In infrastructure construction, especially urban rail transit construction, the "shield" plays an irreplaceable and critical role. Ningbo Rail Transit Group Co., Ltd. of Zhejiang Province (hereinafter referred to as "Ningbo Rail Transit") has independently pioneered and developed the "Quasi-Rectangular Shield" — featuring the largest cross-section, multiple cutterheads with no blind spots, and the highest excavation utilization rate — to address the challenges of operating in narrow and complex underground urban spaces and rare soft marine mud foundations. After a decade of iterative upgrades, it has obtained 42 domestic and international patents, demonstrating strong international competitive advantages.

　　Recently, at the Yangming Hao "Quasi-Rectangular Shield" Technology Achievement Evaluation Meeting held in Ningbo, authoritative industry experts from home and abroad stated that the scientific and technological achievements of the "Quasi-Rectangular Shield" project led by Ningbo Rail Transit have reached an overall internationally leading level. Its successful exploration and practice have made significant contributions to global technological progress in this field; it also holds inestimable value for China's current acceleration of urban organic renewal, better utilization of precious underground space resources, and promotion and application in other industries and international markets.

　　Benchmarking Against the World's Forefront to Achieve "Global Leadership"

　　Rail transit is a new calling card for a city. In August 2008, Ningbo Rail Transit received approval from the State Council, opening the curtain on its "metro city" era. "As a world-renowned port city and an important base for China's advanced manufacturing, Ningbo's entry into the metro era is an important marker of its status as a modern international metropolis," said Yao Yanming, Party Committee Member and Chief Engineer of Ningbo Rail Transit. "To this end, we must benchmark against world-class standards and plan Ningbo's rail transit construction with a high starting point and high standards."

　　To do a good job, one must first sharpen one's tools. Metro construction equipment cannot do without shields (also called "shield tunneling machines"). Shields are known as the "aircraft carriers" of engineering machinery and can be widely used in major engineering and infrastructure fields such as urban rail transit, underground utility tunnels, and railway and highway tunnels.

　　Therefore, during its initial planning, Ningbo Rail Transit proposed a new path combining absorption and introduction with independent research and development, and repeatedly went abroad to conduct in-depth research on world-renowned shield machine manufacturers and metro operators. "Engineering equipment, especially for underground space construction, depends on local conditions and adaptation to local circumstances," said Zhu Yaohong, then Deputy Commander-in-Chief and Chief Engineer of Ningbo Rail Transit.

In its independent research and development, Ningbo Rail Transit, targeting local geological structures and underground pipeline characteristics, worked with authoritative professional research institutions and manufacturers at home and abroad, conducting repeated experiments and practices, and successfully developed "Ningbo-exclusive" shield tunnel technology with internationally advanced standards. In just four years, it successfully completed three urban metro lines. The advanced planning and design, rapid construction speed, and excellent engineering quality are among the best at home and abroad.

　　However, at that time Ningbo Rail Transit used single-tube single-track shield technology, which sometimes struggled to adapt to narrow and cramped underground construction spaces. This problem became even more acute during the construction of Ningbo Metro Line 4. Line 4 is an urban tourism line with a total length of 35.95 kilometers, of which 2 kilometers are located directly beneath the old Cuibaili neighborhood, where the available underground clear width is less than 14 meters. Working in such a narrow underground space is extremely rare.

　　Faced with this world-class challenge, after repeated demonstration and analysis, Ningbo Rail Transit decided to attempt to develop the emerging single-tube double-track "rectangular shield." Huang Guibin, Party Secretary of Ningbo Rail Transit Construction Branch, said that to address the urgent need in the Cuibaili section, they initially considered gritting their teeth and buying a finished "rectangular shield" from abroad, but the other party declined on the grounds of "no current sales plans." When it comes to bottleneck high-end equipment and key technologies, one cannot count on or rely on foreign introduction.

　　So Ningbo Rail Transit joined forces with top universities, research institutions, and manufacturing giants, gathering a large number of top engineers and senior scholars to build an integrated platform for industry-academia-research-application collaboration and carry out coordinated research. In this arduous decade-long climb toward the peak of science and technology, Ningbo Rail Transit and its platform partners not only pioneered the world's largest cross-section "Quasi-Rectangular Shield" but also continuously improved its technology and functions, obtaining 42 domestic and international patents and 4 provincial and ministerial science and technology progress awards, becoming a true "global leader."

　　"A Decade to Sharpen a Sword" — Turning the Construction Site into an International-Level Research Arena

　　In the "decade to sharpen a sword" process, Ningbo Rail Transit mobilized elite personnel and established a research task force. Numerous technical experts stood firm and fought hard, turning the construction site into an international-level research arena.

　　"This is a highly difficult and high-tech project, but we have the confidence and ability to meet the challenge," said Huang Yi, a senior engineer and frontline leader. He introduced that among those participating in the coordinated research were professional institutions and universities such as Shanghai Tunnel Engineering Design Institute, Shanghai Tunnel Engineering Co., Ltd., and Tongji University, all giving full play to the collaborative advantages of the research platform and leveraging their respective strengths to form complementary synergies.

　　Regarding the shield's structural shape, Shen Zhangyong, senior engineer at Shanghai Tunnel Engineering Design Institute and design leader of the project, said that during preliminary demonstration, the team considered that since neither circular nor rectangular shapes worked, could an elliptical shape be considered? However, stress calculations and various experimental results showed that the structure with more reasonable force distribution and better resistance could not be a simple ellipse, but rather four curves of different arcs joined and assembled into something like a horizontally placed oval duck egg — which is precisely the origin of the "Quasi-Rectangular Shield."

　　Liu Xian, a professor at Tongji University who led the underground pressure experiments for the "Quasi-Rectangular Shield," said: "Such experiments require enormous human, material, and financial resources. Countries around the world, including many internationally renowned underground engineering projects, generally conduct only one-time experiments. But in close cooperation with Ningbo Rail Transit, we carried out seven prototype structural laboratory tests and field verification tests for the 'Quasi-Rectangular Shield.'"

　　Shanghai Tunnel Engineering Co., Ltd. is a leading comprehensive enterprise in China with all-around capabilities in shield tunneling machine research, manufacturing, and construction. "For this unprecedented super-large cross-section 'Quasi-Rectangular Shield' project, we initially wanted to invite foreign experts to participate, but after learning about the situation they waved their hands and said it was too much for them, so we did everything possible to crack this hard nut ourselves," said Zhu Yanfei, Chief Engineer of Shanghai Tunnel Engineering Co., Ltd. The assembled segments of this "Quasi-Rectangular Shield" weigh up to 9 tons, which traditional mechanical arms cannot bear. We innovatively adopted a serial loop-arm assembly technology to cleverly solve this problem.

　　With all parties on the research platform going all out, in just nine months, the world's first largest cross-section "Quasi-Rectangular Shield," 11.83 meters wide and 7.27 meters high, was finally unveiled, and Ningbo named it "Yangming Hao."