History of tunnels

A tunnel is an underground or underwater passage that is primarily horizontal. A tunnel may be for foot or vehicle traffic, for rail traffic, or for a canal. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers. A tunnel is relatively long and narrow; the length is often much greater than twice the diameter.

Some 3000 years ago, when our ancestors started discovering techniques of building stable and strong bridges, they also discovered new way of connecting two points of land – tunnels. This discovery was initially used not for transport of goods and people across harsh terrains, but for defensive purposes in the vicinities of important military or royal posts (tunnels below castles). Babylonian and Persian architects were the first who saw the potential of large underground networks of tunnels called kareez. These irrigation tunnels were used to transport water underground trough deserts, enabling life in some of the most hostile lands on planet. In Babylonia, royal families enjoyed fresh water from Euphrates that was delivered to them through incredibly built 900m long tunnel that was lined with bricks.

Greeks and Romans took all the knowledge of Babylon and Ancient Egypt, and improved it. With tunnels they were able to transform marches, transport water through mountains, and create pedestrian tunnels trough very harsh terrains. To this day historians wonder how much workforce was involved in the construction tunnel between Naples and Pozzuoli that was created around 36 BC. This incredible structure was 4800 foot long, 25 foot wide and 30 foot high, and it even had ventilation shafts. Less than 100 years later in 41 AD, Romans used around 30,000 workers to build even larger tunnel that was 5.6 km long.

In European Middle Ages, tunnels were almost exclusively used for mining or for military. After public transportation they finally started to grow under the influence of Renaissance and trading with distant lands. Hundreds of smaller tunnels were created between mid-1600s and 19th century, but by then new driving force of tunnel construction came – railroads. This new form of transport soon enabled spreading of tunnels across entire world.

Choice of tunnels vs. bridges

Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in tunnels’ favor. Boston’s Big Dig project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, redecorate.

Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather and shipping during construction, aesthetic reasons (preserving landscape and scenery).

However, there are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard Road Tunnel in Switzerland in 2001.