How come tunnels don't collapse?
How come tunnels don't collapse? Because they're stronger that way. Tunnels are circular to take advantage of the characteristics of soil and rock. Soil and rock are resistant to crushing forces (compressive forces). By rounding the ceiling, the weight of the soil and rock from the top to are balanced over the entire tunnel, increasing its strength.
Do tunnels ever collapse?
Although this is also seen to a certain extent in bridges, it is not to the extent seen in tunnels. Events such as earthquakes and floods, which are the major causes of collapse of other structures, account for only about 10% to 20% of all tunnel collapses.
How do you dig a tunnel without it collapsing?
Because we dig while reinforcing them. Once a tunnel is dug, the walls are sprayed with concrete and immediately hardened. Also, steel frames called steel support and steel rods called rock bolts are used to reinforce the tunnel so that it can be dug without collapsing.