Preface 
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. A previous study (licentiate) by Mehdi Bagheri was focused on size of blocks and stability analyses using analytical tools. It was however concluded, that there is no existing analytical model today to estimate block stability with good accuracy. This Ph D project is a continuation of the licentiate research with the following results; a better understanding of block failure mechanisms, development of a better design tool to solve block equilibrium (safer and more economical design), identification of key parameters, and definition of the need for characterization. The two year Ph D project was financed by The Rock Engineering Research Foundation, BeFo. The work was performed by Mehdi Bagheri, who is a Ph.D. after presenting his Thesis under the supervision of Professor Håkan Stille. A reference group has assisted the project and was composed of Beatrice Lindström from Golder Associates, Mats Holmberg from Tunnel Engineering, Rolf Christiansson from SKB, Jonny Sjöberg from Itasca Consulting Group, Anders Fredriksson from Sweco, Jimmy Töyrä and Thomas Dalmalm from the Swedish Transport Administration, Erling Nordlund and Kelvis Pérez from LTU, Lars O Ericsson and Miriam Zetterlund from Chalmers University of Technology, and Tomas Franzén and Mikael Hellsten from BeFo. 
Stockholm in September 2012 
Per Tengborg