I dug a big hole in the city, and what I found was...

When you walk on the streets of a city, you will see tall buildings and busy traffic on the ground. You look up at the sky and down at the road, but few people think about what is under the city? What secrets are there in the underground world that we cannot see?

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Today, let’s lead everyone to “dig three feet underground” to dig deep into the structures and buildings underground in the city!

What's underground in the city?

Urban underground facilities are an important part of the city, including various pipelines, sewers, underground passages, underground parking lots, underground shopping malls , etc. Some large cities also have subways. The existence of these facilities is crucial to the operation and development of the city. Drive the excavator and let's dig on the ground in the city together~ (Pure imagination, please do not practice)

Start digging↓

What's the good of building it underground?

There are so many underground buildings in the city, so why do people go to such great lengths to build them underground? This is because underground facilities have the following advantages:

1. Save land

As cities expand, the resulting lack of construction land and the contradiction between urban development are becoming increasingly acute, especially in busy commercial areas. The development of underground space saves land resources and can ease the tight situation of urban land use. Moreover, houses are built underground, which can reduce the density of ground buildings, expand the green area, and improve the surface ecological environment.

2. Alleviate traffic congestion

The underground vertical space is very large, and transportation lines such as subways are built three-dimensionally underground. There are fewer intersections and no interference with each other, which can greatly save commuting time.

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3. Anti-noise and anti-interference

The thick soil layer underground can isolate noise and some other influences, and building houses underground is quieter and more comfortable. In addition, some precision scientific research equipment is sensitive to external noise signals such as cosmic rays, so building them deep underground can prevent interference.

4. Used for protection during special periods

Compared with ground buildings, underground space is more closed, has much higher defense capabilities than ground buildings, and has stronger post-disaster repair capabilities. It can be built into the city’s disaster prevention and air defense space, greatly improving the city’s air defense support capabilities.

5. Warm in winter and cool in summer, energy-saving and environmentally friendly

Compared with above-ground buildings, underground space has poorer ventilation and smaller temperature differences. It is warm underground in winter and cool underground in summer. This advantage saves the consumption of power resources and alleviates the shortage of power during peak periods to a certain extent.

Why not build underground skyscrapers?

The core reason is that digging underground is expensive. The cost of a basement within 7 meters deep is about 2,000 yuan per cubic meter, while the cost of a 100-meter high-rise building is only 1,000 to 1,500 yuan per cubic meter.

From the perspective of architectural design, building deeper underground buildings is also difficult. The lateral soil pressure and groundwater pressure on the outer walls of underground buildings will increase with depth. The greater pressure is a challenge to the pressure-bearing of building materials and the design of house structures. Building ultra-deep underground buildings is a labor-intensive and time-consuming project, and there are certain risks. Moreover, excessive underground development will also reduce the ground load, which may bring the risk of collapse to the surface buildings.

From the perspective of usage function, waterproofing, moisture-proofing, ventilation, and lighting of underground buildings are also key and difficult issues for both construction and maintenance. Even if these problems are solved, drainage, sewage disposal, and fire prevention must be considered to meet living requirements. It is difficult to achieve good living conditions underground.

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Therefore, if residential buildings are excavated hundreds of meters underground, it is basically science fiction based on existing technology .

Future Underground City?

There is a saying in the international architectural community: “The 19th century was the century of bridges, the 20th century was the century of high-rise buildings, and the 21st century is the century of underground space.

With the rapid increase in the amount of underground space, the total scale of underground space use in China's large and medium-sized cities has reached the forefront of other similar cities in the world. China has developed into a powerful country in the use of underground space in the world. In terms of existing underground facility construction, the use of underground space in China is based on underground parking, underground transportation, and civil air defense evacuation, with underground drainage pipes, subways and underground commercial centers as the focus . At present, the construction of underground urban rail transit, underground commerce, and underground parking lots has entered a development period.

In the Chinese hard science fiction blockbuster "The Wandering Earth", the "underground city" built by humans for survival is impressive. With the development of cities, underground space is becoming increasingly important in the development of modern cities. What kind of underground buildings can we build?

Image source: Stills from The Wandering Earth 2

Generally, the development depth of urban underground space is only 100 meters. Without considering the actual cost, it is possible to build at a depth of 1,000 meters in a certain sense.

Looking up, the world's tallest building is the Burj Khalifa in Dubai, with a standard height of 828 meters. Looking down, heading deeper, the deepest well on Earth is the Odoptu OP-11 oil well in Russia, with a depth of 12,345 meters. The average thickness of the earth's crust is 17,000 meters, and below it may be all lava. This depth is almost the limit of human habitation in imagination. In comparison, the space underground is more vast, with greater room for development, and it also brings us endless imagination. But in addition to the construction problems mentioned above, we don't know what other problems are waiting for us at a deeper level...

As we continue to invade the sky, we are also slowly shifting our attention to the exploration of underground space. With the continuous advancement of technology, perhaps in the near future, underground cities will no longer be a fantasy, and even magical "high-rise" buildings will appear.

References

[1] Zhang Zhi, Wu Menghan. A review of the research and development of urban underground space in China based on CiteSpace[J]. Safety and Environmental Engineering, 2023, 30(01): 25-34.

[2] Du Yulu, Wang Zhichao. Discussion on the necessity of developing and utilizing urban underground space[J]. Sichuan Building Materials, 2016, 42(01): 100-101+109.

[3] “Guiding Opinions of the Ministry of Housing and Urban-Rural Development on Strengthening the Construction of Urban Underground Municipal Infrastructure”, December 30, 2020

[4] Design Standard for Pedestrian Underpass in Shenzhen SJG 68, 2019 edition

[5] “Design and Construction Specifications for Automobile Gasoline and Gas Stations” GB50156-2012, 2014 edition

[6] How deep is the subway usually underground? What factors determine its depth?, Zhihu

[7] Official website of the Qin Shi Huang Mausoleum Museum, https://www.bmy.com.cn/

Planning and production

Source: Institute of Physics, Chinese Academy of Sciences

Editor: Yinuo

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