Comparing rockfall scar volumes and kinematically detachable rock masses

Scenario-based risk assessment for rockfalls, requires assumptions for different scenarios of magnitude (volume). The magnitude of such instabilities is related to the properties of the jointed rock mass, with the characteristics of the existing unfavourably dipping joint sets playing a major role. The critical factors for the determination of the maximum credible rockfall volume in a study site, the Forat Negre in Andorra, are investigated. The results from two previous analyses for the rockfall size distribution at this site are discussed. The first analysis provides the observed size distribution of the rockfall scars, and it is an empirical evidence of past rockfalls. The second one, calculates the kinematically detachable rock masses, indicating hypothetical rockfalls that might occur in the future. The later gives a maximum rockfall volume, which is one order of magnitude higher, because the persistence of the basal planes is overestimated. The tension cracks and lateral planes interrupt systematically the basal planes, exerting a control over their persistence, and restricting the formation of extensive planes and large rockfall failures. Nonetheless, the formation of basal planes across more than one spacings of tension cracks is possible and small step-path failures have been observed too. Concluding, the key factor for the determination of the maximum credible volume at the study-site is the maximum realistic length of the basal planes, penetrating into the rock mass, their spacing, and, if applied, the contribution of the rock bridges to the overall rock mass resistance.Peer ReviewedPostprint (author's final draft

Evaluation of maximum rockfall dimensions based on probabilistic assessment of the penetration of the sliding planes into the slope

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 joints, hence their size is restricted. The methodology requires as an input the spacing of the intersecting joint sets. Its application to a granodiorite slope confirms that for the study site, there is a maximum volume of rockfalls, excluding the possibility of large stepped failures and rupture of rock bridges. The maximum calculated persistence for the two existing sliding planes in the study site is, respectively, 28.0 m and 48.5 m. The maximum calculated sliding plane surfaces are, accordingly, 282.5 m2 and 289.3 m2. These results are compared against the observed scar dimensions at the study site, which have been retrieved alternatively, by processing a LiDAR point cloud. The results from the two alternative sources are similar, indicating that the methodology can be efficiently used to assess the sliding plane persistence and the expected maximum rockfall magnitude at the study site.Peer ReviewedPostprint (published version

Los desprendimientos rocosos, impacto y análisis cuantitativo del riesgo

Los desprendimientos rocosos son los procesos de inestabilidad de laderas que causan un mayor número de víctimas en España, así como importantes pérdidas económicas. En esta contribución se presenta la cuantificación del impacto de los desprendimientos rocosos obtenida a partir del vaciado documental, fundamentalmente prensa escrita. Se han recopilado 990 eventos ocurridos entre 1802 y 2021 que han causado víctimas (cerca de 1400 muertos y más de 2000 heridos). El análisis del inventario permite evaluar el impacto de los desprendimientos rocosos y su evolución en contextos muy diversos como las laderas edificadas, las excavaciones a cielo abierto, los ferrocarriles, las carreteras y en el entorno natural. Se constatan dos tendencias contrapuestas. Mientras que la peligrosidad y el riesgo disminuyen a medida que se acumulan las actuaciones estabilizadoras y preventivas en los entornos de las poblaciones y líneas de ferrocarril, el número de incidentes se incrementa por el aumento de la movilidad en carretera y la mayor frecuentación en el medio natural (mayor exposición). En otro apartado se realiza una primera estimación de las inversiones realizadas por distintas administraciones en la estabilización desmontes y laderas rocosas y para reducir el riesgo por desprendimientos. Finalmente, se presenta un ejemplo, detallado paso a paso, para mostrar el potencial de los inventarios de desprendimientos rocosos para evaluar de manera cuantitativa el riesgo directo e indirecto en carreteras.Este trabajo se enmarca dentro de las actividades del proyecto GeoRisk con referencia PID2019-103974RB-I00 y que está financiado por MCIN/ AEI/10.13039/501100011033.Postprint (published version

Effect of rockfall fragmentation on exposure and subsequent risk analysis

Rockfalls are frequent natural processes in mountain regions with the potential to produce damage. The quantitative risk analysis (QRA) is an approach increasingly used to assess risk and evaluate the performance of mitigation measures. In case of the fragmentation of the falling rock mass, some of the hypothesis taken in the QRA estimation for rockfalls have to be modified since a single block or rock mass can produce several fragments thus modifying the runout probability, the impact energies and exposure of the elements at risk. In this contribution, we present a procedure to account for the exposure in QRA analysis along linear paths using the fragmental rockfall propagation model RockGIS (Matas et al. 2017). The procedure is applied at the “Monasterio de Piedra”, Spain as part of a QRA.Postprint (published version

Metodología integrada para la evaluación de riesgos en la Red de Carreteras de Gipuzkoa

Se presenta una metodología de carácter integradorpara evaluar el nivel de riesgo de fallo o rotura en estructuras y taludes anclados,terraplenes, desprendimientos en laderas y desmontes, corrientes de derrubios y deslizamientos que ha sido aplicada a la Red de Carreteras de Gipuzkoa. En la presente comunicación se proporcionan los detalles del cálculo del riesgo para desprendimientos rocosos y estructuras de contención ancladas.Postprint (published version

Rockfall occurrence and fragmentation

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.Peer ReviewedPostprint (published version

Magnitude and frequency relations: are there geological constraints to the rockfall size?

The final publication is available at Springer via http://dx.doi.org/10.1007%2Fs10346-017-0910-zThere 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 was 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 m2 and a maximum exposed surface of 200 m2. The analysis of the geological structure of the rock mass at the Solà d’Andorra makes 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.Peer ReviewedPostprint (author's final draft

Quantitative analysis of risk from fragmental rockfalls

The final publication is available at Springer via http://dx.doi.org/10.1007/s10346-018-1087-9Rockfalls are ubiquitous diffuse hazard in mountain regions, cliffs, and cutslopes, with the potential of causing victims and severely damaging buildings and infrastructures. A vast majority of detached rock masses break up when impacting the ground, generating multiple trajectories of rock fragments. In this paper, we present the quantitative risk analysis (QRA) of fragmental rockfalls. Fragmentation in rockfalls requires the redefinition of the probability of reach and the evaluation of the effect of multiple rock blocks trajectories on the exposure. An example of QRA was carried out at the Monasterio de Piedra, Spain, using RockGIS, a rockfall propagation model that takes fragmentation into account (Matas et al. Landslides 14:1565–1578, 2017). The results show that fragmentation has a significant but contrasting effect in the calculation of risk. The risk is reduced if the slope where blocks propagate is sufficiently long and gentle. The reason for this is that, compared to the unfragmented rock masses, the new fragments generated travel shorter distances with lesser kinetic energy. The effect disappears in case of large rockfalls. Conversely, the risk increases if the rock fragments propagate over steep slopes. The reason is that few blocks stop along the way while the generation of a cone of fragments increases the exposure. Our simulations also show that assuming a continuous flow of visitors or segregating the flow in groups of different number of people has only a minor influence on the results. Finally, we observed that the capability of the protection barriers to stop rockfalls of up to a few tens of cubic meters increases with fragmentation.Peer ReviewedPostprint (author's final draft

Contribución de las técnicas dendrogeomorfológicas a la estimación de la frecuencia-magnitud de desprendimientos

Para evaluar la peligrosidad de los movimientos de ladera es necesario conocer cómo varía la frecuencia temporal de los movimientos con su magnitud (tamaño). Sin embargo, la información directa sobre la ocurrencia de movimientos de ladera es habitualmente escasa. Las técnicas de dendrocronología aplicada a los procesos geomorfológicos se han mostrado eficientes en la reconstrucción de series temporales de deslizamientos. En la presente comunicación se muestra que dichas técnicas pueden ser utilizadas también para reconstruir la magnitud de los movimientos. Aplicando estas técnicas, se ha obtenido la curva de magnitud-frecuencia de los desprendimientos de uno de los canales más peligrosos del Solà d’Andorra (Andorra la Vella, Principado de Andorra).Postprint (published version