During underground excavations for civil or mining engineering purposes, the variations in rock mass quality are important, especially for the design of the most suitable support to be applied to ensure stability. The aim of this investigation is to model the expected behavior of the ground, and thus to predict the scenarios indicating potential variations in the quality of the rock mass during underground excavation. When considering the rock mass rating (RMR) values observed at the excavation face in six study cases, which together total more than 27 km in length of underground excavation by drilling and blasting (D&B), and based on the observed RMR values at the face, the most probable value (1–100) is estimate for the RMR index at the five subsequent front advances. It is concluded that, up to about 20 m ahead of the current face, the quality of the rock mass for the next advances is close to the quality observed at the present face, and that, with increasing distance, there is a greater deviation of RMR values with respect to the quality observed at the current face.