Prototype of a wire-rope rockfall protective fence developed with three-dimensional numerical modeling

This study uses a numerical procedure, previously validated with data from full-scale experiments, to investigate the performance of a modified prototype wire-rope fence to provide protection against rockfall. The cost-reducing modifications are increased post spacing and fewer wire netting layers. The numerical procedure provides the nonlinear response of the prototype under various impact conditions and insights into each component’s role in dissipating impact energy. A simple but effective method to increase fence capacity is also developed. Finally, the use of two units of the prototype to protect a wide area is investigated employing the numerical procedure.     

Rockfalls into forests: Analysis and simulation of rockfall trajectories — considerations with respect to mountainous forests in Switzerland

Rockfalls are a major threat to settlements and transportation routes in many places. Although the general protective effect of forests against rockfalls is currently not questioned, little is known about the ideal properties of a forest stand that provides good protection. Therefore, in this study the question was assessed of how mountainous forests may influence rockfalls of single boulders. An actual rockfall trajectory was measured, recorded, analysed and simulated with a rockfall model. Rockfalls into different forest scenarios were also modelled for the site. Results showed that the actual rockfall event can be well simulated. Furthermore, a completely forested slope reduces velocity and energy of the falling blocks much better than a sparsely forested slope. For the profile discussed in this paper, the largest effect upon falling 3 m³ blocks was obtained with a high forest containing 350 trees per ha. The results confirmed common assumptions on ideal properties of a protective forest stand against rockfalls.     

Quantitative assessment of the residual risk in a rockfall protected area

Quantitative Risk Assessment (QRA) has become an indispensable tool for the management of landslide hazard and for planning risk mitigation measures. In this paper we present the evaluation of the rockfall risk at the Solà d’Andorra slope (Andorra Principality) before and after the implementation of risk mitigation works, in particular, the construction of protective fences. To calculate the risk level we have (i) identified the potential rockfall release areas, (ii) obtained the volume distribution of the falling rocks, (iii) determined the frequency of the rockfall events, and (iv) performed trajectographic analysis with a 3D numerical model (Eurobloc) that has provided both the expected travel distances and the kinetic energy of the blocks. The risk level at the developed area located at the foot of the rock cliff has been calculated taking into account the nature of the exposed elements and their vulnerability. In the Forat Negre basin, the most dangerous basin of the Solà d’Andorra, the construction of two lines of rockfall protection fences has reduced the annual probability of loss of life for the most exposed person inside the buildings, from 3.8×10⁻⁴ to 9.1×10⁻⁷ and the societal risk from 1.5×10⁻² of annual probability of loss of life to 1.2×10⁻⁵.     

Rockfall susceptibility zoning at a large scale: From geomorphological inventory to preliminary land use planning

Rockfall, up to several hundreds of cubic meters, is a frequent and rapid landslide which menaces extensive areas in mountainous territories. Rockfall susceptibility zoning map at a large scale (1:5000–1:25 000) can be the first tool for land use planning in order to manage rockfall risk. A methodology allowing to analyze susceptibility in extensive areas with optimum cost/benefit relationship is needed. This work analyzes rockfall susceptibility in an extensive rocky mountain of the Principality of Andorra (Pyrenees Mountains), first on the rock slope and then on the exposed area located below. The rockfall record, obtained by means of geomorphological analysis, supplies the main data to analyze the susceptibility on the rock slope. An additional historical inventory verifies the accuracy of rockfall sizes recorded by means of the geomorphological analysis. According to the classification recommended by the Guidelines of Joint Technical Committee, the density of rockfall features on the rock slope assesses susceptibility in four levels. Subsequently, susceptibility on exposed areas has been analyzed by means of reach probability of rock blocks analysis using empirical models. Data acquired from thirteen recent events, from 1999 to 2004, have been used to verify the accuracy of the two empirical models mainly used (reach angle and shadow angle). Five reach probability limits (1, 0.5, 0.25, 0.01, and 0) establish boundaries between susceptibility levels. The resulting rockfall susceptibility zoning map allows: (a) to identify land areas and human elements exposed to rockfalls and, (b) to establish several exposition levels. This map can be a useful and cost-effective tool for administrations responsible to manage natural risk in order to guide urban grow in extensive areas or decide upon work programs based on in-depth analysis (hazard and risk).     

Susceptibility analysis for slides and rockfall: an example from the Northern Calcareous Alps (Vorarlberg, Austria)

In this paper a tool for semi-quantitative susceptibility assessment at a regional scale is presented which is applicable at areas with complex geological setting. At a study area within the Northern Calcareous Alps geotechnical mappings were implemented into a Geographical Information System and analysed as grid data with a cell size of 25 m. The susceptibility to sliding and falling processes was considered according to five classes (very low, low, medium, high, very high). Susceptibility to sliding was analysed using an index method. The layers of lithology, bedding conditions, tectonic faults, slope angle, slope aspect, vegetation and erosion were combined iteratively. Dropout zones of rockfall material were determined with help of a Digital Elevation Model. The movement of rolling rock samples was modelled by a cost analysis of all potential rockfall trajectories. These trajectories were also divided into five susceptibility classes. The susceptibility maps are presented in a general way to be used by communities and spatial planners. Conflict areas of susceptibility and landuse were located and can be presented destinctively.     

Ground reinforced embankments for rockfall protection: design and evaluation of full scale tests

Infrastructures such as roads and railways as well as urbanised areas, in mountainside regions, can frequently be endangered by rockfalls and, therefore, need to be protected against the impact of rolling blocks. Among the various protection works that can be used, ground reinforced embankments can be considered a feasible technique. A set of full-scale tests on embankments made of ground reinforced by geogrids are presented and discussed. The experiments were performed in a specifically designed and constructed test facility, where concrete blocks up to 9,000 kg in weight were thrown onto a geogrid reinforced embankment at a speed of about 30 m/s. Several embankments made of different geogrid types, different soils and construction layouts were tested at different impact energy levels, permitting a quantitative assessment of the resistance impact of these structures. The experimental results were compared with those obtained from a dynamic finite element method numerical model, and a good agreement was obtained.     

Toward objective rockfall trajectory simulation using a stochastic impact model

The accuracy of rockfall trajectory simulations depends to a large extent on the calculation of the rebound of falling boulders on different parts of a slope where rockfalls could occur. The models commonly used for rebound calculation are based on restitution coefficients, which can only be calibrated subjectively in the field. To come up with a robust and objective procedure for rebound calculation, a stochastic impact model associated with an objective field data collection method was developed and tested in this study. The aims of this work were to assess the adequacy of this approach and to evaluate the minimum amount of field data required to obtain simulation results with a satisfactory level of predictability. To achieve these objectives, the rebound calculation procedure developed was integrated into a three-dimensional rockfall simulation model, and the simulated results were compared with those obtained from field rockfall experiments. For rocky slopes, the simulations satisfactorily predict the experimental results. This approach is advantageous because it combines precise modelling of the mechanisms involved in the rebound and of their related variability with an objective field data collection procedure which basically only requires collecting the mean size of soil rocks. The approach proposed in this study therefore constitutes an excellent basis for the objective probabilistic assessment of rockfall hazard.     

Spatial analysis of rockfall activity, bounce heights and geomorphic changes over the last 50 years – A case study using dendrogeomorphology

Tree-ring series have been used to reconstruct 50 years of rockfall behavior on a slope near Saas Balen (Swiss Alps). A total of 796 cores and 141 cross sections from 191 severely injured conifer trees (Larix decidua Mill., Picea abies (L.) Karst. and Pinus cembra L.), combined with a series of aerial photographs, were used to investigate the evolution of the forest stand so as (i) to reconstruct past rockfall rates; (ii) to analyze the spatial behavior of maximum bounce heights; and (iii) to analyze the spatial comportment of rockfall activity over the last five decades.Tree-ring analysis permitted the reconstruction of the age distribution at the study site; results were in perfect agreement with the afforestation process shown in the aerial photographs. The oldest are located in the lower, central part of the study site; the youngest individuals at its uppermost lateral boundaries. Reconstructed rockfall rates reveal strong interannual variations and single event years with increased activity, namely in 1960/1961 and 1995. Spatial analysis of the maximum bounce heights indicate highest values at the lateral boundaries and lowest heights in the lower central part of the forest stand, where a big boulder seems to shield trees growing below it. The spatial analysis of past rockfall activity shows high active zones at the uppermost north-facing boundaries of the forest and least active zones in the lowermost central part of the studied stand. The high rockfall activity at the slope is expressed by a mean rockfall rate of > 1 event m^− 1 y^− 1.     

崩塌落石运动学的模拟研究

长江三峡链子崖危岩，目前正在进行全面治理，本文以链子崖东侧猴子岭崩塌落石为典型实例，开展了地质力学与数值模拟，研究了崩塌落石的运动学特征，并以防治方案提出了建议。     

滚石运动的计算方法及其研究中的不确定性

在多山地区, 滚石事件具有突发性和多发性, 而滚石的运动具有多样性(如自由落体、滑动、滚动和抛体运动等)和随机性等特征。由于影响因素的多样性、运动的随机性和研究的不确定性等原因, 目前滚石运动特征的准确合理预测仍面临较大问题。由此, 也导致了滚石运动的计算结果与实际出入较大, 造成大量防治措施失效而产生滚石灾害的现象。作者在总结前人滚石运动计算方法的基础上, 分析了不同方法的适用性, 结合实例分析, 作者提出了滚石运动研究中具有不确定性, 即认知不确定性和偶然不确定性, 并提出:建立滚石运动研究的不确定性观点、理论和方法是推动滚石运动科学预测和防灾减灾合理决策的有效路径之一。相关结论为滚石运动的研究和防护工程实践中选择适宜的计算理论、方法, 以及进行科学的防灾减灾决策提供参考。