The effects of extreme precipitations on landslide hazard in the pyroclastic deposits of Campania Region: a review
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L. Picarelli I L. Olivares I E. Damiano I R. Darban I A. Santo
The effects of extreme precipitations on landslide hazard in the pyroclastic deposits of Campania Region: a review
Abstract In the last ten thousand years, the explosive activity of some volcanic centers has blanketed with pyroclastic soils the major part of Campania Region, Southern Italy. Experience shows that the sloping deposits may be mobilized by long-lasting precipitations, leading to rapid and sometimes catastrophic flow-type slope movements. This paper summarizes the present knowledge on this subject based on the results of both laboratory tests and experiments and on a review of the main data provided by field surveys. In particular, a careful examination and analysis of the available elements highlight the key role played by lithological and morphological details on failure and post-failure mechanisms, thus affecting landslide hazard. In fact, it is shown that deposition mode and grain size, slope angle, and morphology strongly affect the type of post-failure movement, which may take the features of a debris avalanche, of a debris flow, or of a flowslide and may lead (or not) to soil liquefaction, a mechanism that strongly affects both displacement rate and run-out. Keywords Pyroclastic soil . Rainfall . Slope failure . Liquefaction . Debris avalanche . Debris flow . Flowslide Foreword Pyroclastic soils are typical products of the explosive volcanic activity and consist of fine-grained particles, pumices, and small lava clots. These soils are widespread in areas around volcanic centers (Shoji et al. 1993); in particular, in some tropical countries, they occupy more than 50% of the ground surface. After ejection, the material deposited by surges and/or pyroclastic flows (ash-flow deposits) accumulates over the volcano’s flanks or quite at short distance. Moreover, the finest component may be pushed away from the vent within gas clouds. Transported by wind, this soil mantled plains and hills (Machida and Arai 1992), the depositional area depending on wind direction at the time of eruption (air-fall deposits). While the material deposited by surges and by pyroclastic flows is usually poorly sorted including either coarse grains or fine particles, air-fall sediments are layered, consisting of pumice and of ash layers; any layer, in turn, presents a uniform grain size. Some layers of pyroclastic deposits may be rich in organic matter as evidence of long pauses in the volcanic activity or may be bonded as a result of soil alteration. In some cases, intense weathering can modify the physical-chemical features of particles, leading to plastic clayey materials. All this suggests that the features of pyroclastic soils (uniform/layered, bonded/non-bonded, plastic/non-plastic) may strongly vary from point to point and so the hydro-mechanical response to external input. Experience shows that pyroclastic soils may pose severe engineering problems due to some special intrinsic features, such as (i) quite a high porosity and (ii) high fragility of the singl
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