Fig. 15

Conceptual model of the EDZ (after Bossart et al. 2002). The EDZ fracture network related to tunnel excavation can be subdivided into an inner and outer shell. The inner shell, with an average extent of 1 m from the tunnel circumference, is typically characterised by extensional fractures sub-parallel to the tunnel wall and smaller-scale shear fractures. These fractures developed mainly in the tunnel walls (sidewall, ceiling, floor) and tunnel faces. Extensional fractures are often linked by smaller-scale shear fractures, resulting in an interconnected fracture network. The outer shell, between 1 and ca. 2 m from the tunnel circumference, is composed of individual extensional fractures that are not, or only partially, interconnected