ABSTRACT

The arch of hills which shuts in that La Spezia Gulf (Liguria Region), has been recently involved in the reactivation of ancient landslides. These movements still cause extensive damages to many buildings such to declare a state of emergency. Through numerous geognostic surveys we have been able to recognize the high degree of rock masses fracturing, their lithological heterogeneity, the presence of landforms related to ancient and deep gravitative movements and trick debris covers. A geotechnical, kinematic and geomechanical characterization of the landslides and rock masses was carried out. Different constitutive models was applied to the different materials to simulate their mechanical behaviour. The a 3D numerical modeling was performed to attempt explaining t he mechanisms causing theobserved landslides, their evolution in term of displacements and deformations. The finite- difference method was applied to estimate the stress- strain response of the slope to the variations of tensional state. An integrate model was performed considering the results of all in situ surveys and numerical modeling: the analyzed model shows main shear strain along the discontinuities deriving from plastic deformation of the mass causing the slope failure. According to the considerations derived from the model, some remedical works are already in progress on the slope.

1. INTRODUCTION

The hills near the Marinasco-Stra village, above La Spezia town, were involved in the mid-90s by landslides reactivation, caused by public works, causing damage to several buildings, so that a state of emergency was declared. In order to increase knowledge about the slope stability, these studies were conducted: a geological and geomorphological survey, a geological – technical, kinematic and geomechanical characterization of rock masses and a 3D numerical modeling with the finite differences method. The modeling allowed the examination of most probable, both superficial and deep, sliding surfaces, gathered from geognostic survey and monitoring.

2 GEOLOGICAL AND GEOMORPHOLOGICAL FRAMEWORK

The slope consists of a succession of low ridges and rounded tops, located between250 and 300 meters a.s.l.; in the area the rocks cropping out are mainly sandstones interbedded with thick argillite layers. These sandstones belong to the Mt. Gottero Sandstones Fm. (Upper Campanian-Maastrichtion) (Elter, 1960) and consist of proximal arenaceous turbidites or more rarely arenaceous- politic including quartz-feldspatic greywackes or feldspatic greywackes, often very coarse, grey or light brown. They locally are represented by an irregular sequence of layers, made of minutes lithic fragments dipped in a politic matrix, interbedded with argillitic layers of variable thickness. The sandstones prevail at the top of the slope, the shales and the siltstones at the bottom.

The Mt. Gottero formatting is affected by continuous fracture systems in the NW-SE and N-S directions, referable to the same tectonic phase that gave rise to the lifting of this portion of the Apennines, including the entire western promontory of the gulf of La Spezia. The structural elements, particularly the discontinuitie's surfaces (direct and inverse faults) originated during compressive and extensional tectonic phases, have become the preferential sliding surfaces of mass gravitational movements, at least at the top of the slope.

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