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Interview with Academician of the Chinese Academy of Engineering

Chen Xiangsheng, Director of Shenzhen University Future Underground City Research Institute

The world's largest column-free underground transfer hub

——Shenzhen Gangxia North Underground Hub

The Shenzhen Gangxia North Underground Hub has a total area of ​​220,000 square meters. It is the world's largest column-free underground transfer hub, which can realize "four-line transfers" for the subway and will realize "five-line transfers" in the future.

The Gangxia North Hub is equipped with 23 entrances and exits, and 8 sunken plazas are designed inside, forming an urban complex integrating transportation, shopping and leisure. The ceiling of the Gangxia North underground hub is a huge circular skylight with a diameter of 9 meters, which is called the "Eye of Shenzhen".

Wu Xiaoli:The Gangxia North Hub was built with RMB 7 billion, and the original budget was RMB 3 billion. Why did so much money become needed later?

Chen Xiangsheng:It only costs about 3.5 billion to build the subway, but this 3.5 billion only built the subway transfer and realized its functions, but the underground space was not expanded. We hope that this hub and all the surrounding buildings will be interconnected and seamlessly connected, and at the same time there will be commercial development. If only the original 3.5 billion was built, this station would not have the ability to generate revenue, and the operation cost could not cover the operation and maintenance of this space. We have added a 60,000 square meter commercial area, so basically only half of the commercial income can cover the full life cycle cost of the entire Gangxia North Hub.

Wu Xiaoli:The Gangxia North Hub will be open to traffic in 2022. How is the operation going at present?

Chen Xiangsheng:Very good. The business is just starting now. The east side is already fully occupied, and there are more than 40,000 square meters in the process of investment promotion. Last month, the maximum passenger flow reached 340,000 people a day.

Wu Xiaoli:What size of transportation hub is this in the world?

Chen Xiangsheng:In terms of urban rail transit, it is already one of the largest hubs in the world, basically among the top three.

Wu Xiaoli: Where are the other two hubs?

Chen Xiangsheng:For example, Shinjuku in Japan, but it is a hub together with the high-speed rail, so it is difficult to calculate; and La Defense in Paris, France, it also has more than 200,000 passengers per day, which is slightly smaller than the Gangxia North hub; and New York Plaza also has about 200,000 to 300,000 passengers per day.

Shenzhen Gangxia North Underground Hub

How did Shenzhen become a global benchmark for underground space?

Wu Xiaoli:Shenzhen has proposed that by 2035, Shenzhen’s underground urban space utilization level will become a global benchmark. How do you understand this?

Chen Xiangsheng:The entire Shenzhen area is less than 2,000 square kilometers, with more than 20 million people. Last year, Shenzhen's GDP exceeded 3.2 trillion. With such a large area carrying so many people and so much GDP, Shenzhen is already the highest in the mainland, no doubt about it. Last week, Shenzhen announced that the per capita underground space area is 6.5 square meters. Can you imagine, if there is no underground space, more than 20 million people per capita 6.5 square meters, all put on the ground, how congested would Shenzhen be?

In 1980, Shenzhen looked like an oasis from the air. By 2005, almost all of Shenzhen’s rivers were polluted. The then Shenzhen Municipal Party Committee Secretary proposed four unsustainable issues: land resources, water resources, urban space and transportation. If we continue to develop in an extensive manner, Shenzhen will not be able to support the more than 20 million people under management and the GDP output value of 3.2 trillion last year, and we will fall behind. Therefore, if Shenzhen wants to further develop and increase its per capita GDP, it must upgrade its industries. If there is no land for industrial upgrading, it must develop underground space and free up more ground space for greening and parks.

Therefore, building a subway is building a city. Building a subway is not just about building a means of transportation. It is a reshaping of the entire urban space, a transformation towards a better ecology, environment, soil and water conservation, and transportation.

Chen Xiangsheng: If this is not done well,

The road may collapse and people may die...

Shanghai: Is it impossible to build a subway in silty sandy soil?

Chen Xiangsheng once worked at the China Coal Science Academy, studying the ground freezing method. It was this research direction that brought him into contact with the subway in 1994.Chen Xiangsheng, who was studying at Cambridge University at the time, received a letter from Academician Liu Jianhang saying that during the construction of Shanghai Metro Line 1, landslides were common due to the saturated water-containing soft soil layer.

A foreign expert once said, "Building a subway in Shanghai is like finding a fulcrum in the universe to move the earth. It is impossible."After returning to China, Chen Xiangsheng joined the construction team of the Shanghai Metro.

Chen Xiangsheng:Academician Liu Jianhang went to Germany and the UK to find technology suitable for Shanghai's strata. At that time, foreign people recommended Academician Liu Jianhang to the Coal Research Institute to find someone who was engaged in freezing technology. I was still in Cambridge at that time. There were no telephones or emails like now. He wrote a letter to the British Cementation Company to find me. So when I returned to China, I went directly to Shanghai and put the freezing method into practice in the Shanghai Metro with Academician Liu Jianhang. However, applying freezing technology to cities is not only about strength control, but also about deformation control. High-rise buildings have different foundation forms and will collapse if they are not careful.

Shanghai is a silty sandy stratum, which expands by 8%-9% when frozen. After construction, it will thaw again, and shrink by 10% when thawed. At this time, the ground will sink, or the surrounding buildings will deform horizontally. We didn't have much experience at the time, and we suffered a lot.

Wu Xiaoli:We know that you and Academician Liu spent 9 days and 9 nights in the subway construction project. When it comes to future safety, have you ever thought that you might not be able to solve it?

Chen Xiangsheng:We accompanied Academician Liu on duty underground, because that place involved the roads on the ground and the surrounding primary schools. If it was not done well, the roads might collapse, the primary schools would collapse, human lives would be at stake, and the impact would be huge... Therefore, Academician Liu said that we would be on duty to give everyone confidence and encouragement. Later, we quickly passed the emergency rescue project, and thus we established a new standard for freezing the connection channels of China's subway construction in Shanghai.

Qianhai Bay: What should I do if chopsticks are stuck in tofu?

The first phase of the Shenzhen Metro line passes through Qianhai Bay. Due to technical limitations, the area within 50 meters on both sides of the metro line is a safety protection zone. This means that after the metro is completed, there can no longer be any underground buildings on one-third of the land in Qianhai, which will affect ground buildings.

Chen Xiangsheng:Qianhai is a reclaimed area, and they had to dig a foundation pit of more than 40 meters. As the pit was dug, the two sides moved inward and deformed, suddenly wrapping up like dumplings.

Wu Xiaoli:That's dangerous.

Chen Xiangsheng:Yes. So building a subway in reclaimed land is like sticking chopsticks in tofu, which is very troublesome. Because the subway is under water, the subway needs to float up during excavation, and the water force needs to float it up, so it is very important to control deformation during the excavation process. If there is any mistake, it is easy to cause a big problem.

Wu Xiaoli:How did you control this deformation problem later?

Chen Xiangsheng:We conducted experimental research for 12 years and finally made a breakthrough in this technology, forming a complete set of technologies for the development and utilization of underground space near subways. We excavate and support at the same time. This support is provided by a servo system. If it moves a little bit, I will increase the force to push it back to prevent it from moving inward, and make its displacement almost zero, so that the surrounding buildings will not be deformed.

The core of this technology is that the original subway line was 80 meters on each side, and in Shenzhen it was 50 meters on each side. This part of the space cannot be used. We have solved this problem. As long as the subway structure is safe, excavation can be started.

In Qianhai, we have achieved the goal of digging underground space only two or three meters away from the subway. Under this circumstance, all the plans for Qianhai have been implemented. This is also the first time in the world, and this technology also won the Zhan Tianyou Award.

We calculated in Qianhai that if there are two floors of underground space, the cost per square meter will increase by about 2,000 to 3,000 yuan, and if there are three floors, it will be around 3,200 yuan. But it is worthwhile to spend the extra money to buy the land.

Reflections on the Zhengzhou subway flooding incident: subway design in extreme environments

Wu Xiaoli:Regarding the Zhengzhou subway flooding incident in 2021, did you go to the site to investigate and understand where the problem lies?

Chen Xiangsheng:I think this is a sensitive question, but I can answer it appropriately. The Yellow River is a "hanging river" in Zhengzhou city, which means that the river is higher than the ground. Originally, the terrain of Zhengzhou city was relatively high in the west, and the east was a newly developed city. The prevention of rainstorms and floods may not be based on the rainstorm defense and planning in extreme environments. In addition, the rainstorms did not fall uniformly throughout the city, but moved in patches, just like pouring water from a tank. That area was relatively low, so it entered from there. Therefore, this is also what we should pay attention to in subway design in extreme environments in the future.

Wu Xiaoli:In addition, is earthquake resistance in underground spaces also important?

Chen Xiangsheng:This is a very critical issue. Earthquakes are divided into shear waves and vertical waves. Earthquake damage is mainly caused by shear waves, which are horizontal vibrations, so it is relatively less damaging to the underground. However, we have to consider whether the structure and soil can cooperate to withstand the vibration when there is horizontal movement such as an earthquake. This is why we have established a set of design standards and specification guidelines for underground defense in the mainland over the years based on new requirements, the interaction between new underground structures and surrounding soil.

Chen Xiangsheng: Building rail transit is not

To encourage more people to take rail transit

Wu Xiaoli:You said that you hope to make people who take the subway feel happy. Where do you think their happiness comes from?

Chen Xiangsheng:I wonder what the residents are thinking now that our country's economy has developed to this level? So I took the subway and listened to what the passengers said, some praised it, some complained. So can we learn from Hong Kong when planning the subway and build buildings around the interchange hub to create an urban complex?

An urban complex is one that has nurseries, kindergartens, primary schools, hospitals, nursing homes, libraries, shopping places, and leisure places. In addition to going out for fun, people no longer have to take the subway, and there is no need to rush to send children to school.

The purpose of building rail transit is not to encourage people to take rail transit more often, but to reshape the urban space, concentrate it in one place, build subways in that place, reduce people's carbon footprint, and shorten the time and space perspective. Sometimes people get bored at home and want to relax somewhere. Once they go to the underground complex, they can relax, lie down, read books, buy things, and see various art exhibits...

Will the underground city in "The Wandering Earth" become a reality?

Wu Xiaoli: Some science fiction movies, such as "The Wandering Earth" and "The Three-Body Problem", depict humans moving to underground cities or castles; some big names in the technology world, such as Musk and Zuckerberg, have begun to build underground castles. What do you think of these science fiction imaginations? What do you think the underground city space of the future should look like?

Chen Xiangsheng:Of course, everyone has various ideas about underground space. We humans know less than 5% of the existing world, and we don't know about 95% of the world. The solar system will be gone in another 5 billion years, how many years can the earth survive? How can humans leave behind human seeds? One way is to go to outer space, so we have to go to Mars and the moon to leave seeds of life; on the other hand, if we go underground, we may encounter extreme environments and extreme weather, and we need space underground to ensure survival. But I personally think that in the time we can see today, it is extremely unlikely that humans will live underground. First, the cost of living underground is too high. It is too expensive to create an environment underground that is the same as on the ground. Why do we want to go to the park, swim, and climb mountains? Because we are used to natural and open environments.

Wu Xiaoli:Human beings need sunshine and rain.

Chen Xiangsheng:Sunlight and rain have always been our living environment. It is too difficult to create such conditions underground, so I don't agree with us living underground. The underground can be used as a place to escape. If there is no great demand, it is not worthwhile to move underground. It must also be matched with economic and social development.

Wu Xiaoli:In April 2018, you founded the Future Underground Cities Research Institute. What is the main research topic of your institute?

Chen Xiangsheng:The earth has entered a period of extreme weather changes and earthquakes are about to occur frequently. If the underground space is still operated according to the normal conditions, I am worried that there will be some safety hazards in the future under extreme environments and extreme weather conditions. This is the original intention of my creation. In fact, the entire underground space application, China is a late-developed country, and the underground space majors of various universities in the mainland are different. Some may focus on the quality of underground space, some on the environment of underground space, or the business of underground space, etc., which are all different.

The Future Underground City Research Institute that I founded covers all professional fields except electromechanical equipment. I hope to use the power of the country and work together to study underground space.

Dungeons in Film and Television

Why is it difficult to recruit students for civil engineering majors?

Wu Xiaoli:You studied civil engineering. What advice would you like to give to future civil engineering students?

Chen Xiangsheng:It is very difficult for mainland universities to recruit students for civil engineering programs nowadays. The first reason is that the work is too hard and many students are unwilling to go. The second reason is that China used to be a "infrastructure maniac," but as infrastructure tasks have gradually decreased, the number of needed talents has also decreased accordingly. However, China will still have to build civil engineering infrastructure in extreme environments and under extreme conditions for at least the next 30 to 40 years, especially in some areas in the southwest, northwest, and central China, such as airports, tunnels, and water diversion projects.

Wu Xiaoli:So we have to tell students that civil engineering is a very important subject in the future.

Chen Xiangsheng:The requirements of this generation of civil engineers are completely different from those of the previous generation. If students of the information explosion generation are still taught knowledge in the traditional way, there will be problems in their education.

We have entered the stage of digital civil engineering. Students’ perspectives cannot be limited to their own civil engineering majors. Instead, they must have systematic thinking to solve the extreme environments and extreme weather faced by mankind.

In this case, I want to cultivate students' new philosophical thinking ability, to look at problems systematically, and then find their own entry point and think about what they can do for mankind. This is the student I want to cultivate. Be humble, do things with awe, and treat everyone with kindness. Especially in this era, I hope that children will have compassion and love in their hearts, so that when they enter society, they can go far and steady.