Many tunnels are circular, though, and this is usually because many tunnels are excavated with large drills, and the drills are circular. There are some strictly square tunnels, such as those made by lowering boxes underwater.
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Circular tunnels are the most popular shapes in urban underground transportation systems when mechanised tunnelling is used for the tunnel excavation. However, circular tunnels have a small space utilisation ratio.
In tunnel engineering, various tunnel shapes are adopted [3], although the circular tunnel is usually preferred due to convenient construction and function layout requirements.
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.
The alignment of a tunnel constructed from two ends and meeting somewhere is the middle is achieved via old fashioned surveying methods. If the tunnel breaks through to the surface, surveying stations are accurately established on the surface of the Earth at each end of the tunnel.
Underground structures last a significantly longer life time than above-ground structures. Approximately about 50 years for above-ground, over 100 years for tunnels.
This is said to be less for superstition and more for safety. In the past, tunnels used to be only a single-lane wide, so drivers would honk to let cars coming from the opposite direction know they were driving through.
If for no other reason, the horn can alert those folks to get to safety. either to alert people when they enter the tunnel(like MOW, or trespassers) or in some cases, to be friendly at kids(or railfans) at the tunnel entrance.