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Earth Sciences
Geology

Rockfall Occurrence and Fragmentation

Profile image of Jordi CorominasJordi Corominas

2017, Advancing Culture of Living with Landslides

https://doi.org/10.1007/978-3-319-59469-9_4
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23 pages

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Abstract

Rockfalls are very rapid and damaging slope instability processes that affect mountainous regions, coastal cliffs and slope cuts. This contribution focuses on fragmental rockfalls in which the moving particles, particularly the largest ones, propagate following independent paths with little interaction among them. The prediction of the occurrence and frequency of the rockfalls has benefited by the rapid development of the techniques for the detection and the remote acquisition of the rock mass surface features such as the 3D laser scanner and the digital photogrammetry. These techniques are also used to monitor the deformation experienced by the rock mass before failure. The quantitative analysis of the fragmental rockfalls is a useful approach to assess risk and for the design of both stabilization and protection measures. The analysis of rockfalls must consider not only the frequency and magnitude of the potential events but also the fragmentation of the detached rock mass. The latter is a crucial issue as it affects the number, size and the velocity of the individual rock blocks. Several case studies of the application of the remote acquisition techniques for determining the size and frequency of rockfall events and their fragmentation are presented. The extrapolation of the magnitude-frequency relationships is discussed as well as the role of the geological factors for constraining the size of the largest detachable mass from a cliff. Finally, the performance of a fractal fragmentation model for rockfalls is also discussed.

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Rockfalls are slope instabilities very frequent and harmful in mountainous areas. They cause damage in infrastructures (roads and railways), buildings, vehicles and people.Several testswerecarried out to understand better these events. The field activities comprised real scale tests and the characterization of natural events in the N-E of Spain, mainly in the Pyrenees range. Moreover, in order to understand the behaviourof the blocks during the fall real scale tests were carried out. We dropped a total of 124 rock blocs under controlled conditions. Prior to the block release and during their propagation downslope, several geomatic techniques wereused to monitor the volumes, shapes and trajectories ofthe original blocks and their fragments (due to breakage); it is worth to highlight the videogrammetry to determinethe trajectoriesof the blocks. In order to survey the natural rock walls,source of the rockfalls, the so-called massive data capture by photogrammetry (both terrestrial and ...

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A fractal fragmentation model for rockfalls
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The impact-induced rock mass fragmentation in a rockfall is analyzed by comparing the In Situ Block Size Distribution (IBSD) of the rock mass detached from the cliff face and the resultant Rockfall Block Size Distribution (RBSD) of the rockfall fragments on the slope. The analysis of several inventoried rockfall events suggests that the volumes of the rockfall fragments can be characterized by a power law distribution. We propose the application of a three-parameter Rockfall Fractal Fragmentation Model (RFFM) for the transformation of the IBSD into the RBSD. A Discrete Fracture Network model is used to simulate the discontinuity pattern of the detached rock mass and to generate the IBSD. Each block of the IBSD of the detached rock mass is an initiator. A survival rate is included to express the proportion of the unbroken blocks after the impact on the ground surface. The model was calibrated using the volume distribution of a rockfall event in Vilanova de Banat in the Cadí Sierra, Eastern Pyrenees, Spain. The RBSD was obtained directly in the field, by measuring the rock blocks fragments deposited on the slope. The IBSD and the RBSD were fitted by exponential and power-law functions, respectively. The results show that the proposed fractal model can successfully generate the RBSD from the IBSD and indicate the model parameter values for the case study. 1. Introduction: A fragmental rockfall is characterized by the separation of the rockfall mass into smaller pieces after impact upon the ground surface (Evans and Hungr, 1993). The generated individual rock fragments move as independent rigid bodies, which propagate with different velocities and follow divergent trajectories (Figure 1). Fragmental rockfalls are distinguished from rock avalanches as, during the latter, the mass of fragments moves in a flow-like mode. The deposit of a fragmental rockfall consists of blocks of different sizes scattered on the ground surface (Figure 1). In the case of large fragmental rockfalls (thousands to tens of thousands of cubic meters) a more or less continuous Young Debris Cover (YDC) is formed. The size distribution of the deposit depends on the fragmentation degree. Rock fragmentation is the progressive change in the particle size of an initial rock mass by application of actions. Incorporating the fragmentation in rockfall analysis is fundamental in many aspects. For hazard and risk assessments, its effect on the number, magnitude and intensity (kinetic energy) of the rock blocks is major (Corominas et al. 2012; Ruiz-Carulla et al. 2015, Jaboyedoff et al. 2005). As the rockfall mass splits into pieces, the number of blocks is multiplied (Corominas and Mavrouli, 2013) and their energy changes (Agliardi and Crosta, 2003). This affects their trajectories and run-out, as well the encounter probability with elements at risk (e.g. building, people, vehicle) and their destructive potential. Therefore, a rockfall analysis considering large unbroken rockfall masses may produce unrealistic results (Okura et al. 2000; Dorren 2003) and instead the fragmentation should be taken into account. Fragmentation may also affect the propagation mode in rockfalls, rockslides and rock avalanches, modify the location of the front of the deposit and the position of the final center of gravity (Haug et al. 2016). The design of protection structures such as barriers and rockfall sheds also requires data for the number and size of the rock blocks after fragmentation. Although, rock fragmentation has been frequently observed during rockfalls it is rarely considered for the rockfall analysis. This can be attributed to the complexity of the physical process. Many parameters may have an influence such as the presence of discontinuities in the boulders including their persistence, aperture and orientation at the moment of the impact, the intact and joined rock resistance, the impacting energy, rigidity of ground conditions, impact angle and velocity (Dussage et al. 2003; Wang and Tonon, 2010). Further references on the advances regarding the fragmentation process and analysis are given in section 2 of this paper.

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Analysis of Rockfalls by Means of a Fractal Fragmentation Model
Jordi Corominas

Rock Mechanics and Rock Engineering, 2019

We present the performance of the rockfall fractal fragmentation model (RFFM) developed by Ruiz-Carulla et al. (2017) and based on Perfect (1997). The RFFM combines disaggregation of the initial rock mass and breakage of the blocks. The model has been upgraded as to meet the mass balance, and to generate both a continuous decreasing and scale variant distribution of fragments volumes. The input of the model may be either a single block or a rock mass characterized by its In situ Block Size Distribution (IBSD). The measured fragment size distributions of seven inventoried rockfall events, are used to calibrate the model. The results of the simulations fit well to the measured volume distributions. Our findings indicate that fragmentation is better characterized by the whole volume distribution of fragments generated and the increase of new surface area of the rock fragments. A relation has been observed between the potential energy of the first impact, the new surface area of fragments generated, and the model parameters. Although a greater number of parametric analyses and calibration exercises are required, this relation is proposed as a first approach to model rockfall scenarios.

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Template for the full paper of WLF 4 Rockfall occurrence and fragmentation
Jordi Corominas

2017

Rockfalls are very rapid and damaging slope instability processes that affect mountainous regions, coastal cliffs and slope cuts. This contribution focuses on fragmental rockfalls in which the moving particles, particularly the largest ones, propagate following independent paths with little interaction among them. The prediction of the occurrence and frequency of the rockfalls has benefited by the rapid development of the techniques for the detection and the remote acquisition of the rock mass surface features such as the 3D laser scanner and the digital photogrammetry. These techniques are also used to monitor the deformation experienced by the rock mass before failure. The quantitative analysis of the fragmental rockfalls is a useful approach to assess risk and for the design of both stabilization and protection measures. The analysis of rockfalls must consider not only the frequency and magnitude of the potential events but also the fragmentation of the detached rock mass. The la...

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3D Analysis of a Fragmental Rockfall
Jordi Corominas

Understanding and Reducing Landslide Disaster Risk, 2020

Fragmentation in a rockfall event influence the total number of fragments, the mass distribution, and the impact energies and runouts. Then, the probability of impact and hazard characterization should consider fragmentation. A fractal fragmentation model has been proposed in order to reproduce the phenomenon. The Rockfall Fractal Fragmentation Model has been implemented in a 3D rockfall propagation simulator named RockGIS. We present the analysis of a fragmental rockfall that occurred in Mallorca, Spain. Fieldworks are carried out in order to obtain the block size distribution of the rockfall deposit. A 3D terrain model is obtained using UAV surveys and digital photogrammetric techniques. The obtained 3D point cloud is cleaned of vegetation and used to create a Digital Elevation Model (DEM). The fragmentation model parameters and the propagation simulator coefficients have been calibrated to accomplish both, the resultant block size distribution and the runout distance of the block...

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RockGIS: a GIS-based model for the analysis of fragmentation in rockfalls
Nieves Lantada

Landslides, 2017

is a mass instability event frequently observed in road cuts, open pit mines and quarries, steep slopes and cliffs. After its detachment, the rock mass may disaggregate and break due to the impact with the ground surface, thus producing new rock fragments. The consideration of the fragmentation of the rockfall mass is critical for the calculation of the trajectories of the blocks and the impact energies, for the assessment of the potential damage and the design of protective structures. In this paper, we present RockGIS, a GIS-Based tool that simulates stochastically the fragmentation of the rockfall, based on a lumped mass approach. In RockGIS, the fragmentation is triggered by the disaggregation of the detached rock mass through the pre-existing discontinuities just before the impact with the ground. An energy threshold is defined in order to determine whether the impacting blocks break or not. The distribution of the initial mass between a set of newly generated rock fragments is carried out stochastically following a power law. The trajectories of the new rock fragments are distributed within a cone. The fragmentation model has been calibrated and tested with a 10,000m 3 rockfall that took place in 2011 near Vilanova de Banat, Eastern Pyrenees, Spain.

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    Jordi Corominas

    Landslides

    There exists a transition between rockfalls, large rock mass failures and rock avalanches. The magnitude and frequency relations (M/F) of the slope failure are increasingly used to assess the hazard level. The management of the rockfall risk requires the knowledge of the frequency of the events but also defining the worst case scenario, which is the one associated to the maximum expected (credible) rockfall event. The analysis of the volume distribution of the historical rockfall events in the slopes of the Solà d'Andorra during the last 50 years, shows that they can be fitted to a power law. We argue that the extrapolation of the F-M relations far beyond the historical data is not appropriate in this case. Neither geomorphological evidences of past events nor the size of the potentially unstable rock masses identified in the slope support the occurrence of the large rockfall/rock avalanche volumes predicted by the power law. We have observed that the stability of the slope at the Solà is controlled by the presence of two sets of unfavorably dipping joints (F3, F5) that act as basal sliding planes of the detachable rock masses. The area of the basal sliding planes outcropping at the rockfall scars were measured with a Terrestrial Laser Scanner. The distribution of the areas of the basal planes may be also fitted to a power law that shows a truncation for values bigger than 50 m 2 and a maximum exposed surface of 200 m 2. The analysis of the geological structure of the rock mass at the Solà d'Andorra make us conclude that the size of the failures is controlled by the fracture pattern and that the maximum size of the failure is constrained. Two sets of steeply dipping faults (F1 and F7) interrupt the other joint sets and prevent the formation of continuous failure surfaces (F3 and F5). We conclude that due to the structural control, large slope failures in Andorra are not randomly distributed thus confirming the findings in other mountain ranges.

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    Evaluation of Maximum Rockfall Dimensions Based on Probabilistic Assessment of the Penetration of the Sliding Planes into the Slope
    Jordi Corominas

    Rock Mechanics and Rock Engineering

    There is intrinsic difficulty in the investigation of the largest volume of rockfalls that is expected in an area, which lies in the small number of large events, in registrable times. The maximum credible rockfall size has been associated with the properties of the rock mass discontinuities, as they delimit detachable rock blocks, and in particular with the penetration of those discontinuities that comprise rockfall sliding planes. In highly fractured rock masses, the evaluation of the penetration remains an issue. A probabilistic methodology is proposed, to measure the penetration of potential sliding planes into the interior of a rocky slope. The main hypothesis of the method is that the sliding plane persistence is interrupted along its two directions, at the intersection with two lateral discontinuity sets, as the latter displaces the former. Due to the displacement, the sliding planes are formed by quasi-planes that contain a maximum number of spacings of the intersecting join...

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    Combining Ground Based Remote Sensing Tools for Rockfalls Assessment and Monitoring: The Poggio Baldi Landslide Natural Laboratory
    Giandomenico Mastrantoni

    Sensors (Basel, Switzerland), 2021

    Nowadays the use of remote monitoring sensors is a standard practice in landslide characterization and monitoring. In the last decades, technologies such as LiDAR, terrestrial and satellite SAR interferometry (InSAR) and photogrammetry demonstrated a great potential for rock slope assessment while limited studies and applications are still available for ArcSAR Interferometry, Gigapixel imaging and Acoustic sensing. Taking advantage of the facilities located at the Poggio Baldi Landslide Natural Laboratory, an intensive monitoring campaign was carried out on May 2019 using simultaneously the HYDRA-G ArcSAR for radar monitoring, the Gigapan robotic system equipped with a DSLR camera for photo-monitoring purposes and the DUO Smart Noise Monitor for acoustic measurements. The aim of this study was to evaluate the potential of each monitoring sensor and to investigate the ongoing gravitational processes at the Poggio Baldi landslide. Analysis of multi-temporal Gigapixel-images revealed t...

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    Machine Learning-Based Rockfalls Detection with 3D Point Clouds, Example in the Montserrat Massif (Spain)
    Oscar Gratacós

    Remote Sensing

    Rock slope monitoring using 3D point cloud data allows the creation of rockfall inventories, provided that an efficient methodology is available to quantify the activity. However, monitoring with high temporal and spatial resolution entails the processing of a great volume of data, which can become a problem for the processing system. The standard methodology for monitoring includes the steps of data capture, point cloud alignment, the measure of differences, clustering differences, and identification of rockfalls. In this article, we propose a new methodology adapted from existing algorithms (multiscale model to model cloud comparison and density-based spatial clustering of applications with noise algorithm) and machine learning techniques to facilitate the identification of rockfalls from compared temporary 3D point clouds, possibly the step with most user interpretation. Point clouds are processed to generate 33 new features related to the rock cliff differences, predominant diff...

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    Rockfalls: analysis of the block fragmentation through field experiments
    Nieves Lantada

    Landslides, 2022

    Fragmentation is a common feature of rockfall that exerts a strong control on the trajectories of the generated blocks, the impact energies, and the runout. In this paper, we present a set of four realscale rockfall tests aimed at studying the fragmentation of the rocky blocks, from the global design of the field procedure to the data analysis and the main results. A total of 124 limestone, dacite, or granite blocks ranging between 0.2 and 5 m 3 were dropped from different heights (8.5 to 23.6 m) onto four slopes with different shapes (single or double bench) and slope angles (42º to 71º). The characteristics of the blocks, in particular the size, surface texture and joint condition, were measured before the drops. The trajectories of the blocks and both the initial and the impact velocities were tracked and recorded by means of three high-speed video cameras. A total of 200 block-to-ground impacts have been studied. On average, 40% of the blocks broke upon impact on the slope or on the ground, making it necessary to measure the fragments. The initial and final sizes of the blocks/fragments were measured by hand with tape, though photogrammetric techniques (UAV and terrestrial) were also used for comparison purposes. The information gathered during the field tests provides a deep insight into the fragmentation processes. On the one hand, the high-resolution slow-motion videos help to describe when and how the block breakage takes place and the spatial distribution of the pieces. On the other hand, it is possible to compute the block trajectories, the velocities, and the energy losses using videogrammetry. The results include, for instance, a block average fragmentation of 54% and 14% for the limestone and granitoids, respectively; the systematic inventory of the size fragments, which may be used for fitting the power law distributions; and after each breakage, the total angle of aperture occupied by the fragments has been measured, with values in the range 25º-145º. To figure out the different behavior of the blocks in terms of breakage/no breakage, each block-to-ground impact has been characterized with a set of parameters describing the energy level, the robustness of the substrate, and the configuration of the block contact at the impact point, among others. All these terms are combined in a function F, which is used to adjust the field data. The adjustment has been carried out, first, for the whole 200 events and later for a subset of them. The procedure and the results are described in the paper. Although the discrimination capability of F is moderately satisfactory, it is very sensitive to the test site and setup. It must be highlighted that these field tests are a unique source of data to adjust the parameters of the numerical simulation models in use for rockfall studies and risk mitigation, especially when fragmentation during the propagation is considered.

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