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.

Tunnels are remarkably strong because of their curved or arched shape, which utilizes the principles of physics to distribute external pressure. In an arch, the weight of the surrounding earth, rock, or water is not pushed straight down; instead, the force is directed outward and downward along the curve of the tunnel wall and into the surrounding ground. This is known as the "arch effect" or "compression." Essentially, the deeper a tunnel is buried, the more the surrounding pressure actually helps "lock" the structure into place, making it surprisingly resistant to external forces, including earthquakes. In 2026, engineers enhance this natural strength using "Shotcrete" (sprayed concrete) and massive steel "rock bolts" that tie the tunnel lining directly into the bedrock, creating a unified, load-bearing system. Furthermore, the circular shape of a tunnel is ideal for resisting "hydrostatic pressure" in underwater tunnels, as it distributes the weight of the water evenly across the entire surface, preventing any single point from becoming a structural weakness.