Geotechnical Characterization and Modeling, Via Particle Method, of the Sciara del Fuoco Volcanic Debris, Stromboli Island, Italy
by
Andrea Uttini
Dipartimento di Scienze Chimiche e Ambientali, Università dell'Insubria, Via Vallegio 11, 22100, Como, Italia
Coauthors: Tiziana Apuani, Marco Masetti, Luigina Vezzoli, Claudia Corazzato

The Stromboli island is one of the most active volcanoes in the world; it belongs to the Aeolian Archipelago in the Tyrrhenian sea off the Italian southern coast. It rises approximately 2.6 km above the sea floor with the summit at 924 m above the sea level. In the past 100, 000 years, Stromboli has evolved as a complex stratovolcano. Its structural evolution has been complicated by destructive phases as vertical caldera collapses, gradual slope erosion and four large sector collapses affecting the NW flank (Pasquarè et al. 1993; Tibaldi, 2001), which alternate with growth phases. These collapses resulted in the creation of the Sciara del Fuoco horseshoe-shaped depression.

Our interest has been focused on local and general stability of the Sciara del Fuoco recent volcanic debris, that can be mobilized evolving in granular flow running along the Sciara del Fuoco into the sea, in relation to the present volcano activity. These debris consist of ejecta, ranging from meter-sized bombs to lapilli and ash, related to the recent volcanic activity. They appear unconsolidated to depths of some tens of meters with a slope at the limit equilibrium angle. These loose deposits represent a potential unstable mass that can be mobilized evolving in granular flow running along the Sciara del Fuoco into the sea. This phenomenon can lead to the formation of tsunami waves that can easily reach the Stromboli and Ginostra villages, as occurred in the latest landslide events of December 2002 and January 2003.

This paper presents the first steps toward the stability analysis of volcanic debris via numerical modeling: 1) geotechnical characterization of materials 2) calibration of the numerical model based on experimental geotechnical data.

1) The involved materials has been characterized by the following activities; a) sampling of materials collected at depth between 0-1 m at the base of the Sciara del Fuoco depression and in the summit area and, for comparison, in the Fossetta and Rina Grande-Schicciole depressions, where similar phenomena has been registered, b) measure of physical properties by standardised laboratory tests, c) consolidated-undrained triaxial compression tests. According to the Unified Soil Classification System (U.S.C.S), the studied deposits consist mainly of gravel and sands (SP or SW), with a coefficient of uniformity CU < 7.3; without silt or clay fractions. The natural water content is W < 6.3 %; the specific gravity of the solid soil particles is Gs=28.5-30.2 kN/m3; the maximum and minimum dry unit weight, determined from the grain size fraction less than 9.5 mm are, respectively, 12.9-14.8 kN/m3 and 16.6-17.4 kN/m3, from which porosity has been computed n=40-55 %. Consolidated-undrained triaxial compression tests yield for the peak and residual values of cohesion and shear strength angle: peak values cp=0, phip=43^°-51^° and residual values cr=0, phir=39^°-49^°. The reported values do not account for the grain size fraction larger than gravel.

2) Calibration of numerical model. The volcanic debris has been analysed by the distinct element theory and in particular the two dimension-based code Pfc2d (Itasca), defined as a discrete element code on the classification of Cundall and Hart (1992), has been chosen. The code allows finite displacements and rotations of discrete bodies, including complete detachment, and recognizes new contacts automatically as the calculation progresses. In this study it has been assumed that the debris behaves as an assembly of rigid particles whose movement depends on the inter-particle forces acting at each contact.

In order to create a conceptual model which well represents the actual rheology of materials and its influence on slope dynamics, a set of biaxial tests upon the synthetic material has been simulated by numerical modeling and compared with the experimental triaxial compression tests (Dolezalova et al 2003). To define the numerical model, the 2d porosity has been calculated from the 3d experimental porosity (Hainbuchner et al 2003), considering the particle size distribution obtained by the laboratory grain size analysis. The sensitivity analysis has been focused on finding the particle mechanical parameters that better represent the rheology of the volcanic debris at the macroscopic scale. The deformation in granular synthetic material is controlled by the normal and shear contact stiffness (kn ad ks), and the shear strength is mainly controlled by the material 2d porosity, particle size distribution and particle friction coefficient (m). The relationship between the particle friction coefficient and the overall material friction angle is not yet well defined in literature (Oger et al. 1997). The analysis confirms that a specific correlation must be determined for different materials as a function of the acting stress state and taking into account the scale effect. This relation represents the first goal in modeling debris slope instability by particle numerical methods.

References

Cundall, P. A., and R. Hart, Numerical Modeling of Discontinua, J. Engr. Comp., 9, 101-113, 1992.

Dolezalovà M, Czene P., Havel F., Micromechanical modelling of stress path effects using Pfc2D code, Numerical Modeling in Micromechanics via Particle Methods, Konieetzky (ed.), 173-182, 2003

Hainbuchner E., Potthoff S., Konietzky H., Kamp L., Particle based modeling of shear box tests and stability problems for shallow foundations in sand, Numerical Modeling in Micromechanics via Particle Methods, Konieetzky (ed.), 151-156, 2003

Itasca Consulting Group, Inc, PFC2D - Particle Flow Code in 2 Dimensions, Theory and Background, 1999

Oger, L.; Savage, S.B.; Corriveau, D.; Sayed, M., Yield and deformation of an assembly of disks subjected to a deviatoric stress loading Mechanics of Materials 27, 4, 189-210, 1998.

Pasquarè, G., L. Francalanci, V. H. Garduno, and A. Tibaldi, Structure and geologic evolution of the Stromboli volcano, Aeolian Islands, Italy, Acta Vulcanologica, 3, 79-89, 1993.

Tibaldi, A., Multiple sector collapses at Stromboli volcano, Italy: How they work, Bull. Volcanol., 63, 112-125, 2001.

Date received: July 21, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-45.