EGU 2021 : Submitted abstracts

The virtual EGU assembly took place from 19 to 30 April 2021. We submitted several abstracts to present the main topics and projects we are working on in our group.

Quantification of erosion rate in crop field gullies from point clouds with two different methods : the case study of Savigny crop field (North of Lausanne, Switzerland)
Landslide investigation using Remote Sensing and Geophysics
Arduino based low-cost short-range terrestrial LiDAR Scanner
Oxidative weathering deterioration of black shale and its bedding shear failure modeling
Thermal imaging and rock slabs deformation due to daily solar radiation
Implementation of InfraRed Thermographic surveys in complex coastal areas: the case study of Polignano a Mare (southern Italy)
Preliminary statistical analysis of the Ticino landslide inventory
Development of Rainfall threshold model for the watershed/sub-watershed landscape at data scarce situation – a case study of mid-hill region, Nepal.
Glacial Lake Outburst Floods Early Warning System to save lives and livelihood of the Nepal Himalaya communities: A case Study of Imja Glacial Lake, Nepal
The role of freezing-thaw cycling in rock samples topography evolution and rock cliff retreat
Cross-Bedding and Structural Mapping for Rockfall Assessment of a Tunnel using Hi-Resolution LiDAR (Fribourg, Switzerland)
2D quantitative analysis of fractures from high-resolution photos for the geomechanical characterization of rock masses
Shear banding in elasto-plastic slumping process on a GPU: mechanical and hydromechanical MPM solver
Assessment of multitemporal INSAR data for establishing regional landslides inventories

1- Quantification of erosion rate in crop field gullies from point clouds with two different methods : the case study of Savigny crop field (North of Lausanne, Switzerland)

Charlotte, W., Tiggi, C., Marc-Henri, D., Li, F., and Michel, J.

The spreading use of remote techniques is in our daily life benefits to ease and/or speed up the acquisition and analysis of geographical data that can be meaningful for risk assessment or for taking decisions for prevention measures.

Here is presented one of the possible applications for the Unmanned Aerial Vehicle (UAV) acquisition, to evaluate the volume of eroded soils in a crop field due to washout after heavy rains. The case study is located North of Lausanne (Switzerland), in the village of Savigny. It is a crop field with a gentle slope where we can clearly see washout gullies appearing after rainfalls. A great number of small water streams disappeared for more intense agriculture which is the case here : According to topographical maps, a small stream was flowing in the past but disappeared after 2004. It is interesting to see that after important rainfalls, gullies appear that could correspond to old small stream patterns.

The data acquisition survey of October 30th, 2020 was done by means of a DJI Phantom 4 RTK flying at an altitude of about 20m and the Pix4d Capture planning mission application. To process the obtained 800 images, the new Pix4D Matic software was tested to get a fast dense point cloud with GSD ~1 cm accuracy, a DEM and an orthophoto. The dense point cloud was then analyzed with two compared methods to estimate the washout volumes, which are (1) inverse Sloping Local Base Level; and (2) Point cloud segmentation based on normal vectors and curvatures.

As a result, these two methods gave a first estimation of the eroded volume of around 15m3 over a surface of 9 hectares which corresponds to an erosion rate of 1,7m3/hectare. These remote and non-destructive techniques are fast and easy compared to conventional field surveys, and the data acquisition and processing could be automated. In conclusion, these techniques provide a relatively low-cost time-series datasets processing to monitor and quantify the ongoing gully erosion.

Further investigation would be to keep recording the volume and erosion rate estimations after important rainfalls, when clear new gullies appear and to record in the meanwhile the rainfall intensity. This could help assess in a second step the relationship between the erosion rate and the rainfall intensity and control if this relation follows a power-law function. Such a study could also give some clues about the possible impact of climate changes on erosions in crop fields.

2- Landslide investigation using Remote Sensing and Geophysics

Derron, M.-H., Bossuat Pahud, D., Thuegaz, P., Bertolo, D., and Jaboyedoff, M.

Over 2700 gravitational movements are recorded as polygons in the inventory of the Aosta region (3261 km2, Northern Italy). The surface affected by gravitational processes is about 20% of the overall surface area of the Aosta region and corresponds mostly to deep seated slope deformations, landslides and rock slope collapses. In addition, a complete set of multitemporal INSAR data has been recently made available for the same area (SqueeSAR processing by TRE, for both ascending and descending orbits, from October 2014 to February 2020).

In a first step, the distribution of INSAR data was analyzed with respect to landcover and radar geometric deformations. Main outcomes are:

About 732’000 points were found by INSAR corresponding to a total average density of ~220 pts/km2.
0% of points have velocities below 1 mm/y, 20.4% between 1 and 10 mm/y, and only 0.6% more than 1 cm/y.
The landcover is forested over 30% of the surface, covered by low vegetation on steep slopes for 46% and unvegetated for 24%, Points density are respectively 146, 200 and 369 pts /km2.
Less than 5% of the Aosta region is affected by radar layover or shadowing. But, considering the slope direction as possible vector of displacement, 60% of INSAR velocities are underestimated of 50% or more when projected on the line of sight of the satellite (of course most of the time these are not the same slopes for ascending and descending orbits).

In a second step, we assessed the information provided by INSAR for the landslides recorded in the IFFI inventory:

29% of the polygons of the IFFI inventory do not include INSAR pts. However, those are mostly small zones, corresponding to only 9% of the total surface mapped as affected by gravitational movements. Most of large instabilities have INSAR points. 52% of the polygons have INSAR points from both ascending and descending orbits, and 19% from only one orbit.
68% of IFFI polygons have all their INSAR velocities slower than 5mm/y (for both orbits). It doesn’t mean automatically that these instabilities are dormant or slow moving, because for about half of them INSAR velocities strongly underestimate expected real velocities because of unfavorable projection on the line of sight of the satellites.
55 instabilities show INSAR velocities between 50 and 10 mm/y, and 31 faster than 10 mm/y.

Finally, an independent inventory was made using only the INSAR data and then compared to the IFFI inventory. In order to handle the data, a minimum velocity 2.5 mm/y was selected.

1437 instabilities were mapped in this inventory, covering 308 km2, for 2702 instabilities over 604 km2 in the IFFI inventory.
About 60% of the moving area detected looking only at the INSAR data are visible on only one orbit (ascending or descending).
62 clusters of INSAR points with velocities higher than 1 cm/y and not in the IFFI polygons were detected. Among them, 4 sites with significant extensions will require further geological investigations.

3- Arduino based low-cost short-range terrestrial LiDAR Scanner

Gygax, C., Derron, M.-H., and Jaboyedoff, M.

The search for low-cost equipment solutions in geomatics and other domains is a theme that is increasingly addressed by a growing number of researchers. Today, the open-source resources and the availability of cheap electronic equipment and easy to program microcontrollers to manage them (e.g. Arduino) make this type of research accessible to everyone.

The goal of this project is to assemble, program, test and evaluate a low-cost short-range terrestrial LiDAR scanner, i.e. a device that can scan a surface with a laser and represent it in 3D as a point-cloud. An initial prototype was assembled and programmed from low-cost electronics and mechanical components partly ordered and partly 3D printed, at a total cost of around USD 340. Conceptually, the operation of the device is simple: two stepper motors drive a laser sensor on two axes (horizontal and vertical), and a distance measurement for each of the motors positions is taken. These components are controlled by an Arduino Mega 2560, a powerful microcontroller known for its simplicity and versatility, which also receives the measurements and stores them on a SD card. A smartphone application was also developed to send scanning parameters to the LiDAR via Bluetooth. This first prototype detects on average 150 points/second at a maximum distance of about 40 m with an average error of 2 cm and a maximum resolution of less than 0.012° (1 point every 2.9 mm at a distance of 15 m).

Initial tests of the device in the laboratory and in the field are encouraging. In order to obtain a better-performing device, some mechanical components will be improved (to make the device more robust and reduce vibrations), a better-performing laser sensor installed (less error and higher maximum distance of at least 100 m) and a small solar panel coupled , so that the device can be tested in the field on several consecutive days.

This device will have two main applications: 1) it will be used for continuous monitoring in areas where the probability of destruction is too high to put a commercial device thousands of time more expensive; 2) it is planned to develop a DIY kit to be used by students in geosciences to understand the principles of laser scanning.

4- Oxidative weathering deterioration of black shale and its bedding shear failure modeling

Sun, C., Derron, M.-H., Jaboyedoff, M., and Wu, X.

This work investigated the oxidative weathering deterioration of black shale along a bedding slip zone and how it affects the bedding shear failure in the Xujiaping landslide, southern Sichuan Province in China. Many dissolved pits were found on the limestone, and part of the black shale in the slip zone is mud-like and clastic, showing local shear failure, which can be one of the main reasons of slope instabiliy. The microstructure of black shale under oxidative weathering condition was observed by scaning electron microscopy (SEM), characterized by dissolved pores, weathering crust (iron sulfate) of pyrite crystals, and the filling gypsum crystal in the bedding foliation. The deterioration mechanism was expanded: (i) rock-forming and carbonate minerals were especially prone to dissolution by sulfuric acid from black shale oxidation in the slip zone, and (ii) volume expansion due to the crystallization force of precipitated minerals caused further fracture expansion and deformation. Therefore, two theoretical models were developed that use stoichiometric calculations of pyrite and calcite to determine the dissolution rate and the rock structure after chemical weathering; and establish a rock structure model characterized by foliation weakening of gypsum crystallization. In order to analyze the landslide failure, discrete element method (DEM) is used to analyze the black shale shear failure mechanism of the two degradation models after oxidative weathering. It will be useful to better understand how these oxidative weathering deterioration contribute to bedding shear failure in natural hazards.

5- Thermal imaging and rock slabs deformation due to daily solar radiation

Derron, M.-H., Maillard, L., Fei, L., Jaboyedoff, M., and Guérin, A.

In the Yosemite National Park, it has been shown that large granitic exfoliation sheets can be subject to spectacular daily deformations (with cracks opening and closing with a magnitude of up to 15 mm over 24 hours). These thermal deformations, observed during hot summer days, are known to contribute greatly to rock falls. However, it is questionable whether this kind of deformation only occurs with exfoliation flakes (which have very particular shapes), or if it can be observed on more common rock faces geometries. Moreover, does this phenomenon only occur during hot summer days or also in other seasons?

To answer these questions, cracks and slabs in two sedimentary rock walls were monitored over 24-hour cycles, in summer and winter. The first site is in the massive limestones of the old quarry of St-Triphon (in the Swiss Prealps), the second one in the cliff of la Cornalle (near Lausanne), an intercalation of poorly consolidated sandstone and marls. Air and contact temperature loggers, a pyranometer to measure the incident solar radiation and crackmeters were used in situ. Thermal images were acquired every 20 minutes (surface thermocouples sensors and aluminum reflectors are used to constrain the surface emissivity and the environmental radiative temperature).

First it was shown, that during sunny days, the amplitude of the daily variation of the rock surface temperature is as large in winter than in summer (up to 30°C). As expected, this amplitude is larger in the detached slab than in the massive rock mass. In both sites, the deformation measured in the cracks reach about 0.2 mm. Depending on the slab geometry and its “attachment points” with the main rock mass, an increase of temperature can correspond to a closing or to an opening of the cracks. In conclusions, however these daily deformations are about two orders of magnitude smaller than those measured in the Yosemite big walls, they appear to occur also in “common” rock faces all around the year. On the long term, these deformations will contribute to the rock weakening at sub-surface conditions.

6- Implementation of InfraRed Thermographic surveys in complex coastal areas: the case study of Polignano a Mare (southern Italy)

Loiotine, L., La Salandra, M., Andriani, G. F., Apicella, E., Jaboyedoff, M., Parise, M., and Derron, M.-H.

InfraRed Thermography (IRT) spread quickly during the second half of the 20^th century in the military, industrial and medical fields. This technique is at present widely used in the building sector to detect structural defects and energy losses. Being a non-destructive diagnostic technique, IRT was also introduced in the Earth Sciences, especially in the volcanology and environmental fields, yet its application for geostructural surveys is of recent development. Indeed, the acquisition of thermal images on rock masses could be an efficient tool for identifying fractures and voids, thus detecting signs of potential failures.
Further tests of thermal cameras on rock masses could help to evaluate the applicability, advantages and limits of the IRT technology for characterizing rock masses in different geological settings.
We present some results of IRT surveys carried out in the coastal area of Polignano a Mare (southern Italy), and their correlation with other remote sensing techniques (i.e. Terrestrial Laser Scanning and Structure from Motion). The case study (Lama Monachile) is represented by a 20 m-high cliff made up of Plio-Pleistocene calcarenites overlying Cretaceous limestones. Conjugate fracture systems, karst features, folds and faults, were detected in the rock mass during field surveys. In addition, dense vegetation and anthropogenic elements, which at places modified the natural setting of the rock mass, represent relevant disturbances for the characterization of the rock mass. In this context, IRT surveys were added to the other techniques, aimed at detecting the major discontinuities and fractured zones, based on potential thermal anomalies.
IRT surveys were carried out in December 2020 on the east side of the rock mass at Lama Monachile site. Thermal images were acquired every 20 minutes for 24 hours by means of a FLIR T-660 thermal imager mounted on a fixed tripod. Ambient air temperature and relative humidity were measured during the acquisition with a pocketsize thermo-hydrometer. A reflective paper was placed at the base of the cliff to measure the reflected apparent temperature. In addition, three thermocouple sensors were fixed to the different lithologic units of the rock face. These parameters, together with the distance between the FLIR T-660 and the rock face, were used in order to calibrate the thermal imager and correct the apparent temperatures recorded by the device, during the post-processing phase. Successively, vertical profiles showing the temperature of the rock face over time were extracted from the thermograms. Thermal anomalies were correlated with stratigraphic and Geological Strength Index profiles, obtained by means of field surveys and Structure from Motion techniques. The presence of fracture and voids in the rock mass was also investigated.

7- Preliminary statistical analysis of the Ticino landslide inventory

Gutierrez, C. A., Jaboyedoff, M., Pedrazzini, A., and Derron, M.-H.

The canton Ticino, Switzerland, with an alpine setting and humid climate, is exposed to a great number of natural events, among which are gravitational movements. The StorMe inventory, which is compiled by the Forestry Section from the Republic and Canton of Ticino, contains all the relevant information regarding recorded natural events during the last 20 years, including rockfalls, debris flows, floods, landslides and avalanches (Galfetti et al., 2019).

Annually, millions of Swiss francs are invested for risk management for natural events (Galfetti et al., 2019): base studies, precautionary measures (such as mitigation works, and the protection and maintenance of woods) and monitoring. Considering landslide inventories are important sources of information for hazard and risk assessment, it is crucial to exploit the existing data in order to gain a better understanding of the specificities of the processes present at a regional scale.

The most significant statistical properties of landslides derive from geometrical parameters such as landslide area or volume; which can be used to calculate the size, frequency and potential distribution of future landslides as well as the contribution of sediment yield to erosion.

Here, a statistical analysis was carried out using the StorMe inventory and additional data, in order to better understand the spatial and temporal distribution of events in the study area, their geometric characteristics (distance and angle of propagation, volume) and their relation to the soil/rock type, land use, and climate. The inventory consists of both spatial attributes (points and polygons) and linked attributes. Additional input data included topographical, geological, land cover and previously created hazard maps; both in raster and vector formats.

Preprocessing of the available data included the calculation of derived attributes (slope, curvature, elevation, area, perimeter, among others…) and the joining of spatial and textual data. Bivariate and multivariate statistical analyses were carried out first on the whole inventory (including time series, spatial distribution, volume distribution, frequency–area distribution and inventory quality) and then analyses on controlling factors (mainly elevation, slope, lithology and land cover) for each different type of process was carried out.

Preliminary analysis results show a few general trends regarding chiefly the distribution of landslide types, volumes, propagation distance, and reach angle (Farböschung); as well as some local anomalies. More in-depth analysis using machine learning will be carried out in the future in order to determine main controlling factors for each movement type in the study area.

8- Development of Rainfall threshold model for the watershed/sub-watershed landscape at data scarce situation – a case study of mid-hill region, Nepal.

Devkota, S., Kc, D., Jaboyedoff, M., and Acharya, G.

Landslides are common in the mid-hill region of Nepal where the terrain slopes are steep and consist of fragile geo-morphology. In Nepal, the casual and triggering factors of the landslides are respectively the underlying geology, intense rainfall and unplanned construction of rural roads is highly recognized, which is however less known and limited in study. Establishment of rainfall threshold for landslides at the watershed landscape is data driven, which is scared in the context of Nepal. The only available long term daily rainfall and sparsely available historical landslides date has been used to develop the rainfall threshold model for the two watersheds in central and western mid-hill regions respectively the Sindhukhola and Sotkhola in Bagmati and Karnali Provinces of Nepal. The watersheds are located in two distinct hydro-climatic regions in terms of rainfall amount and intensity. Historical daily (monsoonal) rainfall data of over four decades (1970-2016) were analyzed available from the Department of Hydrology and Meteorology (DHM)/Government of Nepal and five days’ antecedent rain was calculated. With the limitedly available temporal landslides data, correlation was examined among the 5-days antecedent rain (mm/5days) and daily rainfall (mm/day) portrayed the rainfall threshold (RT) model (Sindhukhola=180-1.07R_T5adt and Sotkhola = 110-0.83*R_T5adt). Utilizing the five days’ antecedent rain fitted into the model, results the threshold rainfall. Deducting the five days’ antecedent rains to the RT described the threshold exceedance (R) for the landslides. The model can be plotted in simple spreadsheet (landslides date in Y-axis and threshold exceedance R in X-axis) to visualize the changes in the threshold exceedance over time, whenever the threshold exceedance progressively and rapidly increased and crossed the threshold line and reached to the positive (> 0) zone, the plots allows for the landslides warning notice. In case of the threshold exceedance is further increased there is likely to have landslides in the watersheds. The model was validated with the 35 dated landslides recorded in monsoon 2015 in Sotkhola watershed. The result indicated that the model preserves 72% coefficient of determination (R²) where there were landslides in the watershed during 2015 monsoon. Due to the simplicity and at the data scarce situation, the model was found to be useful to forecast the landslides during the monsoon season in the region. The model; however, can be improved for better performance whenever the higher resolution time-series landslides data and automated weather stations are available in the watersheds. Linking this model to the proper landslide susceptibility map, and the real time rainfall data through mobile communication techniques, landslide early warning system can be established.

9- Glacial Lake Outburst Floods Early Warning System to save lives and livelihood of the Nepal Himalaya communities: A case Study of Imja Glacial Lake, Nepal

Kc, D., Khatri, T., and Sharma, R.

Nepal, a mountainous country, is experiencing multiple disasters, majority of which are induced by Climate Change. Erratic rainfall, extremely high temperature during summer, cold waves are some of them. Nepal will experience the impacts of climate change through an increase in temperature, more frequent heat waves and shorter frost durations in the future (5AR IPCC). Nepal is witnessing the increased maximum temperature of 0.56^oC per decade and the increment of the temperature is even higher in the mountain region (ICIMOD 2019). One of the major impacts of Climate Change among others, is glacier retreat and Glacial Lake Outburst Floods (GLOFS). Nepal has already experienced more than 26 GLOFS (UNDP and ICIMOD 2020), originated both from Nepal and China, Tibet.

The Imja Glacial Lake is located at 27° 53′ 55“ N latitude, 86° 55’ 20” E longitude and at an altitude of 5010 m in Everest Region of Nepal Himalayas. Imja was identified during 1960s as a small supra lake, was later expanded to an area of 1.28 Km², 148.9 meter deep, holding 75.2 million cubic meters of water in 2014. Lake lowering by 3.4 metres and establishment of early warning system was done in 2016 by the Government of Nepal and UNDP with the support of Global Environment Facility. Hydro-met stations & GLOF Sensors in the periphery and downstream of Imja Lake and automated early warning sirens in six prime settlements in the downstream of Imja watershed linking with dynamic SMS Alert system along 50 km downstream of Imja Dudh Koshi River have been have been linked with community-based DRM institutions at local government level. This initiative is important for preparedness and response of GLOF Risk Reduction in the Imja Valley, benefitting 71,752 vulnerable people, both local and the tourists visiting the Everest Region of Nepal.

Early Warning System of Tsho Rolpa Glacial Lake, the biggest Glacial Lake of Nepal is another example in the such system. New inventory of Glacial Lakes has identified 47 critical lakes as priority lakes for GLOF Risk Reduction in Koshi, Gandaki and Karnali basins. In the new context of federal governance system, the role of federal, province and local government and communities is crucial for achieving the targets of Sendai Framework for Disaster Risk Reduction , particularly target “g” and SDGs 11 and 13 through integrating the targets in the regular planning and its’ implementation for resilient and Sustainable Development of Nepal.

10- The role of freezing-thaw cycling in rock samples topography evolution and rock cliff retreat

Fei, L., Derron, M.-H., Choanji, T., Jaboyedoff, M., Sun, C., and Wolff, C.

Freezing-thaw weathering is recognized as one of the most significant factors in the fatigue of rock mass in areas where the temperature periodically fluctuates around the freezing point.
A one-year monthly SfM monitoring program from December 19, 2019, to January 7, 2021, was done to detect rockfall activity on a rockslide cliff composed of marl-sandstone at La Cornalle, Switzerland. More than one hundred rockfall events were detected during this period with the volumes varied from 0.005m³ to 4.85m³.
We texture all the rockfalls on the 3D SfM model. It is shown that most of them are mainly located in three areas: the top of the cliff, the foot of the cliff, and the medium-left part of the cliff. The common feature of these three parts is that the layers are more or less overhanging with dense fractures around them. At the same time, the meteorological data collected by a weather station on site is correlated with the rockfall events to figure out the relationship between each other. Actually, about 30% of total rockfall volume fell during winter on this site. The triggering factor of rockfall during winter is related to freezing-thaw cycling. This kind of weathering can be understood as an interplay between rock properties and its dynamic environment.
In order to make clear the role of freezing-thaw played on the rockfall generation, an on-site 24h monitoring measurement program that consists of two crack meters, one rock thermal sensor, and thermal camera monitoring is installed in January 2021. Those datasets will help to understand how the crack grows with the changing temperature. In addition, freezing-thaw cycling laboratory experiments for the rock samples taken from different areas of the cliff will be done with an environmental test chamber. The topography of the rock samples before and after the experiments will be acquired by a 3D handheld scanner. This work will benefit to reveal the rock surface evolution during the freezing-thaw cycling in a dynamic environment with varied humidity and number of cycles.
In conclusion, the combination of on-site measurements and laboratory freezing-thaw experiments will provide a good basis for a better understanding of the rockfall triggering mechanism led by physical weathering.

11- Cross-Bedding and Structural Mapping for Rockfall Assessment of a Tunnel using Hi-Resolution LiDAR (Fribourg, Switzerland)

Choanji, T., wolff, C., Fei, L., Loiotine, L., Gutierrez, A., Sun, C., Derron, M.-H., Carrea, D., and Jaboyedoff, M.

Lithology identification and discontinuity mapping are necessary for rockfall hazard assessment in tunnels. However, the restricted exposure and variability of rock face orientation in tunnels ought to be taken into account. Therefore, using Light Detection and Ranging (LiDAR) technique may significantly contribute to this task.

A historical carved tunnel in the Upper Marine Molasse (a poorly consolidated sandstone) of the City of Fribourg (Switzerland) was then investigated by fieldwork and LiDAR. Interestingly, it appears that in addition to joints and layering, some specific sedimentary structures, i.e. cross-bedding, have an important role in the tunnel roof stability. Cross-bedding is a sedimentary structure that can be identified clearly by the geometry of layer within one or more beds in a series of rock strata that does not run parallel to the plane of stratification.

In order to detect and analyse these sedimentary structures, the intensity of the backscattered LiDAR signal is analysed using the Oren-Nayar reflectance model, which considers range, incidence angle, scanned surface geometry (i.e. roughness). It provides corrected values of intensities that make possible to distinguish and identify geometry of cross-beddings in the tunnel.

An analysis of structural discontinuities was also performed using Coltop Software which identified joint sets developed inside the tunnel. Based on this approach, lithology characterizations, orientation of each discontinuity and bedding structures could be identified in point clouds confidently for understanding the mechanisms of potential rockfalls in the tunnel.

12- 2D quantitative analysis of fractures from high-resolution photos for the geomechanical characterization of rock masses

Wyser, E., Loiotine, L., Wolff, C., Andriani, G. F., Jaboyedoff, M., and Parise, M.

The identification of discontinuity sets and their properties is among the key factors for the geomechanical characterization of rock masses, which is fundamental for performing stability analyses, and for planning prevention and mitigation measures as well.
In practice, discontinuity data are collected throughout difficult and time-consuming field surveys, especially when dealing with areas of wide extension, difficult accessibility, covered by dense vegetation, or with adverse weather conditions. Consequently, even experienced operators may introduce sampling errors or misinterpretations, leading to biased geomechanical models for the investigated rock mass.
In the last decades, new remote techniques such as photogrammetry, Light Detection and Ranging (LiDAR), Unmanned Aerial Vehicle (UAV) and InfraRed Thermography (IRT) have been introduced to overcome the limits of conventional surveys. We propose here a new tool for extracting information on the fracture pattern in rock masses, based on remote sensing methods, with particular reference to the analysis of high-resolution georeferenced photos. The first step consists in applying the Structure from Motion (SfM) technique on photos acquired by means of digital cameras and UAV techniques. Once aligned and georeferenced, the orthophotos are exported in a GIS software, to draw the fracture traces at an appropriate scale. We developed a MATLAB routine to extract information on the geostructural setting of rock masses by performing a quantitative 2D analysis of the fracture traces, based on formulas reported in the literature. The code was written by testing few experimental and simple traces and was successively validated on an orthophoto from a real case study.
Currently, the script plots the fracture traces as polylines and calculates their orientation (strike) and length. Subsequently, it detects the main discontinuity sets by fitting an experimental composite Gaussian curve on histograms showing the number of discontinuities according to their orientation, and splitting the curve in simpler Gaussian curves, with peaks corresponding to the main discontinuity sets.
Then, for each set, a linear scanline intersecting the highest number of traces is plotted, and the apparent and real spacing are calculated. In a second step, a grid of circular scanlines covering the whole area where the traces are located is plotted, and the mean trace intensity, trace density and trace length estimators are calculated.
It is expected to test the presented tools on other case studies, in order to optimize them and calculate additional metrics, such as persistence and block sizes, useful to the geomechanical characterization of rock masses.
As a future perspective, a similar approach could be investigated for 3D analyses from point clouds.

13- Shear banding in elasto-plastic slumping process on a GPU: mechanical and hydromechanical MPM solver

Jaboyedoff, M., Wyser, E., and Podladchikov, Y.

Strain localization plays an important role in the mechanical response of a slumping mass and defines the overall behaviour of such process. We study strain localization with the help of the Material Point Method (MPM), which is well-suited to simulate large deformation problem.

We implemented both mechanical and hydromechanical (i.e., we assume fully saturated conditions of the material) MPM-based solvers within a rate-dependent formulation framework under a GPU architecture. We selected an explicit MPM formulation enriched with the Generalized Interpolation Material Point (GIMP) variant, which fixes a major flaw of MPM, i.e., the cell-crossing error. To avoid spurious oscillation of the pressure field (due to the use of low-order elements) for both solid and liquid phase, we used an element-based averaging technique. This minimizes volumetric locking problems. This numerical framework allows to study high-resolution two-dimensional elasto-plastic (i.e., Mohr-Coulomb plasticity) problems in an affordable amount of time. The solvers were written in a CUDA C environment on a single Nvidia GPU. We report a speed-up factor of 500 compared to a similar MATLAB implementation.

Our results showcase a contribution of pore water pressures over shear banding. In particular, we report a significant influence of the liquid phase over the steady thickness of the shear bands and their location. Pore pressures add a viscous contribution to the elasto-plastic rheological model we choose, i.e., Mohr-Coulomb.

As a future perspective, even high resolution could be achieved considering the extension of the actual implementation toward a multi-GPU solver using MPI.

14- Assessment of multitemporal INSAR data for establishing regional landslides inventories