Characterization of avalanche loading on impacted structures: a new approach based on inverse analysis Philippe BERTHET-RAMBAUD, 1,3* Ali LIMAM, 2 Djebar BAROUDI, 2 Emmanuel THIBERT, 1 Jean-Michel TAILLANDIER 1 1 Erosion Torrentielle, Neige et Avalanche (ETNA), Cemagref, Domaine universitaire, 2 rue de la Papeterie, BP 76, 38400 Saint-Martin-d’He `res Cedex, France E-mail: rambaud@groupemnd.com 2 Unite ´ de Recherche en Ge ´nie Civil (URGC), Institut National des Sciences Applique ´es de Lyon (INSA Lyon), 20 avenue Albert Einstein, 69621 Villeurbanne Cedex, France 3 Laboratoire Re ´gional des Ponts et Chausse ´es, Centre d’E ´ tude des Tunnels, 25 avenue Franc ¸ois Mitterrand, 69674 Bron Cedex, France ABSTRACT. Experiments have been conducted on the French full-scale experimental site at Lautaret pass to improve our understanding of the action of snow avalanches on obstacles. The ultimate objective is to provide realistic pressure distribution models suitable for use in civil engineering design and to eliminate the restrictive assumptions currently used in this field. We focus on the feasibility of using the inverse method to quantify the action of the avalanche from its effects on realistic structures rather than from sensors placed directly in the flow. This approach takes into account the interactions between the flow and the obstacle and ensures that the result is effectively the action experienced by the obstacle. The inverse analysis procedure is developed and validated using both numerical and laboratory tests. In situ tests carried out at the Lautaret site to determine the avalanche action at different scales confirm the reliability of this original approach. Its intrinsic characteristics make it especially suitable for application to different structures to provide new knowledge in this complex field. 1. INTRODUCTION In terms of avalanches, the winter of 1999 was catastrophic in the European Alps, confirming the urgent need to improve the design of avalanche protection structures and buildings subject to snow-avalanche action (Glass and others, 2000). Over the years, civil engineers have continued to improve building design calculations and techniques, developing new tools to take into account complex loading conditions. However, these tools require precise knowledge of ava- lanche action characteristics. In this paper, we present a new experimental method to determine what structures effect- ively undergo from a civil engineering point of view when impacted by a snow avalanche. Many studies have been conducted concerning the action of avalanches on structures (Lang and Brown, 1980; Schaerer and Salway, 1980; McClung and Schaerer, 1985; Schaer and Issler, 2001) and propose pressure measurement methods and values. They correspond mainly to point values meas- ured by pressure sensors impacted by the freely flowing avalanche. Two main limitations make it difficult to use these ‘flow’ data for civil engineering purposes on the scale of a real structure. First, depending on their size, pressure sensors are often directly subjected to the heterogeneous character of avalanche flow on a decimetric scale, for instance by the impact of snow blocks, resulting in large spatial and temporal variations that are not relevant from a civil engineering point of view. Furthermore, real structures interact with the ava- lanche, forming an obstacle that creates a stagnation zone and lateral or vertical deviations depending on the structure’s behaviour and shape. These interactions are not fully taken into account by the ‘flow’ approach, even if recent work (Gauer and others, 2007; Sovilla and others, 2008) provides supplementary data on the spatial variability of the pressure. However, only a few studies have focused on large obstacles (Kotlyakov and others, 1977; Norem and others, 1985), and, in practice, avalanche experts continue to evaluate avalanche action from the maximum pressure P obtained by a hydrodynamic analogy based on Equation (1) and involving the density  and velocity V of the flow (Ancey, 2006): P ¼ k 1 2 V 2 : ð1Þ The k factor theoretically represents the so-called drag factor which depends on the flow regime and obstacle geometry and is used to account for the interaction between the flow and the obstacle. Considering the difficulties in defining and estimating suitable values of V and , which vary with position and time, and the questionable validity of the hydrodynamic analogy, this k factor often becomes a rough safety factor in civil engineering projects (Ancey, 2006). Furthermore, as demonstrated experimentally by Sovilla and others (2008), Equation (1) assumes that the main source of energy in the flow acting on the obstacle is kinetic energy, which may not be true for low-velocity avalanches with Froude numbers near or lower than 1. Under such con- ditions, the k factor is used to correct any deviation from Equation (1). In addition, Equation (1) provides only a single static pressure value, neglecting the spatial distribution of the pressure and especially its dynamic variation with time which can be crucial to the actual behaviour and strength of rigid structures. The best way to determine the action of an avalanche on an impacted structure is to introduce a suitable experimental Journal of Glaciology, Vol. 54, No. 185, 2008 * Present address: MND Engineering, Alpespace, 74 voie Magellan, 73800 Sainte He ´le `ne du Lac, France. 324