Improvements in our understanding of rockfall ...

Universitat de Barcelona - Facultad de Geología - Departamento de Geodinámica y Geofísica Programa de doctorado en Ciencias de la Tierra - Bienio 2004 - 2006 Grup de recerca en riscos naturals RISKNAT

Improvements in our understanding of rockfall phenomenon by Terrestrial Laser Scanning

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Emphasis on change detection and its application to spatial prediction

____________________________________________

Antonio Abellán Fernández

Supervisor:

Dr. Joan Manuel Vilaplana Fernández

Barcelona, July 2009

The financial support of the Spanish Ministry of Science and Education (pre-doctoral grant 2004-1852) is gratefully acknowledged. This work was funded by the Geomodels Institute, the Natural Park of the Garrotxa Volcanic Field and following projects: MEC CGL2006-06596 (DALMASA) and TopoIberia CSD2006-0004/Consolider-Ingenio2010.

A mi familia y amigos Con mención especial para B.

PREFACE This PhD thesis describes research carried out by Antonio Abellán Fernández as part of the Earth Sciences Doctoral Program in the Geodynamics and Geophysics Department (Faculty of Geology, University of Barcelona) during the period 2004 - 2009. The research was supervised by Dr. Joan Manuel Vilaplana Fernández, head of the RISKNAT group in the Geodynamics and Geophysics Department, University of Barcelona. Part of this research was carried out in collaboration with Dr. Nicholas J. Rosser, professor at the Institute of Hazard and Risk Research, Geography Department, University of Durham, UK and Dr. Michel Jaboyedoff, head of the Institute of Geomatics and Risk Analysis (IGAR), Faculty of Geosciences and Environment (GSE), University of Lausanne. The core of this dissertation is a compendium of four research papers, of which two have been published, one has been accepted and one has been submitted for publication. i.

Publication A (chapter 2): Abellán, A., Vilaplana, J.M., Martínez, J., 2006. Application of a long-range Terrestrial Laser Scanner to a detailed rockfall study at Vall de Núria (Eastern Pyrenees, Spain). Engineering Geology, 88(3-4), 136-148.

ii.

Publication B (chapter 4): Abellán, A., Vilaplana, J.M., Calvet, J., García-Sellés, D. Asensio, E., (submitted to NHESS). Rockfall monitoring by Terrestrial Laser Scanning. Case study of the basaltic rock face at Castellfollit de la Roca (Catalonia, Spain).

iii.

Publication C (chapter 5): Abellán, A. Jaboyedoff, M., Oppikoffer, T., Vilaplana, J.M., 2009. Detection of millimetric deformation using a terrestrial Laser scanner: Experiment and application to a rockfall event. Natural Hazards and Earth System Science, 9, 365372.

iv.

Publication D (chapter 6): Abellán, A., Vilaplana, J.M., Calvet, J., Blanchard, J. (accepted in Geomorphology): Detection and spatial prediction of rockfalls by means of terrestrial laser scanning monitoring.

ABSTRACT Minor-scale rockfalls (up to 100 m3) are the most frequent type of landslides on steep slopes, such as rock faces, coastal cliffs and the sides of transportation corridors (Guzzetti et al., 2004; Copons and Vilaplana, 2008; Lim et al., 2009). Since their impact energy can be very high (Agliardi and Crosta, 2003; Dorren and Seijmonsbergen, 2004), rockfall phenomena present a common risk to infrastructures, buildings and/or populations in steep mountainous terrain. The overall aim of this PhD is to improve our understanding of rockfall phenomena by means of terrestrial Light Detection and Ranging (LIDAR) technology. Specifically, we used a Terrestrial Laser Scanner (TLS), ILRIS-3D model (Optech). This new technology acquires dense 3D information of the terrain, with high accuracy (σ = 0.72cm at 50m), high resolution (mm-cm order) and very high data acquisition speeds (up to 2,500 points/second). The TLS instrument is currently being used in different applications related to rockfalls, such as the generation of a high-resolution, high-accuracy Digital Elevation Models (DEM) of steep slopes, the characterization of 3D discontinuities that play a key role in rockfall detachment, the monitoring of rock slopes, the study of rockfall magnitude-frequency relationships, etc. (e.g. Slob and Hack, 2004; Bauer et al., 2005; Rosser et al., 2005; Lim et al., 2006; Jaboyedoff et al., 2007; Oppikoffer et al, 2008a; Sturzenegger and Stead, 2009). In relation to these applications, one of the most interesting challenges is the possibility to link spatial and temporal prediction of rockfalls (Rosser et al., 2007; Abellán et al., 2009; Abellán et al., accepted). The selected study areas are characterized by highly variable geomorphological scene, lithologies, geomechanical behaviour, hazard and risk levels, etc. The areas were: (i) a sector of the steep gneissic slopes at Vall de Núria (Publication A); (ii) the basaltic cliff at Castellfollit de

la Roca (Publication B); and (iii) the main scarp of an old landslide at Puigcercós, mainly formed by marls and sandstones (Publication D). In addition, an experimental test in artificial scenery (Publication C) allowed the instrumental and methodological issues to be better understood. This PhD research focuses on the detection of changes using TLS (Publications B, C and D). Sequential datasets were compared to detect 3D temporal variations of the terrain. Using this method, it is possible to detect sudden changes in the morphology of the slope and small-scale deformations. Measurement was considerably improved (up to a subcentimetric scale) by applying a Nearest Neighbour (NN) averaging technique. The results of this research mainly consisted of: (a) the detection of rockfalls (location, geometry, magnitude and frequency) that occurred in Castellfollit de la Roca and Puigcercós during the monitoring period (22 and 10 months, respectively); (b) the simulation, in artificial scenery, of small-scale deformation prior to failure (slope creep, Terzaghi, 1950) using a methodology that is able to detect subcentimetric deformation; (c) the detection of this deformation in natural slopes (Castellfollit and Puigcercós); (d) the spatial prediction of rockfalls. The possibility of using TLS to spatially detect pre-failure deformation in natural slopes and to predict the location of a rockfall, constitutes the main contribution of this research. These results have significant implications for rockfall risk management, specifically as regards the possible implementation of the findings in an early warning system.

Keywords: rockfalls; Terrestrial Laser Scanner (TLS); monitoring; change detection; precursory deformation; spatial prediction; early warning.

RESUMEN Los desprendimientos de rocas de pequeñas dimensiones ( 0.1 m occurred in 2008). Free fall height ranges from 10 to 25 m. Short runout distances.

Exposure (existence of vulnerable elements)

Two vulnerable elements: the rack railway and walkers on the Queralbs-Núria path.

Risk

A recently constructed tunnel means that the train is no longer exposed to rockfall. However, walkers face a certain degree of risk.

The most frequent volumes range 3 from 0.5 to 1.5 m . The annual rockfall frequency (4 rockfalls/ year) is considerably lower than that of the Puigcercós study area (see Chapter 6). Free fall height ranges from 30 to 40 meters. Short runout distances. Two rock slab failures (magnitudes 3 around 1000m ) occurred in the last 30 years.

There are two vulnerable scenarios: the top (Castellfollit de la Roca Village) and the bottom of the cliff. No permanent vulnerable elements were found at the latter. The considerable cliff retreat rate may endanger the houses located on the edge. The risk is lower for the bottom of the cliff.

There are no permanent vulnerable elements in the study area. The old village of Puigcercós was abandoned prior to the occurrence of the 1881 landslide.

Reduced risk due to low exposure.

Table 1.1. Summary of the main characteristics of the different pilot study areas. A detailed analysis (except for risk and exposure assessments) is discussed in subsequent sections (see Chapters 2, 4 and 6).

Antonio Abellán Fernández

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SECTION I. Introduction

1.5 Overview of the publications

num. of publications

The number of publications discussing the use of TLS in the Earth Sciences field has grown considerably during the last five years (2003-2008). As an example, Figure 1.1 shows the number of publications in the aforementioned field found in the “Science Citation Index Expanded (SCI)”.

40 35 30

laser+scan*+terrest* laser + rockfall

25 20 15 10 5 0 2000

2001

2002

2003

2004

2005

2006

2007

2008 year

Fig. 1.1. Number of publications indexed by the SCI Expanded (ISI Web of Knowledge, www.isiwebofknowledge.com). Analysed period from 2000 to 2008. Keywords (in topic) = (laser scan* terrest*) or (laser rockfall); Subject Areas = geography or geology; Current year (2009) is not included in this figure due to an incomplete record.

An overview of the four PhD thesis publications is given below. PUBLICATION A 1 (see Chapter 2) deals with the main application of the terrestrial laser scanner to rockfall studies. The test area (Vall de Núria, Eastern Pyrenees, Spain) was selected as it is the site of frequent rockfall activity (Rendón, 2004; Fernández and Vilaplana, 2004) that affects the rack railway. Two rockfalls of dozens of cubic meters each damaged the rack railway and isolated the Vall the Núria resort in April and June 2003. This highlights the rockfall hazard and risk in the area. To improve our understanding of rockfall hazard in the Vall de Núria, we needed to study the source and propagation areas. At that time, the TLS attracted our attention, as this innovative technology could acquire dense 3D information with centimetric accuracy. Although literature on the application of TLS to natural hazards was scarce (e.g. Hunter et al., 2003; Rowlands et al., 2003), we launched a TLS field study in a section of this valley. The dense point cloud obtained with this technology was the basis for the following applications: (a) a high accuracy & resolution digital elevation model (DEM) that considerably improved the subsequent rockfall simulation (trajectory 1 Abellán, A., Vilaplana, J. M., and Martínez, J.: Application of a long-range Terrestrial Laser Scanner to a detailed rockfall study at Vall de Núria (Eastern Pyrenees, Spain), Eng. Geol, 88(3–4), 136–148, 2006

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Chapter 1. General Overview

and energy); (b) accurate 3D location, volume calculation and orientation of the family of discontinuities that conditioned one of the rockfall events mentioned above. This first publication was of interest to my PhD as it opened up a wide range of potential applications of the instrument. To deepen my research, I had to select and develop just one application. Finally, the following topic was selected: rockfall monitoring (detection and prediction) through a multi-temporal comparison of TLS datasets. At the end of 2005, innovative research on the application of TLS to monitoring a cliff section on the coast of North Yorkshire (UK) was published (Rosser et al., 2005). Monthly acquisition of datasets enabled the Durham University group to calculate the volume and the rockfall frequency with a previously unobtainable precision. I started working on this topic by acquiring datasets on the emblematic basalt cliff at Castellfollit de la Roca (Natural Park of the Garrotxa Volcanic Field, Catalonia, Spain). A few months later I had the chance to apply for an academic visit to Durham University. Consequently, for 3 months I was supervised by Dr. Nicholas J. Rosser, the author of the aforementioned publication (Rosser et al., 2005). The method developed by this group, i.e., change detection using TLS, was the basis for the further development of this PhD thesis. PUBLICATION B 2 (see Chapter 4) shows the results of monitoring the basalt cliff at Castellfollit de la Roca for 22 months (from March 2006 to January 2008). The vulnerability of the houses at the top of the cliff was the main reason for carrying out this research. Two types of mass movement were detected in the monitoring period, which enabled us to make a short-term estimate of the frequency of minor scale rockfalls: (a) detachment of single basalt columns, with magnitudes below 1.5 m3 and (b) detachment of group of columns, with magnitudes from 1.5 to 150 m3. These results were temporally biased: a longer study period was required. As a result, this approach was combined with historical records to obtain a more representative estimation of the rate of retreat of the cliff. Undoubtedly, the most important finding in this study area was the detection of a centimetric deformation prior to a 50m3 rockfall (April 2007 event). This deformation was interpreted as a slope creep phenomena (Terzaghi, 1950), i.e. a precursory deformation prior to the occurrence of a mass movement. Nevertheless, this centimetric movement was disguised by same-order-of-magnitude instrumental noise. Thus, a filtering technique was needed. At this time, there were no publications in the literature on the use of TLS to detect precursory deformations. Therefore, the prediction of the event was impossible technologically and conceptually.

2 Abellán, A., Vilaplana, J.M., Calvet, J., García-Sellés, D., Asensio, E., submitted. Rockfall monitoring by Terrestrial Laser Scanning. Case study of the basaltic rock face at Castellfollit de la Roca (Catalonia, Spain). Nat. Hazards Earth Syst. Sci.

Antonio Abellán Fernández

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PUBLICATION C 3 (see Chapter 5) was carried out in collaboration with the Institute of Geomatics and Risk Analysis (IGAR), University of Lausanne (Switzerland), and was supervised by Dr. Michel Jaboyedoff. An outdoor experiment was performed to ascertain whether the TLS instrumental error was small enough to detect precursory, small scale (mm-cm) deformation. The experiment consisted of an induced displacement of three objects relative to a stable part. Millimetric changes were not detected using conventional methods. However, the accuracy of the measurements was considerably improved through an averaging technique (25 Nearest Neighbours). This study allowed us to accurately detect the location and magnitude of small displacements from a stable part, which indicated that precursory displacements on real rock slopes could also be detected. Subsequently, the technique was applied to the precursory deformation observed in the Castellfollit de la Roca datasets (April 2007 event). This deformation was clearly detected. The implication of these results very important: if millimetric displacements prior to a rockfall can be detected through a TLS, the spatial prediction of rockfalls could be a real possibility. Nevertheless, in this case, the precursory deformation was detected through a back analysis procedure. Thus, it was not a real prediction. At this stage of the PhD thesis, the detection of precursory deformation in additional study areas was necessary. Finally, PUBLICATION D 4 (see Chapter 6) shows the application of the TLS to rockfall detection and prediction at the main scarp of Puigcercós landslide. The first part of the research was carried out using the same approach as that described in publication B: change detection by TLS. As a result, we detected more than 40 rockfalls (from 0.1 to 100 m3) in a 9 month period (from October 2007 to July 2008). This was a much higher rockfall frequency than previously expected. The second and most relevant part of this publication dealt with detecting a centimetric precursory deformation in different parts of the slope. In contrast to the back analysis procedure used in publication B, the early detection of a precursory deformation was the basis for the spatial prediction of rockfalls. The research carried out in the Puigcercós test area allowed us to deepen our understanding of precursory deformation in minor scale rockfalls. The possibility of linking spatial and temporal prediction through the detection of precursory deformation could be of paramount importance to rockfall early warning systems. Nevertheless, more case studies are needed of different materials, structural conditions and different failure mechanisms.

3

Abellán, A., Oppikofer, T., Jaboyedoff, M., Vilaplana, J.M., 2009. Detection of millimetric deformation using a terrestrial LIDAR: Experiment and its application to a rockfall event. Nat. Hazards Earth Syst. Sci., 9, 365–372, 2009

4 Abellán, A., Vilaplana, J. M., Calvet, J., Blanchard, J., 2009_accepted. Detection and spatial prediction of rockfalls by means of terrestrial laser scanning monitoring. Geomorphology.

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CHAPTER 2 Applications of TLS to rockfall studies Publication A: Abellán et al., (2006). Engineering Geology

Application of a long-range Terrestrial Laser Scanner to a detailed rockfall study at Vall de Núria (Eastern Pyrenees, Spain) A. Abellán a,*, J.M. Vilaplana a,, J. Martínez b a b

RISKNAT group & GEOMODELS Institute, Depart. of Geodynamics and Geophysics, Univ. of Barcelona, Spain Serfocar S.L.: Servicios de Fotogrametría y Cartografía. C/ Saragossa, 95-97. 08006-Barcelona, Spain

Submitted: December 2004 - Revised: December 2005 - Accepted: September 2006 – Published: December 2006

Abstract. In this study we show the application of a long-range Terrestrial Laser Scanner (TLS) to a detailed rockfall study in a test zone at Vall de Núria, located in the Eastern Pyrenees. Data acquisition was carried out using TLS-Ilris3D, the new generation of reflector-less laser scanners with a high range, accuracy and velocity of measurements. Eight scans were performed at 3 stations to acquire coordinates of almost 4 million points. The results from the acquired data are a high accuracy Digital Elevation Model (DEM) and the reconstruction of the joint geometry. The former is used for inventory of rockfalls and for more accurate rockfall simulation (trajectories and velocities). The latter allows us to model the geometry and volume of the source area in recent rockfalls. Our findings suggest that TLS technology could be a tool of reference in rockfall studies in the near future. © 2006 Elsevier B.V. All rights reserved.

Keywords: Rockfall; Terrestrial Laser Scanner; DEM; Rock slope; 3D geometry modelling; Eastern Pyrenees

Abellán et al. (2006)

Chapter 2: Applications of TLS to rockfall studies

2.1 INTRODUCTION

result that a greater density of information using terrestrial sensors is obtained.

Rockfall is the relative free falling or precipitous movement of a newly detached segment of bedrock from a cliff or other very steep slope (Bates and Jackson, 1987). Since rockfall is the fastest type of landslide (Varnes, 1984), it presents a common risk to transportation and structures in steep mountainous terrain (Pfeiffer et al., 1995; Corominas et al., 2003).

One of the terrestrial sensors is the new long-range Terrestrial Laser Scanner (TLS). This is a technology with considerable potential in the characterization and monitoring of slope instability due its maximum reach (103m) and accuracy (7mm at 50m).

In recent years considerable advances have been made in the analysis of rockfall susceptibility (Marquínez et al., 2003; Baillifard et al., 2003; Günther et al., 2004), in the modelling of rock trajectories on a 3D slope (Agliardi and Crosta, 2003; Crosta and Agliardi, 2004; Dorren and Seijmonsbergen, 2004), as well as in the risk management of rockfalls (Guzzetti et al., 2004; Copons et al., 2005). Many of these studies use topographical maps and Digital Elevation Models (DEMs) derived from aerial sensors, i.e. aerial photography and airborne LIDAR (Light detection and Ranging). These sensors achieve maximum density of information when the incident ray is perpendicular to topography, typically subhorizontal surfaces. By contrast, the instabilities due to rockfall usually occur on vertical slopes with the

TLS was used at Vall de Núria (Eastern Pyrenees), an area currently affected by rockfalls (Rendón and Vilaplana, 2004). In our study we show how TLS can help us to improve the estimation of parameters of interest in rockfall studies: TLS can produce high resolution DEMs, which can be employed for inventory of rockfalls, monitoring of mass movement time evolution and more accurate numerical simulation of rockfall trajectories and velocities. Furthermore, data obtained from TLS allows the reconstruction of joint geometry and an estimation of the volume of blocks that can fall from steep inaccessible rock slopes.

2.1.1 Study area The study area is located in a high mountain valley, Vall de Núria, Eastern Pyrenees, Spain (Fig. 2.1). There is only one form of transport in this area: the

Fig. 2.1. Study area in Vall de Núria, Eastern Pyrenees, Spain. Pilot area is enclosed by dotted line.

Antonio Abellán Fernández

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SECTION I. Introduction

rack railway. Vall de Núria is a popular mountain resort, especially with skiers and ramblers. It caters for more than 250,000 visitors annually. A small test area was selected on the railway track (300×500 m) in order to calculate the 3D geometry of the slope using TLS. This area forms part of a steep rock slope made up of heavily fractured Paleozoic gneisses of Núria (Santanach, 1974). This area was selected given its exposure to frequent rockfalls, resulting in considerable damage to the railway in recent years (Rendón and Vilaplana, 2004) (Fig. 2.2), specifically in 2003: 3 March, 4 April and 15 June (henceforth, events A, B and C respectively). This scenario of natural risk has been the subject of recent studies in an attempt to evaluate the phenomenon of rockfall in the whole valley (Rendón et al., 2004; Fernández and Vilaplana, 2004) and to implement preventive and corrective measures.

1.2 Terrestrial laser scanner The long-range TLS is a new instrument that massively captures coordinates of ground points in 3D with high velocity and accuracy. TLS began to be used in the 1990s for mobile robot navigation (Singh and West, 1991; Hancock et al., 1998), in the construction of metric scale 3D models, such as

Publication A: Eng. Geol., 88(3–4), 136–148

sculptures (Beraldin et al., 2000) and industrial applications (Sequeira et al., 2003). Given the rapid development of technology, the maximum distance of the laser is continually being improved. This has been accompanied by an increase in TLS applications, including calculation of 3D models of large surfaces (architecture, archaeology, topography) and recently the characterization and monitoring of natural hazards, e.g. volcanoes (Hunter et al., 2003) and landslides (Rowlands et al., 2003; Bitelli et al., 2004). TLS consists of an instrument for measuring distance (laser) and a scanner. The laser beam is focused and is reflected directly on the land surface, obviating the need for the existence of intermediate prism reflectors. The TLS used in this work is the Ilris3D from OPTECH (Fig. 2.3). It can reach an accuracy of 7mm at a distance of 50 m, which is comparable to a reflectorless total station. TLS is able to acquire a mean of 2000 points/s (see the website http://www.Ilris-3D.com). This rapid data acquisition enables us to acquire 3D coordinates of millions of points in only 15 min. These points can be transferred to CAD software (Computer Aided Design) for subsequent visualization and data treatment. After scanning in the study area, the instrument stores data in a memory card. The file is structured

Fig. 2.2. Rack railway and recent rockfalls in the study area. (a) Event B: 2003/04/04. (b) Event C: 2003/06/15

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Chapter 2: Applications of TLS to rockfall studies

Fig. 2.3. Close up (3a) and front view (3b, from www.ilris-3D.com) of long range Terrestrial Laser Scanner Ilris-3D

in three columns with the coordinates (x, y, z) of the total number of points scanned, and a fourth column corresponding to the value of intensity (l) of the land surface on which the laser beam is focused. The maximum range depends on the reflectivity of the materials on the slope and on the angle of incidence. Even though range of 1500 m can be theoretically reached for targets with reflectivity of 80%, the maximum range for natural slopes is in the order of 1000mowing to the low reflectivity of the ground. Table 2.1 shows the technical features of TLS-Ilris3D. A comparison between the existing TLSs on the market (manufacturers: Cyra, Mensi, Optech, Riegl, Z and F) can be found in Staiger (2003). TLS-Ilris3D is equipped with a LCD (Liquid Crystal Display), where the zone to be scanned is visualized (Fig. 2.3a). The user can define the necessary parameters for data acquisition (spacing between points, dimensions of the area to be scanned, etc.) in a PDA (Personal Digital Assistant) connected to the instrument.

treatment: this was undertaken by triangulation (Delaunay), trigonometrical calculations, interpolation (Kriging) and rockfall simulations; (c) calculation of results: high resolution DEM, trajectories and energies, inventory of rockfalls, Dip and Dip Direction of joints and volume of the blocks in the rock slope; (d) validation of results: this was performed with direct measurements of the joint orientation in the field by means of a compass,

Characteristic Performance Range Data sample rate

Antonio Abellán Fernández

350 m (4% target) 800 m (20% target) 2,000 points/second

Accuracy Target registration accuracy Modeling accuracy Depth resolution

4 mm 3 mm 3 mm

Size Scanner (L × W × H) Scanner weight

312 × 312 × 205 mm 12 kg

Output

Metafile consisting of XYZ, Intensity, digital photo data, operator setup parameters and notes.

Spot Spacing

S = 0.026R, where S = spacing (mm) R = range to target (m)

Laser Spot Size

D = 0.17R+12, where D = diameter of spot (mm) R = range to target (m) (e.g. 29 mm at 100 m)

2.2 METHODOLOGY Fig. 2.4 illustrates the methodology used for the application of TLS in our rockfall study at Vall de Núria. This consists of the following steps: (a) data acquisition: this was carried out using TLS-ILRIS 3D with 4 million 3D points (x, y, z); (b) data

Value

Table 2.1: Terrestrial Laser Scanner Ilris3D technical specifications (www.ilris_3D.com)

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Publication A: Eng. Geol., 88(3–4), 136–148

Fig. 2.4. Methodology used in our study. Step 5 is currently under construction (see section 2.2).

and through direct observations of trajectories, volume of the blocks and location of the detachment area of rockfalls occurred during 2003 (Events A, B and C). The results that were not validated were revised in step 2 (data treatment). The ultimate aim of the study is hazard assessment (Step 5). This step is currently under construction given that event frequency has not yet been estimated.

2.3 DATA ACQUISITION A minimum of 3 stations: E1, E2 and E3 were necessary to cover the study area. From these stations, scanning with TLS-Ilris3D was performed. In order to obtain a complete dataset for the study area, 8 scans were performed as follow: E1 (E11, E12), E2 (E21, E22, E23, E24) and E3 (E31, E32).. The views from each of the stations and to the scanning areas are shown in Fig. 2.5.

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The total scan time from the sum of the 8 scans was less than 2 h. One working day was employed for mobilization, setting up and acquisition of nearly 4 million points using TLS. If a total station had been used to carry out the same work, at least two weeks would have been necessary. Moreover, the point cloud obtained in this way would have been much smaller. The data acquisition is summarized in Table 2.2. After selecting the area to be scanned, the TLS performs a preliminary reading of the mean distance to the surface of the slope with the aim of identifying different point spacing. Subsequently, one of these point spacing is selected and acquisition of coordinates is carried out. In accordance with Alba et al. (2005), one of the main problems in laser scanning data processing is dealing with huge point clouds. For this reason, a balance must be reached between information density acquired with TLS and information that is necessary. In our study we selected point spacing from 6 to 20 cm.: E11: 10cm, E12: 12cm, E21: 15cm,

E22: 13cm, E23: 20cm, E24: 6cm, E31: 20cm, E32: 20cm. The scan with the highest density of points (E24) corresponds to the detachment area of event C, PhD thesis

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Chapter 2: Applications of TLS to rockfall studies

Fig. 2.5. Digital Elevation Model (a) and photographs of the study area seen from the stations E2 (b), E1(c) and E3(d). A total of 8 scans were performed from each of the stations: E1 (E11, E12), E2 (E21, E22, E23, E24) and E3 (E31, E32). The scanned areas are enclosed by dotted lines.

which was studied in greater detail (6 cm). Point spacing from 10 to 20 cm. was good enough to generate a high accuracy DEM for rockfall simulation. We used optimized software (polyworks, http://www. innovmetric.com) to handle large data sets obtained from the laser scanner. With this program, the union of the different scans was carried out using a user supervised identification algorithm of the common areas. Owing to our limited experience in data acquisition using TLS, the scans E12 and E32 were unsuccessful because of instrument errors with the result that the overlapping area was not sufficiently large to join all the scans. For this reason, the georeferencing of those scans was carried out by the identification of control points in the point cloud and in the photographs and the subsequent acquisition of its coordinates with a reflectorless total station (Leica TRC 705). Antonio Abellán Fernández

In the subsequent visualization of the complete scanned data, it is possible to observe areas without 3D land information (shaded areas). In order to avoid the appearance of these shaded areas, additional scans from different angles and heights should be performed or aerial and terrestrial laser techniques should be combined (Ruiz et al., 2004).

2.4 RESULTS slope and rockfall modelling From 4 millions 3D points we obtained on the one hand the superficial geometry of the slope (DEM), and on the other hand the geometry of joints. Our initial hypothesis was that the bigger and the more accurate the 3D point cloud, the more detailed the topography of the slope.

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Publication A: Eng. Geol., 88(3–4), 136–148

Likewise, the more detailed the topography (defined by DEM) and the more precise the delimitation of the source area of the rock blocks, the more accurate the rockfall modelling. Characteristic

Value

Total points Scans Stations Maximum range Minimum range Accuracy

3.979.436 8 3 870 m 180 m 3 mm @100m

Table 2.2. Data acquisition using TLS-Ilris3D.

2.4.1 Inventory of rockfalls Using CAD software allows us to visualize the slope in 3D, rotate the axis to obtain a better visualization and to make a zoom or move away from a specific detail (Fig. 2.6). This visualization from 3D point cloud or DEM renders geological and geomorphological features more accurately

than in a topographic map. For example, we can locate in the point cloud the rockfall source (bedrock outcrops, potentially unstable rocks, indicators of recent rockfall, etc.), the rockfall trajectory and all the relevant information for the subsequent rockfall simulation. 2.4.1.1 Coordinates of the detachment area Fig. 2.7 shows the precise location of the source area of event A at the northern outlet of the Fenech tunnel. The range of coordinates of this area in the local system of TLS (8370 m