Constructing the World’s Longest and Most Useful Tunnels

Sometimes the best route between two locations runs underground. In their most basic sense, tunnels are underground passageways that are closed off except for openings at the entrance and exit at each end. In the United States, the National Fire Protection Association defines “tunnel” as an underground structure with a design length greater than 75 feet and a diameter greater than 5.9 feet.

Some tunnels serve as aqueducts to supply drinking water, to provide sewer service, or hydroelectric stations. Others route water for telecommunications cables, to connect buildings, or for convenient passage of people and equipment. Still other tunnels provide safe crossings for wildlife, allow for covert smuggling of supplies, and provide military barriers for safety.

Tunneling Construction Methods

There are two basic techniques commonly used to advance the construction of a tunnel. The full-face method involves excavating the diameter of the tunnel while moving forward in it. This technique is often used for small tunnels and when tunnels run through hard ground. The other basic method is the top-heading and bench method, which requires workers to first dig a small top tunnel and then begin excavating below the floor of the top tunnel. By using this method, engineers can test the stability of the rock and gauge the tunnel’s safety before moving forward with excavation.

Other common tunneling methods include the following:

• Bottom-up and Top-down “cut and cover” methods for shallow tunnels
• Tunnel boring machines to automate processes and cut costs
• Shafts to provide temporary access during excavations
• Sprayed concrete techniques to provide safe support to the tunnel lining

 

 

 

 

Soft Ground v. Hard Rock

When engineers encounter soft ground, the excavation must be timbered for support as the work moves forward. Workers often cut two parallel excavations in this instance, so that the side walls are constructed first. When carving a tunnel through sand, gravel, mud, or clay, workers must test the “stand up time” to gauge how long the ground will safely stand by itself. Since this amount of time is short when the ground is soft, workers must push an iron or steel cylinder shield into the soft soil to support the earth while excavating is in progress.

On the contrary, hard rock is typically removed for tunneling by blasting. Tunnels cut through rock sometimes don’t even require lining. Workers typically use a type of scaffold to quickly place explosives and move to safety. Most hard rock has a long “stand up time,” however, engineers typically use bolts, steel beams, and sprayed concrete to support points of fractured rock and breakage.

Blasting v. Fire-Setting

One alternative to blasting is fire-setting, a tunneling technique that heats the tunnel walls with fire and then cools them with water. In this method, large chunks of rock break away due to the rapid expansion and contraction caused by sudden temperature changes. This method was commonly used from prehistoric times to the Middle Ages. Engineers often plan to add a permanent concrete lining to the tunnel when employing this method.

Great Tunnels of the World

• Yerba Buena Island Tunnel in California – largest single-bore tunnel in the world
• Laerdal Tunnel in Norway – longest road tunnel in the world
• Channel Tunnel in England – longest undersea tunnel in the world
• Fenghuo Mount Tunnel in China – highest railway tunnel in the world
• Gotthard Base Tunnel in Japan – longest operational railway tunnel in the world

*Photo credit: Störfix and Johann Jaritz via WikiMedia Commons