Dynamic Slope Stability Analysis of Mine Tailing Deposits: the Case of Raibl Mine

Over the last few years, many embankments and levees have collapsed during strong earthquakes or floods. In the Friuli Venezia Giulia Region (North-Eastern Italy), the main source of this type of risk is a slag deposit of about 2x106m3 deriving from galena and lead mining activity until 1991 in the village of Raibl. For the final remedial action plan, several in situ tests were performed: five boreholes equipped with piezometers, four CPTE and some geophysical tests with different approaches (refraction, ReMi and HVSR). Laboratory tests were conducted on the collected samples: geotechnical classification, triaxial compression tests and constant head permeability tests in triaxial cell. Pressure plate tests were also done on unsaturated slag to evaluate the characteristic soil-water curve useful for transient seepage analysis. A seepage analysis was performed in order to obtain the maximum pore water pressures during the intense rainfall event which hit the area on 29th August 2003. The results highlight that the slag low permeability prevents the infiltration of rainwater, which instead seeps easily through the boundary levees built with coarse materials. For this reason pore water pressures inside the deposits are not particularly influenced by rainfall intensity and frequency. Seismic stability analysis was performed with both the pseudo-static method, coupled with Newmark’s method, and dynamic methods, using as design earthquake the one registered in Tolmezzo (Udine) on 6th May 1976. The low reduction of safety factors and the development of very small cumulative displacements show that the stability of embankments is assured even if an earthquake of magnitude 6.4 and a daily rainfall of 141.6mm occur at the same time.