The process of spalling failure around a deep excavation in a rock mass is one of the main problems in tunnel construction. It consists in a brittle failure, produced by the crushing of compressed rock, during which strain localization appears as a shear crack. Both the tensile damage and the confinement reduction, produced by the excavation of the tunnel, play a fundamental role in the production of the spalling failure [
]. To better understand and to prevent this failure, a plane strain model able to evaluate the structural response of the rock around the excavation is here proposed. In particular, under the hypothesis of circular tunnel (of radius
), within an undefined rock mass having an initial hydrostatic pressure σ
, the stress σ
in the tangential direction is evaluated during the progressive reduction of the radial stress σ
(Fig. 1a). This reduction represents the loss of confinement produced by the excavation.