There are three steps to a tunnel's success.Today, engineers know that there are three basic steps to building a stable tunnel. The first step is excavation: engineers dig through the earth with a reliable tool or technique. The second step is support: engineers must support any unstable ground around them while they dig. The final step is lining: engineers add the final touches, like the roadway and lights, when the tunnel is structurally sound

Based on the setting, tunnels can be divided into three major types

Soft-ground tunnels...
are typically shallow and are often used as subways, water-supply systems, and sewers. Because the ground is soft, a support structure, called a ,tunnel shield must be used at the head of the tunnel to prevent it from collapsing

Rock tunnels
require little or no extra support during construction and are often used as railways or roadways through mountains. Years ago, engineers were forced to blast through mountains with dynamite. Today they rely on enormous rock-chewing contraptions called tunnel boring machines
Hoosac Tunnel interior
North Adams, Massachusetts

Image of the Hoosac Tunnel

 

Forces that act on Solid-Rock Tunnels.

Rock Tunnel: Forces
The rock walls are very dense and can support themselves. Some sections of the rock are less dense than other sections. These loose chunks of rock push on the sides of the tunnel.

Underwater tunnels are particularly tricky to construct, as water must be held back while the tunnel is being built. Early engineers used pressurized excavation chambers to prevent water from gushing into tunnels. Today, prefabricated tunnel segments can be floated into position, sunk, and attached to other sections

Tunnel segment being floated into position
Boston Harbor, Massachusetts

Ted Williams Tunnel
 TO BE CONTINUED