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1 Introduction Large rockslides are characterized by complex spatial and temporal evolution, with non-linear displacement trends and significant effects of seasonal or occasional events. Forecasting landslide motion and collapse is a fundamental task for hazard zonation and the design of risk mitigation structures. Consequently, the analysis and modeling of the involved phenomena are very important. One of the most deeply investigated landslides of the whole Alpine arc is Mont de La Saxe landslide (Fig. 1), located within a deep-seated gravitational slope deformation (DSGSD). This landslide is located at the upper part of the Aosta Valley, NW Italy. The rock slide dimension is of about 8x10 m, extends between 1400 and 1870 m a.s.l., over an area of 150'000 m with a horizontal length of about 550, maximum width of 420 m, and average slope gradient of 37°. The area is subjected to snow fall during the winter (average equivalent rainfall 810 mm ca., data— Mont de La Saxe meteo station at 2,076 m a.s.l.) with a total average precipitation of about 1,470 mm (at the rock slide crown area) and a real evapo-transpiration of about 370 mm. Aim of this work consists in analyzing this instability by means of an experimental campaign and numerical modelling. The target of this analysis consists in forecasting the landslide displacement and the possible failure. 2 Experimental tests The samples used for experimental tests derived by full core recovery are characterized by a metasedimentary sequence. The petrographic characterization have been performed by XRD (X-Ray Diffraction), XRF (X-Ray Refraction) and SEM (Scanning Electron Microscope) with microprobe in addition to laboratory tests on samples from shear zones. Samples from shear zones have different characteristics in terms of thickness of the shear zone, grain size, mineral composition and weathering.
Summary: Recent progress in the understanding of some fundamental issues associated with deep seated landslides from massive rock slope failure, i.e., slow to extremely slow moving massive natural rock slopes characterized by failures deformation which occurs at great depth, is addressed in this lecture. The Beauregard Landslide located in northwestern Italy in the Aosta Valley (Dora di Valgrisenche river) forms the reference case study. The interest stems from the presence, at the toe of this extremely slow moving landslide, of a concrete arch gravity dam which is being continuously loaded and has caused some closure of the arch with deformation and cracking developing on the downstream side. The discussion is on advanced laboratory testing of rock samples taken at depth along the basal sliding surface of the landslide, slope monitoring by state of the art approaches such as ground based interferometric synthetic aperture radar (InSAR), and numerical modelling developed and calibrated to simulate the behaviour patterns observed through monitoring. It is shown how these studies have contributed to the understanding of the underlying mechanisms and behaviour of deep-seated landslides. The aim is to gain the necessary confidence in predicting the likely development scenarios, assessing risk, and finding possible preventive/remedial measures. 1. introduction Deep-seated landslides from massive rock slope failure (MRSF) are slow to extremely slow moving massive natural slopes characterized by failures deformation which occurs at great depth in excess of 100 m and up to 250–300 m , . In cases such deformation takes place along a basal sliding surface which is described as a zone of sheared and cataclastic rock, locally reduced to a soil-like material with silt and clay. Deep-seated landslides are also known as deep-seated gravitational slope deformations (DSGSD), which occur on high relief energy hill-slopes, with size comparable to the whole slope, and with displacements relatively small in comparison to the slope itself. DSGSD exhibit on the ground surface typical morphologic and structural features such as double ridges, ridge depressions, scarps and counterscarps, trenches, and open cracks, etc. . Triggering and causal mechanisms of these landslides include post-glacial slope unloading and changes in ground water flow, regional tectonic stresses, earthquake ground shaking, fluvial erosion at the toe, etc. The importance of deep-seated landslides is that they occur in many parts of the world, are considered to be a major geological hazard, and impact very significantly on infrastructures and on society . It is to recognized that many aspects of deep-seated landslides are poorly understood and need be investigated in order to gain the necessary confidence for anticipating and predicting their behaviour, assess the risk, and find possible preventive/remedial measures. This is indeed due to the many complexities of the phenomena involved which encompass the understanding of the underlying mechanisms, the initial and post failure behaviour, and the different secondary processes resulting from instability . In this lecture we will be principally concerned with the understanding of the underlying mechanisms and initial behaviour of deep-seated landslides.
Kalenchuk, K. S. (Queen’s University) | Hutchinson, D. J. (Queen’s University) | Diederichs, M. S. (Queen’s University) | Barla, G. (Politecnico di Torino) | Barla, M. (Politecnico di Torino) | Piovano, G. (Politecnico di Torino)
ABSTRACT: The Beauregard Landslide is a deep-seated gravitational slope deformation located in the Aosta Valley (Dora di Valgrisenche river) in northwestern Italy. Numerical simulations of the Beauregard Landslide use three-dimensional mixed continuum-discontinuum methods to explore the role and importance of sophisticated geometric interpretations in analyzing landslide mechanics and to test model sensitivity to shear zone strength parameters. 3DEC (3-Dimensional Distinct Element Code) has been used to generate complex threedimensional landslide geometries. The landslide and surrounding, undisturbed, rockmass are defined as distinct continuum blocks which interact along discrete discontinuities representing landslide shear surfaces. The full three-dimensional geometries of these shear surfaces are interpreted from geological and morphological data using a rigorous statistical interpolation approach. This study aims to improve landslide hazard management by recreating observed slope deformations which vary across the landslide footprint. The simulated deformations from models are compared to observed deformations from real slope monitoring data to assess the validity of modelled slope behaviour. 1 INTRODUCTION To effectively manage hazards associated with massive, slow moving landslides, it is necessary to understand geomechanical factors controlling slope kinematics. These factors, including material strength, slope geometry and groundwater conditions, are rarely homogeneous across the extent of a landslide mass and usually change over time. Detailed site investigation is required for thorough landslide analysis. This should include studies of site specific geology, geomorphology and hydrogeology, as well as slope monitoring to assess howdifferent regions of a massive landslide exhibit spatially discriminated magnitude and direction of deformation, as well as modes of instability. Based on detailed interpretations of site specific conditions sophisticated three-dimensional numerical models can be developed, and then trained to reproduce observed slope behaviour. Once models are calibrated to reproduce observed slope deformations, mitigation techniques such as slope drainage can then be numerically tested.
ABSTRACT: This paper refers to the characterization of a geo-structural context traversed by a Aosta-Monte Bianco motorway tunnel. The tunnel was excavated in metamorphic Alpine rocks (mainly calcschist, micaschist and metabasite). Two different characterization methods have been used: a) outcrop geologic mapping and structural analysis with both weighted subjective sampling and systematic sampling; b) systematic pilot hole structural survey. The outcrop survey allowed one to reliably extrapolate the surface lithologic contacts to the tunnel level. All the structural data have been treated with analytical and computational (simulation) procedures that are useful for geometrical rock mass modelling. A comparison between the surface and underground explorations show an acceptable agreement for joint distribution and at a lesser extent for the fracturing degree. The characterization obtained from the surface survey also gives a suitable appraisal of the tunnel stability condition. RESUME: Cet article concerne la caracterisation du contexte geologique et structural traverse par Ie tunnel de I'autoroute Aosta-Monte Bianco. Le tunnel a ete creuse dans I'amas metamorphique Alpin (principalement constitue par chaux-schistes, micaschistes et metabasites). On a applique deux differentes methodes de caracterisation: a) cartographie geologique et analyses structurales par echantillonage soit systematique soit par analyse subjective ponderee; b) releve structurel systematique par un trou pilote. Le releve geologique a permi d'extrapoler en profondeur, au niveau du tunnel, les contacts geologiques mis en evidence en surface. Toutes les donnees structurales ont ere analysees par procedures analytiques et simulations numeriques, utilisables dans la modelisation geometrique de l'amas rocheux. Une comparaison entre les explorations effectues en surface et en souterrain, a montre une acceptable agrement pour ce qui concerne la distribution des joints et une moindre agrement pour ce qui concerne Ie degre de fracturation. La caracterisation obtenue par releve de surface a, de plus, montre une prevision fiable de la stabilite du tunnel. ZUSAMMENFASSUNG: Es wird uber getroffene Geologie und Felsgefuge im Aosta - M. Bianco Tunnelbau berichtet: Der Tunnel wurde im Metamorphischen Alpengebirge, d.h. in Kristallinen Kalk-und Glimmerschiefern und Metabasit aufgefahren. Zwei verschiedene Methoden wurden zur Charakterisiesung des Gebirges angewandt: a) eine geologische Kartierung und Gefugeanalyse, durch systematische oder statistisch erwagene Proben; b) Die Erfassung des Felsgefuges, durch Veruschsstollen. Die geologische Aufnahme erlaubte die Aufschlusse vom Tage, d.h. aus der grundderflache, in die Tiefe, d.h. nach der Tunneltrasse, zu extrapolieren. Alle Gefugedaten wurden durch analytisches Verfahren und numerische Simulation. hinsichtlich der geometrischen Modellierung des Gebirges, ausgewertet. Ein Vergleich zwischen den uber tagigen und unterirdischen durchgefuhrten Aufschlussen bestatigte eine Ubereinstimmung der Trennflachen verteilung und auch eine Korrespondenz der Zerkluftung. Dier Erkundungen au der Grundoberflache erviesen sich sinnvoll lind benutzbar fur eine Vorhersagung der Standfestigkeit des Tunnels. 1. INTRODUCTION The construction and performance of underground openings greatly depend on the geological and structural conditions of the rock mass. The design and construction choices must therefore rely on a reliable anticipation of the main characteristics of the underground environment. Estimating the width of different lithologies and the location of tectonic contacts along a tunnel route is essential for a correct choice of the excavation methods and points out the zones where heavy supports may be required. The description of the natural joint field is of fundamental importance to make a link between the foreseeable ground condition and the possible excavation behaviour, above all in competent structured rock masses.
ABSTRACT Heavy increases in traffic over the past 10 years and the resulting problems caused by exhaust fumes have led to greater corrosiveness of the atmosphere in road tunnels, and the problem of corrosion of installations is worsening. Fasteners also are affected, and their corrosion creates a safety hazard. In 1981, a general modernization was planned for the Mont Blanc Tunnel and cladding for the tunnel wall. The tunnel connects the Aosta Valley of Italy with the Upper Savoy of France between Courmayeur and Chamonix. It is the key section of one of the most important routes across the Alps. More than 15,000 vehicles travel through the tunnel daily and ~ 50% are heavy trucks. Fastening systems were included in the field tests. Types 304 (UNS S30400)(1) and 316 (UNS S31600) stainless steels (SS), the materials available for anchors at that time, were tested. After only 12 months, it was evident that neither material was resistant to corrosion under the existing conditions. In view of this finding, a major testing program began in 1987. During these tests, SS and nickel (Ni) alloys were examined, in addition to coated carbon (C) steel, aluminum (Al) alloys, and copper (Cu) alloys. Samples were evaluated after exposure periods of 9, 11, 19, 24, and 36 months. Laboratory tests were run at the same times. Based on results of the tests, materials were proposed for use in fastening the tunnel wall cladding, as well as in the lighting and the cable trays that were being replaced during modernization of the French section of the tunnel. The objective of the present work was to evaluate the corrosion behavior of the SS and Ni alloys with respect to the specific corrosive conditions in the tunnel.