltrepò Pavese area (Northern Italy). The two test sites are in the same climatic area but are characterised by different geological features. In fact, the first objective is to compare the behaviour of two different soils, namely a clayey-sandy silt (CL) and a silty clay (CH), under similar meteorological events. Soil-atmosphere interaction is studied on the basis of long-term (about 87 and 42 months for the two test sites, respectively) monitoring data of both volumetric water content and soil water potential, recorded at different depths along two vertical soil profiles in the first two metres from ground level. Field measurements, together with meteorological data such as precipitation and air temperature, allow for clear identification of the seasonal fluctuations of unsaturated soil hydraulic properties. To infer detailed information, the recorded data were processed and relationships between soil water potential and water content were investigated. Different time spans, from several months to a few days, even including single rainy events, are considered to show the hydraulic soil behaviour. The hysteretic cycles of water content with respect to soil water potential and non-equilibrium flow are highlighted. In particular, the measured soil water potential is in the range of 0–800 kPa and of 0–1500 kPa for the CL and CH soil, respectively. At both sites, the observed hysteretic cycles are more frequent in the hot season (summer) than in the cold season (winter) and tend to reduce with depth. The experimental results are compared with the soil water characteristic curves (SWCCs) to assess whether and to what extent the SWCCs are reliable in modelling the hydraulic behaviour of partially saturated soils, under atmospheric forcing, at least in the considered climatic contexts

Scenarios of lateral collapses of the Vavilov seamount in the central Tyrrhenian sea

The strong EW asymmetry observed for the Vavilov seamount could be the result of an ancient collapse from its western flank. Two scenarios for the pre-collapse morphology of the seamount are built up by reshaping the western flank of Vavilov and modelling it as the eastern one. In scenario 1 the detached mass is assumed to slide down on the present bathymetry, i.e. the sliding surface outside the detachment niche is identified with the today’s abyssal plane to the west of Vavilov. Instead, in scenario 2 it is assumed that the difference between the depth of the western (less deep) and eastern (deeper) abyssal planes is due to the deposition and subsequent reworking of the collapsed material. Therefore, in scenario 2 the identified deposit is removed from the present bathymetry, while the western flank is reshaped approximately like in scenario~1. The motion of the landslide is simulated by the model UBO-BLOCK2, developed by the Tsunami Research Team of the University of Bologna, that treats it as a grid of blocks sliding under the effect of friction, drag forces and interaction forces. The influence of various parameters on the model's output is discussed for simulations that regard scenario 1. For scenario 2, the agreement between the hypothesised deposit and the one computed through the simulations is used to tune the model parameters, and in particular the sea bottom friction coefficient, that is the one that most affects the landslide motion. The conclusion of the thesis is that scenario 2 supports reasonably well the hypothesis that the Vavilov seamount experienced a collapse of the western flank, that the collapsed material filled the western abyssal plane causing an average decrease of the sea floor depth by hundreds of meters, that most of the material accumulated not too far from the feet of the Vavilov’s seamount, which is consistent with the available seismic lines