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    Building Shanghai’s Deepest Station

    From combustible gas to land subsidence, the construction of Shanghai’s deepest metro station was rife with difficulties largely unknown to the public — until now.

    Editor’s note: This is the third in a series to mark the Shanghai Metro turning 30. Read part one, part two.

    New Year’s Day, 2019. Millions of Chinese were at home celebrating the most important holiday in the Chinese calendar. But that morning, a team of engineers were racing to the center of Shanghai.

    The wall separating the new underground tunnel of Yuyuan Garden station and the adjacent Renmin Road tunnel, only 1.2 meters thick, had burst open.

    The stakes were high: Renmin Road is one of Shanghai’s major thoroughfares. This meant the most effective way for the engineers to fix the wall, from Renmin Road itself, was also impossible, due to the disruption to traffic it would cause.

    Construction had already been halted once after substantial deposits of flammable methane were discovered deep underground. To make things worse, many of the superiors of Chen Biao, manager of the Yuyuan Garden Line 14 station construction project, including the president of the Shanghai Metro, had also left their families to assess the situation.

    “In the end, you can’t take full control of what will happen tomorrow, and the day after,” recounts Chen.

    Born and raised in Shanghai, Chen studied railway engineering at university before joining the Shanghai Metro’s engineering team in 1991. He has helped build several metro lines over the years, including Line 1, the first line on the system.

    Now 59, Chen remembers the new Yuyuan Garden station as one of the most difficult projects he has worked on. Not only is it Shanghai’s deepest station, it also connects the city’s historic center to its financial hub, across the iconic Huangpu River.

    Today, Yuyuan Garden station has a daily ridership of more than 90,000 people — more than New York City’s busiest subway station, Times Square.

    Construction of the new station had begun three years earlier. As project manager, Chen supervised it from beginning to end. He assisted the local government with land acquisition and the tender process. He reviewed all the engineering plans submitted by the construction firms. But most importantly, he was responsible for safety and risk control.

    Engineers in Shanghai are some of the best in the country due to the city’s rich history of construction under challenging geological conditions. Chen and his team’s rapid response that New Year’s Day meant that the breach in the separation wall did not result in a major incident. “We were able to get away with it thanks to our close monitoring,” said Chen.

    In fact, they had been taking on the project’s inherent challenges from the very beginning. These challenges have remained largely unknown to the public, with much of public attention limited to the award-winning interior design of the new station.

    The first major decision they had to make was the depth of the new station. Yuyuan Garden is situated just a few hundred meters away from the Huangpu River, central Shanghai’s biggest river, meaning that the new tunnel would have to cross a set of underground structures, including the foundations of the river’s flood control wall and the Bund tunnel.

    With the deepest foundation piles reaching as far as 48 meters underground, Chen and his team had no choice but to build the new station at a record depth of 36 meters so that incoming trains could make it to the station after diving under the piles. For safety and energy consumption reasons, official regulations limit subway inclination to a maximum gradient of 3%.

    The new station, literally having to reach deep into the ground for its passengers, was not going to be easily built.

    This became clearer just over halfway down the excavation when workers noticed gas coming out of the foundation pit — the open space being excavated. An expert team was brought in to identify the gas: It was a mixture of methane, ammonia, and carbon monoxide.

    The discovery of poisonous and flammable gas was concerning due to the depths they were working at. Methane gas explosions have been responsible for some of China’s worst mining disasters

    “It was easy to have a feeling of depression when you go this deep and look up from the bottom,” said Chen. “There was no chance of running if the pit collapses or explodes.”

    It seemed that a river had flowed through the site in ancient times. Rivers can deposit substantial amounts of greenhouse gasses in the surrounding soil due to the decomposing organic matter they carry. Urban rivers in particular produce high levels of methane because of additional human pollution.

    And with Shanghai being one of the world’s largest and most densely populated lowland coastal watersheds, this meant high levels of methane in the soil. 

    It took almost a week to pump out most of the gas, during which workers required gas masks and had to carry oxygen tanks as they worked in the pit. Chen ordered the use of gas detectors to continuously monitor concentrations from then on. Work would be suspended whenever safe levels were exceeded.

    The site’s location in the heart of the metropolis also meant that excavation could only happen in a small area. There was already a Yuyuan Garden Line 10 station, but a new station was needed on Line 14, the east-west metro line, to ease the pressure on public transport around some of the most densely populated areas of Shanghai.

    “It was like building a grand religious site within a snail’s shell,” recounts civil engineer Huang Xiaoping, citing a common Shanghai adage.

    The deep excavation also ran the risk of damaging crucial underground pipelines and road surfaces in the city’s oldest neighborhoods. Historic landmarks surrounded the excavation site in every direction: the City God Temple to the east, the Shanghai Museum to the west, and remnants of the old city wall to the south.

    With many historical buildings nearby, the usual problem of the ground sinking around metro construction projects became even more pressing. When groundwater is pumped out, a process known as dewatering, the reduction in pore-water pressure — the pressure of groundwater held between soil or rocks in the gaps — can result in the land sinking.

    This can cause serious damage to buildings resting on the deformed soil, leaving old buildings especially vulnerable.

    “Surrounding structures, such as roads and buildings, will be immediately put at stake once the ground sinks too much, and it could be brought about by pumping the water out too much or too fast,” said Chen.

    In China, most subway excavations are 15–30 meters deep, where high levels of groundwater already pose a significant risk. And in Shanghai, water intrusion has caused signal failures in the metro system before. So serious was the risk of water leakage that Chen had all the joints of the continuous wall shared with the adjacent Renmin Road reinforced with steel plates in advance.

    To make matters worse, it’s particularly challenging in Shanghai to build deep stations due to its soft soil. The city sits on a thick layer of soft clay and alluvial silt deposited in the Yangtze River Delta in the Late Pleistocene and Holocene eras. Early Soviet observers described building cross-river tunnels in Shanghai as analogous to “drilling holes in tofu.”

    For Chen and his team, it meant that the further down they dug, the more difficult it was to control deformation of the foundation pit. At the same time as excavation under soft soil conditions having the potential to cause damage to nearby infrastructure, this infrastructure can also cause the soil around the foundation pit to deform, threatening the safety of engineers working inside the pit.

    The risk is even higher when excavating in close proximity to an existing metro system. As the surrounding soil is already compromised, this can lead to deformation of the adjacent metro tunnel and even cracking of its lining structure. What’s more, the problem is made worse by rain.

    Fortunately, Shanghai engineers are among the most experienced in the country at deep excavations in soft soils surrounding existing subway tunnels. Such engineering projects date back to 1993, when the massive New World shopping complex was built atop People’s Square station.

    Leveraging his decades of experience, Chen closely monitored the levels of land subsidence and pit deformation throughout the construction. Recharge wells were installed to make sure water pumped out of the ground could be transported back into the ground if needed.

    In the end, all the precautionary efforts paid off. When construction of the station finished in 2020, a comprehensive evaluation of the surrounding area found that land subsidence caused by the new station was limited to just 5 millimeters, in line with city targets.

    “You only feel true relief after you … see the whole situation and use your knowledge and experience to confirm it’s in a good shape,” said Chen.

    “From design to management, I’m sure it will provide a lot of useful experiences for building a similar station in the future.”

    A year away from retirement, Chen is currently working on his last Shanghai Metro project: the new north-south Line 19. He will not see the project to completion, however, making Yuyuan Line 14 the last station he builds for the city he loves.

    Editor: Vincent Chow.

    (Header image: Passengers stand on an escalator inside Yuyuan Garden Line 14 station in Shanghai, 2022. Fan Jianlei/VCG)