Simplified methodology to design flexible debris flow barriers

Alberto Grimod, Giorgio Giacchetti, Alessio Savioli, Marco Cerro

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Landslides and Geohazards

ABSTRACT: Debris flows are highly mobile flows of mixed material and are trigged by the rapid build-up of water within the slope, saturating the ground. Several debris flow mitigation systems may be foreseen. Nevertheless, it is nowadays evident the increasing of flexible debris flow barriers, which can be easily installed in remote areas. The paper presents a simplified calculation approach to design these debris flow fences. All the components of the structures, such as, the interception structure, the diameter of the longitudinal ropes and their vertical space, the length of the energy dissipater devices (brakes) and the type and length of lateral anchors, can be designed. The model returns restraining forces and rope stresses that can be used for an appropriate barrier design. RÉSUMÉ Les débris flow sont des coulées très mobiles de matières mélangées et sont déclenchées par l'accumulation rapide d'eau dans le talus, saturant le sol. Différentes typologies de mitigation contre les coulés de débris peuvent être adoptées, mais il est évident que dans ces dernières années les écrans flexibles contre les éboulements (débris flow barriers) sont devenus très populaires, car ils peuvent être installés dans des endroits presque inaccessibles. Cet article présente une méthode simplifiée pour le calcul de ces typologies de barrières. Toutes les composantes, comme la structure d'interception, le diamètre et l'espacement vertical des câbles longitudinaux, la longueur des dissipateurs d'énergie (frein) et la typologie et la longueur des ancrages latéraux, peuvent être conçus. Le model de calcul permet de définir les pression qui agissent sur la structure et les forces qui agissent sur les câbles afin de pouvoir faire un design approprié. 1 INTRODUCTION The kinetics of a landslide (how movements are distributed through the displaced mass) is one of the main criteria to classifying landslides (Cruden & Varnes, 1996). According to Cruden & Varnes (1996) it is possible to define five distinct landslides, characterized by a different kinetic: fall, topple, slide, spread and flow. 1.1 Flow A flow is a spatially continuous movement in which surfaces of shear are short-lived, closely spaced, and usually not preserved. The distribution of velocities in the displacing mass resembles that in a viscous liquid. The lower boundary of displaced mass may be a surface along which appreciable differential movement has taken place or a thick zone of distributed shear (Cruden & Varnes, 1996). Flows are often activated by heavy rainfalls. Thus, the material slides downslope increasing its volume. The dimension of the transported solid particles may reach metric dimensions. The velocity of the flow can vary depending on the water ratio content, grain size and slope gradient. According to Hungr et al. (2001) the following main categories of flows may be distinguished: debris flow, earth flow, debris avalanches, mud flow. The movement expands along preferential ways, such as natural draining systems, creeks, etc., that allow the flow traveling for miles, therefore their negative effect can be perceived far away from the starting zone (Figure 1). 1.2 Debris flow The category 'debris flows' includes the most common flow types that are capable to transport a huge amount of material with different sizes. During an event, the total material moved toward the accumulation zone may define the magnitude of the debris flow. The magnitude is rarely related to the volume of the initial landslide. Often, the initiating slide is small and the bulk of the volume transported to the deposition area results from entrainment of material along the path. Thus, it is the flow mechanism that primarily determines the total volume of a debris flow. Such aspect is extremely important to scale the event and allow correlating it to the run-out distance and maximum discharge (Hungr et al., 2005) The rheology of the debris flow varies with the time. The increasing of the debris front raises the discharge that drives the flow downstream. Thus, the peak discharge rises as well. Generally the biggest particles constitute the forward-face of the debris flow, whereas the small ones create the central core. The tail of the flow is basically composed of water and very small material (Figure 2).

RÉSUMÉ: ified methodology to design flexible debris

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Cite this article:
Alberto Grimod; Giorgio Giacchetti; Alessio Savioli; Marco Cerro (2014) Simplified methodology to design flexible debris flow barriers in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper301,author = Alberto Grimod; Giorgio Giacchetti; Alessio Savioli; Marco Cerro,title = Simplified methodology to design flexible debris flow barriers,year = 2014}