Mr. Topias Siren, Kalliosuunnittelu Oy Rockplan Ltd., Finland
Mr. Daniele Martinelli, Kalliosuunnittelu Oy Rockplan Ltd., Finland
Mr. Lauri Uotinen, Kalliosuunnittelu Oy Rockplan Ltd., Finland
Mr. Guido Nuijten, Kalliosuunnittelu Oy Rockplan Ltd., Finland Ms. Riitta Lehmusjärvi, Posiva Oy, Finland
ONKALO is a rock characterisation facility for future geological nuclear waste disposal purposes. The excavation of ONKALO is currently at the bottom level, at around 430 metres from ground level. At these depths, it is important to be able to predict the extent and probability of rock spalling. When secondary stresses at the excavation boundary exceed the rock mass strength, a brittle failure occurs that is often called “spalling”. The spalling phenomenon occurs as a strong compressive pressure induces crack growth behind the excavated surface and buckling of thin rock slabs. In this environment spalling is an event that can create problems, not so much for the overall stability of all of the excavations, but rather in particular areas that can cause unnecessary and unintended over-excavations and hazards."