Geomechanical model of an alpine rock mass
Rock Engineering 2009 - Rock Engineering in Difficult Conditions
Gian Paolo GIANI, Andrea MERRI,
The model refers to an area of about 50 km2, extended in the Italian western Alps along the tha Spluga Valley where different civil surface and underground works are planned. The regional geological setting is related to the Pennidic Nappe arrangement, characterized by the emplacements of sub-horizontal gneissic body resulting from the Mesoalpine isoclinalic folding of crystalline basements, emplaced throw East, and separated by matasedimentary cover units. The investigated geological unit, “Tambò” is overlapped by its meta-sedimentary cover and by the “Suretta” unit .The tectonic contact gently dips to the E–NE. The valley, furrowed by the Liro Torrent, follows a N–S-striking tectonic lineament, almost parallel to the nappe contact. A number of more then 70 sites, well distributed in the studied area, were chosen and detailed structural and geomechanical field surveys have been performed according to the ISRM suggestion methods to characterize the rock masses, its intact rock and discontinuities. The procedure identified the number of joint sets and their representative orientation, the set spacing, the type of movement, the amount of dilation, the degree of alteration, the roughness coefficient, the presence and nature of infill.
The work tries to assess how the geometrical and mechanical properties of the rock masses can be regarded as The spatial distribution of these properties is a function of the geological and structural phenomena at which the rock masses have been subjected.
The definition of specific analytical relations to evaluate the spatial variability of the mentioned mechanical properties, can represent an useful tool: first in implementing the comprehension of the geological and geomechanical features related to the rock mass history, then to suppose quantitative geomechanical parameters, where no direct investigations are available. The main investigate features are the orientation, roughness, wall strength, spacing and persistence of the principal joint sets. Rock mass quality indexes, such as the Rock Mass Rating (Bieniawski, 1989; ) or the Geological Strength Index (Hoek et al., 2002) have been evaluated in each survey site. Then, by means of specific correlations acquired using geostatistical techniques, the rock mass properties were estimated in the rest of area.
The estimation of the rock mass quality distribution of a rocky body that should be excavated allows to initially optimize the tunnel path, in order to minimize major instability problems or prevent excavation difficulties.