Cyclic shear strength of a volcanic sandy silt susceptible to flowslide Paper N. UCBPO

The paper provides insight into the cyclic behaviour of volcanic sandy-silt material susceptible to flowslides. Flowslides and liquefied debris flows are typical phenomena in the Calabria region (Italy) following intense and/or prolonged rainstorms. In particular, a catastrophic event occurred in April, 2000, which caused very destructive impacts on the existing down-hill structures. Such phenomena involve sandy-silt materials of volcanic origin, covering the steep slopes located along the calabrian coast of the Messina Strait, near the city of Reggio Calabria. These shallow deposits display key properties of volcanic materials, such as low plasticity, high porosity, low density and tendency to collapse upon load application. The experimental site has been investigated through detailed stratigraphic borings, trial pits and Standard Penetration Tests (SPT). Undisturbed samples were carefully recovered by block and tube samples from material exposed in carefully dug trial pits. To establish a more consistent theoretical frame for interpreting the observed cyclic behaviour, undisturbed samples were initially subjected to one dimensional compression tests and isotropically consolidated drained and undrained triaxial compression tests. Laboratory undrained cyclic simple shear tests in a modified NGI type apparatus provide a valuable means of assessing material susceptibility to liquefaction or cyclic failure of the recovered samples. Cyclic test results were carefully analysed with the aim of identifying the influence of combination of high limit liquid and low plasticity index on the observed behaviour. The results gathered in the present research in terms of undrained cyclic resistance vs. SPT, fall well in the range of other field performance-based data of silty sands, sandy silts and silts with fine content (FC) equal to or higher than 35%, at a given equivalent earthquake magnitude.