Buckling of high natural slopes: The case of Lavini di Marco (Trento-Italy)

Abstract

Layered limestones with marly–clayey interbeds are widespread in mountain regions. Where tectonics has not changed the original structure they frequently form homocline slopes, that since the last deglaciation have evolved through planar block slides along bedding joints. A typical dip slope showing this evolution is represented by the left flank of the Adige River valley south of Rovereto (North-Eastern Italy). Even though layers never daylight at the slope toe, the exceptional persistence and weakness of bedding joints allows buckling of layers, which occasionally collapse causing rock avalanches. Before monitoring started, deformations were believed to be no longer active, this conviction being strengthened by the high safety factors provided by limit equilibrium analyses. SAR satellite interferometry performed between 1990 and 2002 and probe inclinometer measures indicate that the slabs located uphill from the buckle folds are slowly sliding. Since some doubts have been raised about rock mass behaviour, slope geometry and hydraulic conditions assumed in the analyses, buckling was back-analysed using the Distinct Element Method (DEM) and the Discontinuous Deformation Analysis (DDA). These treat the rock mass as a discontinuum and account for deformation and failure of the rock material. To perform numerical analyses, rock mass structure, strength and stiffness were determined through extensive in situ and laboratory investigations. For this purpose field surveys, borehole core logging and geophysical surveys were performed and laboratory geotechnical tests on the rock material, clayey interbeds and bedding joints were conducted. Discontinuous analyses confirmed that buckling deformations are definitely possible if some conditions are present (high water pressure, minor flexures). Modelling also indicate that buckling deformations may evolve into failures and should therefore be taken into account in evaluating long term stability of the slope.