Geophysical, geotechnical and hydrological investigations of a small landslide in the French Alps

Geophysical, geotechnical and hydrological surveys were conducted in the French Alps on a small landslide in black marl material to assess the stability. The geophysical survey, consisting of electromagnetic, geo-electric and relative differences in water content.

The data on the basal slip surface, together with geotechnical parameters (cohesion and angle of internal friction), were used to make a stability analysis. This analysis revealed that a groundwater level of 4 m below the ground surface is the critical threshold for reactivating the landslide.

The hydrological investigation, which entailed measurements of saturated permeability and pressure heads, indicated that instability depended on long-term wet conditions with little evapotranspiration.     

Long-term monitoring of a large deep-seated landslide (La Clapiere,South-East French Alps): initial study

The large-scale deformation of high mountain slopes finds its origin in many phenomena (inherent parameters, external stresses) with very different time constants (instantaneous to geological scale). Gravitational effect, tectonic forces and water infiltration are generally the principal causes of slope instability. However, it can be very difficult to distinguish which cause is dominant and which are their respective effects. To gain a better understanding of the complex processes taking place during the evolution of an unstable slope and separate the causes responsible of the landslide dynamic, an observational study based on geodetic, meteorological, seismological and electrical data has been performed on the La Clapière rockslide (Southern French Alps). This deep-seated landslide (DSL) is known for many years as one of the largest and fastest rock slide in Europe (60 million m³ of highly weathered metamorphic material, moving at 1 to 3 m year^?1). The set-up of the “Observatoire Multidisciplinaire des Instabilités de Versants” (OMIV, http://omiv.osug.fr) in 2011 has allowed the production and availability of an important and original data set over several years of accurate monitoring. Thus, for the first time, the long-term study of geodetic data permitted us to highlight acceleration phases in the general movement of the landslide that affect its dynamic. These modifications are associated with variations of the velocity by a factor 3 to 6. The characterization of the origin of these variations was possible due to the comparison with meteorological, electrical and seismological data. Based on these various signals, we were able to establish correlations and contributions of meteorological water infiltration in the dynamic evolution of the La Clapière slope. We determine several response times to the meteorological stress for seismic endogenous events (mainly rockfalls), the resistivity of the ground (quasi-instantaneous) and the kinematics of the slope (from 2 weeks to 2.5 months). Moreover, our results strongly suggest the existence of rainfall threshold of 3.5?±?1 mm day^?1 from which the number of seismic endogenous events is highly increased.     

Differential single-frequency GPS monitoring of the La Valette landslide (French Alps)

Recently, Global Positioning System (GPS) surveying techniques have been increasingly employed to monitor landslide movement. Here we present an application of GPS to monitor the La Valette landslide, located in the Ubaye Valley in the southern French Alps. This complex landslide is composed by an upper rotational part, a central part with generally translational movement and a lower part, which occasionally transforms into mud flows during intense rainfall events. Displacement rates average a few centimeters per month, with velocity peaks of some centimeters per day during periods of strong activity. GPS data presented in this study were acquired with two single-frequency GPS receivers Magellan connected to multipath-resistant antennas. The data were processed with the Magellan software MSTAR. Nine points have been set in the studied area, seven of them in the moving area, one in a stable area near the landslide and one on the facing slope, which is used as reference point. For each observation, one GPS receiver is placed on the reference point for the whole day and the second one is placed on each monitored point for 1-h sessions. The distance between the base and monitored point ranges from 480 to 1660 m. Eight survey campaigns were made between October 2000 and October 2002, to follow the evolution of the surface displacements. The maximum cumulative 3-D displacement observed in the area was about 21 m during the period in the center part of the landslide, corresponding to an average rate of movement of about 3 cm/day. The accuracy achieved during the GPS measurements has been evaluated to be about 2.4 cm in E–W direction, about 11 cm in N–S direction and about 7.4 cm in elevation in the worst case. The GPS results have been compared with traditional surveying techniques (EDM) carried out on the same site by RTM Service (Restauration des Terrains en Montagne). The velocities obtained by the two methods are similar. The advantage of the GPS technique is the collection of data for the three spatial coordinates (x, y, z) of each point, which allow to calculate the 3-D displacement vector. These measurements have been combined with SAR interferometric data in order to analyse the temporal evolution of the different landslide sectors.     

Dendrogeomorphic reconstruction of past landslide reactivation with seasonal precision: the Bois Noir landslide, southeast French Alps

The purpose of this study was to reconstruct spatiotemporal patterns of past landslide reactivation in a forested area of the Barcelonnette Basin (Bois Noir landslide, Southern French Alps). Analysis of past events was based on tree ring series from 79 heavily affected Mountain pine (Pinus uncinata Mill. ex Mirb) trees growing near or next to the landslide body. Dendrogeomorphic analysis focused on the presence of compression wood and growth reductions, with the first reaction being used for a dating of past reactivations with seasonal precision. A total of 151 growth disturbances were identified in the samples representing eight different stages of reactivation of the landslide body between 1874 and 2008. The spatiotemporal accuracy of the reconstruction is confirmed by historical records from neighboring sites and by aerial photographs. The onset of compression wood formation allows identifying five stages of landslide reactivation during the dormant season or the very beginning of the growing season of trees, i.e., between early October and late May, and three stages toward the end of the growth period. Monthly rainfall data from the HISTALP database demonstrate that the rainfall during four out of the eight reactivations are characterized by summer rainfall totals (July?CAugust) exceeding 200?mm, pointing to the important role of summer rainstorms in the triggering of events at the Bois Noir landslide body.     

Geophysical surveys to study a landslide body (north-eastern Sicily)

Almost the entire Italian territory is prone to hydrogeological risk mostly due to landslides and flooding. The high frequency of extreme weather events in areas prone to geological–hydraulic hazards contributes significantly to increasing the risk for cities and its infrastructure, but above all for the people living there. Therefore, it is vital to research into rapid monitoring techniques to be applied following a disaster such as a landslide, so that important background information, useful for planning interventions aimed at hazard mitigation, can be obtained. This work seeks to identify the depth of the sliding surface of a landslide affecting the municipal area of Tripi (village in the Messina province, Sicily), through two geophysical field surveys (MASW and HVSR) undertaken in the same area at an interval of about 5 years. The MASW surveys have enabled to reconstruct the distribution of shear wave velocity with depth. The HVSR surveys have provided information on resonant frequency and directional effects. The data integration obtained by the two methods allowed to reconstruct impedance contrast sections indicating the depth of the sliding surface of the landslide.

     

Deformation pattern in a heterogeneous material: Folded and cleaved sedimentary cover immediately overlying a crystalline basement (Oisans, French Alps)

Particularly well exposed structures in folded and cleaved sedimentary cover immediately overlying a crystalline basement have been studied. Chemical analysis (X.R.F. and microprobe) reveal pressure solution process and give the possibilities of measurement of mass transfer. Study of fluid inclusion veins has determined the temperature pressure conditions: thermal effect of the basement and decrease of temperature and pressure with the age of various synkinematic veins.Characteristic examples of the behaviour of a heterogeneous material during coaxial and non-coaxial deformation are shown:

1. (1) Successive different asymmetrical folds, various cleavages and fractures appear in a shear zone parallel to the main fabric with variations of thickness and rock behaviour.

2. (2) Evolution of cleavage in such a shear zone (with or without slipping) is linked to the relations between the rotation of contraction direction and the rate of the cleavage process.

3. (3) Fold axes changed from the horizontal y direction to the vertical (or E—W transversal to the crystalline massif) X direction, with increase of the (X/Z) and (X/Y) ratios (obtained by fossils and reconstructed fold shape). This strain is always heterogeneous and the most deformed zone frequently evolves to discontinuities with slip.

4. (4) Indentation exists on all scale: from hard object (100 μ, with parenthesis form of pressure solution cleavage apparent on map distribution of various element) to basement block (with variation of strain value in the indented cover).

A model of the evolution of the deformation of sedimentary cover immediately overlying a crystalline basement is given in conclusion.     

Geophysical evidence for a large impact structure on the Falkland (Malvinas) Plateau

A large, roughly circular structural basin is recognised on the Falkland (Malvinas) Plateau to the NW of West Falkland (Gran Malvina) Island (S 51°00′, W 62°00′). The basin, seen in seismic‐reflection profiles and evident as a large negative gravity anomaly, has a diameter of ~250 km. The age of the basin is estimated to be Late Palaeozoic. It is completely buried by younger sediments and has no topographic expression on the sea floor. We propose that the basin and geophysical anomalies, especially the combination of a large circular negative gravity anomaly with a rim of positive anomalies, and a marked circular series of positive magnetic anomalies in the same area, may be best explained by the presence of a large buried impact structure.     

The curved translation path of the Parpaillon Nappe (French Alps)

The Parpaillon Nappe is one of the two Helminthoid Flysch nappes emplaced on the external Dauphinois zone of the Western Alps. A structural analysis of the nappe is presented. Two superposed deformations D1 and D2 are described, that are mainly characterized by large-scale recumbent folds whose axes are quasi-orthogonal: NE-SW for D1 and NW-SE for D2. Their vergence is northwestward for D1 and southwestward for D2. During the D2 deformation, the nappe was separated into two units, one of these being thrusted over the other. An analysis of incremental strain using quartz and calcite fibre growth indicates that D2 follows D1 without discontinuity. Therefore the superposition of D1 and D2 structures is interpreted as a progressive deformation instead of two distinct phases of deformation. The emplacement of the nappe is discussed under two aspects, the relations between displacement and strain and the role of gravity. It is concluded that the translation has been twofold, first towards the NW and then towards the SW, and that the displacement result essentially from gravity forces. Kinematic implications for the Alpine collision are suggested.     

From biological to lithological control of the B geochemical cycle in a forest watershed (Strengbach, Vosges)

There is a fast growing interest in understanding the coupling between mineralogical and biological processes responsible for the migration of elements through continental ecosystems. This issue has fundamental impacts at the soil/plant scale because it can explain the tight links between soil and plant development and at the watershed scale because it gives a direct access to the water quality. In the present study, we performed an extended investigation of the bio-geochemical cycle of boron, which is an element known to be suitable for investigating water/rock interactions and vegetation cycling. New B data are provided along the hydro-bio-geochemical continuum in a forest ecosystem (Strengbach basin, Vosges, France), from rainwaters down to the outlet of the basin including systematic analyses of throughfalls, soil solutions, springs and brooks scattered in the watershed. At the watershed scale, we evidence a relationship between the B isotopic composition of river waters and the weathering regime outlining a predominant control of the parent rock mineralogy on the B geochemical behavior. At the soil/plant scale, it appears that the B geochemical cycle is controlled by the vegetation cycling, which is characterized by an uncommon, easy to distinguish, B isotopic composition (δ¹¹B ranging from about +30‰ to +45‰). Each year the amount of B being involved in the vegetation cycle is about four times greater than that of B being exported out of the watershed. At 10 cm depth in soil, where the plant roots are expected to be the most active, we observe a marked seasonal oscillation of the B isotopic values, which is interpreted as resulting from the vegetation activity. A mass balance calculation based on the assumption that that ¹⁰B is preferentially accumulated in the biomass tends to indicate that the soil/plant system does not behave at steady state with respect to B.Because of the very distinct B isotopic signature of vegetation and minerals in soil, box modeling allows to quantify the part of the B fluxes involved in the vegetation cycling and the mineral reactions, respectively. This calculation reveals a clear correlation between the amount of B derived from soil weathering and the amount of B absorbed by plant roots. This result clearly supports the idea that a coupling exists between mineral weathering and plant activity, for the study of which B isotopes appear particularly suitable.     

Landslide kinematics inferred from in situ measurements: the Cliets rock-slide (Savoie,French Alps)

This paper presents an analysis of two large rock toppling/sliding events which occurred in January 2014 and February 2019 at the Cliets unstable slope (Savoie, French Alps). To understand the mechanism involved and its control by external forcings, a multi-technique analysis approach is used combining geological observations, meteorological data analysis, topographic measurements and simple physical modeling. The pre-failure stage of the events is more particularly analyzed. No direct relationships are found between triggering factors and surface motion though a kinematics analysis highlights the transition toppling-sliding. It showed that, at first order, this transition occurred 4 years before the first failure of 2014, while it happened 2 months before the second failure of 2019. From this date, the environment is considered like a block sliding on an inclined plane. By applying a frictional model (Helmstetter et al. in Journal of Geophysical Research: Solid Earth 109(B2), 2004), we illustrated that the two events belong to an unstable velocity-weakening sliding regime. The time to failure (Voight in Science 243(4888):200–203, 1989) is forecasted with the model, and the results are consistent with the observations. They confirm that the gravitational factor is predominant over the triggering factors for the two events.

     

Within-Plate type Meta-Volcaniclastic Deposits of Maastrichtian-Paleogene age in the Grande Motte unit (French Alps,Vanoise) : a first record in the Western Alps and some implications

Abstract

Metamorphosed mafic rocks probably corresponding lo ancient basaltic ashes (meta-tuffs) have been discovered in Kastern Vanoise. These rocks are interbedded within the Maastrichtian-paleocene formations of the Grande Motte unit. Their geochemícal characteristics are consistent with a within-plate affinity. A remote origin of these materials, as airfall ashes, is possible. These volcanic rocks arc coeval with eruptive alkaline products known is the Southern Alps. They have formed in a local extensional and/or strike-slip environment involving the internal Alpine realms around the Cretaceous-Tertiary boudary.     

云南腾冲地区粘土质硅藻土公路滑坡形成机理 与防治优化设计

腾冲地区分布的粘土质硅藻土是一种特殊性的岩土体,具有孔隙大、密度低、含水量高,富含膨胀性粘土矿物、结构性强等特点。近年来已发生一系列与粘土质硅藻土相关的滑坡等地质灾害,造成了较大损失。在粘土质硅藻土工程地质特性测试分析的基础上,对腾冲地区粘土质硅藻土滑坡的分布特征和形成机理进行了研究,认为粘土质硅藻土滑坡以中小型滑坡为主,受公路切坡、采矿、降雨等因素影响较大;公路边坡滑坡多发生在坡体的中部和后部,在牵引作用下部分滑坡可发生大范围滑动。粘土质硅藻土具有膨胀性和极强的结构性,其边坡稳定性较差。以新修建腾泸公路边坡滑坡为例,对S238省道冯家大山滑坡的形成机理进行了探讨,认为受粘土质硅藻土膨胀性、结构性等的影响,人类工程扰动、降雨等引起边坡表部土体出现强度分层、膨胀性增强和力学强度降低,从而导致原支护结构破坏;在此基础上,对冯家大山边坡进行了优化设计和数值模拟分析,提出了综合防治建议。     

Historical reconstruction and evolution of the large landslide of Inza (Navarra,Spain)

Natural Hazards - In the winter of 1714–1715, a large rotational landslide originated on the northern slope of the Sierra de Aralar (Navarra, Spain), generating an earthflow that slowly...     

Failure mechanisms and triggering factors in calcareous cliffs of the Subalpine Ranges (French Alps)

In order to enhance the detection of prospective rock falls in calcareous cliffs, 25 rock falls have been described in a more detailed way than for an inventory. They are representative of middle size rock falls (10 to 100,000 m³) occurring in the French Subalpine Ranges, at an elevation between 200 m and 2000 m. Structural conditions of the rock masses, morphology of the initial cliff surface and the scar, possible failure mechanisms and processes have been studied. Typical failure configurations have been identified, based on the attitude of the failure surface, in relation to the bedding planes and the cliff surface. Irregular cliff morphology appears to be another important susceptibility factor. In most cases, the classical comparison of the average planes of the main joint sets with the average plane of the slope could not define the potentially unstable masses. Rather, those ones are due to joint planes that deviate from their mean set plane or to irregularities of the cliff surface. The proposed investigation method to detect prospective rock falls mainly consists in observing stereoscopic aerial photographs in order to look for critical configurations. Once a critical mass has been detected, its failure probability for a period of the order of one century must be evaluated (or its life expectancy). The main factor to consider for this purpose appears to be the proportion of rock bridges in the potential failure surface.

The triggering factors of rock falls in our study area have been investigated, by analysing an inventory of 46 rock falls. Statistical tests have been carried out to study the relation between rock falls and daily rainfall, freeze–thaw cycles or earthquakes. A good correlation has been obtained with freeze–thaw cycles, a slight correlation with rainfall and no correlation with earthquakes. This suggests that ice jacking could the main physical process leading to failure by causing microcrack propagation.     

Calculating the long-term displacement rates of a normal fault from the high-resolution stratigraphic record (early Tethyan rifting, French Alps)

Displacement rates of normal faults deduced from stratigraphic data are often unreliable. Here we calculate the velocity of motion on a normal fault from the variations in accommodation potential on both sides of the fault within a high‐resolution time‐frame established by biostratigraphy and physical stratigraphy. Our example is the Ornon normal fault bounding the Early Jurassic Bourg‐d'Oisans Basin formed during Tethyan rifting. We show that motion on the fault was discontinuous when examined at high resolution and over a long time interval. During a first interval (Hettangian to Sinemurian Arietites bucklandi zone) a low rate of displacement (=202–423 m Myr^?1) coeval with diffused extensional deformation throughout the sedimentary basin is observed. A second interval of localized deformation (Early Sinemurian Caenisites turneri zone) is characterized by higher rates of displacement on the fault (1846 m Myr^?1). Our results concur with recent numerical models identifying the main stages of extensional deformation.     

Geotechnical and geophysical characterization of the Bouira-Algiers Highway (Ain Turck,Algeria) landslide

The Ain Turck (Bouira) landslide, in north-center Algeria, is one of the numerous instabilities recorded along the Lakhdaria-Bouira section of the 1200-km-long east-west Algerian highway. The locality of Ain Turck is known for its unstable slopes characterized by a very rough morphology with steep slopes (20 to 25%). This slide threatens the inhabitants of the Ibournanen village, located down the unstable slope, where parts of some houses have fallen into ruin, while others are cracked. It is characterized by an active movement extending over a more or less important slope, of the order of a hundred meters. The land mobilized by this movement corresponds to the layer of shale clays and clays overlaid by a backfill, placed there following the east-west highway works. Geological, geomorphologic, and geotechnical analysis allows determining the soil instability probably related to earthworks during the construction of the highway section a few years earlier, followed by a particularly rainy season in 2012. Acquisitions of ambient seismic noise and H/V ratio processing, as well as the acquisition of an electrical resistivity profile at the instability site, have reinforced our preliminary interpretations of depth and geometry of the sliding surface.     

Structural evolution of a nappe complex,southern Vanoise massif,French Penninic Alps

The Arpont-Parrachée region in the southern Vanoise massif comprises a stack of minor fold- and thrust-nappes that were emplaced during subduction and closure of the Piémont ocean basin in Late Cretaceous to Eocene time. The stack includes the Arpont nappe, composed mainly of pre-Permian schist metamorphosed to blueschist facies early in the Alpine history, and several sheets of Permian to Eocene metasedimentary rocks. Nappe formation, recumbent folding, and associated ductile deformation postdated the high-pressure metamorphic peak, and probably involved translation to the northwest. The rocks were then refolded by large- and small-scale folds trending roughly E-W. These deformational events were accompanied by a decrease in metamorphic pressure, indicating uplift. They were followed by regional greenschist-facies metamorphism, which caused breakdown of high-pressure parageneses, annealing of microstructures, and widespread growth of albite porphyroblasts. The entire nappe pile was then refolded by large- and small-scale folds overturned towards the southeast. Reorientation of small-scale structures with increasing strain by this event indicates a large component of ESE-directed shear, which culminated in the formation of anastomosing ductile shear-zones.