Parametrization of a dry retaining wall on a terraced slope in Valtellina (Northern Italy) and stability analysis

The mechanical characterization of dry retaining walls is a key issue for the stability analysis of slopes in Valtellina, where vineyard cultivated terraces have already been involved in rapid mass movements. The study presents the solution adopted to approach the problem by numerical modelling, focusing on the difficulties in the parameterization of dry walls. While geotechnical field and laboratory measurements allowed to define the backfill soil properties following conventional procedures, no standards are proposed for dry walls. The walls are likened to equivalent rock masses, where blocks with different shapes and dimensions, are separated by “discontinuities” characterized by aperture, filling and roughness. Direct observations and images analysis allowed to assign to the walls a Geological Strength Index, applying the Hoek & Brown criterion, and to calculate the wall equivalent values of cohesion and friction angle. The performed stability analysis is supported by a previous hydrological model, which allows to define a temporary perched groundwater level when a rainfall is simulated. The infiltration phase has been calibrated and validated comparing the in situ water levels, recorded by continuous piezometric datalogger, with the simulated ones, using as input rainfalls registered with a meteorological station. Then two different rainfall scenarios, with similar duration and return period, are reproduced: the first caused three mass movements in 1983 while the second had no instability consequences. Once the hydrological models have been reconstructed, the stress-strain modeling is performed to verify the worth of the geomechanical parameters assigned to the wall, and eventually calibrate them. The present work emphasizes the importance of direct measurements and monitoring activities to develop reliable conceptual models for numerical analysis of groundwater flow and stability in an anthropogenic impacted geological context. Moreover it highlights the importance of field measures to reduce the uncertainty of parameters that are almost impossible to be measured directly.