Integrated Monitoring and Assessment of Rockfall

Marco Scaioni, Diego Arosio, Laura Longoni, Monica Papini, Luigi Zanzi Politecnico di Milano, Polo Regionale di Lecco

(email: {marco.scaioni, diego.arosio, laura.longoni, monica.papini, luigi.zanzi}@polimi.it)

Abstract

This paper will present a new methodology for the assessment and monitoring of rockfalls through an integrated multi-disciplinary approach. Current solutions include classical surveying instruments (e.g. total stations, GPS) integrated to sensors for monitoring local deformations (e.g. strain-gauges, deformometers), which are used to achieve information about a limited number of critical points of a rock slope, complemented by geological inspection and qualitative analysis. The innovation of this approach is firstly based on the use of some new sensors which allow to increase the achievable information: terrestrial remote sensors – laser scanner and ground-based interferometric SAR – would allow the measurement of deformations of whole surfaces instead of single points, ground penetrating radar the exploration of rock sub-surface, digital photogrammetry the automatic measurement of crack deformations, seismic and acoustic sensors the detection of vibrations and sounds which could be pre-signal of a rockfall. The second stage of the research involves the integration of different techniques to exploit the full achievable data. This means either the integrated use of sensors and the development of expert systems to integrate different measurements and to make decisions. All activities will be carried out through the setup of some test fields in the Alpine area, where all investigation techniques will be tested.

Keywords: Engineering Geology, Natural Hazards, Rockfall, Georadar, Ground-Based InSAR, Seismic Sensors, Terrestrial Laser Scanning

1. Introduction

1.1 Relevance and impact of rockfall disasters

The investigation of potentially unstable mountain slopes is today a primary need to increase natural and anthropic risk prevention and forecasting. The continuous expansion of human habitats, the presence of transport routes in valleys, melting of alpine permafrost as a consequence of global warming, and exceptional climatic events are amplifying the risk of catastrophic mountain-slope failures, landslides and, more in general, hydro-geological instability. Among the many natural hazards in mountainous regions, rockfalls are frequently occurring processes that are characterized by their suddenness and difficulty of prediction [5].

The most part of worldwide countries are interested by this concern, that involves areas with major and minor relieves, with a manifold impact at social level, on national and regional

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economy, and on the environment. These grounds show somehow this research topic is nowadays relevant, and operational solutions to cope with it might have a great direct and indirect fall-down on the whole society.

Figure 1: A case of rockfall happened at Fiumelatte di Varenna (Northern Italy); on the left is shown the rock face where in november 2004 several big masses fell and hit a railway station

and a house

1.2 State-of-the-art of rockfall prevention

The problem of preventing or reducing damages consequent to rockfalls is complex, due to the very large number of feasible scenarios, with the local morphology of the site providing an additional degree of freedom. Due to this complexity, several competences are needed to address at the best methods and investigation techniques.

The state-of-the-art on analysis, prevention, and monitoring of rockfalls accounts for several studies which mostly concern only a limited aspect of the whole problem. Solutions applied for deep-seated landslides, based on either terrestrial and aerial observations [4], cannot be easily extended to rock face investigations, due to the presence of vertical and sub-vertical faces. Here the main role is currently played by classical surveying and monitoring instruments (e.g. robotic total stations, deformation and displacement sensors) with results complemented by geological inspection and qualitative analysis, as well as the weather observation and forecasting. On the other hand, measurements are registered at predefined times, according to the magnitude of the rock displacements and to the acquisition rate of the adopted instrument. In case of permanent monitoring systems, some measurements per hour could be taken, while in case the instrument needs to be periodically repositioned, the frequency might become weekly or monthly. Moreover, traditional monitoring techniques are based on the definition of a safety threshold for every measurement. When this is not respected, an alarm will be activated and emergency procedures will be called for. By this approach the data integration is seldom exploited.

The complexity of rockfall assessment and monitoring requires not only to focus on specific issues, but to establish a close cooperation between experts skilled in several research fields. Indeed, only a multi-disciplinary environment would allow a real integration between different technologies and methods. Unfortunately, until now the cooperation has been generally limited to geologists and geotechnical engineers, with very few openings to expertises in measurement and data acquisition systems. Thus it’s possible to state that a multi-disciplinary approach represents the real new frontier of this research field.

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1.3 The GPE-PROMETEO project at Polimi

In 2005 the Politecnico di Milano (Italy) university launched an internal project (PROMETEO) focusing on 6 different frontier research fields on the theme of hazard management and public protection (see the website of PROMETEO for information about all involved topics – [16]). The basic aim was to establish some multi-disciplinary investigation teams collecting different resources (instruments, knowledge and people) working inside the university. Here we limit ourselves to deal with one of the sub-projects (GPE), where the problem of rockfall is concerned, among others. The acronym GPE means “first emergency management” (“Gestione della Prima Emergenza”, in Italian), and is focused on establishing criteria for optimizing aid actions just after disasters due to the hidro-geological desease (or to other reasons, e.g. earthquakes) resulting in the destruction of buldings and human artifacts, and involving buried people. One of the specific tasks of this project is to establish the safety conditions for intervention of the emergency teams, based on the evaluation of the so-called residual risk. In case of a landslide or a rockfall, this means the capability of understanding in a quick time and possibly with limited resources, if the happened phaenomenum is not completely ended and might occur again. This issue becomes even more complex when dealing with rockfalls, subject that has only been partially investigated till now, as reported in subsection 1.2. This goal generated the need for a wider investigation about rockfall in GPE, which is focused to open new frontiers in this research field based on a multi-disciplinary approach, called IMARF (“Integrated Monitoring and Assessment of RockFall”).

On-going research activities under GPE are organized in two main sections. The first one concerns testing and development of new technologies, sensors, and data processing techniques for rock face monitoring. This aspect will be the specific subject of the paper and it will be dealt with in section 2. The second one is based on establishing a methodology to apply different investigation and monitoring instruments and method by an integrated approach, which can be summarized by the following items:

1) Considering in a given region all sites possibly interested by rockfalls, in each of them different investigation tools are applied to locate sensible areas where rocks might fall down.

2) Relationships should be found between observed processes (cracks, deformations, vibrations, sounds,…), morphology and rockfall triggering. This goal can be reached by the analysis of several cases, either from literature and past experiences, and from new testing activities.

3) Criteria to define which sites should be monitored and which techniques should be applied have to be set up.

4) Different sensing techniques need to be tested in on-the-field labs (see next section 4 for more detail).

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5) Some risk-based operational guidelines for setting an effective monitoring strategy and for emergency management and rescue related to a given rock face have to be written.

As it can be seen by the listed items, the IMARF research program requires several activities and specializations to be continuously applied for a period of at least 5 years. In the following pages of this paper we would like to deal with the problem of monitoring the rock face stability, being this task already on-going at our university.

2. Overview of adopted monitoring techniques

In recent years new instruments and techniques for deformation monitoring based on ground remote sensors have appeared (Terrestrial Laser Scanning and Ground-Based Interferometric SAR), whose application to rockfall monitoring represent a current challenge. On the other hand, the development of Ground Penetrating Radar and related data analysis methods offers powerful tools to investigate about sub-surfaces. This could be successfully exploited to detect discontinuities under a rock face, and then to locate areas of possible mass detachments. Similarly, digital photogrammetry is expected to allow monitoring of deformations on the surface of the rock (e.g. in case of cracks). Eventually, distributed sensors (microseismic or acoustic) could allow to sense small sounds or vibrations which might be a signal of an upcoming rockfall.

New techniques need to be widely tested on sites really interested by rockfalls, in order to evaluate their potential and operational effectiveness. On the other hand, these are expected to be integrated both mutually among them as well as to traditional monitoring systems.

2.1 Terrestrial Laser Scanning

Terrestrial Laser Scanning (TLS) is a quite recent technique (first instruments appeared about 10 years ago) able to directly acquire in a quick time 3-D unspecific points describing the surface of a given object, with an accuracy lower than ±1 cm [15]. The availability of Long-Range sensors, capable to realistically operate up to a range of 500 m, and the possibility of integrating data acquired from different points of view, allow the geometric survey of geological sites of big dimensions [1]. For this reason, this tool is ideal for the determination of the local morphology of a slope (see item 1, sub-sec. 1.4), considering the possibility to integrate also LiDAR data as well as terrestrial and aerial imagery, extending the achievable information.

However, the most challenging task concerning the use of TLS for rock face analysis is monitoring. Indeed, this technique allows to measure a huge number of points with a very high spatial resolution (also 1 point every few square cm) which is not comparable to that of any other instruments (also Ground-Based Interferometric SAR), as presented in the following sub-sec. 2.2). On the other hand, considering the intrinsic precision of range measurements by laser scanners, summed up to the accuracy of georeferencing (task needed to transform all data acquired from different positions and epochs into the same reference system), the accuracy of the acquired 3-D points of a rock face is not enough to forecast rockfalls. Indeed, these might

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occur also after displacements of a few mm, which cannot be observed by directly comparing TLS measurements. In recent years, several papers were published dealing with the monitoring of buildings by TLS (see e.g. [12]), the most of these overcoming the problem of low accuracy by exploiting the regularity that surfaces of structures generally present. Thus they interpolate the georeferenced point-cloud captured by TLS by geometric shape (planes, cylinders, polynomial surfaces,…) so that the acquisition noise can be strongly filtered and deformations detected from the comparison of interpolating surfaces taken at different epochs.

Unfortunately, the application of the same strategy to rock face monitoring is not trivial, first because regular surfaces seldom exist here, secondly because the complexity of sites and the long-ranges involved make very critical the georeferencing. In the activities of IMARF, some results achieved during a parallel research on dam monitoring will be translated to the case of rock faces [2]. These are based on 2 main solutions, whose effectiveness is to be tried during on-going tests:

1. to improve the georeferencing, the laser scanner is accurately repositioned over a fixed removeable pillar, locked to a stable permanent foundation on the ground (see Figure 2);

2. to reduce the measurement noise, small (the size depending on the rock face regularity) portions of the rock face are interpolated by simple surfaces (flat or parabolic).

Moreover, the application of change-detection [20] based on TLS data will be adopted for measurement of the total mass of rock which has detached from a given face between two different epochs.

During this project, further investigations are carried out in order to evaluate parameters influencing the accuracy of TLS measurement, and in particular: roughness and colour of the surface, angle of incidence laser beam, intensity of sun lighting. Similarly to somewhat was carried out by other researchers on the sensor calibration of some TLS models, the scanner adopted in our testing is under experimental calibration [11].

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Figure 2: On the left a GB-In SAR system IBIS-L by IDS (Pisa, Italy); on the right a long-range terrestrial laser scanner Riegl LMS-Z429i mounted over a steel pillar for precise repositioning.

2.2 Ground-Based SAR Interferometry (GB-InSAR)

Synthetic Aperture Radar (SAR) interferometry is a methodology that has been used for a wide range of applications among which the measurement of ground displacements. The combination of the SAR technique, exploiting the movement of the physical radar antenna along a straight trajectory, and differential interferometric analysis, comparing phase information of reflected waves collected in different time periods, provides high resolution radar images allowing displacements evaluation with a sub-millimetre accuracy.

In recent years, satellite, airborne, and Ground-Based Interferometric SAR (GB-InSAR) techniques have been successfully employed for terrain monitoring (landslides, glaciers, subsidences, and volcanic slopes deformations [19]) and for civil engineering concerns (bridges, dams, towers and buildings monitoring). Both systems can be used in all weather conditions, and they allow to obtain information from all the region covered by the antenna beam, to perform measurements during night and without the need to access the area undergoing examination. Nevertheless, ground-based investigation offers some benefits when compared to satellite-borne platforms: a remarkable flexibility in the acquisition design can be achieved (i.e. suitable to almost any application) and the system can usually be deployed in a straightforward and time-effective way.

The possibility to analyse nearly vertical instable rock faces (very steep slopes are not visible in satellite images) and the capability to perform fast and frequent measurements (satellites pass over the same ground area after a time period related to their orbits) make GB-InSAR a very promising technique for addressing rockfall monitoring and management of the first emergency. In Figure 2 an example of a GB-InSAR system is shown.

Moreover, the acquisition of TLS scans concerning the same rock face monitored by GB-InSAR is expected to improve the results which can be obtained from both techniques separately. Indeed, precise deformation measurements carried out by GB-InSAR will be used to refine TLS data. On the other hand, laser scanning measurements will allow to solve for phase ambiguities

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of the radar system or to explain the consequent loss of coherence. Eventually, integration of these remote sensors would improve the localization of controlled points on the rock face and should make possible the repositioning of GB-InSAR instrument to carry out periodic measurement at different epochs, without the need of its permanent installation.

2.3 Image-based techniques for monitoring of crack deformations

Enlargement of cracks in rock faces is one of the more evident signal of failure, thus their monitoring is a useful tool of prevention. Different methods are currently available to perform this task, all of them capable of a precision better than that needed to detect mass detachment. This could be evaluated in the order of ±0.1 mm, even though it depends on local conditions. An important classification can be made according the automation degree of the adopted method: (i) several sensors (e.g. deformometers, comparators) require to be handled in correspondence of a pair of reference point across the crack, then they are able to measure the variation with respect to a previous epoch; (ii) automatic systems (e.g. strain-gauges or fiber-optic deformometers – see [14]) based on sensors which are permanently positioned over each crack and linked to an acquisition/energy supply unit via a serial or parallel cable connection. The use of (i) or (ii) approach presents evident advantages and drawbacks, and the selection is mainly based on the possibility to access the slope in safe conditions. In IMARF a solution has been already developed which is based on the analysis of a sequence of images captured by a digital camera at different epochs (see [3] for details). Either deformations across and along the crack direction can be measured, thank to a pair of targeted plastic labels which are permanently fixed on both sides of the existing (or foreseen) fissure. Images can be acquired periodically by a digital camera, but also a continuous monitoring by a videocamera is feasible, according to the local geometry of the site.

2.4 Ground Penetrating Radar

Dip, shape, filling, orientation and penetration depth of fractures in a rock-mass are important parameters in geomechanical modelling as well as for slope-stability analysis. Ground Penetrating Radar (GPR) is indeed a powerful non-destructive tool to image the presence of discontinuities in the sub-surface. Many studies involving GPR investigations have already been carried out on rock faces and very steep slopes ([8], [10], [17]). Multi-frequency and multi-polarization surveys were performed to assess sub-surface rock conditions in order to delineate and locate internal fractures.

To overcome the fact that surface-based georadar method is not appropriate for the reflection imaging of steep-dipping features (this is primarily due to the unfavourable radiation patterns of most georadar antennas), borehole data were also collected but final results showed little significance because of azimuthal invariance of borehole antennas.

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Rather complex processing algorithms have been developed to deal with data collected in presence of rugged terrain, undulating topography and big boulders [6].

Data collection in such difficult contexts (rough terrain, steep and uneven slopes) is definitely demanding, thus improvements and customizations (e.g. wireless technology to move the instruments efficiently on the slope) to the acquisition system are essential. For instance, data from GPR could be coupled with TLS techniques to position the GPR traces on a numerical model of the rock face and to define the 3-D geometry of potentially unstable blocks.

Previous experiences of our research group with GPR applied to the investigations on limestone rock quarries have been very encouraging [13]. Preliminary experiments in different test-sites with high frequency antennas were performed to evaluate the propagation of the radar signal inside a limestone rock mass and its ability to resolve thin discontinuities. The achieved results were positive.

Finally a new acquisition configuration with a low frequency antenna has been tested to explore the possibility of collecting data with an air gap between the antenna and the investigated rock mass. However, this solution still requires further study.

2.5 Seismic/Acoustic techniques

Seismic investigation of an unstable slope may involve passive short-to-long term monitoring of microseismic events and refraction seismic experiments [18].

There is usually a good correlation between fracture propagation inside the rock mass as well as slope displacement and the rate of the microseismic activity. The monitoring strategy basically foresees the installation of a permanent seismic network (surface and borehole sensors) to develop a site history and to provide a predictive capability based on temporal changes in the rate of the microseismic activity and/or on temporal changes in the recorded waveform characteristics.

On the other hand, the purpose of 3-D tomographic seismic surveys is to determine the broad scale distribution of highly fractured rocks (dry cracks, fracture zones, and faults on a wide variety of scales), which is expected to be represented by low P-wave velocities [7]. The 3-D velocity model has a great importance when incorporated in the microseismic events localization procedure. It has been shown that the quality of the velocity model dramatically affects localization errors [9].

At present our researches are addressing lab and on site tests for the evaluation of different data acquisition and inspection solutions, e.g. seismic sensors able to detect microseismic events (geophones, piezoelectric and MEMS - Micro Electric Mechanical System - accelerometers), and devices tailored to the detection of acoustic emissions generated inside the rock mass.

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The final goal would be developing a wireless seismic network able to operate autonomously in a challenging environment (harsh weather conditions, difficult access and constraints for power supply), allowing an efficient, low-cost gathering and transfer of acquired information that will be handled with brand new dedicated and power-aware software for a fast and efficient data processing (e.g. filtering, detection, pattern recognition and localization algorithms).

3. Analysis and integration of multi-source data

The integration of different data is the core of IMARF project and concerns two different levels. The first one is related to the use of multisource data during the data processing stage, in order to improve the quality of achievable information. Example of this have been already reported at sub-sections 2.2 (TLS and GB-InSAR) and 2.4 (TLS and GPR).

The second one concerns the integration of different final results that are obtained from every monitoring system, in order to recognize and to predict cases when the risk of rockfall is too high and decisions must be taken (e.g. evacuation of population which could be hit). Typically, monitoring sensors work through the definition of thresholds: when a signal or a measurement goes out the safety field, an alarm is activated. This concept still holds for integrated monitoring as well, even though the setup of suitable values for every threshold is an open problem (see sub-sec. 1.4). However, the added-value derived from the IMARF approach is not only limited to the availability of several systems sensible to processes which might address to possible rockfalls (deformations, sounds, etc.), because this also accounts for correlations between different signals. This extension would allow to detect high risk situations which might occur also in case every of the single sensors’ alarm thresholds is still satisfied. Strategy will involve pre-alarm thresholds which could be activated by different monitoring systems, each of them triggering a specific emergency procedure. These might consists in analysing data acquired by other systems to look for correlations, or to start new investigations by adopting techniques for remote deformation measurement (GB-InSAR and/or TLS) or GPR measurement. The integrated analysis of all collected data after a pre-alarm status will give a final risk evaluation based on the estimation of the possible total volume of detached rock mass. At this stage an external alarm procedure involving Civil Protection forces will be called for.

4. On-the-field testing

The IMARF’s approach requires to experiment different innovative technologies to the aim of assessing and monitoring the stability of a rock face. Moreover, also already known monitoring and investigation instruments need to be tested in this context, in order to optimize, improve and possibly standardize their use. The testing stage will involve test-sites of different size and complexity, which will be dedicated to experiments concerning specific sensors only, or to the whole integrated monitoring system. In this case, tests will be not limited to assess performances and capabilities of each technique, but will extend to the full IMARF procedure.

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Currently some initial test fields have been selected in the Lecco mountain area, in the nearby of a county road which is continuously affected by rock falls in different areas. In a second stage, three different rock faces which have been recently affected by rockfalls (see Figure 1) or which are likely to be (see Figure 3) will be equipped by an integrated monitoring system.

Figure 3 - Two test-sites for on-the-field experimentation of integrated monitoring, located in the nearby of Lecco town (Northern Italy): on the left the Navegno rock face, on the right the

Rialba Tower.

5. Conclusions and future activities

As a first follow-up of the research, we expect the assessment of innovative technologies requiring both a metrological analysis and further investigation and improvement to grant effectiveness in the envisaged application. The second main objective of the research is to develop risk-based operational guidelines for emergency management and rescue under specific site conditions. This activity will be based on innovative modeling and analysis approaches, such as: definition of a quantitative method for the evaluation of the contribution of the new monitoring systems to the effectiveness of emergency management plans and, in general, to improve the safety of population; development of a decision support system for real-time emergency management, specifically to assess the stability of a given site, integrating experts’ judgements and observed data.

Acknowledgements

Acknowledgements are addressed to all researchers involved in the PROMETEO project who are cooperating to this activity (especially Proff. C. Alippi, P. Trucco, and O. Grande), and the technical personnel of Polo Regionale di Lecco (Politecnico di Milano) for supporting on-the-field measurement campaigns.

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