In design of underground openings, especially rock tunnels and rock caverns, it is important to determine the stability of the structure with a certain amount of the confidence. Due to the fact of relatively large uncertainties to be handled in design of rock structures, compared to most other civil designs, it is necessary to assess uncertainties in the models and material used. Design of underground structures in Sweden shall, since 2009, follow the European Euro code and based on this, the ultimate limit state design is used. Therefore, the designer needs to know how to properly assess model uncertainty in an ultimate limit state design. Block stability remains today a common issue in tunnel engineering and is the purpose of this project.