Abstract
Abstract. An integrated investigation was carried out on the volcanic soils involved in the landslide phenomena that occurred in 2006 at Mt. Vezzi on the island of Ischia (southern Italy). Chemical (soil pH, organic carbon content, exchangeable cations and cation exchange capacity, electrical conductivity, Na adsorption ratio and Al, Fe and Si forms), physical (particle and pore size distribution, pore structure), hydrological (soil water retention, saturated and unsaturated hydraulic conductivity), mineralogical and micromorphological analyses were carried out for three soil profiles selected in two of the main head scarps. The studied soils showed a substantial abrupt discontinuity in all the studied properties at the interface with a buried fine ash layer (namely, the 2C horizon), that was only marginally involved in the sliding surface of the landslide phenomena. When compared to the overlying horizons, 2C showed (i) fine grey ash that is almost pumice free, with the silt content increasing by 20 %; (ii) ks values 1 order of magnitude lower; (iii) a pore distribution concentrated into small (15–30 μm modal class) pores characterised by a very low percolation threshold (approximately 15–25 μm); (iv) the presence of expandable clay minerals; and (v) increasing Na content in the exchange complex. Most of these properties indicated that 2C was a lower permeability horizon compared to the overlying ones. Nevertheless, it was possible to assume this interface to be an impeding layer to vertical water fluxes only by the identification of a thin (6.5 mm) finely stratified ash layer, on top of 2C, and of the hydromorphic features (e.g. Fe / Mn concretions) within and on top of the layer. Although Mt. Vezzi's soil environment has many properties in common with those of other Campania debris-mudflows (e.g. high gradient, north-facing slope, similar forestry, and volcanic origin of the parent material), the results of this study suggest a more complex relationship between soil properties and landslides and emphasise the role of vertical discontinuities as noteworthy predisposing factors.
Highlights
The Italian territory is vulnerable from a hydraulic and geological standpoint, owing to its distinctive geological, geomorphological and climatic factors, along with anthropic pressures
The results showed two main types of aggregates that build this horizon: aggregate type I, which has internal massive microstructure, and aggregate type II, which has a micro fibric structure; both were characterised by high porosity (43.9 and 60.7 %, respectively) made by small pores
The inter-aggregate porosity, which was investigated by means of microtomographic analyses, shows remarkable differences between the Bw and 2C horizons in terms of the following: (i) total porosity larger than 30 μm (36.2 and 6.7 %, respectively); (ii) mode of the pore size distribution (210 and 150 μm, respectively); (iii) maximum pore size, which is larger in Bw (≤ 2 mm) than in 2C (≤ 1 mm); and (iv) connected porosity
Summary
The Italian territory is vulnerable from a hydraulic and geological standpoint, owing to its distinctive geological, geomorphological and climatic factors, along with anthropic pressures. Landslides are frequent and widespread (Guzzetti et al, 1994; Guzzetti and Tonelli, 2004) and the damage they cause is severe. In the last 50 years (1964–2013), all 20 Italian regions have suffered fatal events. Data from CNR-IRPI (Italian National Council of Research-Research Institute for Geo-Hydrological Protection), released by La Stampa (2014), show floods in 989 municipalities, which caused 1629 casualties (710 killed, 72 missing and 847 injured), and landslides in 1332 towns, which produced 3042 casualties (1297 killed, 15 missing and 1731 injured). Campania is the region with the third-highest number of landslide events (363) in Italy. One of the most catastrophic historical events occurred at Sarno on 5 and 6 May 1998: debris-mudflows detached from the slopes of Pizzo D’Alvano and extended 3–4 km into the surrounding lowlands, causing the loss of 161 human lives and severe destruction.
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