Event tree analysis for mountain roads under rockfall hazard

Rockfall is one of the most hazardous and dangerous landslide phenomena, which can significantly affect mountain roads. To the knowledge of the Author, limited studies focus on the quantitative risk assessment to pedestrians (hikers) and vehicles along mountain little traffic roads. A method tailored to these elements at risk is herein presented and applied to a real case. The calculation method is based on the Event-Tree Analysis, through which all the scenarios which can lead to a fatality or injuries are investigated. An application on a study case in the Italian Alps illustrates the potentialities of the methodology

Multiple Lines of Rockfall Net Fences: A Design Proposal of the System

Net fences constitute a valuable and widely adopted solution for mitigating rockfall risk. Nevertheless, in complex morphological situation with several rocky clifs, a single line cannot be efective, and a double line of net fences is thus often required. At present, the existing design procedure has been conceived for a single line, only. In the present work, a design procedure for double line of net fences is introduced, considering the double line as a system, in which the upslope line stops, or at least decelerates, the great majority of the blocks, while the downslope line stops the remaining part. As a system, the effects of actions and resistances should be evaluated as a whole. The integrative trajectory analyses are required to optimise the design in terms of necessary energy absorption capacity, height of the lines, and maintenance planning. This system is suitable for mitigating rockfall risk, especially in very complex situations

Debris flow interaction with open rigid barriers A DEM-LBM approach for trapping efficiency and impact force analysis

Debris flow is a dangerous landslide phenomenon occurring after intense rainfall in mountainous regions. It can be defined as a very rapid flow of heterogeneous material of different grain sizes with high water content. Due to its multi-phase nature, in which solid, fluid and air continuously interact, debris flow is a complex phenomenon, difficult both to analyze and to simulate. Because of its rapidity and unpredictability, it can cause loss of lives and extended damages to environment and structures. Thus, efficient mitigation measures are often desirable. Due to the complexity of the phenomenon, the design of barriers is still a challenging problem. Since a proper regulation does not exist, several of them have been designed only by imitating previously built barriers that have exhibited the proper functions during past events. Moreover, different types exist. The present thesis focuses on structural mitigation measures, with particular reference to open rigid barriers. Several Authors suggested that these barriers have to lower the kinetic energy of the flowing mass and to retain coarse sediments, allowing water and fine particles to pass. The main aspects to consider in the design of such barriers are: (1) the filter size problem, i.e. the size of the outlets, (2) the forces exerted on the barrier by the flowing mass during and after its impact. Thus, the present thesis addresses such two problems through a novel numerical method. An existing DEM-LBM code (Leonardi et al., 2015) has been enhanced with a complete friction model, which allows the creation of stable structures among grains. The result, a 3D continuum- discrete two-phase code, is able to consider the three-dimensional behaviour of the granular mass, the influence of the fluid phase, and their effects when they impact on the barrier. The new code has been validated and adopted to study the clogging mechanisms and the outlet geometry that promotes a retention of coarse grains. First, a monosized dry granular mass has been released under the effect of gravity in an inclined channel, at end of which the barrier is set. A complete parametric study on a single outlet barrier has been performed to provide the bases for furthersimulations on multiple-outlets barriers. The influence of the impact angle, of the channel slope, and of the normalized outlet width on both the trapping efficiency and the impact force has been evaluated and critically discussed. Then, progressively weakening the assumption of dry monosized mass, more realistic configurations have been analyzed. On one hand, bidisized dry granular simulations have been performed accounting for the presence of fine particles. On the other hand, a fluid phase, representing water and fine particles, has been added to the monosized dry granular mass. Interesting outcomes have been obtained on both trapping efficiency and impact forces. Starting from the dry monosized material and a single outlet barrier, a geometrical setting which provides a complete clogging of the barrier has been found. For opening width lower than 5 times the mean particle radius, the trapping efficiency is almost 100%. This result can be extended to the multiple-outlets barrier case if the width of the barrier piles is at least 6 times the mean particle radius. Moreover, introducing a bidispersion in grain size, the efficiency of the retaining function of the barrier is preserved up to a 70% in volume of small particles. The addition of a fluid phase, for solid volume fraction greater than 5%, does not affect the results. Considering the impact forces, high stresses are localized in the outlet neighbourhood, and their intensity increases by increasing the outlet width. The presence of bidispersion lowers the global impact forces, almost independently from the fraction of fine particles. Comparing the dry cases with those in which the fluid is added, it is noted that, in the first seconds after the impact, the presence of the fluid slightly lowers the impact forces due to the solid phase. Then, the fluid phase mainly transfers its momentum to the clogged solid phase, rather than directly to the barrier

Combined Effect of Pore Water Overpressure, Far-Field Stresses, and Strength Parameters in Wellbore Stability

Studying the stability of wellbores drilled to access reservoirs can be a challenging issue in overpressured basins because accurate estimation of the far-field stresses, overpressure, and rock strength parameters can be difficult to achieve. Unexpected overpressures can induce tensile stresses around the wellbore, and the selection of an appropriate strength criterion and rock properties play an important role in determining the limit mud pressures. This study focuses on the stability analysis of wellbores by considering the mutual interaction of far-field stresses, fluid overpressure, and strength parameters of isotropic rock. We performed sensitivity analyses with the Mohr–Coulomb and Hoek–Brown criteria in two overpressured fields (North Sea basin and Browse basin) to highlight the influence of the uncertainties related to the rock strength. We defined an effective stress path (ESP) failure line to analyze the failure limit condition in the tension and compression zone. The analysis results indicated that the Hoek–Brown criterion better describes rock failure conditions, especially in the tension zone. Furthermore, we suggested using two different frictional components of strength for the tension and compression zone because it is a conservative approach, particularly at high overpressures. The mud pressures obtained from the uniaxial radial/tangential (HF) conventional analysis give different failure limits with respect to the ESP approach suggested in this study. These differences are low, and the mud weight margin can be low. In addition, we carried out numerical simulations with FLAC to investigate the extent of failure. The results indicated that the mud weight margin between the onset of local failure and borehole collapse is very low at high overpressures. Finally, the geomechanical analysis of wellbore stability in overpressured basins indicated the need for improving the accuracy in determining the strength parameters of the rock

Una procedura speditiva per la valutazione dello stato di conservazione delle barriere paramassi a rete

Le barriere paramassi a rete sono sistemi multicomponente per la mitigazione del pericolo da caduta massi. Come misure di protezione passive, sono utilizzate per intercettare e arrestare la propagazione dei blocchi di roccia a seguito del loro distacco da una parete o da un pendio. Durante la loro vita utile di progetto, le barriere sono soggette a fenomeni di invecchiamento, corrosione, e carichi da impatto; questi causano perdite di efficienza o, addirittura, di efficacia.. Una procedura multi-matriciale per la valutazione dei danni, già proposta per le reti in aderenza, è qui adattata e personalizzata per questi sistemi. Considerando le barriere paramassi come come sistemi multicomponente e individuando tutti i possibili meccanismi di danneggiamento sono studiati gli effetti di ogni potenziale danno sul comportamento complessivo. Applicazioni del metodo proposto a casi reali sono qui illustrati

A quick-assessment procedure to evaluate the degree of conservation of rockfall drapery meshes

Drapery meshes are protection devices installed on a cliff for mitigating rockfall hazard. They can prevent the detachment of rock fragments and control the dynamics of the falling blocks. During their design working life, drapery meshes are subjected to ageing phenomena, corrosion and impact, which can invalidate the purposes of the protection devices. A novel procedure based on a multi-hierarchical assessment of the damages is proposed. The approach is tailored for two technologies well diffused in the Alps. The main components of the system are identified and the effects of each potential damage on the overall behaviour are taken into account through risk analysis approached. A site campaign serving to test the procedure is described. The proposed approach can be modified to consider other drapery mesh installation types

Event tree analysis and comparison for mountain roads under rockfall hazard

Rockfall is one of the most hazardous and dangerous landslide phenomena, which can significantly affect mountainous roads. To the knowledge of the Authors, limited studies focus on the quantitative risk assessment to pedestrians and vehicles along mountain little traffic roads. A method tailored to these elements at risk is herein presented and applied to a real case. The calculation method is based on the Event-Tree Analysis, through which all the scenarios which can lead to a fatality or injuries are investigated. An application on a study case in the Italian Alps illustrates the potentialities of the methodology

Reliability analysis of rockfall net fences capacity

Rockfall protective structures are widely used in hazardous environment to protect inhabited settlements and roads. These infrastructures are provided in kits, i.e., built in parts and assembled onsite and their performance is assessed through EAD 340059-00-0106. The choice of the most appropriate product must follow reliability considerations. Since rockfall is generally modelled as a probabilistic phenomenon, a deterministic analysis on rockfall net fence capacity is inadequate. We propose a reliability based approach for assessing the safety of an installation, based on several variables associated to rockfall phenomenon and to the uncertainties related to the installation of the protection structure