ANALYSIS OF THE STABILITY VARIATION OF A SLOPE CROSSED BY FOREST FIRE ROMINA SECCI, SERGIO V. CALCINA, GAETANO RANIERI & GABRIELE URAS Department of Civil Engineering, University of Cagliari, Environmental Engineering and Architecture DICAAR, Cagliari, Italy ABSTRACT This paper deals the study of the stability variation of a hill slope crossed by a forest fire. In order to perform the stability analyses, two geotechnical models were derived by the integration of the results of geotechnical surveys (NSPT measurements and direct shear tests) with electrical resistivity tomographies. Classical approach based on Limit Equilibrium Method is used to determine the safety coefficient. Furthermore, the effects of the forest fire on the slope stability have been discussed modifying the 2D models and introducing an ultra-shallow thin layer with the shear strength parameters determined on burned soil samples, collected after the fire crossing. In particular, this analysis has shown a marked reduction of safety factor at the interface between the burned soil layer and the underlying material for both geotechnical models, considering the infinite slope method with several saturation conditions of the shallow layer. KEYWORDS: Electrical Resistivity Tomography, Fire Effect, Geotechnical Modeling, Slope Stability, Soil Erosion INTRODUCTION The assessment of the slope stability can be treated by a basic approach founded on modeling techniques, considering the physical and mechanical laws of force and moment equilibrium. Several studies have documented the collapse susceptibility of slopes through the integration of the results of geotechnical investigations. However, a preliminary analysis requires specific laboratory test in order to determine the principal parameters of the mechanical soil behaviour, together with a careful analysis of the geomorphologic features of the site [1]. This paper deals the spatial variability analysis of the slope stability trough mechanical models, derived from the results of laboratory tests and the interpretation of geophysical surveys. In a first step, many topographical information are taken from the GIS database, i.e. the digital terrain model for the elevation data are analyzed. After the acquisition of the topographic, geometrical and mechanical information it was possible to apply deterministic models, where the collapse mechanism of the ground is simplified by using the Mohr-Coulomb failure criterion and adopting a rigid perfectly plastic constitutive law. Also this study aims at assessing the effects produced by the passage of wildfires on the susceptibility to instability phenomena. In fact, as is well known, the repeated fire transition predisposes to erosion phenomena [2-4] and consequent desertification [5]. However, the direct effect on the mechanical parameters of soils is not clear. The impact of wildfires on soil erosion is supposed to operate through their effect on soil organic matter and, thus, it depends strongly on fire severity. Soil erosion is then little affected by low-severity wildfires but it markedly increases following high-severity wildfires. All burned top soils becomes from strongly to very strongly water repellent [6]. Furthermore, debris flows are usually associated with shallow landslides and are mainly generated during heavy rains on slopes characterized by low or poor vegetation cover that can induce phenomena of instability on localized zones of the slope [7]. Several works shows that the materials mobilized by shallow landslides are the main source of detrital sediments and they can generate successive debris flows or can induce shallow flows with high-level of solid transport [7-12]. Debris flows increase their run-off rate, by International Journal of Civil Engineering (IJCE) ISSN(P): 2278-9987; ISSN(E): 2278-9995 Vol. 3, Issue 1, Jan 2013, 41-50 © IASET