Evaluation of rock mass strength criteria

Sammanfattning: Knowledge of the rock mass behaviour in general, the failure process and the strength in particular, is important for the design of all kinds of underground excavations. One of the most common ways of estimating the rock mass strength is by using a failure criterion. The existing rock mass failure criteria are stress dependent and often include one or several parameters that describe the rock mass properties. These parameters are often based on classification or characterisation systems. A comprehensive literature review of existing classification/characterisation systems and rock mass failure criteria has been performed. As the application of this licentiate thesis is for hard rock masses some limitations have been stated on the systems and criteria. The limitations of the classification/ characterisation systems are that they should present a result that is relevant for the strength, give a numerical value, have been used after the first publication and be applicable to hard rock masses. Based on the literature review, it was concluded that the uniaxial compressive strength, block size and shape, joint strength and a scale factor are the most important parameters that should be used when estimating the rock mass strength. Existing rock mass failure criteria and classification/characterisation systems have been evaluated through the use of case studies. The aim of the case studies was to identify robust systems and criteria, parameters having the strongest impact on the calculated rock mass strength and those giving a large interval of the result. The case study revealed that the rock mass quality (Q-system), rock mass Number (N-system), Rock Mass index (RMi) system, Yudhbir – Rock Mass Rating (RMR76) and Hoek-Brown – Geological Strength Index (GSI) seem to be the most suitable systems and criteria to use when determining the rock mass strength. None of the systems or criteria complies with the requirements of a method to determine the rock mass strength and a better rock mass strength estimation method should be developed. This requires more case histories where the determined/estimated rock mass strength from the criteria/systems can be compared to the measured/determines rock mass strength.