Casagli, Nicola

Falorni, G.

Fukuoka, H.

Nocentini, Massimiliano

Tofani, Veronica

Wang, F.

English

Florence Research

Geophysical Research Abstracts,Vol. 10, EGU2008-A-09391, 2008SRef-ID: 1607-7962/gra/EGU2008-A-09391EGU General Assembly 2008© Author(s) 2008Instability conditions of the landslides triggered by the2006 rainfall event in Ischia Island, ItalyV. Tofani (1), F. Wang (2), N. Casagli (1), G. Falorni (1), M. Nocentini (1), H.Fukuoka (2)(1) Department of Earth Sciences, University of Florence, Italy (veronica.tofani@unifi.it ), (2)Research Centre on Landslides, DPRI, Kyoto University, JapanIschia is an active volcanic island located in the Tyrrhenian Sea, approximately 30 kmWSW from the city of Naples in Southern Italy. On 30 April 2006, following severalhours of rainfall, four small soil slips-debris flows were triggered on the slopes of Mt.Vezzi (ca. 400 m a.s.l.), in the SE portion of the island. The flows caused the deathsof 4 people, forced the evacuation of another 250 inhabitants and destroyed severalbuildings.The steep slopes of Mt. Vezzi are conditioned by the volcanic activity of the last 10.000years: a 1 – 2 m thick soil derived from the weathering of pyroclastic material overliesthick fallout deposits produced by the last eruptions. The debris flows initiated as soilslips at the soil – fallout interface, in hollows with slope gradients ranging between35˚ - 40˚, and quickly transformed into flows that reached the floodplain at the base ofthe hill, finally coming to a stop in a low gradient road. The flows started at elevationsranging between 310 and 350 m and stopped at approximately 80 m, correspondingto travel angles of 20-24˚. The volumes of the single events ranged from 2000 – 5000m3.Two field campaigns carried out immediately following the events were focused oncollecting geotechnical data in the source area (angle of internal friction, unit weight,water content, grain size, etc.). Material shear strength was determined both in thelaboratory on reconstituted samples and in-situ by means of Borehole Shear Tests(BST) coupled with a tensiometer for the measurement of matric suction. Soil strengthwas assessed both in partially and totally saturated conditions.Soil hydraulic conductivity was quantified by means of in-situ constant head perme-ameter tests. These highlighted a marked contrast in permeability between the soil andthe underlying pumice layer. Saturated hydraulic conductivity values ranged from 1.2x 10−5 m/s in the former to 6.2 x 10−8 m/s in the latter.Soil slip triggering conditions were analyzed on the basis of precipitation data relatedto the 24 hour period prior to the event using two different approaches: a coupledseepage-stability analysis and a geotechnical modelling by means of the ring shearapparatus DPRI-No.7.The former approach consists in the modelling of changes in positive and negativepore pressures during the event by a finite element analysis in transient conditions,using the software SEEP/W (Geo-Slope Int.). Positive and negative pore pressure dis-tributions obtained from the seepage analysis were used as input data for the stabilityanalysis. This was performed with SLOPE/W, applying the limit equilibrium method(Morgenstern-Price) for each of the time steps used in the seepage analysis. The Mohr-Coulomb criterion in terms of effective stress was used in the case of positive porepressures, while the Fredlund et al. (1978) criterion was applied in the case of nega-tive pore pressures. The results show that the distribution of pore pressure conditionsare largely influenced by the high permeability contrast between the soil pyroclasticcover and the underlying pumice layer, by the soil type and by the limited thicknessof the cover. The instability condition are reached 10 hours after the beginning of therainfall event with a value of pore water pressure along the shear surface of about 7kPa.The ring shear apparatus DPRI-No.7 is a geotechnical instrumentation developed atthe Disaster Prevention Research Institute at Kyoto University. The shear apparatushas an inner diameter of 27 cm and an outer diameter of 35 cm, the maximum sam-ple height is 11.5 cm. The apparatus has two kinds of shearing controlling modes:shear speed-controlled mode and shear stress-controlled mode. By means of a gap-controlling system with a precision of 0.001 mm, the shear box can be kept in fullyundrained condition.Seven tests have been conducted on Ischia samples: 5 stress-controlled tests at natu-rally drained conditions and 2 speed-controlled tests at undrained conditions. Givencertain values of normal load and back pressure, shear displacement, shear resistanceand pore water pressure were measured. The simulation of the triggering conditions onIschia samples highlights that the drop of shear resistance occurs when the pore waterpressure on the failure surface reaches a value of about 6.5/7 kPa, in good agreementwith SEEP/SLOPE modelling.

Instability conditions of the landslides triggered by the 2006 rainfall event in Ischia island, Italy

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Instability conditions of the landslides triggered by the 2006 rainfall event in Ischia island, Italy

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