Bodies of water have always presented challenges to engineers. In the early days, rivers were powerful avenues of commerce. Sooner or later, though, people just wanted to get to the other side of them.
Boats, such as ferries, were the earliest and most obvious solutions. Eventually, engineers began to build bridges. Soon, though, there were people that wanted to tunnel under bodies of water. Other than hiring a crack team of moles and beavers, how could this be done?
As early as 1818, a French engineer named Marc Brunel invented a device that allowed workers to tunnel under rivers without worrying about water and mud ruining their work. Brunel's “tunnel shield" was a big rectangular iron wall with lots of small shutters in it.
Workers would open the shutters one at a time to dig out a few inches of dirt. After a bit of progress was made, the whole shield would be pushed forward. As the shield advanced a few inches at a time, workers would build a thick brick wall behind it that would become the shell of the tunnel.
This was very time-consuming work, of course. For example, it took workers nine years (from 1825 to 1843) to build a 1,200-foot tunnel below the Thames River in London. This became the first underwater tunnel in the world.
Technology has advanced a lot since Brunel's day. Today, underwater tunnels are often created with humongous tunnel-boring machines (TBMs) — sometimes called moles. These machines cost millions of dollars, but they can create large tunnels in a very short time.
A circular plate with disk cutters rotates to cut through rock as the machine inches forward slowly. As the machine excavates the tunnel, it also helps to build the walls that will eventually support the tunnel.
France and England used 11 massive TBMs to create — in just three short years — the three tubes that make up the 32-mile Channel Tunnel. Also called the Euro Tunnel or Chunnel, these tunnels now connect the two countries under the English Channel.
Another new method of creating underwater tunnels is the cut-and-cover method. To use this method, builders dig a trench in the riverbed or ocean floor. They then sink pre-made steel or concrete tubes in the trench. After the tubes are covered with a thick layer of rock, workers connect the sections of tubes and pump out any remaining water.
This method was used to create the Ted Williams Tunnel, which connects the southern part of Boston with Logan Airport. The 12 giant steel tubes that were sunk in the trench were each 325 feet long and already contained fully-constructed roads!
Engineers are always coming up with new ideas. Based upon experimental rock-cutting methods, tomorrow's underwater tunnels might be built with the help of high-pressure water jets, lasers or ultrasonic sound machines.
New technologies could enable the building of tunnels that once seemed impossible. For example, some engineers would like to build a Transatlantic Tunnel to connect New York with London. The 3,100-mile tunnel could house a train that could travel 5,000 miles per hour. The trip that now takes 7 hours by plane could someday take less than one hour!