The project and construction of roads in mountainous regions occasionally need large excavations in rock masses involving great dimensions in extension. This is the case of a highway itinerary in the north of Portugal from where a single characteristic section was selected to perform numerical 2D finite element analysis. The mechanical behavior of rock slopes results from the combination of properties from intact rock blocs defined by a discontinuity system. However an isotropic model was chosen for the calculations as a first step in the slope stability study. Some interesting conclusions can be made from the numerical results obtained.
The stability of rock slopes is traditionally estimated using 2D and 3D limit equilibrium methods. However these methods present some disadvantages and cannot simulate global instability situations in rock slopes. Due to development of computer efficiency numerical methods are increasingly gaining popularity in slope stability engineering. The slope factor of safety can be obtained through the gradual reduction of rock shear strength from its actual value to the failure situation. The application of this method to a rock slope poses several problems as one has to considerer separately the strength of different rocks and joints that constitute the whole rock formation. The reduction of a single parameter characteristic of the rock formation would be more adequate to obtaining a global safety factor for the rock slope stability. The GSI (Geological Strength Index), that was introduced by Hoek (1994) and Hoek, Kaiser and Bawden (1995), is a method for estimating the reduction in rock mass strength for different geological conditions. From this index the parameters describing the rock mass strength can be estimated. The consideration of a GSI value in failure could be a method for obtaining a global safety factor in rock slopes. The construction of a main itinerary in the North of Portugal constitutes an opportunity to test and compare some limit equilibrium techniques and numerical calculations using the finite element method.
The IP3 highway between Regua and Vila Real crosses a shist graywacke zone in the North of Portugal which is litologically composed of alternated series of metagraywacke and smooth phyllite, with a layer of black slate at the base and some limestone levels. As main tectonic accidents, faults show strikes with orientation N 110°E-N13°E dominant, and N040°E-N070°E. The joint network distribution has a similar orientation with steep dip angles varing from 70°N to vertical, being quite normal to stratification joints. This rock formation was attributed a GSI value of 40 based on some geotechnical elements obtained from the field. This geomechanical classification has proved to be a useful link between in situ geological conditions and mechanical behavior. There are several excavations along the itinerary from which a single one was chosen to perform a 2D numerical analysis with software Phases2 developed by Rocscience (1999). The analyzed excavation section has a maximum height of 41 m with benches height varying between 6 and 12 m.
