Natural instability phenomena in granitic rock masses: slopes in decomposed granite, boulder fields and irregular large granitic boulders

Abstract

Based on the experience gathered by the authors, this article presents a review of the instability phenomena typically associated to natural slopes in granite terrains, focusing on conditions encountered in the northwest of the Iberian Peninsula, particularly in the region of Galicia, characterized by a very humid oceanic climate with high rainfall. Accordingly, a brief introductory review of geomorphological aspects of granitic terrains is presented, in which different slope morphologies are defined that can give rise to varied instability phenomena. Three types of potentially unstable slope environments are defined, namely high areas with large ellipsoidal shaped individual granite boulders, mid-slope regions formed by groups of average size granitic boulders and mild slopes formed by highly or completely decomposed granite, locally known as ‘jabre’. In all these cases, the weathering of granite materials plays an important role, so we briefly explain how the granite rock masses are weathered and eventually eroded. Subsequently, and for each of these three cases, the authors present a typical description of the phenomena to be studied, the characteristic instability mechanisms observed and that need to be analyzed and the most appropriate characterization techniques to be applied. A case study concerning the stability analysis of a particular scenario of every type of instability phenomenon is then presented.