Evaluation of methods for rock mass characterization and design of rock slopes in crystalline rock

Abstract: Construction of rock slopes is needed in many civil work projects. It is for example very common in road and railway cuts, but other applications include excavation for tunneling or building foundations, where perhaps sensitive constructions are present in the immediate vicinity. In Sweden the majority of the rock is hard crystalline bedrock of relatively good quality, and the fracture orientation have a large effect of the stability of the rock slope.If the geology is not properly considered for when the design of the slope is carried out, it can result in slope failure, with severe consequences. This applies especially if the rock slope is high, but unwanted effects like increased excavation and construction costs, could occur also in smaller slopes if the risks are not identified and managed. However, it is difficult to standardize design of rock slopes in fractured hard crystalline rock because of the uncertainties and variations in the geological conditions during the design phase.Rock mass characterization systems like Rock mass rating, RMR (Bieniawski 1989) and the Qsystem (Barton, Lien och Lunde 1974) are commonly used to describe the general rock mass quality. Whilst a good rock mass quality is generally easier to construct in, stability problems do occur due to structural geology in rock slopes even in good rock mass quality. The application of these systems in rock slopes can be problematic as they do not describe the geometry of the slope and how the fractures daylight in the slope face. Instead, stereonets can be used to visualize this, but fracture parameters of large importance for stability (persistence, roughness and alteration) are traditionally not presented in a stereonet analysis. Additionally, these parameters and the structural geological conditions can be difficult to predict and can vary significantly over short distances, why it can be difficult to forecast and predict failure in the design face.Slope instability due to large sliding and wedge failures have been observed in a large number of slopes in crystalline rock, and a standard method for design of rock slopes is lacking. This has given rise to the research question of how best to describe rock mass conditions, how to design slopes in crystalline rock and how to manage these risks during construction.To investigate this, three rock slopes where large failure had occurred were selected for the case study and were mapped, characterized and analyzed. The results from the case study showed that all rock slopes investigated had in common that failure had occurred along fractures of large persistence. This highlights the importance of evaluating the geology in the correct scale and suggest that lineament studies and other elevation based interpretation are perhaps more useful than previously thought. It also came to light that the planning and design process is inflexible which in many cases lead to large risks being overlooked or left unnoticed.A flow chart for risk classification was produced and suggested for aid in decision making aiming at simplifying project management of rock slopes, as well as aiding in visualization of the risk that rock slopes can be associated to.