Franck Bourrier

A Reliability-Based Approach for the Design of Rockfall Protection Fences

This paper proposes a method for improving the design of rockfall protection fences and accounting for the variability of loading cases. It is based on a probabilistic reliability analysis and can combine loading cases from rockfall propagation simulations with numerical simulations of the structure response to the block impact. The advantage of such a reliability-based approach is that statistically relevant results can be obtained concerning the fence’s efficiency in stopping the block with a limited number of simulations. This method was employed in a study case, involving a low-energy tree-supported fence placed on a forested slope. The trajectory simulations were conducted using Rockyfor3D and the fence was modelled using a three-dimensional discrete element method model. For demonstration purposes, two parameters were considered: block velocity and the block’s angle of incidence before impact. The probability of the fence stopping the block was evaluated accounting for the variability of these two parameters separately and together, either considering these variables as non-correlated, or as correlated. The value of this approach is demonstrated in terms of computation cost. In addition, the results revealed the importance of accounting for both these parameters in designing the structure as well as in estimating the residual hazard downslope from the protective structure.

Predicting load path and tensile forces during cable yarding operations on steep terrain

Cable yarding systems constitute an adapted solution for steep-slope harvesting in mountain forests. However, it requires many specific skills for both forest managers and operators. The objectives of this research were to: (1) develop a CableHelp model for the set-up of cable yarding systems where inputs are operational field data and outputs are load path and tensile forces, and (2) to validate it with field experiments. The results show a high accuracy between the data predicted by the model and field measurements. Furthermore, this work stresses the importance of taking into account both the mainline effect and the friction between skyline and intermediate supports to properly calculate the skyline tension and load path. The CableHelp model shows great adaptability and ensures highly accurate predictions for any position on the line profile and for different configurations: single-span or multiple-span profiles, uphill or downhill yarding and for different kinds of carriage. A direct application of this research is to optimize the set-up of cable lines in order to reduce equipment wear, as well as operating cost, while respecting operator safety.

Experimental analysis of the response of fresh wood stems subjected to localized impact loading

The forest is a well known and efficient natural protection solution against rockfall. To compensate for a loss of the forest’s protective function after windstorms or maintenance tasks, some of the felled trees can be left in an oblique position on the slope as wooden protection structures. No studies have been conducted on the efficacy of these devices and particularly their resistance to rock impacts and their energy dissipation capacity. The dynamic response of fresh stems to impact was analyzed by laboratory impact experiments with a Mouton-Charpy pendulum. The experimental results allowed the definition of different impact types related to the occurrence of nonlinear processes associated with partial rupture of wood fibers. The physics controlling the stem loading force and displacement were shown to be mainly associated with inertial effects during the early stages of the impact. Second, when the stem displacements become larger, the stem response becomes quasi-static. Based on these results, a practical approach for assessing the capacity of wooden structures made of fallen trees to resist rock impact was proposed and evaluated.

Impacts of land-use and land-cover changes on rockfall propagation: Insights from the Grenoble conurbation

Several studies have debated the incidence of global warming on the probability of rock instability, whereas the impacts of land use and land cover (LULC) changes on rockfall propagation and associated hazards have received comparably little interest. In this study we evaluate the impacts of LULC changes on rockfall hazards on the slopes above the village of Crolles (Chartreuse massif, Grenoble conurbation, French Alps) through a three-level approach: (i) diachronic landscape analysis for four different periods of the past (i.e. 1850, 1956, 1975, and 2013), (ii) computation of 3D rockfall simulations taking explicitly account of reconstructed LULC changes, and (iii) resulting changes in rockfall hazards over time. We reveal that the disappearance of viticultural landscapes (relating to the decline of cropping areas during the interwar period) and intense afforestation of the steepest upper portion of the slope resulted in a significant increase of rockfall return period associated to a gradual decrease of mean kinetic energy at the level of the urban front of Crolles. According to the Eurobloc methodology, the degree of hazard decreased significantly despite the continuous and rapid urban sprawl on the slopes. These results underline that forests can indeed have significant protection function but also call for a more systematic inclusion of LULC changes in hazard assessments in the future.

Comparing numerical and experimental approaches for the stochastic modeling of the bouncing of a boulder on a coarse soil

ABSTRACT This paper proposes numerical investigations for the stochastic modeling of a boulder impacting a coarse granular soil. The soil is considered as a noncohesive granular medium using a discrete element method and the soil model is calibrated compared to results from half-scale experiments. Based on this numerical model, an extensive numerical simulation campaign is carried out. The statistical analysis of the numerical results allows the definition of a stochastic bouncing model that quantifies most of the variability of the numerical results. Comparisons with classical bouncing models in the field of trajectory analysis highlight that this model expands classical approaches. The comparison of results from real-scale experiments to trajectory simulations based on the stochastic model show the relevance of the proposed approach to modeling rockfall trajectories.

Accounting for the variability of rock detachment conditions in designing rockfall protection structures

Abstract This study is based on the analysis of the residual rockfall hazard at the elements at risk and accounts for the variability of the rock release parameters influencing the trajectory. The design of protection structures is conducted in two phases: a functional design phase consisting of quantifying the structure height from the rock passing height distribution and a structural design phase where the structure required capacity is assessed from the rock passing energy distribution. This framework is used on a well-documented study site for identifying the effects of the definition of the rocks release conditions, limited to the rock volume and falling height, on the design and efficiency of protection fences. The rock volume is modeled using a random variable, with different probabilistic laws. A probabilistic method is also used to analyze the effect of the rock volume distribution. These sensitivity analyses are conducted using a point estimate method for saving computation time. In this work, the initial falling height is shown to have a negligible influence on both the functional and structural designs of the fence. On the contrary, the rock volume range appears to be the leading parameter. The influence of the distribution law is shown to be of second order. The proposed approach may be extrapolated to other uncertain or variable parameters, as well as to other types of passive rockfall protective structures.

Effects of fungal decay on elasticity and damping of small-diameter silver fir logs assessed by the transverse vibration resonant method

A number of studies have shown the ability of the vibration resonant method (VRM) to measure the modulus of elasticity (MOE) and the damping ratio (

Effect of Forest Presence on Rockfall Trajectory. An Example from Greece

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