This paper presents a new geomechanical classification for slopes, the SFi (Slope Failure Index) system based on the detailed examination of more than 700 road cuts. The SFi system adopts different evaluation codes based on the different geotechnical conditions. In the SFi system, the rock masses are divided into five types on the basis of strength parameters, soil depth and block size ratio: Discontinuous jointed rock mass (JRM), continuous heavily fractured rock mass (HRM), continuous competent rock mass (CRM), continuous incompetent rock mass (IRM) and soil-like mass (SLM). Parametric studies were performed to determine the factors causing instability and controlling the failure processes. The factors can be classified into internal and external factors. Internal factors can be subdivided into main and sub factors. The basic SFi value is evaluated from the summation of the main factors and multiplication of sub factors. Subsequently, the total SFi value is defined as the sum of basic SFi values and the ratings of external factors. It also provides some simple rules between instability modes and the required support measures.
Failure modes in cut slopes are triggered by combinations of several failure factors for each ground condition. In practical application, the factors affecting slope instability are classified into internal factors induced by the slope material itself and external factors related to surrounding conditions (Table 1). Slope instability factors play a different role on slope behavior depending on the ground conditions. In the case of moderately weathered and jointed rock masses, orientations of discontinuities have a critical effect on the slope instability, whereas frequency (or density) of joints is more important than the orientation in highly weathered or fractured rock masses (Singh 1973, Hoek & Bray 1981, Einstein et al. 1983, Goodman & Kieffer 2000).