Shafts (Fig. 30.11) are vertical wells or passages constructed along the alignment of a tunnel at one or more points between the two entrances. A shaft is provided for the reasons listed below.
Working shafts These are provided for the expeditious construction of tunnels by tackling the same at a number of points. These are generally vertical and of a minimum size of 3.7 m x 3.7 m or of a diameter of 4.30 m.
Ventilation purposes In order to ensure better ventilation, these shafts are generally inclined and have a girder size of about 1.2 m diameter.