Monthly Archives: September 2013

Thomas Zufferey: Analyse et monitoring des mouvements gravitaires du versant sud-est du massif de l’Arpille

Thomas Zufferey
Supervisor: Prof. Michel Jayboyedoff
Expert: Andrea Pedrazzini

The present study concerns the deep-seated gravitational slope deformation (DSGSD) of the south-east slope of the Arpille massif. This rectangular mountain is part of the Aiguilles-Rouges external crystalline massif in the centre of the Swiss Alps close to the city of Martigny in the Rhône valley. Its glacial history is complex and left many traces. Two main lithologies compose the slope: crystallines rocks from Aguilles-Rouges massif, like orthogneiss and paragneiss, and limestones from the Mont-Blanc cover. In the area surrounding the tectonic contact between those two lithologies, rocks are laminated by a vertical alpine schistosity. This particularity affects the lower part of the south-east slope. This cleavage is totally toppled over the limit of the DSGSD. This is one of the main characteristics of the geometry of the Arpille DSGSD. The aim of the study is to develop a global understanding of this phenomenon. The results are based on in-situ investigations as well as geomorphological and geological analyses. Numerical and geometrical models have been developed to understand the role of pre-existent structures in the formation of the DSGSD and associated morpho-structures.

Damien Roth: Analyse microclimatique du Vallon de Nant

Damien Roth
Co-supervisors: Prof. Michel Jayboyedoff, Pascal Horton, Jean-Michel Fallot

This paper aims to treat and analyze a data set coming from four weather stations setup by Lausanne University’s IGAR (Institut de Géomatique et d’Analyse du Risque) in Prealps, within the Vallon de Nant.

This work has two main goals:

  • Processing the data set of all four stations from the 19th of December 2007, setup date of the first station, to the 15th of August 2010.
  •  Analyzing temperature data coupled with wind data, azimuth and speed, in order to understand the peculiarity of observed phenomena.

Processing and pooling the data set was performed using Matlab®, which enabled better availability of data for analysis. Processing allowed us to exclude outlying values from the data set and brought to light a few temporal problems in the data set of several stations. It also made us aware that some data contained in the stations database were actually coming from another site.

The analysis’ aim to understand the phenomena happening within the Vallon de Nant showed several temperature inversions, mostly at low altitudes, as well as wind direction changes which we couldn’t fully explain. It also highlighted the difficulties that some precipitation sensors experience at high altitudes and in harsh winter conditions.

Martin Franz: La falaise des Bornes du Diable : de l’Instabilité au Tsunami.

Martin Franz
Co-supervisors: Prof. Michel Jaboyedoff, Prof. Yury Podladchikov, MSc. Clément Michoud
Expert: Dr. Marc-Henri Derron

The case study, “les Bornes du Diables” cliff, 160 meters high, is located directly above the Mauvoisin dam’s reservoir in south western Switzerland. This study is conducted due to recent rock falls that reached the lake and damaged an alpine road and due to the clear presence of back cracks. The important rock fracture, the thin schistosity and the highly weathered surface contribute to a Geological Strength Index betweeb 35 and 45. The entire case study is located in the Penninic domain. The “marbres phyllteux roux” (foliated marbles) and the base complex that compose the cliff belong to the Tsaté nappe. The albitic gneiss is part of the Mt.Fort nappe. The boundary between both these nappes is the base of the cliff itself.

In order to assess the consequences of a potential collapse of the instability for the road, the dam and the reservoir of Mauvoisin, the aim of this study is to characterize the structural settings, to quantify the movements occurring in the area as well as to compute the propagation of a possible landslide generated impulse wave.

The structural analysis is performed using both field observations and TLS point clouds processed with Coltop3D. Seven discontinuity sets are identified. A kinematic analysis results in the identification of three failure mechanisms they are wedge and planar sliding as well as toppling.

Three discontinuity sets match the observed back cracks. They divide the cliff into five compartments basally delimited by the contact between the Mt Fort nappe and the Tsaté nappe. These blocks volumes range from 2’000 m3 to 500’000 m3. The mane mechanisms controlling the blocks are planar sliding, toppling and a composite mechanism regarded as a step-like sliding (pseudo-circular failure).

In 2009 and 2010, DGNSS measurements were performed on seven determined points in order to characterize recent movements. Five of these measurements are “RTK fixed” it allows a 2 mm horizontal and a 4 mm vertical precision. The suitable calculated displacements measure 3 to 7 cm.

To determine the longer trend displacements, simultaneously orthorectified aerials measurements and stereophotogrammetry are respectively applied to 1979 and 2000 aerials and 1961 and 2009 aerials pairs. ERDAS IMAGINE® software and more specifically the LPS and StereoAnalyst® components were used. The resulting displacements measured are of around 1 cm per year.

Using both movement quantification and structural analysis allows explaining the complete potential collapse mechanism through a 3D static model. It is supported by a model performed with the UDEC® software. The model supports the failure mechanism and the displacements measured.

The potential collapse could generate an impulse wave in the dam’s reservoir that could lead to catastrophic consequences. The wave’s characteristics i.e. its height, run-up and over topping are calculated using the VAW model (ETH Zurich). In addition the wave propagation is calculated using a 2.5D model developed on MATLAB®, which is based on the shallow water equation. The results show the propagation and associated consequences of such a wave. Those latter are characterized by a 3 m high overtopping, in the condition of the lake at its maximum level.

Vincent Dauphin: Evaluation de la vulnérabilité des bâtiments d’habitation individuelle à Delémont ; vers une nouvelle approche de la gestion du risque d’inondation.

Vincent Dauphin
Co-supervisors: Prof. Michel Jayboyedoff, Marc Choffet

The vulnerability, when it’s not defined quantitatively, tally with a rate of damage. However, this definition is not commonly accepted, which is certainly harmful to the consideration of its fair value. In a context of risk management bound to flooding hazard, consideration of the vulnerability may bring different concrete solutions, as the increasing number of publication can prove it. However, it has difficulties to impose itself as an applicable concept to reduce the risk at an acceptable level.

The existing literature also leaves a part of vulnerability to the little investigated floods, it is the quantitative approach. The possibility to use a tool recently developed to quantify the amount of damages potentially reached by a building of individual house, according to the height of water reached in the building, to explore this leaved part. The obtained result is under the shape of a curve of amount of damages and height of water describing and quantifying in a precise manner the constructions materials, caoatings and equipments in the building. The tool attribute a damage capacity to each elements described and its replacement value. However this data capture is relatively tedious if a high number of building must be analysed. Thereby the principal idea of this achieved work is to propose a typology of the built reflecting as a satisfied manner the reaction in front of floods for all kinds of buildings.

Once the typology established for each building on the zone of study – the maximal extension zone of a flood of Sorne, on the municipality of Delémont (Jura, Switzerland) – which is confronted to different buildings analysed in details. Finally quick damages on average, depending on the type of buildings, have been obtained to be spread to the totality of a portfolio composed of a high number of individual residential buildings in the zone of study.

However the vulnerability is bounded to the chance of a risk study. An analysis work of chance of the zone of study trying to define the heights of water that can reach the buildings is presented for the study zone. The second step of the practical approach aims the validation of the typology and the quantification of the vulnerability of the objects that can be find in the portfolio. Once the chance and the vulnerability are quantified, it is possible to quantify the risk, integrating the total potential loss into the approach (issued of the insurance database.)

All the concepts bounded to the management of the evaluation of the risk of flood is presented in the dedicated theoretical part. In the practical part it quickly has needed to face a major problem, the incompatibility between insurances data of the buildings and the georeferenced data of the GIS. Indeed the insurances data are organised around the address to define the building with which they are attached, while the SIG data use either the geographical coordinates, or the specific identifiers. The tedious work of compatibility between the two types of data was made manually.

In the end, the approach that is proposed – although exploratory –l allows to quantify the flood risk by each building of the portfolio by the quantification of the three elements of the equation risk : the vulnerability, the hazard and the total potential loss.

Nicolas Sarrasin: Etudes d’instabilités rocheuses dans le Saastal et le Mattertal (Valais, Suisse)

Nicolas Sarrasin
Supervisor: Prof. Michel Jayboyedoff
Assistant: Andrea Pedrazzini

A lot of alpine areas are characterized by ground movements. These may represent a danger for people and infrastructure. These phenomena are described and studied for a long time. However the advent of new technologies can improve these analysis.

LiDAR (Light Detection and Ranging) represents a new technology which modernizes the traditional approach of field measurements. LiDAR is therefore defined as a powerful tool able to take remote measurements through laser technology. It allows an analysis in three dimensions adapted to the characterization of rock instabilities. The processing of information gathered makes possible a multitude of tests. LiDAR is so considered a promising tool for the study of risks and natural hazards.

This Master Thesis will use LiDAR technology to analyze the structure of mountain ranges. This method will also highlight the movements of blocks and it will serve as the calculation basis of susceptibility to fracture in rock walls. The target of this study is to identify rockfalls using semi-automatic approaches based on LiDAR surveys. A kinematic analysis of the structure will also be undertaken to arrive at a definition of the susceptibility of rupture for massive Jegihorn of Gabelhorn and Durlochhorn (Mattertal and Saastal, Valais). All analysis performed will aim to highlight areas prone to failure mechanisms and each of the specific results will be compared to observations made on the ground.

Maria Elena Fonticoba: Etude structurale et GPR de la région de Varneralp (Valais)

Maria Elena Fonticoba
Co-supervisors: Prof. Michel Jayboyedoff, Prof. Dr. François Marillier
Assistants: Andrea Pedrazzini, Jehanne Correia-Demand

This work aims the understanding and characterization of a possible deep seated gravitational slope deformation in Sierre (Valais, Switzerland). A catastrophic rock avalanche occurred a long time ago causing the deposit of about 1 km3 of material in the Rhone valley close to the town of Sierre.

The uphill area of this collapse is the subject of this master thesis. The area is occupied by the pastures of Varneralp and underwent significant displacements. Their chronology relative to the collapse has not yet been established.

This work has two main objectives:

  • The characterization of the mechanisms and magnitude of displacements.
  • The study area is used to test the contribution of GPR to highlight fractures in deep seated gravitational slope deformations.

This study was carried out using different methodologies:

  • A structural analysis based on field observations proves the presence of deep and surface deformations. 5 joints families have been highlighted in the region. Two of them are actively involved in landslide mechanisms.
  • The creation of a geomecanical model has been used to check the chronology of deformations. The movements in Varneralp seem to appear after the collapse start.
  • 20 GPR profiles have been carried out. Two joint networks have been highlighted. They correspond to joint families which are present in the region. Despite of acquisition difficulties, the GPR has brought complementary information.

Jacques Bechet et Julien Duc: Etude et quantification des processus et dynamique d’érosion à Draix

Jacques Bechet et Julien Duc
Co-supervisors: Prof. Michel Jayboyedoff, Alexandre Loye
Expert: Dr. Marc-Henri Derron

The Roubine catchment located in the experimental research station of Draix (south French Alps) is situated in the black marls formation, a substratum particularly prone to weathering processes. Since 25 years this small watershed (0.13ha) has been dedicated to the monitoring and the study of hillslope erosion processes at the scale
of the elementary gully.

Since 2007, a monitoring of topographic changes of the entire Roubine catchment has been performed with terrestrial laser scanner (LIDAR) in order to observe and quantify the sediment production and the evolution of the gully in terms of sediment availability and transport capacity vs. precipitation and runoff generation. LIDAR data have been acquired periodically based on the seasonal activity with a very high point cloud density ensuring a resolution of the DEM at the centimeter scale. The topographic changes over a record spanning of 4 years are analysed by comparing the time series of LIDAR data.

Quantitative observation of the seasonal erosion activity and a fine-scale analysis of the sediment flux contributing to an important recharge of tributary gullies and rills with debris into the channel network could be performed. According to the transport capacity generated by the runoff during the season, freshly-built sources of regolith are removed at different periods of the year, forming transient deposits in the main reach when routed downward, evolving from a transport-limited to supply-limited regime through the year. The acquired high resolution point cloud enable to perform a local scale morphostructurales analysis in terms of stability and sediment production rate of the catchment slopes supplying the rills and gully with sediment.

This study aims to quantify the erosion in the Roubine’s watershed, through the comparison of the different LIDAR time series. Comparison of LIDAR Data mass balance with the measures taken by the limnigraph located at the end of the catchment was processed. Our results show that Terrestrial laser scanner is a relevant method to quantify the erosion. The results have been used for the realization of a gully’s analogical model in laboratory.

These experiments attempt to document toward a better understanding of erosion processes, which affect the badland-type terrain, by illustrating the mode of sediment production and temporal storage/entrainment in the adjacent slopes of the Roubine major gullies.

Stephanie Jaquet: Forest cover trends and landslides occurrence in selected areas, Dolakha District, Central Nepal

Stephanie Jaquet
Supervisor: Prof. Michel Jayboyedoff
Experts: Dr. Marc-Henri Derron, Dr. Jane Carter

The purpose of this thesis is to study forest cover changes and degradation trends as well as to make risk assessments for selected study cases in Dolakha district, Central Nepal over the period 1979-2009.

The forest cover is an important issue in Nepal. It decreased for decades; however, since the Forest Act in 1993, Community Forests have been established in all districts in Nepal. Among other benefits, this change in the forest management is supposed to reduce the deforestation rate. Although the deforestation rate is still negative, -1.35 % annually (Petley, 2007), the present study shows a general increase of the forest cover in the area of study (Central Nepal). In 2009, forest covered about 46.3 % of the study areas compared to only 28 % in 1992. The study shows a clear increase of forest cover in gullies, formerly degraded areas, expanded forest areas and next to rivers. Community forests are essential for mountain populations as forests bring the communities income and reduce erosion. Where improved forest policies and rules are applied, better forest management has made visible improved changes in forest cover.

Risk has been assessed in three different areas with different types of landslides. In Thang
Thang (Suspa Village Development Committee (VDC)), there is a slow-moving landslide and potential debris flows. The risk is high in this area mainly due a high potential for debris flow partly caused by an uncemented irrigation canal. In case of heavy rainfall the debris flow will be a sudden event, which cannot be prevented and monitored like a landslide. In addition, poor forest management and lack of resources have made this village very vulnerable to all hazards. In Kothepole and Phedi (Suspa VDC), landslides which occurred in 1975 are nowadays covered by forests even if landslides are still slowly active. In Khariswara (Gairimudi VDC), a sudden large landslide occurred the 15th of September, 2010 during our field trip. Our risk assessment shows that risk depends mainly on distance to the scarp. Even though risk is high, the high education of the people of Khariswara and their good organization have reduced their exposure. Indeed, they were actively monitoring the landslide three times a day during the rainy season. The last case study is located in Eklebari (Bhirkot VDC) where there is a slow moving landslide, which is threatening mainly houses upslope without posing a great risk for people. A district road crosses the same landslide downslope and its more active parts are threatening the road. The road has already been blocked several times in the past decades and more recently for 14 days in 2010. The lack of proper maintenance is making the road even more susceptible to landslides as the steep slope is undercut. Finally, in the north of this landslide, there is a school which is threatened by debris flows, mitigation measures have been taken but they are not sufficient to protect school buildings.

Proper mitigation measures are difficult to undertake mainly because of the lack of resources. Gabion walls are the most commonly used method with drainages and bioengineering to mitigate roads. Thus forests are a good way to mitigate shallow landslides. This study shows how landslides and forests are linked. If forest degradation is considerable, erosion is more likely and slopes are more prone to landslides and gully erosion. Forests are important for stabilizing slopes and protecting infrastructures from rock falls. Furthermore, even though in some cases (i.e. deep-seated landslides) forests cannot help to reduce landslide hazard, it can at least provide people with fodder, timber and income.

Marie Charrière: Granulometrical, geological and morphological description of the Frank Slide deposit (Alberta, Canada)

Marie Charrière
Supervisor: Prof. Michel Jaboyedoff
Expert: Ing. Geol. Corey Froese

The Frank Slide (Alberta, Canada) is one of the rock avalanches which was the most studied throughout the History of the research on this kind of landslides. Indeed, it was already analyzed just after its occurrence in 1903, where 70 people died. Nevertheless, a complete description of its deposit was never conducted.

Hence, this study proposes the results of such an analysis based on field survey and numerical methods. The description of the deposit is conducted following several parameters. The spatial pattern of the surface’s granulometry, resulting from a field sampling, is complete by a remote sensing analysis which uses morphological filtering. It appears that globally the bigger blocks are found in the central part of the deposit. Granulometrical indicators, such as the diameters corresponding to each volumetric frequency and the uniformity and curvature coefficients, present a partially sinusoidal spatial distribution in the longitudinal direction of the propagation.

In addition, the spatial distribution of the geological formations, which conserve the original stratigraphic sequence, is enhanced. It allows proposing a mode of collapse and propagation of the mass. The hinge area, which is supposed to have been the weakest
point of the Turtle Mountain anticline, seems to have been the starting point of a shearing mechanism. After the failure, during the motion, the normal limb overrode the inverse limb. Thus, the Banff and the Lower Livingstone formations, which constituted the hinge, travelled at the front of the moving mass and reached the farthest distance. In this configuration, the Upper Livingstone formation, which was initial located at the top of
Turtle Mountain, must have been positioned at the end of the rock avalanche during the propagation. This can explain its presence in the proximal part of the deposit. The Mt-Head formation was initially positioned at the toe of the fell mass. It was hence overridden by the other part of the moving mass. Its actual position on the deposit surface suggests that it gradually stopped during the motion.

This study is also constituted of morphology’s analysis of the deposit. Based on GIS results such as, among others, shaded relief map, roughness index map, azimuth map and plan curvature map, it highlights the presence of morphological features like longitudinal ridges, flowbands, hummocks and compression ridges. Moreover, a heterogeneity of the flow in enhanced by the distinction of the three different behaviors. This allows understanding the process of transport.

The Sloping Base Level Method (SLBL) is also used in this study. It permits to reconstruct the original topography of the mountain and to simulate the spatial distribution of the deposit’s thickness. Furthermore, it leads to an upward revaluation of the volume of the Frank Slide avalanche to approximately the double of the conventional value of 30 millions cubic meters.

Florian Humair: Turtle Mountain anticline (Alberta, Canada): Rock slope stability related fracturing

Florian Humair
Co-supervisors: Prof. Michel Jaboyedoff, Dr. Jean-Luc Epard
Expert: Ing. Geol. Corey Froese
Assistant: Andrea Pedrazzini

The aim of this study is to improve the general understanding of the Turtle Mountain anticline and its fracturing pattern as well as evaluating the implications of these latter two features in potential rock slope failures.

Turtle Mountain anticline is located in southwest Alberta (Canada) and is part of the Livingstone Thrust sheet of the Foothills. The latter form the eastern part of the Canadian Rockies which are located in the easternmost part of the Canadian Cordillera. The study area became well known when a 30 Mm3 rock avalanche of massive limestone and dolostone, known as the Frank Slide, affected the eastern mountainside of Turtle Mountain on April 1903. It resulted in more that 70 causalities and buried part of the Frank village.

This study focuses on the structural features of the anticline, comprising the characterization of the fold geometry and formation mechanism as well as the related fracturing pattern using a high resolution digital elevation model (HRDEM) combined with a detailed field survey. The investigations allow describing the anticline as an eastern verging fold with multiple hinges that merge at depth. Field evidences revealed the expression of both flexural slip and tangential longitudinal strain folding mechanisms involved in the development of the fold.

Ten discontinuity sets are identified in the study area. The local variations of the sets are estimated in order to separate the study zone into homogenous Structural domains.

Based on the different tectonic phases affecting the region, the potential origin of the detected fractures is interpreted. It results that the origin of the four major discontinuity sets, in the sense that they are spatially homogenously represented, is attributed to the folding phase. Except the bedding plane and a pre-existing joint set, the other discontinuity sets are interpreted as resulting of post-folding deformations.

The conduction of a Rock mass condition analysis allows pointing out the influence of the fold geometry revealing a general decrease of the Rock mass strength with the proximity of the axial surface of the anticline. It also illustrates the importance of the high fracturing density of the hinge area on the weathering of the mountain. A predictive modelling of the GSI (SVR by Matlab) completes and supports this analysis.

A field and remote sensing analysis of the large open back cracks of crown area of the mountain (between North Peak and Third Peak) is performed in order to investigate the deformation modes affecting the rock compartments. It illustrates the control of the fold related fractures in the cracks’ geometry and allows highlighting four main sectors characterized by distinct deformation modes. Additionally, it is estimated that the cumulative displacement of the top of the mountain reaches at least 10% due to the opening of the cracks.

Using both field measurement and numerical modelling (UDECTm), a slope stability analysis of a rocky spur NE of North-Peak is performed,. The results show that a volume of 0.2 Mm3 is likely to collapse following a bi-planar sliding (or either a wedge sliding in a lesser extent) failure mechanism. The joint sets involved in the failure mechanism are those which developed during the folding phase. The potential unstable volume is also calculated using the Sloping Local Base Level (SLBL) and there is a good accordance between both methods.

Globally, this study highlight that the fold related fracturing plays an important role in the destabilization of the mountain. Indeed, due to its homogenous spatial distribution in the study area as well as its geometrical configuration, it constitute an important predisposing I factor leading to potential present day rock slope failures.