Rockfall Hazard Research Articles

Mechanisms of Block Instability at the Toe of a Slowly Deforming Rock Slope

Steep alpine rock slopes undergoing deformation may give rise to concurrent landslide hazards of different type and magnitude. The underlying mechanisms of instability are often challenging to investigate due to their inherent complexity; furthermore, they can occur on poorly accessible terrain, preventing the collection of data by means of traditional field techniques or even inhibiting awareness of hazards. This paper focuses upon one such case, in which a major transportation corridor running along the floor of the Aosta Valley (Western Italian Alps) is affected by significant—and until recently unknown—rockfall hazards promoted by a previously collapsed rockslide still deforming slowly at elevations almost 600 m above the road. In particular, two large discrete blocks (volume > 103 m3) lie precariously at the toe of the slide and could fall downslope at extremely rapid velocity. The design of countermeasures for the stabilization or removal of these blocks would require the assessment of their mechanical interaction with the bedrock and degree of internal fracturing (i.e., possible pervasive damage within the blocks). We perform this task by first exploring potential kinematic styles and damage patterns at failure according to a series of preliminary finite-element models. We then use detailed displacement measurements from remote sensing and in situ monitoring, in conjunction with repeat topographic surveying from a terrestrial laser scanner (TLS) and a drone laser scanner (DLS), to reconstruct the actual kinematics of the blocks. The results substantiate the hypothesis that instability is primarily controlled by transient degradation of friction on a through-going basal rupture surface. Development of a large tensile fracture in one of the two blocks is inferred to be conditioned by increased non-planarity of the slipping joint in comparison with the other block. We highlight that optimized integration of cutting-edge rock slope investigation tools can help address otherwise unresolved key aspects of complex instabilities in steep mountainous areas.

An Analysis of Slope Stability in the Penipe–Baños Road by Applying Empirical Methods, Kinematic Analysis and Remote Photogrammetry Techniques

The purpose of this work is to analyze the stability of four slopes along the Penipe–Baños road, which is situated in the provinces of Chimborazo and Tungurahua and where there are occasionally rockfalls that hinder passage and endanger road users. The methodology used to conduct the analysis was based on data collection with the help of remote techniques such as structure from motion, which allows us to obtain slope data using photogrammetry. Empirical methods such as slope mass rating, Q-slope, the kinematic method and the Rockfall Hazard Rating System method were used. These methods were evaluated with Rocfall3 software for the analysis of the fall trajectory of rock blocks. The results of this work show that the slopes studied do not represent a greater risk to the road than other slopes close to those studied, but these could not be analyzed due to their lack of accessibility and the danger of obtaining data under those conditions. The study of these different methods demonstrates the reliability of low-cost, remote techniques in the facilitation of analysis in places with similar conditions.

Development of a risk mapping along a railway

The Ramal Trem Turístico (RTT), a 17 km long railway between Ouro Preto and Mariana, a centenary line, currently in operation as a tourist attraction with historical importance. The railroad was built along a steep region, with hundreds of slopes, and eventually slope instabilities compromised the tour. In linear infrastructure projects, a risk assessment is an important tool for risk management, mapping it is the first step. After slope instabilities in 2019, a risk map was elaborated early 2021 for RTT. Risk is obtained by multiplying the probability of a certain event to occur by its consequences. Define its value in a quantitative way demands geotechnical investigation, for precise failure probability. Regardless of its importance, its determination needs high resources, specially for linear infrastructure in a region with heterogeneous geology and pedology. Therefore, for the risk mapping of the RTT, a semi-empirical methodology was adopted, based on the Rockfall Hazard Rating System (RHRS), originally developed for rocky slopes, and adapted for the registration of slopes in soil and in landfill. The aim of the method is to register a series of slope characteristics to be assigned a specific score, framing the slopes within pre-defined risk classes. This methodology was applied to this 17 km railway and identified 286 slopes. In January 2022 extreme rainfall triggered slopes instabilities at RTT. A critical analysis of this event shows a satisfactory result for the applied methodology. Risk mapping is an important tool for risk management, helping to prioritize investments in mitigation measures.

Comparison of Rating Systems for Alberta Rock Slopes, and Assessment of Applicability for Geotechnical Asset Management

In 1999, Alberta Transportation and Economic Corridors (TEC) implemented the Geohazard Risk Management Program (GRMP) to identify, assess, monitor, and prioritize the mitigation of risk resulting from geohazard events at specific sites along the provincial highway network. The GRMP was developed to address a variety of geohazard types including rockfall hazards that occur at natural and constructed (cut) highway backslopes. An evaluation of various methods for the condition assessment of rockfall geohazards, including TEC’s current GRMP risk rating system, has been completed with the intent of better understanding the suitability of each method as TEC transitions to a formalized GAM program. The GRMP risk rating values for selected rockfall geohazard sites along highway corridors in Alberta were compared to values developed from the results of five established rock mass and rock slope rating systems. The results of this study demonstrate that TEC’s current GRMP risk rating system is a viable tool for the condition assessment and performance monitoring of rockfall geohazards, which could be utilized within a formalized GAM program, further benefitting from years of recorded application in Alberta. Of the other rating systems tested, the rockfall hazard rating system (RHRS) showed a strong correlation with the GRMP risk rating while Q-Slope, the Geological Strength Index (GSI) and Rock Mass Rating (RMR) correlation were marginal but displayed a potential for use as condition assessment tools. The work presented in this paper provides the first evaluation of rock slope rating systems for rockfall hazards along corridors in Alberta, directly comparing them to the slope performance as observed by TEC in a quantitative manner.

Rockfall monitoring with a Doppler radar on an active rockslide complex in Brienz/Brinzauls (Switzerland)

Abstract. We present and analyze a rockfall catalog from an active landslide complex in Brienz/Brinzauls of the Swiss Alps, collected with a new Doppler radar system. This radar system provides a complete and continuous time series of rockfall events with volumes of 1 m3 and greater since 2018 and serves as automatic traffic control for an important main road. In the period between January 2018 and October 2022, 6743 events were detected, which is 2 orders of magnitude higher activity than in stable continental cliffs. A few percent of all rockfall events reached the shadow area, which hosts an important road and agricultural area. The Doppler radar data set allows us to investigate the triggering factors quantitatively. We found that the background rockfall activity is controlled by seasonal climatic triggers. In winter, more rockfalls are observed during thawing periods, whereas in summer the rockfall activity increases with hourly rainfall intensity. We also found that, due to the geological setting in an active landslide complex, increased rockfall activity occurs clustered in space and time, triggered by local displacement hotspots. Thus, monitoring spatial and temporal variations of slope displacement velocity is crucial for detailed rockfall hazard assessment in similar geological settings.

Assessment and Mechanism Analysis of Forest Protection against Rockfall in a Large Rock Avalanche Area

Trees in forests can obstruct falling rocks and serve as a natural barrier to reduce the velocity of falling rocks. Recently, there has been growing interest in utilizing forests to safeguard against potential rockfall. Nevertheless, there is a dearth of research regarding the impact of rock size and forest structure on forest preservation against rockfall. This study takes the Jiweishan rock avalanche that occurred in China in June 2009 as an example to discuss the protection mechanism of forests against rockfall in rock avalanche disasters. Three sizes of rockfalls from the Jiweishan rock avalanche were simulated and analyzed with and without forests using Rockyfor3D software. The findings indicate that forests can mitigate the energy impact of falling rocks. Especially in the debris flow area of rock avalanches, the protective effect of trees on small-sized falling rocks is most obvious, reducing the runout distance and damage range of the debris flow. Moreover, the protective effect of forest structures on rockfall risk was explored. It was found that broad-leaved forests had the best protection against falling rocks, followed by coniferous broad-leaved mixed forests, and coniferous forests had the worst protective effect. Furthermore, increasing forest planting density and tree diameter at breast height (DBH) can result in better protection against rockfall. Thus, rational planning of forest species and planting density in areas of rockfall can effectively reduce the threat of rockfall risk. The research ideas in this study can provide a basis for evaluating the mitigation of rockfall hazards by forests and provide a reference for constructing and planning protective forests in rockfall and rock avalanche hazard areas.

Lessons learnt from a rockfall time series analysis: data collection, statistical analysis, and applications

Abstract. Historical rockfall catalogues are important data sources for the investigation of the temporal occurrence of rockfalls, which is crucial information for rockfall hazard and risk assessments. However, such catalogues are rare and often incomplete. Here, we selected and analysed seven catalogues of historical rockfalls in Austria, Italy, and the USA to highlight existing relationships between data collection and mapping methods and representativeness of the resulting rockfall records. Heuristic and simple statistically based frequency analysis methods are applied to describe and compare the different historical rockfall catalogues. Our results show that the mapping strategy may affect the frequency of the assessed rockfall occurrence and the completeness and representativeness of the related time series of historical rockfalls. We conclude by presenting the advantages and limitations of the application of different frequency-based methods for analysing rockfall catalogues and providing recommendations for rockfall mapping. We furthermore present non-parametric statistical methods for dealing with typically small rockfall datasets, which are particularly suited for the characterization of basic rockfall catalogues. Such recommendations should help in the definition of standards for collecting and using temporal rockfall data in hazard and risk assessments.

Classification of rock slope cavernous weathering on UAV photogrammetric point clouds: The example of Hegra (UNESCO World Heritage Site, Kingdom of Saudi Arabia)

The analysis of three-dimensional point cloud data is becoming one of the most used approaches to assess instabilities processes affecting rock slopes. With the increased collection of point cloud data, there is an increasing demand for rapid computational point cloud segmentation techniques to format data for rock fall risk analysis. However, there is a scarcity of semantic segmentation research focusing on geological data and the practical issues they pose. In this study, we propose a novel application to automatically define the spatial extent and intensity of weathered rock slope areas. The studied rock slopes are in Hegra Archaeological Site (al-Hijr / Madā ͐ in Ṣāliḥ), the first UNESCO World Heritage area inscribed in the Kingdom of Saudi Arabia (KSA) in 2008. Here, cavernous weathering processes, with honeycombs and tafoni features, affect the carved rock slope and increase the rockfall hazard. With the spread and improvement of close-range methods, such as Terrestrial Laser Scanner (TLS) and Unmanned Aerial Vehicle Digital Photogrammetry (UAV-DP), it is possible to obtain fast and accurate high-resolution point cloud data of the studied scenario, i.e., rock mass. This paper presents the automated mapping results obtained by a combination of explainable artificial intelligence (XAI) algorithms and three-dimensional geometric analysis of a point cloud using the geometric feature tools available for free on CloudCompare (CC) open-source software. Three-dimensional data were obtained using the UAV-DP technique. The proposed workflow allows, using random forest modelling (RF), to quantify the cavernous weathering extent and estimate the relative intensity on the rock slope, faster than using traditional time-consuming manual segmentation processes. The RF model was able to accurately forecast the cavernous weathering regions (accuracy of up to 85%), with a slight propensity to underpredict, as indicated by the somewhat low recall (∼ 63%), and a limited number of false positives, as indicated by the high precision (up to 84%). This approach is easily reproducible at low cost in terms of computational, human, and financial resources. The results highlight the reliability of the proposed method for the preliminary assessment of cavernous erosional features with the aim of future assessment, monitoring, and deeper analysis of rock-carved slopes.

A Design Scenario Approach for Choosing Protection Works against Rockfall Phenomena

Proximity remote sensing techniques, both land- and drone-based, allow for a significant improvement of the quality and quantity of raw data employed in the analysis of rockfall phenomena. In particular, the large amount of data these techniques can provide allows for the use of probabilistic approaches to rock mass characterization, with particular reference to block volume and shape definition. These, in return, are key parameters required for a proper rockfall hazard assessment and the optimization of countermeasures design. This study aims at providing a sort of guide, starting from the data gathering phase to the processing, up to the implementation of the outputs in a probabilistic-based scenario, which is able to associate a probability of not being exceeded with total kinetic energy values. By doing so, we were able to introduce a new approach for the choice of design parameters and the evaluation of the effectiveness of mitigation techniques. For this purpose, a suitable case study located in Varaita Valley (Cuneo, Italy) has been selected. The area has been surveyed, and a model of the slope and a digital model of the rock faces have been defined. The results show that a 6.5 m3 block has a probability of not being exceeded of 75%; subsequent simulations show that the level of kinetic energy involved in such a rockfall is extremely high. Some mitigation techniques are discussed.

Lidar-Derived Rockfall Inventory—An Analysis of the Geomorphic Evolution of Rock Slopes and Modifying the Rockfall Activity Index (RAI)

Rockfall presents a significant risk to the safety and economy of communities and infrastructure in mountainous regions. The recently-developed Rockfall Activity Index (RAI) utilizes high-resolution terrestrial lidar-derived digital elevation models (DEMs) of rock slopes to categorize a slope face into seven distinct morphological units, or “RAI classes”. This paper focuses on a comprehensive study conducted at four sites in Alaska, USA, where a robust lidar-based five-year inventory of 4381 rockfall events was analyzed. The primary objective was to investigate variations in failure characteristics, such as cumulative magnitude–frequency distributions, non-cumulative power–law parameters, average annual failure rates, and average failure depths, among the different RAI classes. The findings demonstrate that the seven RAI classes effectively differentiate the rock slope based on unique mass-wasting characteristics. Furthermore, the research explores spatial and temporal variations in these failure characteristics. Based on the study’s outcomes, recommendations are provided for modifying the RAI parameters for each RAI class, specifically the annual failure rate and average failure depth. These modifications aim to enhance the accuracy and effectiveness of rockfall hazard assessments.

Rockfall hazard evaluation in Sargalu village: North of Iraq

Rockfall hazard evaluation in Sargalu village: North of Iraq

Accounting for the effect of forest and fragmentation in probabilistic rockfall hazard

Abstract. The presence of trees along the slope and block fragmentation at impact strongly affect rockfall dynamics and hazard as a consequence. However, these phenomena are rarely simulated explicitly in rockfall studies. We performed rockfall simulations by using the 3D rockfall simulator Hy-Stone, modeling both the presence of trees and fragmentation through specific algorithms implemented in the code. By comparing these simulations with a more classical approach that attempts to account implicitly for such phenomena in the model parameters and by using a new probabilistic rockfall hazard analysis (PRHA) method, we were able to quantify the impact of these phenomena on the design of countermeasures and on hazard. We demonstrated that hazard changes significantly when accounting explicitly for these phenomena and that a classical implicit approach usually overestimates both the hazard level and the 95th percentile of kinetic energy, leading to an oversizing of mitigation measures.

Potential Rockfall Source Identification and Hazard Assessment in High Mountains (Maoyaba Basin) of the Tibetan Plateau

Potential rockfall source areas are widely distributed in the high mountain areas of the Tibetan Plateau, posing significant hazards to human lives, infrastructures, and lifeline facilities. In a combination of field investigation, high-precision aerial photogrammetry, and numerical simulation, we took the Maoyaba basin as an example to explore a rapid identification method for high-altitude rockfall sources. An automatic potential rockfall source identification (PRSI) procedure was introduced to simplify the process of rockfall source identification. The study revealed that rockfall sources are concentrated in areas with intense frost weathering. Our identification results were validated using rockfall inventory data detection from remote sensing images and field investigation. Of the rockfall source areas identified by the PRSI procedure, 80.85% overlapped with the remote sensing images result. The accuracy assessment using precision, recall, and F1 score was 0.91, 0.81, and 0.85, respectively, which validates the reliability and effectiveness of the PRSI procedure. Meanwhile, we compared the rockfall source distribution of five DEMs with different resolutions and four neighborhood areas. We discovered that, in addition to high-resolution DEMs (i.e., 1 m and 2 m), medium-resolution DEMs (i.e., 5 m, 12.5 m) also perform well in identifying rockfall sources. Finally, we conducted a hazard assessment based on Culmann’s two-dimensional slope stability model and rockfall hazard vector method. Appropriate protective measures should be taken at high-hazard sections to safeguard pedestrians, vehicles, and related infrastructure from rockfalls.

Quantitative correlation between rock fall and weather seasonality to predict changes in rock fall hazard with climate change

Quantitative correlation between rock fall and weather seasonality to predict changes in rock fall hazard with climate change

Characterizing large rockfalls using their seismic signature: A case study of Hongya rockfall

A preliminary, yet timely characterization of rockfall hazards is essential for effective emergency response and mitigation measures. Such characterization is, however, particularly challenging for events occurring on high-mountain slopes in remote locations, for which precursory information remains often undetected. Large rockfalls are known to produce seismic signatures that, if properly interpreted, provide key information on the magnitude and destructiveness of the event. To achieve this, we establish a seismic signal-based assessment scheme and demonstrate its capability by taking a large event – the 5 April 2021 Hongya rockfall (Sichuan, China) – as a case study. First, we show how a rockfall can be distinguished from an earthquake and a rockslide by analyzing its seismic signatures. Then, we demonstrate how the kurtosis-based method can be used to rapidly detect the initiation of a rockfall and determine the seismic wave velocity accordingly, as well as how the arrival-time-based location method can be used to locate the event. The rockfall volume can be estimated from the magnitude of radiated seismic energy. Furthermore, we characterize the different phases in seismograms and link them to the typical stages of rockfall, i.e., precursory small rockfalls, impact and fragmentation, sliding and entrainment, and gravity deposition stages. Finally, a discussion and suggestions are provided for further improving the robustness of the assessment scheme. Our results show that the seismic signal-based scheme presented in this study is suitable for characterizing large rockfalls and has the potential for rapid and effective emergency management.

Automated Delimitation of Rockfall Hazard Indication Zones Using High-Resolution Trajectory Modelling at Regional Scale

The aim of this study was to delimit potential rockfall propagation zones based on simulated 2 m resolution rockfall trajectories using Rockyfor3D for block volume scenarios ranging from 0.05–30 m3, with explicit inclusion of the barrier effect of standing trees, for an area of approx. 7200 km2 in Switzerland and Liechtenstein. For the determination of the start cells, as well as the slope surface characteristics, we used the terrain morphology derived from a 1 m resolution digital terrain model, as well as the topographic landscape model geodataset of swisstopo and information from geological maps. The forest structure was defined by individual trees with their coordinates, diameters, and tree type (coniferous or broadleaved). These were generated from detected individual trees combined with generated trees on the basis of statistical relationships between the detected trees, remote sensing-based forest structure type definitions, and stem numbers from field inventory data. From the simulated rockfall propagation zones we delimited rockfall hazard indication zones (HIZ), as called by the practitioners (because they serve as a basis for the Swiss hazard index map), on the basis of the simulated reach probability rasters. As validation, 1554 mapped past rockfall events were used. The results of the more than 89 billion simulated trajectories showed that 94% of the mapped silent witnesses could be reproduced by the simulations and at least 82% are included in the delimited HIZ.

Classification of Risks for Landslides in Slopes and Hillsides of Volcanic Nature in Macaronesia and Their Application to the Canary Islands

Due to their genesis, volcanic rocks present some singularities that make their geotechnical characteristics significantly different from other more common types of massifs, such as sedimentary and metamorphic rocks. The formation mechanisms of volcanic rocks are varied, rapid, and, in general, of high energy. These processes give this type of rock a geotechnical behaviour and geomechanical properties that are totally different from those of other nonvolcanic materials, derived from their high heterogeneity and anisotropy. There are voids and cavities due to the alternation of strata of different competences and resistances, or susceptibility to erosion, and discontinuities and joints of very different genesis (of thermal origin, by mechanical forces, by erosive processes or by shrinkage—recrystallization). The phenomenology of the instability of blocks and stones is variable, which makes it very difficult to establish simple and concrete methodologies or procedures to study and analyse this problem. To date, the estimation of the risk of this type of phenomenon has been quantified using empirical methodologies; this approach is considered to be the most operative in responding to such a complex phenomenology in which a multitude of factors intervene. In the field of roads, the most widely used methods are RHRS and RHRON. Therefore, a new rockfall risk classification based on the RHRS (Rockfall Hazard Rating System) methodology is proposed in this article and specifically applied to the Canary Islands region.

Preliminary estimation of rock-fall lateral dispersion by laboratory test

Lateral dispersion significantly directs the assessment of rockfall hazard and design of countermeasures. In the present study, the dependence of lateral dispersion on different controlling factors has been systematically evaluated by performing laboratory tests using three different rock block types, namely circular block, and two types of elliptical block. The three types of rock block are released onto an inclined surface with the identical initial status. Parallel, anti-parallel, and oblique impact tests set at slope angles of 22.5° and 45°are conducted to study the block-slope interaction of rockfall. Lateral dispersion of rockfall is less influenced by the block shape for the oblique impact, while the post-impact behaviors are greatly affected by the block shape. The key factors influencing the deviation of the post-impact trajectory direction are the slope angle (θ) and direction difference (Δφ). An empirical model is then developed to characterize the deviation distribution of lateral dispersion by 5th and 95th percentile values with the inclusion of the two key factors. Linear function can be used to describe the 5th percentile boundary, while hyperbolic function is good for the 95th percentile boundary, which need to be validated by field tests in the subsequent research.

Dynamic responses and damage behavior of hollow RC piers against rockfall impact

Rockfall impact on bridge piers poses a challenge to the safety of bridge structures in mountainous regions. Hollow reinforced concrete (RC) piers are one of the popular constructional elements in engineering construction, and their impact behavior has not been fully investigated. Therefore, this study aims to explore the dynamic responses and damage behavior of hollow RC piers under rockfall impact. A finite element (FE) model of a hollow RC pier is established and validated against experimental tests. The validated model is then utilized to investigate the impact force, lateral displacement, internal forces, and failure mode of the hollow pier under rockfall impact. A parametric study is conducted to examine the effectiveness of rockfall diameter, impact velocity, axial force, impact elevation, and reinforcement ratio on the dynamic responses and damage modes of hollow piers. The results show that hollow piers under rockfall impact are prone to suffer serious localized damage with concrete spalling at the front panel, which plays a significant role in the pier’s global response. The stress distribution of cross-sections near the impact position is governed by the stress wave propagation and sectional internal force. Increasing the rockfall diameter would change the resistance mechanism of the front panel and enlarge the peak impact force (PIF) due to the influences of side panels and impact energy. The effects of impact elevation, axial force ratio, and reinforcement ratio are marginal on the impact force but significant on the lateral displacement and damage of hollow piers. This study provides a comprehensive understanding of impact performance of hollow RC piers and offers useful insights for their design and protection against rockfall hazards.

Rockfall hazard estimation and related applications for a preliminary risk assessment at regional scale: an example from northwestern Italian Alps

Rockfall hazard estimation and related applications for a preliminary risk assessment at regional scale: an example from northwestern Italian Alps