Design of underground construction projects 
This short paper describes the main components of the design of geotechnical structures with attention to underground construction projects. An exploratory tunnel driven ahead of a long base tunnel with high overburden and expected complex geological, hydrogeological, and geomechanical conditions is considered. A case example of an exploratory tunnel of relatively small diameter in a flyschoid rock mass (a sequence of sandstone and marl layers), driven ahead of the main tunnel crossing the Apennines along the Bologna to Florence new highway, is briefly addressed. The intent is to predict the ground response when a large diameter tunnel will be excavated in the same conditions. In the framework defined by the Italian Technical Norms (NTC 2008) a scheme (Figure 1) is given, which shows that in situ investigations, laboratory and field tests lead to geological and geotechnical characterizations with the objective to describe the geological model and the geotechnical model,  with in mind the need to define in due detail the ground mass response. The attention is posed on the importance of providing the necessary data for the assessment of tunnel stability, in the short and long term, and for the design of the stabilization measures including the final tunnel lining. The design model needed for computation purposes is mentioned next as a means for performing the design analyses. The importance of analyzing in due detail the excavation/construction sequence and the ground/support interaction is stressed, dependent upon the model adopted for the description of the ground behavior. Specific examples are given in Figure 2 (in a schematic form) and in Figure 3 (showing typical rock mass conditions at the tunnel face during excavation) leading to the rock mass represented as a continuum, an equivalent continuum or a discontinuum. The role of geotechnical monitoring during tunnel excavation is addressed next. This consists of a description of the geological, hydrogeological, and geotechnical conditions at the tunnel face, including rock mass classification. By keeping in mind the need to define the rock mass response during excavation, the importance of in situ observation and monitoring through detailed measurement programs is pointed out. The need to compare the results of performance monitoring during excavation with the predictions based on geotechnical modelling is stressed in the framework of the Interactive Observational Design Approach as illustrated in Figure 4. The case of a small diameter exploratory tunnel, excavated by an open type TBM driven ahead of a larger final tunnel, is described. By observing the types of rock block instabilities occurring in the small pilot tunnel (Figure 5), a discontinuum model by the Discrete Element Method is used as a means for predicting the ground response and the failure zones (in terms of block detachment, shearing, and plastic deformations) during excavation of the larger tunnel (Figure 6). In this manner the importance of the rock block size could be well understood in view of the design of the same large tunnel stabilization measures and final lining.