Rock Mass Rating spatial estimation by geostatistical analysis

• Many surface geo-mechanical surveys were carried out in a wide alpine area.
• The estimation of the geomechanical quality by the Rock Mass Rating was achieved.
• Some geostatistical techniques were applied to predict the rock mass quality.
• Variograms show correlations with structural/geological features.
• Sequential Gaussian simulation with a 3D-approach supplies the best results.

This work aims to estimate the Rock Mass Rating of 200 km2 area of the Italian Central Alps, along San Giacomo Valley (province of Sondrio). The regional geological setting is related to the Pennidic Nappe arrangement, which is characterized by the emplacement of sub-horizontal gneissic bodies, separated by meta-sedimentary cover units. The resulting RMR map can be a useful tool to forecast the quality of outcropping rock masses as well as to derive their geomechanical behaviour. Almost 100 geomechanical field surveys have been carried out in the research area, in order to characterize the outcropping rock masses; afterwards rock mass quality indexes have been evaluated in each surveyed site. In order to estimate the Rock Mass Rating values in un-sampled locations, different geostatistical techniques (kriging and simulations) have been applied, using both bi-dimensional and almost three-dimensional approaches. The validation process shows that kriging tends to produce smoothened distributions, while conditional simulations allow respecting local extreme values. Although geostatistical analysis reveals that geomechanical properties show spatial correlations, it is to remind that rock mass quality is strongly related to its geological and structural history.