The development of instability, associated with road construction is the source of important problems, This situation appears to be related to three fundamental circumstances: A complex orography; an abundant rainfall and a large amount of tectonically disturbed sedimentary rocks. The development of unstable slopes depends on the combination of the rock massifs, their strength characteristics, structure, and precipitation- runoff relationship. Moreover, in coastal areas, there are a series of additional aspects which necessarily have to be taken into consideration: the particularly aggressive weathering processes on the coast and significant erosion along highly active cliff bases. This work attempts to improve the geomechanical characterisation of rock masses in the Basque Arc. These form a large complex of very different materials, usually weakened by different sets of joints, which makes it difficult to work with the usual classifications. These materials are especially problematic, since they favour the development of instability and simple behavioural models are not applicable.