Nowadays, with the development of the city, the subway has become more and more dense. As a new type of transportation, it has greatly facilitated people's daily life since its birth.

According to public reports, Shanghai, Beijing, Guangzhou, Chengdu, Wuhan, Shenzhen, and Nanjing have successfully ranked among the world-class metro cities in the total mileage of rail transit, mainly by subway, including light rail and trams. Among them, Shanghai The highest ranking, the total operating mileage of its track reached 825 kilometers.

Then the question also arises. Why do China's subways dig so deep, and no soil is dug out. Why is this?

Among them, the reason is not complicated!

First, soil backfill

In order to ensure that the subway does not collapse, a large amount of soil with strong adhesion is needed to reinforce the road. Therefore, when workers are working, they usually directly use the excavated soil for the construction of the subway.

When the subway is under construction, additional reinforcement of the tunnel needs to be prioritized. Reinforcing the tunnel requires solid supports, as well as a sufficient amount of soil, ore and hardened walls. The soil obtained when digging the tunnel usually needs about 30%, which will be used again for the construction of the subway.

Sometimes, the excavated soil is not enough, and some cement needs to be poured into it, so the subway is dug deep, but there is little feeling of soil being transported out.

If there is any leftover soil from the excavation, it will not be wasted casually, but will be directly transported to other cities for construction. Subway construction teams usually choose to resell them to other construction teams at relatively low prices, or be invested in green belt construction. As a result, the resource utilization efficiency of the entire city will be greatly improved.

Using this method of soil backfill, on the one hand, can save the construction cost of the subway, on the other hand, it can make it more convenient for workers to perform operations, shorten the project, and improve work efficiency.

If the method of transporting soil from other places is adopted, it will not only cost more and take longer, but also easily cause other unpredictable things. Once it encounters bad weather such as rain, it will greatly extend the entire construction period.

Therefore, under a comprehensive consideration, the working people of China have exerted their infinite wisdom, and when conditions permit, they will give priority to this method of collecting mud on the spot to improve the utilization of resources.

Second, shield machine and night earth moving

The shield machine is a tunnel boring machine that uses the shield method. Its specific function is that while the boring machine is excavating, it continuously builds a tunnel support segment around it. In China, tunnels in soft soil are often used. The boring machine is called a shield machine, while the tunnel boring machine in harder formations such as rock is called a TBM.

The use of shield machine can greatly improve the construction speed and save manpower. Because it has a high degree of automation, it also has the advantages of not being affected by the surrounding climate, and can control the land subsidence, so it is also very popular among the construction team.

In addition, when using the shield machine, the circular turntable at the head of the machine, which is covered with blades, will chisel the soil in front, and part of it will be squeezed out, while strengthening the surrounding hole walls.

In this way, using a shield machine will make less soil dig out!

Also, when building the subway, in order to minimize the harassment of city residents, the construction team will surround it with iron sheets, so unless you are looking down from a high place, you will not be able to see the work inside. Yes, you can't even see the sand inside.

In cities during the day, traffic is usually congested, so in order to improve transportation efficiency, when there is really excess sediment that needs to be transported out, it is usually done in the middle of the night, when people are usually sleeping.

And if the dirt is scattered on the road, the cleaners who get up earlier than the sun will clean up the city roads almost before you get up. It was so deep, but I still didn't see the illusion that the soil was transported out.

Third, China's subways are not really deep

Many small partners may think that China's subways are very deep, but in fact, compared with foreign countries, we have already regarded them as relatively shallow.

The underground space is generally divided into shallow layer (0-10 meters), sub-shallow layer (10-30 meters), sub-deep layer (30-50 meters), and deep layer (50 meters).

Generally, the construction limit of urban underground space is 100 meters underground.

But generally speaking, for civil facilities such as subways, considering multiple factors such as economy, safety, and comfort, subways will not be built so deep, and there is no need at all.

Like China's subway construction, it is usually controlled within 30 meters underground, that is, the subway construction is carried out in the shallow and sub-shallow areas. Generally, most of them are within 10 meters, and there are only a few cases within 30 meters. For example, in Shanghai In such a super-large international metropolis, the underground facilities in some core areas may exceed more than 30 meters underground, and the rest are basically gone.

Because in terms of safety and comfort, the subway within 30 meters underground is more comfortable than the subway 30 meters below the ground, and in the event of an accident, the shallow subway people are more likely to escape .

The same is true in terms of economic effects. The deeper the subway construction, the more money it will cost.

Some countries, such as North Korea and Russia, may build subways at 50 to 100 meters deep for the purpose of national defense, while China's subways only need to consider the livelihood of the people, so when construction workers build subways, they must It is built on shallow soil as much as possible.

In this case, the soil dug up during the construction period is not as much as everyone imagined. If there is any surplus, you can use the methods mentioned above, such as: backfilling, selling at a low price, and using it for construction. The green belt and so on are consumed.

China has always been known as an infrastructure maniac. A large amount of infrastructure construction experience has summed up a set of relatively scientific and reasonable construction concepts for China. This concept is comprehensive efficiency, people's livelihood, green, environmental protection, economy, safety, etc. from the dimension.

In the treatment of the sediment excavated from the subway construction, these factors are fully reflected. The backfilling of sediment is efficient and economical; the use of excess sediment to build urban green belts also reflects greenness and environmental protection; Construction in the shallow soil layer reflects safety and people's livelihood!

Therefore, after this scientific and reasonable treatment method, it is not surprising that ordinary people still have the illusion that the subway has been dug so deep and where has the sediment gone!