GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 105 (1976) is a a geomechanical classification system for rock masses, which takes into account 6 parameters: Uniaxial Compressive Strength (UCS) of rock material; Rock Quality Designation (RQD); Spacing of discontinuities; Condition of discontinuities; Groundwater conditions; Orientation of discontinuities. In particular, UCS is one of the most important parameters since it depends on physical properties of the rock (i.e. porosity, micro fracturing, texture) (Pappalardo et al. , 2013). Spacing of discontinuities affects the degree of fracturing, while their orientation plays a significant role in the stability analysis. Each of the six parameters is turned into a value corresponding to the characteristics of the rock. These values are derived from field surveys and laboratory tests. The sum of the six parameters is the “RMR value”, which lies between 0 and 100 and identifies 5 classes of quality, from “very good” to “very poor”. Furthermore, on the basis of RMR, mean cohesion and internal friction angle of the rock mass can be calculated (Bieniawski, 1976) through: c ' = 5 RMRb (2) φ = 0.5 RMRb + 5. (3) Stereographic projections of the discontinuity poles allowed grouping all the surveyed discontinuity into sets for the kinematic and stability analysis (Fig. 3). Kinematic analysis, according to Hoek and Bray (1981), is a graphical procedure allowing the identification of the main failure mechanisms with respect to the geometric relationship between the discontinuity and the slope. If the discontinuity has the same orientation of the slope (±20°) but a lower inclination, planar sliding is the possible failure mechanism; if the discontinuity has the same orientation of the slope (±20°), but an opposite immersion, toppling may occur; if two discontinuities intersect, with a plunge of the intersection lower than the dip angle of the slope face, wedge sliding may be the potential failure mode. Fig. 3 – Location of geostructural measurement stations with corresponding stereoplots showing the pole density with respect to the surveyed discontinuity sets.