1. General

Conventional large-diameter tunnel boring machines (TBMs), with all-round attack, progress through the terrain under the action of thrust cylinders located in the shield between the boring head and the fixed support rings, which are successively installed as they advance.

On the other hand, in the case of small-diameter mini or micro TBMs (diameters inside 1,800 mm), the boring head (or cutting head) and its shield are pushed forward by the train of tubes forming a sliding support, which is advanced by jacks installed in a fixed position in the drive shaft (figure 1 ).

Today, there's a distinction between mini-tunnelers and micro-tunnelers.

The mini-tunneler is used to create a circular structure by excavating...