This paper presents analysis of the mechanical behavior of a tunnel, specifically assessing the degradation of gneiss rock mass excavated over time, in the southeastern region of the state of Rio de Janeiro in Brazil. The underground excavation has a depth of 300m in a rock mass containing three families of discontinuities, the intact rock showing degrees of alteration of its minerals. Scenarios for analyses are evaluated in conditions of very persistent and moderately persistent discontinuities with the purpose of observing their influence on the stability of the tunnel. Three scenarios of in-situ stress conditions are considered. Excavations with and without support elements were also considered. The supports were shotcrete with steel fibers, reinforcement cables and a mixture of the two. Two-dimensional numerical analyses of the stability of the excavation were carried out using the Discrete Element Method (DEM) through the software UDEC (Itasca, 2011). Failure mechanisms were observed plotting maximum shear displacement values in discontinuities. Sensitivity analyses were carried out in order to determine most sensitive parameters in relation to failure conditions.

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