The purpose of this paper is to present an application of the Rock Engineering Systems (RES) methodology for developing a new rock mass classification, sensitive to large-scale instability in natural slopes and suitable for indicating critical sites. In the area under study, located in the Italian Central Alps, and in most natural slopes, analysis is complicated by lack of data, geological complexity, scale of the instability phenomena and the high number of interacting factors. In order to be able to have a structured approach to such complexity, a comprehensive method based on the RES approach was adopted. This is an objective-based approach which allows the utilization of all the information related to the project, tailoring the classification to the actual circumstances. Hence those parameters which are particularly active at the site can be evaluated and the importance of their interactions established.
Following this approach, 19 parameters relating to the general environment and to the rock mass characteristics are considered. Their reciprocal causes and effects are analysed in order to weight each parameter according to its degree of interactivity in the system. The final definition of the rock mass instability index takes into account the variability of the parameter values for different slopes in assigning ratings to different classes of parameter values. In parallel, a predictability rating is computed, according to the presence in the field of a number of ‘indicators of instability’. Both indices allow discrimination of critical slopes, and are in good agreement with field evidence demonstrating the value of the approach.