Pick the rock considered as soft rocks for tunnelling.

Type of hydropower tunnel where water is conveyed under gravity is ______

Which outline is preferred for weak rocks with unequal lateral pressure?

Type of hydropower tunnel where water is conveyed under high pressure to turbines is ______

Geologically, how many classes of tunnels are recognized?

Which type of strata is rare in occurrence?

Which type of rocks requires double lining, i.e., temporary and permanent lining?

The underground routes or passages driven through the ground without disturbing overlying soil cover are called ______