The selection of the excavation method is a key aspect for the design of tunnels. The mechanized excavation by means of Tunnel Boring Machines (TBM) is possible and convenient only in specific geomechanical conditions. The preliminary TBM design becomes therefore relevant in order to define the main features of the TBM, determining the feasibility of this methodology, and considering schedules and costs. The main characteristics of the TBM, i.e. advancement thrust, cutter head torque, shield geometry, cutter head properties and power of the hydraulic equipment, can be established by studying the geological framework and carrying out a proper geomechanical analysis. Other relevant aspects as the drilling machinery for the grouting works and geological investigations as well as the equipment required for the backfilling of the annular void can also be defined at this stage. In specific geomechanical conditions, advanced calculations (typically 3D) could be required. The performance of such models must take into account the interaction between mechanized excavation and the geological setting. Lombardi Ltd. has been actively involved in this engineering framework, carrying out successfully the preliminary TBM design regarding multiple underground works: highway, railway and hydraulic tunnels, rocky and soil ground, with high overburden or in urban areas.