Image: Tunnel to a Lower Budget

This is a diagram overlay of a typical 8' square tunnel as compared to a 10' wide by 8' high air-formed tunnel. It is easy to see that the round tunnel is biggest (2' wider) where it is needed. Any engineer can verify that the round shape is much stronger than the square. The round tunnel walls are 8" thick as opposed to 12" to 16" for the square. Even though it is wider the total amount of wall and roof is about 15% less. Approximately 35% less concrete and reinforcing bars are needed in the construction.