Comparing kinematically detachable rock masses and rockfall scar volumes (original) (raw)
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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.
Risk evaluation of rock mass sliding in El-Deir El-Bahary valley, Luxor, Egypt
Bulletin of the International Association of Engineering Geology, 1990
Tile temple of Hatshepsut is considered as a most interesting ancient architectural monument and its stability is of real concern to human civilization. The temple is completely incised into the rock mass of Gebel Gurnah which is composed of Esna shale overlain by highly fractured Thebes limestone. Recently, two rock fails have occurred. A geotechnical characterization of the site around the temple was carried out. The present work aims to evaluate the probability of further limited circular failures in the Esna shale. This evaluation is achieved by using a near circular failure model based on the behaviour of intact rock, in conjunction with the CSIR rock mass classification system which gives the reduced mechanical properties due to the increase in discontinuity sets and weathering of the rock mass of the mountain around the temple.
Rockfall Occurrence and Fragmentation
Advancing Culture of Living with Landslides, 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 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.
Magnitude and frequency relations: are there geological constraints to the rockfall size?
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.
Template for the full paper of WLF 4 Rockfall occurrence and fragmentation
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...
Landslide Dynamics: ISDR-ICL Landslide Interactive Teaching Tools, 2018
This paper is aimed at presenting a semiautomatic procedure that, coupled with conventional methods, can be useful for a prompt definition of rock fall susceptibility scenarios with civil protection purposes. Due to its landscape morphology (steep slopes and narrow valley), regional seismicity, and rock mass characteristics, the Nera Valley (Valnerina, Umbria Region, Italy) is highly prone to rock falls. In order to cover a wide range of features and investigate the main advantages and drawbacks of the proposed approach, data collection was carried out in three different slopes by means of terrestrial laser scanning (TLS) and geomechanical surveys. Detailed three-dimensional (3D) terrain models were reconstructed to obtain the geometry of the most unstable blocks, to define the position of the main rock fall source areas, and to precisely distinguish the outcropping materials and the position of the elements at risk for reliable runout analyses. Consequently, the proposed approach can positively support proper maintenance and land management programs both in ordinary and in emergency circumstances.
The RockRisk project: rockfall risk quantification and prevention
2017
Rockfalls are frequent instability processes in road cuts, open pit mines and quarries, steep slopes and cliffs. The orientation and persistence of joints within the rock mass define the size of the kinematically unstable rock volumes and determine the way how the detached mass be-comes fragmented upon the impact on the ground surface. Knowledge of the size and trajectory of the blocks resulting from fragmentation is critical for calculating the impact probability and intensity, the vulnerability the exposed elements and the performance of protection structures. In this contribution we summarize the main goals and achievements of the RockRisk project. We focused on the characterization of the rockfall fragmentation by means of the analysis of the fracture pattern of intact rock masses, the development of a fragmentation model and its integration into rockfall propagation analysis. The ultimate goal of the project is to quantify risk due to rockfalls and develop tools for the improve...
In this paper, two analytical procedures which are independent from the existence of empirical data are presented for the calculation of (1) the size distribution of potentially unstable rock masses that expresses the potential rockfall size distribution, including big volumes corresponding to potential rare events with low susceptibility of failure and (2) the in situ block distribution on the slope face. Two approaches are, respectively, used. The first one involves the detection of kinematically unstable surfaces on a digital elevation model (DEM) and on orthophotos and the calculation of the volumes resting on them. For the second one the in situ block volumes formed by the intersection of the existing discontinuity sets are calculated using a high-resolution DEM. The procedures are presented through an application example at the country of Andorra and in particular at the chute of Forat Negre. The results from the first procedure indicate that it is kinematically possible to have mobilized volumes of some thousands of cubic meters; however, these are considered rare events with low susceptibility of failure. The size distribution of potentially unstable rock masses for big volume events was well fitted by a power law with an exponent of-0.5. The in situ block distribution on the slope face from the second procedure, assuming three types of intersection between the joints of the existing discontinuity sets and two extreme cases of discontinuity persistence, was also found to follow a power law, but with an exponent of-1.3. The comparison with the observed in the field block volume distribution on the slope face indicates that in reality discontinuities have a very high persistence and that considering only their visible trace length overestimates volumes, which is conservative.
Techniques of Rockfall Inventory in Earthquake Prone Rock Slopes
Bulletin of the Geological Society of Greece, 2017
A relevant hazard in mountainous regions is the steep rock slopes concentrating rock falls. Although rock falls are characterized by smaller rock volumes compared to other landslide types, can also provoke severe damage to buildings, infrastructures and human life due to their sudden and highly fast movement. The key to understand the processes that result in rock fall onset is an integrated study of the major causing parameters that affect slope stability. A rock slope may be subjected to many forms of triggering factors including tectonic, geomorphic, seismic, climatic or even human induced damages. This contribution provides an overview of the previous and current research related to rock falls and uses case studies of North Peloponnese in order to prove the usefulness of these methods in the Greek territory. Collecting data and production of thematic maps by means of field and remote sensing investigations can yield far more updated results incorporated in hazard assessment tech...
Magnitude–frequency relation for rockfall scars using a Terrestrial Laser Scanner
The analysis of the three-dimensional rockfall scar geometry provides clues for the understanding of the failure mechanisms acting on cliffs, of the conditioning factors, and on the frequency of the events. In this paper, a supervised step-by-step methodology is presented for establishing the statistical magnitude-frequency relation of rockfall scar volumes, using a point cloud from Terrestrial Laser Scanner (TLS) data. The methodology includes a procedure for identifying discontinuity surfaces, calculating the areas of those which are exposed, and the height of rockfall scars. In the estimation of the rockfall scar volume a key issue is the consideration of the minimum spacing of the discontinuity sets to differentiate between step-path surfaces and undulated ones. Having obtained the distributions of both the basal area and height of the scar across the slope, the volume of the rockfall scars is calculated stochastically by multiplication of these two parameters by means of a Monte Carlo simulation. Both distributions of the basal area and of the rockfall scar volume are found to be power-law, with the exponent b ranging from 0.9 to 1.2. The relation obtained might be used as a first approach of rockfall magnitude-frequency curves in large cliffs.