Geomechanical surveys and geostatistical analyses in Valchiavenna (Italian Central Alps)

The present study concerns the forecast of geomechanical features in rock masses and how mechanical properties can be regarded as regionalized variables. It considers an area of about 200 km2, extended in the Italian Central Alps along the Valchiavenna Valley (SO) - San Giacomo and Bregaglia valleys - where different civil and mining works are present. The regional geological setting is related to the Pennidic Nappe arrangement, characterized by the emplacement of sub-horizontal gneissic bodies resulting from the Mesoalpine isoclinalic folding of crystalline basements (“Tambò” and “Suretta” Units) emplaced throw East and separated by metasedimentary cover units. More than one hundred detailed structural and geomechanical field surveys, mainly located in San Giacomo Valley, were carried out in order to characterize the rock masses, in accordance with the I.S.R.M. suggested methods. The procedure allowed to identify the number of joint sets and their average orientations, supplying a quantitative description of the discontinuities in terms of spacing, persistence, roughness, aperture, filling, wall strength and weathering. From the collected data the rock mass quality indexes have been evaluated in each surveyed site. Geostatistical methods have been applied to study the spatial distribution of the main rock masses characteristics, such as joint set orientation, fracture intensity and joint spacing, being the direct survey data local. Where no data are available the rock mass features have been estimated, the results obtained by kriging and conditional simulation techniques are presented.