The rock mass and the development of joints during the tunnel excavation

The term joint has been historically used imprecisely when carrying out geomechanical characterization of rock mass. The current classification systems consider the different fracture planes simply as they are observed visually, assuming independence of dimensions and other fixed patterns with depth. That can lead to over-design underground works. Joint patterns in a rock mass depend on the change of the stress field acting on the rock from its original confinement state. The stress field on the rock mass is not constant and varies with changing distance in any underground excavation. In tunnel analyses, empirical classifications consider the parameters related to the joints in the rock mass as constant variables. Those classifications, constitute more than 75% of the valuation without considering the effect of the development and frequencies of joints according to the change of the stress field. In this work, it is considered the necessity to broaden the definition of joints as non-moving fractures for which the development of the fracture plane depends on the change of the stress field in the rock mass. Regarding the discontinuity term, it is important to differentiate the joints from the stratification and foliation planes, which are terms associated with the classification of rocks according to its origin, particularly for sedimentary and metamorphic, respectively. These are made up of continuous planes that control the mechanical behavior according to the stresses acting in the rock mass. Considering a tunnel excavated in an igneous rock mass, the decompression due to the opening effect differs from a stratified or foliated massif. The first one depends on the development of joints, and the second, mainly on the phenomenon of bending of layers within the rock mass, according to the relative dimensions of layer thickness to tunnel diameter. In this article, several practical examples are presented which support the hypothesis that joints develop as a function of the change in the stress field. Also, by means of examples applying numerical methods, it is shown that the deformations of a rock mass with constant parameters are bigger than those when a zoned rock mass is considered, including changes in the geomechanical characteristics related to the distance in the tunnel excavation. The results have an impact on the thickness of supporting elements, yielding lower construction costs.

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