Shafts are vertical or inclined tunnels. They are either driven downwards or upwards. In the former case, muck and inrushing groundwater has to be conveyed upwards, i.e. against gravity. This can be done, for example, with the help of air lift. If the shaft is driven towards a pre-existing tunnel (so-called bottom access), then muck and water can be conveyed downwards through a pilot borehole. Reaming of the pilot borehole occurs either downwards (‘downreaming’) or upwards (‘upreaming’ or ‘raise boring’, see fig. 11.1). For downreaming with machines the following two methods apply:
A vertical TBM with conical cutting wheel is braced with grippers against the shaft wall and pushes downwards (Wirth). Alternatively the downwards thrust is provided by the own weight of the TBM (weight stack downreaming of Robbins, Fig. 11.2). For large diameters upreaming is risky because of possible instability of the face and the wall, difficult exchange of the disk rolls, limited thrust against the face and water inrush. Note that countermeasures can only be taken after the completion of the shaft.