Landslides of the flow type involving granular geo-materials frequently result in casualties and damage to property because of the long travel distance and the high velocities that these may attain. This was true for the events that took place in Campania Region (Southern Italy) in May 1998, involving pyroclastic soils originating from explosive activities of the Somma-Vesuvius volcano. Although these phenomena have frequently affected various areas of the Campania region over the last few centuries, there were no useful geological and geotechnical references available in the aftermath of the May 1998 events. For this reason Salerno University, which was involved in the scientific management of the emergency, addressed the issue of acquiring data on the geological, geomorphological and hydrogeological features of the slopes where the landslides had taken place. The information acquired made it possible to set up a slope evolution model that is able to interpret, from a geological point of view, past and more recent landslides that had occurred in the same area. As preliminary geotechnical analyses had already validated the above model, more detailed investigations were performed both on the pore pressure regimen of the covers still in place as well as on the physical and mechanical properties of pyroclastic soils, in saturated and unsaturated conditions. The present paper begins by discussing the data acquired during the first phase of the studies and then goes on to illustrate the laboratory results so far obtained with the aid of approximate procedures. These help advance our knowledge of pyroclastic soils within a reasonable time frame, thus improving landslide triggering analysis.

This paper investigates the effects of indigenous vegetation on the shear strength of loose pyroclastic soils of the Campania region (southern Italy); these soils are frequently affected by shallow landslides 1–2 m deep that experience static liquefaction during the post-failure stage. Perennial graminae grasses were seeded in a one-dimensional column 2 m high and filled by pyroclastic soils, allowing the root to grow under atmospheric conditions. A noninvasive sampling procedure was adopted to take the vegetated soil samples, in which the roots were in their natural geometrical distribution. For each rooted sample, the root biomass, RM, was measured and the root volume density, RVD, was calculated. Isotropic consolidated triaxial tests in both drained and undrained conditions were performed on the rooted specimens, as well as on bare specimens as a control. The obtained results showed that the roots generally provided an increment to the soil strength. In drained conditions a reduction in the volumetric deformation was observed, which, under undrained conditions, was reflected in a general reduction of the excess pore-water pressures with a possible inhibition of the static liquefaction occurrence. This study highlights the potential role of grass roots as bio-engineering practice for stabilizing shallow covers of pyroclastic soils.

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The paper provides some remarks on a new suction-controlled simple shear apparatus. The features of the testing device are first outlined and the material used in the experimental programme is described. As a fundamental field of application, wetting tests under simple shear conditions are performed, being representative of the initial stress conditions and stress paths related to the typical triggering mechanisms of rainfall-induced shallow landslides. The benefits of this new device are illustrated, as an alternative or additional tool to other existing equipment. This paper presents a short selection of the measurements obtained using a partially saturated volcanic soil, which is prone to static liquefaction and volumetric behaviour on shearing. The challenges associated with simple shear tests are also discussed using a few examples from the ongoing work