Effect of Groundwater on Landslide Triggering

Landslides belong to the most important natural hazards in all mountainous regions. It is wellknown that water is one of the major triggers of landslides. Numerous landslide studies discuss different effects that water may have on slope stability: decreasing suction, rising groundwater table and subsequent increasing pore water pressure, groundwater exfiltration from the bedrock, seepage erosion, hydraulic uplift pressure from below the landslide, and influence of water on the plasticity of the landslide. Geological and hydrogeological characteristics such as permeability and saturation of the landslide and the substratum determine to a large extent, which of those processes are dominant in causing triggering. The first objective of this study is to investigate the origin, flow paths and effect of groundwater in three different slopes and landslides in Switzerland. These are 1) an artificially triggered shallow landslide in Rüdlingen located in the distal Molasse in the North of Switzerland; 2) the active Pont Bourquin landslide located in a tectonically complex zone in the Western Swiss Prealps; and 3) the Rufiberg, a slope prone to shallow landslides located in the Subalpine Molasse in Central Switzerland. Hydrochemical analyses, groundwater monitoring in observation wells and geophysical profiling are found to be promising methods to trace the origin and flow paths of groundwater in these three slopes and landslides. Based on hydrogeological, hydrochemical and geophysical data, three conceptual hydrogeological models are constructed which show the different effects groundwater may have on landslide triggering: Large joints in the bedrock are draining the loose soil cover in Rüdlingen, which has a stabilizing effect on the slope. At Pont Bourquin, groundwater flow through the fractured bedrock and the formation of perched groundwater in the heterogeneous highly-plastic landslide mass is found to be crucial for the triggering. And at Rufiberg, artesian or uplift pressure that builds up in the bedrock below the potential landslide may influence the triggering. These different case studies show how diverse and complex the hydrogeological pattern in landslides may be. So far, no classification system exists that is based on the dominant hydrogeological characteristics of landslides and slopes prone to landslides. Therefore, the second objective of this study is to develop a classification for the hydrogeological categorisation of landslides and slopes prone to landslides. This classification is based on the parameter "permeability contrast" between the landslide and the substratum (hydrogeological predisposition) and the time dependent parameter "saturation" of the different geological layers. The classification provides a tool to describe the hydrogeology of landslides and to evaluate the potential hydrogeological triggering mechanisms. It can be applied to illustrate the evolution of the saturation of the different geological layers during rainfall and snowmelt. This classification may be applied to estimate, whether a slope is in a critical equilibrium stage depending on the saturation. In the present study, the classification is applied to the three landslides and slopes investigated (Pont Bourquin, Rüdlingen and Rufiberg) and on seven other well-known landslides in the Alps. The classification supports the understanding of complex hydrogeological processes occurring in landslides. It is a framework for the construction of conceptual hydrogeological models of landslides and can be applied as a tool for planning further investigations, numerical modelling and mitigation measures.

Using geotypes for landslide hazard assessment and mapping: a coupled field and GIS-based method

Séverine Bilgot, Aurèle Parriaux

Switzerland is exceptionally subjected to landslides; indeed, about 10% of its area is considered as unstable. Making this observation, its Department of the Environment (BAFU) introduces in 1997 a method to realize landslide hazard maps. It is routinely used but, like most of the methods applied in Europe to map unstable areas, it is mainly based on the signs of previous or current phenomena (geomorphologic mapping, archive consultation, etc.) even though instabilities can appear where there is nothing to show that they existed earlier. Furthermore, the transcription from the geomorphologic map to the hazard map can vary according to the geologist or the geographer who realizes it: this method is affected by a certain lack of transparency. The aim of this project is to introduce the bedrock of a new method for landslide hazard mapping; based on instability predisposition assessment, it involves the designation of main factors for landslide susceptibility, their integration in a GIS to calculate a landslide predisposition index and the implementation of new methods to evaluate these factors; to be competitive, these processes have to be both cheap and quick. To identify the most important parameters to consider for assessing slope stability, we chose a large panel of topographic, geomechanic and hydraulic parameters and tested their importance by calculating safety factors on theoretical landslides using Geostudio 2007®; thus, we could determine that slope, cohesion, hydraulic conductivity and saturation play an important role in soil stability. After showing that cohesion and hydraulic conductivity of loose materials are strongly linked to their granulometry and plasticity index, we implemented two new field tests, one based on teledetection and one coupled sedimentometric and blue methylen test to evaluate these parameters. From these data, we could deduce approximated values of maximum cohesion and saturated hydraulic conductivity. The hydraulic conductivity of fractured rocks was obtained from the analysis of their geometrical properties (fractures density, aperture size and orientation). The other factors were extracted from DEM and hydrologic mapping. Then, we merged these parameters into three groups corresponding to three families of factors (gravitational, hydrodynamic I based on hydraulic conductivity contrast between superficial loose material and hard rock substratum and hydrodynamic II related to susceptibility to suffosion). We added a fourth factor related to the predisposition of the geotype (new classification for geologic formations, based on genetic standards for loose material and on lithologic standards for hard rock) to slope instability processes: the latter enabled us to integrate attributes proper to each geotypes (over-consolidation for ground moraines, stratifications for glaciolacustrine deposits, etc...) and which would be long and complex to integrate to a GIS. Afterward, we implemented an ArcGis® toolbox allowing to obtain automatically cohesion, hydraulic conductivity, saturation and slope from field parameters (granulometry, plasticity, fracturation, geomorphology and drainage) and DEM and to calculate geotype, gravitational, hydrodynamic I and hydrodynamic II factors; combining them, we led to a landslide susceptibility index. To know the relative importance of these four factors, we tried different weightings on four areas in different geologic contexts of Switzerland (flysch, molasse, crystallin) and of different sizes (from 0.1 to 2.5 km2). Finally, we applied this methodology (from field survey to GIS operations) to five other sites in Switzerland to check its validity: in flysch and molassic contexts, more than 90% of actual landslides were classified as “very susceptible to slope instability”. In Triassic area, more than 50% of the actual unstable areas were classified as "stable", which is not satisfying:in this case, it should be necessary to integrate another layer of data to take into account dissolution and plasticity of Triassic formations. The main assets of this new method are that the integration of field measurements in GIS provides for a good transparency in the landslide susceptibility index attribution and that the use of geotypes classification enables to use this method in most of European contexts.

Copernicus GmbH2009

Typologie géochimique des eaux des aquifères carbonatés des chaînes alpines d'Europe centrale et méridionale

This study describes the spatial heterogeneousness of the chemical composition of underground waters, particularly the trace elements, and the carbon-13 isotope in several carbonate aquifers of the alpine belt in Europe. The research forms part of the AQUITYP project which have been worked on the last 14 years by the Geology laboratory of the Swiss Federal Institute of Technology in Lausanne (GEOLEP). The main aim of the project is to define the typology of the aquifers, based on the chemical characterisation of underground waters. The carbonate aquifers has been divided in several groups chosen after geological, geographical and hydrological criterias. The geological criterias are petrography and depositional environments of carbonates. The geographical criteria is the altitude of the catchment area. The criteria hydrology is the type of flow in karstic systems. The first stage of this research consisted of a selection of 87 carbonate aquifers divided in 13 regions of study in the alpine belt between France and Greece. Aquifers were selected in order to guarantee the absence of external contamination in groundwaters. Sampling were carried out in a period of base-flow, when groundwaters are equilibrated with rocks. The analyse of 112 samples of water provided the basis of the observations of the various chemical and isotopic composition of groundwaters of carbonate aquifers. Lixiviation of rocks forming aquifers provides information about geogenic origin of some trace elements. The geological and hydrogeological context of aquifers The aquifers studied are composed of platform and basin limestones, that form part of the paleogeographic domain of the Tethys. One example of Devonian limestone belongs to the complex of the Palaeozoic of Graz is also included. Each aquifer has homogenous petrographic composition, but from one aquifer to another the petrographic composition may differ. In this research are included pure limestones, marly limestones and dolomites. From a hydrogeological point of view the 87 aquifers show different degrees of karstification, they are placed either in regions of mountains (caracterised by an important hydraulic gradient), either in region of plateau. The surface of the catchment areas is variable between ten km2 (for example the karstic plateau in Slovenia) and a few km2 (for example the Parmelan massif from subalpine belt in France). Discharges of springs are also variable, between 1 and 1000 l/s. The chemical characterisation of water The study of the chemical typology of the waters of carbonate aquifers are carried out on non thermal water rich in oxygen (median values: temperature 8.8°C, dissolved oxygen 11.8 mg/l, Eh 0.4 V, pH 7.2). The total dissolved solids differs between 100 and 600 mg/l. Underground waters are calcium bicarbonate type. Within these facies in some cases, the Mg and S04 content can be superior, thus defining two sub-facies. The iodine content in the groundwaters of carbonate aquifers usually shows higher values than the other types of aquifers of the project AQUITYP. Concentration in the groundwaters of carbonate aquifers is variable between 10 and 60 µg/l. The origin of iodine is caused by the oxidation of fossil organic matter present in the carbonate rocks. Trace elements allows to distinguish groups of aquifers composed of pure limestones, dolomite or deep sea limestones : pure limestones. The aquifers composed of pure limestones are marked by a very small concentration of trace elements. This can be explained by the poorness of the detrital materials (clays, Fe-Mn oxides, heavy minerals) and the compact lithology that reduces the surface of water-rock interaction. dolomite. The typical elements of dolomite aquifers are Mg, U and Mo. The high primary porosity and the brittle deformation of this type of rock favorise a better water rock interaction compared to poor reef limestone aquifers. Examples of this group of aquifers have been selected in the Italian Dolomites and in the Zone of Karavanke in Slovenia deep sea limestones. The barium is a geogenic tracer in some aquifers composed of basin limestones. The aquifers of the Maiolica and of the Scaglia in the Apennine and the Malm aquifer (spring of Grandchamp) in the Prealpes in Switzerland have a high level of Ba in groundwater (between 60 and 220 µg/l). The limiting factor of Ba mobility in this carbonate environment is the concentration of SO42- in the water. The SO42- makes the Ba precipitate in shape of barytes. This process explains the presence of barytes in fractures found in the aquifer of the Calcare Massiccio in the Apennines. The content of carbonate-13 in groundwaters allows to make a distinction between waters infiltrated in catchment areas with or without vegetation and soils. This isotope is also interpreted as a tracer of the type of underground flow in Karstic system (rapid flow in Karstic channels or slow flow in little fractures of the rock). For the project an example has been taken in Tanneben karst, in Austria. This karst shows that water with a slow flow from blocks has a higher 13C content in comparison with fast flow channel water. Finally some analysis of waters from the bottom of soils on the Jura belt in Switzerland showed that during little rain periods the content of isotope is comparable to the one of slow flow waters of the unsaturated zone in the karst. This demonstrate that the isotopic exchange with rock is completed at low horizon in the soil. The vanadium is a geogenic tracer in the Malm aquifer of the Jura belt in Switzerland and in the Cretaceous aquifer in the region of Trieste, in Italy. The origin of vanadium differs in these two examples. In the Jura belt the vanadium comes probably from marly layers of limestones of the low Portlandien. In the region of Trieste the element vanadium is present in bituminous carbonate sediments from inner platform (Calcari bituminosi from Paleocene). In conclusion the importance and the limits of the chemical criterion for the typology of aquifers are shown. This method of distinction of groups of limestone aquifers is one of the tools which permit the determination of the origin of water sources, particularly in underground and borehole construction projects.

EPFL1995

Méthodes d'analyse du fonctionnement hydrogéologique des versants instables

Pierre Tullen

Significant progress was made these last decades in the development of hydrogeological methods. However, these new methods are still very little used in the problematics of unstable slopes, mainly because of the hydrogeological complexity, which characterizes them (thin aquifers, discontinuous media, succession of saturated and unsaturated zones, lower permeability, higher hydraulic gradients, evolution of the permeability in time in relation to slope movements, etc.). Consequently, the approach usually developed to study slope instabilities is based on the setting-up of relations between hydroclimatologic parameters and slope movements. This approach exclusively concentrates on causes and effects, without developing the processes induced by these causes and responsible for these effects. This "black-box" type of relation is in some cases satisfactory, but in others, the analysis of the induced hydrogeological processes becomes essential to understand the functioning of the instability phenomenon and to predict the movements. Our project aims at introducing this missing link to allow a better understanding of the hydrogeological processes caused by particular hydroclimatologic conditions. It is a matter of determining the relation between hydroclimatologic phenomena and a critical hydrogeological state, which can lead a phenomenon of instability to reactivation or to a state of rupture. Unfortunately, this relation cannot be approached in the same way for all instabilities throughout the world. Owing to the specificity of each slope, the relation must be established individually, which may lead to a hydrogeological typology of unstable slopes. Accordingly, we propose a methodology comprising a succession of stages that are useful for the understanding of the hydrogeological processes underlying slopes. This approach aims at establishing a hydrogeological assessment on the scale of the slope or, if necessary, on a regional scale, in order to determine groundwater infiltration zones, trajectory and velocity of groundwater run-off as well as outlet areas. The usual ways of doing this are generally the measurement of the river flow rate, a pluviometric follow-up or direct hydrogeological measurements via piezometers. These traditional approaches generally remain limited by problems of space representativeness or life span; on top of this come problems of cost due to the often chaotic morphology of the slope instabilities and the fact that they are not easily accessible. To make up for these various problems, we propose a complete methodological approach to improve the understanding of the groundwater run-off systems for the unstable slopes: A characterization of the geological domain based on geophysical measurements in addition to the borehole data. This approach, original in the specific context of unstable slopes, makes it possible to obtain a three-dimensional representation of the heterogeneity which composes the subsurface grounds in a fast, efficient and economic way; A spatialization of the climatologic observations in order to take into account the distribution of the precipitations while considering the various states (rain and/or snow) that constitute the water contribution to a slope in relation to altitude; A hydrogeochemical characterization of the outlets, which makes it possible to assess the general groundwater circulation system within the slope, and to come to a decision about one or several possible trajectories of groundwater run-off; An isotopic characterization of the outlets, which allows estimating the altitude of infiltration of the groundwater. This characterization can also give some information on the infiltration zones. These methods also make it possible to characterize the general groundwater run-off system. In some cases, it is also possible to come to a conclusion, quantitatively speaking, concerning the mixing between several reservoirs and to estimate the time of residence of the water in the aquiferous system; A qualitative validation of the assumptions on the basis of artificial tracing tests which will attest the existence of hydrogeological connections between a catchment area and an outlet. This approach will also inform about the time of residence of water; A validation of the conceptual model on the basis of a bi- or three-dimensional numerical simulation of the groundwater run-off in order to validate the hydrological and hydrogeological relations. The calibration of these numerical models is based on a set of hydrogeological data, in particular the outlet discharge, the isotopic composition of water and/or their hydrochemical composition. These strictly hydrogeological numerical simulations can then be translated into a mechanical state to evaluate the stability of the slope. The whole of the points suggested by this methodological approach is not always essential to determine the hydrogeological processes that govern each slope. According to the characteristics of each study site and the means at disposal, some could be treated only partly, or not at all, depending on the established objectives. In the end, a hydrogeological typology of the unstable slopes should make it possible to propose a standard methodological approach and specific tools for analysis in direct relationship to the hydrogeological context which characterizes each slope. The methodological approach suggested in this PhD thesis was tested and validated on three representative sites through Switzerland and the alpine arc that present quite distinct characteristics of hydrogeological functioning. They are the slopes of Hohberg in the canton of Fribourg, Triesenberg in the Principality of Liechtenstein and Peney in the canton of Geneva. This validation phase allowed to assess the hydrogeological functioning of the three studied slopes and to evaluate the relation between the hydrogeological processes and the phenomena of instability. An approach such as that presented in this study opens up prospects to carry out means of sustainable stabilization with an aim of solving problems of slope instabilities.

EPFL2002