城市的地下

When you think of a city, you probably think of busy streets and towering skyscrapers . But the city doesn’t just reach up into the sky. It also extends down below the earth. Hidden beneath buildings and roads, a complex network of wires and pipes brings residents electricity, water, and gas and also removes their waste. Underground tunnels house subway systems, storage rooms, or other structures. In many cities, space above ground is so precious and crowded that engineers are finding new, creative ways to use underground space. Singapore is one of those cities. Here, the underground is already used for many purposes, and the city government is looking into ways to build even more underground structures. “You can build up, but there is a limit,” said John Keung when he was CEO of Singapore’s Building and Construction Authority. “The only thing left is to go underground.”

Before you can build anything underground, though, you have to know what’s down there. The underground consists of dirt, clay, sand, and gravel layered over bedrock, the solid rock that circles within Earth’s crust. But the composition and depth of these layers, also called the subsoil profile, is different everywhere on Earth. The subsoil profile can even vary widely in different parts of the same city! In some places, bedrock is very close to the surface or even exposed. In other places, bedrock is very deep. The layers above bedrock may contain very loose materials, like sand or gravel, or very hard, like clay. Water seeps through many of these layers. Dangerous gases, such as methane, can be trapped inside rock or soil formations. If not carefully handled, these gases could cause fires or explosions or could suffocate workers. Brad Cowles oversaw the construction of a subway tunnel in Seattle, Washington. He says, “The thing about tunneling around here is you always know what conditions you have—right here! Three feet away, it can be completely different. ”

Each subsoil profile presents unique challenges. While it’s easier to dig through dirt or sand, these loose materials don’t support structures as well as hard clay or rock. In Singapore, rainfall and high temperatures have weakened the rocks and soil in one area to a depth of forty meters . To build on these weak layers, engineers must use more intricate support systems. For example, they may have to pump cement into the ground to make the soil stronger.

Before starting an underground project, engineers usually drill deep holes, called boreholes, and remove samples. They then test these samples to learn how strong each layer is, how water will move through it, how much the weight of a building will compress that layer, and more. While building, engineers must also be very careful. Disrupting the underground in the wrong way can lead to small tremors or even earthquakes, gas explosions, or sinkholes-where the ground collapses into itself.

The ground is not the solid, stable surface it appears to be from above. It’s a complicated world. But engineers have found ways to dig, drill, and blast open the space necessary for a city to support itself and even expand downward. Let’s go on a journey beneath a city, making stops at structures that exist in most cities as well as ones that could get built   in future cities.

To build the towering high rises and skyscrapers of a large city, builders must first dig down. Directly beneath almost any building, you’ll find its foundation. This structure helps spread the weight of the building out over a large area. It also prevents the building from sinking unevenly or deforming in shape. To understand why a building needs a foundation, imagine stacking blocks on top of loosesand. The tower is likely to fall or shift. But if you first bury a large block so only itstop sticks up, then build on top of that, your tower will be much stronger. The buried block anchors the tower.

There are two types of foundations： shallow and deep.Typically, a shallow foundation is wider than it is deep. Shallow foundations are cheaper because engineers don’t have to dig as far down. They work best for small or light buildings, up to about five stories in height. A foundation of isolated footings (also called individual footings) is the simplest and most common shallow type. First, builders excavate a large hole where the building will go. Then, they construct a series of footings. Each one is a metal column that sits on top of a flat pad of concrete. The columns will hold up the weight of the building above.

Very tall or heavy buildings require a deep foundation, which is usually a pile foundation. A pile is a long rod made of concrete or another strong material. Builders push these rods very deep into the ground. If possible, they will drive the piles down into the bedrock. The piles will act like anchors holding the building in place. Taller, heavier buildings or buildings in areas that experience a lot of wind or earthquakes will typically need deeper foundations or more piles. The amount of weight that each pile can hold depends on the type of soil and rock beneath the building. For example, a pile driven into bedrock will be able to hold much more weight than one driven into sand or clay.

In some places, preparing a foundation is an especially tricky puzzle . The Twin Towers in Kuala Lumpur, Malaysia, couldn’t be built at the site originally chosen because the bedrock there was too close to the surface. So construction moved to a location with deeper bedrock. The tallest building in the world, the Burj Khalifa in Dubai, United Arab Emirates, sits on top of sandy layers filled with salty groundwater that could eat away at metal. And the bedrock below that is weak and very deep. To hold up the skyscraper, engineers had to bury 192 steel piles fifty meters deep and surround those with salt-resistant concrete.

The people and industries in cities consume resources including water, electricity, and gas for cooking and heating . And they need access to phones and the Internet. Pipes and wires carry all of these utilities to and from homes and businesses.

Pipes also carry some types of waste away from buildings . Buildings and roads are designed to help rain water flow towards drain pipes or sewer. From there, the rain water may flow directly into lakes or seas, or it may join waste water from showers, sinks, toilets, and other sources . The wastewater pipes lead to a processing plant, which uses a series of filters and other methods to clean toxic materials out of the water. Then, the clean water gets returned to the environment.

In most cities, all of these different pipes (and sometimes also wires) are buried at a shallow depth, often directly under roads. This is called open cut installation because it requires digging a hole from the surface using backhoes and other trucks, then laying in the pipe. Finally, workers must fill up the hole, then repair the road or whatever other structures were on top.

That’s not ideal because whenever someone has to repair, perform maintenance on, or upgrade any of the pipes or wires, the city has to go through the entire process all over again. The digging disrupts daily life and may damage nearby utilities or other structures.

Phone, Internet, and electric cables may also be strung up on poles, which is ten times cheaper than burying the wires. But in a storm, falling branches often break overhead powerlines, leaving many homes and businesses without electricity.

Singapore, Tokyo, Shanghai, Helsinki, Madrid, and many other cities around the world have begun to replace open cut or overhead installations with utility tunnels. People can climb inside a tunnel at any time to install or repair pipes and wires. No new digging required. Around Tokyo, Japan, six hundred fifty kilometers of concrete tunnels running as deep as fifty meters carry Internet and telephone cables. In Seattle, Washington, a nine-meter-deep tunnel carries electrical cables. Fiel Diaz works on those cables. Often, he’s not alone in the tunnel. “It’s kind of scary here at night,”he said.“These cables become rat and cockroach highways. ” Singapore’s brand new utility tunnel houses the typical things—water, sewer, electricity, and telecommunications . But it also has two futuristic features: pipes that carry cooled water for air conditioning and pipes that use pressurized air to remove garbage from buildings.

If rats and cockroaches can travel underground, why not people? Over 150 cities around the world have an underground subway systems. A typical subway tunnel is about six meters wide, just wide enough for a full-grown giraffe to lie sideways inside it. But you can’t dig just one tunnel. There have to be two tunnels next to each other, so trains can travel in both directions. Many cities have also built roads that go beneath the ground. The Big Dig project in Boston, Massachusetts, included a 2.4-kilometer-long underground highway. It took over ten years to complete the project. Digging out a tunnel for an underground subway or highway is very expensive. The Big Dig cost around three billion dollars . The 1.4 kilometers long utility tunnel in Singapore was built at a cost of eighty-one million dollars.

Many experts believe that moving transportation underground is the best solution to the traffic problems that plague most cities . Famous inventor Elon Musk started The Boring Company in 2017 because he was frustrated with the traffic problems in the city of Los Angeles. He decided it made the most sense to move roads underground. “There’s no real limit to how many levels of tunnels you can have,”he said at a 2017 conference. “The deepest mines are much deeper than the tallest buildings are tall. ” So far, his company has built a tunnel for pedestrians under a convention center in Las Vegas. Singapore, Chicago, and many other cities have also built underground walkways for people to use.

To build a tunnel for utilities, trains, or cars underground, engineers typically use a tunnel boring machine. This is a gigantic, cylinder-shaped machine . At the front end, a round plate called a cutting wheel spins around. Sharp teeth on the wheel break up the soil, which gets fed back through the machine and carried up to the surface on conveyor belts. The cutting wheel is like a giant drill bit for digging out the shape of the tunnel.

Most tunnel boring machines also place support structures. Without support, water could flood the tunnel or the ceiling could collapse. Typically, a boring machine places concrete sleeves along the sides, bottom, and top of the tunnel as it digs. It then seals the sleeves with grout. Many tunnel boring machines are also equipped with gas sensors that shut off the system when harmful gas is detected.

Just about any city service could pote ntia lly move u nde rg ro u nd . Si ngapo re is not the o n ly city experimenting with underground trash removal services. The city of Singapore is already building underground substations. Small, underground nuclear power plants are also a possibility. And some cities have built underground bunkers or bomb shelters to use in case of emergency.

Many cities maintain large reservoirs of water underground. Flooding was a big problem in Tokyo until the city built a huge network of G-Can flood surge tunnels and tanks filled with pumps and other equipment. The network keeps the city dry during heavy rains and typhoons.

Large underground caverns can be perfect for storage. In Singapore, engineers carved out a huge rock cavern big enough to fit a nine-story building. The Jurong Rock Caverns go as deep as 130 meters and are being used to store the ingredients needed to make oil. Engineers used a combination of drilling and blasting to create the caverns.

The city of Helsinki in Finland has built tourist attractions underground. People can visit an underground art museum, church, swimming pool, shops, and more. The possibilities are endless. Richard McLane is the chief mechanical engineer for the Regional Connector Project, a new subway tunnel currently being built in Los Angeles. He asks, “Why is tunneling so addicting?”Then he answers his own question, “It’s like watching civilization in action… The work we do will last generations . ” Indeed. The underground networks that support cities may be hidden. But they are as much a part of urban identity as the skyscrapers that tower above.