... excavating a central drift and then by widening this drift on both the sides. The sequence of excavation of arch portion for the powerhouse cavern has been shown in Figure 1. After completion of the arch portion, lowering of the bench was taken up. Considering the importance of ensuring the stability...

..., and have no covering. They only have a primary support in a few highly localised zones. Madeira Island sprayed concrete portal widening Encumeada tunnel levada Island canal execution intake structure Thickness localised zone marine transportation characteristic hydraulic exploitation...