Tunnel

A tunnel is an underground or undersea passageway.

These structures are typically dug through soil, earth, or rock. They may also be laid under water. Most tunnels are completely enclosed. They usually feature two openings at the ends called portals. Some tunnels may have extra openings for ventilation or access.

While a pipeline is different, some modern tunnels use immersed tube construction. This is a method used instead of traditional boring techniques.

Tunnels serve many different purposes in our world. Many are built for vehicular road traffic or rail traffic. Some are used for canals to move boats. Rapid transit networks often use tunnels for their central portions. Other tunnels act as sewers or aqueducts. Aqueducts are structures used to supply water for drinking or hydroelectric stations.

Utility tunnels are also common. These route steam, chilled water, electrical power, or telecommunication cables. They can even connect buildings for people and equipment to move between them. Special tunnels, called wildlife crossings, allow animals to cross human-made barriers safely.

Building a major tunnel requires careful geotechnical investigation. Engineers must study ground conditions before they start digging. They collect samples using boreholes or geophysical techniques. This helps them choose the right machinery and excavation methods. This planning reduces the risk of hitting unexpected ground. Engineers may select a route based on the best ground and water conditions. They often dig deeper to reach solid rock. Solid rock is easier to support during construction.

Sometimes, a pilot tunnel, or drift tunnel, is driven ahead of the main excavation. This smaller tunnel is less likely to collapse catastrophically. It can also serve as an emergency escape passage.

Engineers must constantly monitor the stand-up time. Stand-up time is how long a new cavity can support itself. This is the time before added structures are needed. Rock and clay configurations often have the greatest stand-up time. Sand and fine soils usually have a much lower stand-up time.

Groundwater control is also vital. Water leaking into a tunnel can decrease stand-up time. This makes the excavation unstable and risks a collapse. Engineers use dewatering pipes to pump water out. Another method is ground freezing. This uses pipes filled with refrigerant fluids to freeze the soil. This creates a frozen barrier to keep water out.

Humanity has been making tunnels for thousands of years. Prehistoric people likely began by enlarging natural caves. One of the first artificial tunnels was built in Babylon around 2200 B.C. It used the cut and cover technique to join a palace and a temple.

Other early examples include the Siloam Tunnel in Jerusalem from the 8th century B.C. The Tunnel of Eupalinos in Greece served as an aqueduct in the 6th century B.C. Much of our early tunneling technology evolved from mining and military engineering. These fields provided the skills needed for civil engineering.

Deciding between a tunnel and a bridge is a complex choice. Tunnels are generally more costly to build than bridges for water crossings. However, bridges can interfere with shipping channels. High bridges or drawbridges can be difficult for large ships to navigate. In cities like Manhattan or Hong Kong, land is very expensive. Tunnels are often preferred because they require a smaller footprint on the shore.

Tunnels can also preserve beautiful scenery and views. The Sydney Harbour Tunnel was built to avoid spoiling the iconic view of the bridge. Some projects, like the Chesapeake Bay Bridge-Tunnel, use both structures.

Tunnels also carry specific risks that engineers must manage. Vehicle fires can produce combustion gases that are dangerous to breathe. To reduce these risks, designers install emergency ventilation systems. They may also build isolated emergency escape tunnels parallel to the main passage. Large projects also require massive amounts of money. They often need government funds and long-term financing through bonds. Planning involves using a work breakdown structure and the critical path method. These tools help managers track the time, labor, and materials needed for success.