The mobility of the Alvera landslide (Cortina d'Ampezzo, Italy)

The Alvera landslide is a slow moving earth flow (some cm per year) in the Italian Dolomites situated east of the town of Cortina d'Ampezzo. It shows a source area and an elongated track zone (transport zone) and a lobe or accumulation zone. In this paper the mobility of this slow moving earth flow is analysed over a 3 year period with daily observations of the groundwater level and displacement measurements to identify critical thresholds in the mobility of this landslide and to assess the temporal trend and variations in the viscosity of the material. The total displacements in this 3 year period (July 1989 until October 1992) amount to 40 cm. Displacements of the Alvera landslide have been measured with deformometers and inclinometers. The analysis of the groundwater records in combination with displacements of the Alvera landslide over a period of three years shows at first sight an irregular response with respect to the onset of movement and the pore pressure fluctuations. In general, movements were detected at groundwater table elevations above the slip surface ranging from 0.7 to 0.9 of the total thickness of the landslide body. By using an expression derived by G. Salt, which puts in relation the inverse safety factor (1/F) and the logarithmic of the velocity (log v) by the constant of proportionality a (named ''viscosity index'' and which has the meaning of viscosity strength), it was found (for different periods of moving incidents and couples of 1/F and log v values) that the obtained trend of the calculated viscosity index values does not allow the critical evolution toward catastrophic movements of the landslide for a specific inverse safety factor value. The analysis of the moving incidents showed also a temporal variation of the viscosity index but there is not a decreasing trend of the viscous strength of the material. In other words the material does not show strain softening effects. There seems to be, however, a positive gradient relationship between the inverse safety factor and the viscosity index.