Tunnel stability assessment by 3D DDA-key block analysis

The key block theory is often the first analysis carried out in assessing potential instability caused by tunneling through jointed rock masses. This study suggests that it is beneficial to include it as the first step for a detailed design analysis by discrete numerical modeling, such as 3D DDA. The procedure is illustrated with an example which is simplified but is general and contains both primary and secondary key blocks. The agreement of the identified key blocks from 3D DDA and the key block theory provides credence for the DDA study, while the DDA, in turn, gives insights on how the failure of the unstable blocks evolves and complements the key block analysis.