Toward a new definition of terms in the rock geomechanical classification and its relation to the numerical analysis of tunnels

It is proposed that terms such as persistence and discontinuity should be eliminated from the rock mass geomechanical classification, since these include the formational planes of the classification of rocks according to their origin, highlighting the continuous planes of the layers in sedimentary rocks and the foliation planes in metamorphic rocks. The term joint, widely known as non-moving fractures, should be redefined for its proper use in geomechanical classification, as non-moving fractures where the extension and frequency depend on the stress state of the rock mass. In a tunnel analysis, the existing empirical classifications consider various parameters related to the joints, analyzed as constant patterns, with valuations up to 75% of the total, without considering the effect of the development and frequencies of joints, according to the change of the stress state. This has repercussions in the tunnels numerical analyzes, where the development and frequencies of joints depend on the state of stress and the stress release by the excavation, so that the geomechanical conditions of the rock mass are the most affected in the vicinity of the excavation and the less affected at a greater distance from the excavation. According to observations in physical models, the deformations around a tunnel tend to form resistance ovals, which can be represented in numerical models in a similar way. The results of the analyzes that consider the variation of the resistance, show a decrease in the stress and deformation towards the cavity. In this research, various practical examples are presented that support the hypothesis that joints develop as a function of the change in the stress state. Through examples by applying numerical methods, it is shown that the deformations of a rock with constant parameters is greater than those obtained with a zoned mass model by changes in the geomechanical characteristics as a function of the distance from the tunnel excavation. The results have an impact on the design and on the costs.