A river of clear water, green banks on both sides. Inside a working shaft 45 meters deep on the south side of the Yangtze River in Nanjing, Jiangsu, a 155-meter-long "steel dragon" lies quietly underground, with a 2,349-meter completed tunnel stretching behind it.

"After 26 months of underground boring and the precise assembly of 11,750 segments, the completed tunnel is solid inside and dense outside, with no seepage or leakage—we have finally reached the moment of breakthrough," said Mu Zheng, project manager of the Nanjing Jianning West Road project of China Railway 14th Bureau Group, gazing recently at the "steel dragon" lying quietly underground. On that day, between the Nanjing Yangtze River Bridge and the Yangtze River Tunnel in Jiangsu, the "Chuangxin" (Innovation) shield machine with an excavation diameter of 15.07 meters completed its reception successfully, and the left-line shield tunnel of the Nanjing Jianning West Road river-crossing passage now lies beneath the riverbed.

Adapting the shield machine to local conditions

The Nanjing Jianning West Road river-crossing passage, in whose construction China Railway 14th Bureau Group is participating, is an important link connecting the main city on the south bank with the national-level new area on the north bank. It is also a key passage for advancing Nanjing's river-inclusive development, accelerating construction of the Jiangbei New Area, and enhancing the Yangtze River's cross-river capacity.

Walking into the Nanjing Jianning West Road smart center, colorful geological diagrams come into view. In the diagrams, the shield tunnel, about 2,349 meters long, has a maximum water depth of 55 meters and bears a maximum water-and-soil pressure of 7.7 bar. It also includes a 280-meter shallow-cover section beneath the riverbed, where the minimum cover thickness is 10.55 meters—less than 0.75 times the tunnel diameter. Where the shield tunnel crosses the south-bank levee, it runs close to the deep channel of the Yangtze, with deep water, steep slopes, and soft geology. At the same time, a 15-meter-class large-diameter slurry shield machine cutting through the reinforced-concrete anti-sliding piles of the levee is the first case of its kind in China, with no prior experience to draw on and extremely high construction risk.

"Facing these challenges, we carried out targeted research and design on the cutterhead type, cutter type, and pressure resistance of the shield machine, equipping it with a atmospheric cutterhead to enable safe and rapid cutter changes beneath the riverbed," said Wang Hui, director of the electromechanical department of the China Railway 14th Bureau Group project. They also equipped the shield machine with a retractable main drive with a maximum extension of 40 centimeters, greatly reducing the risk of cutter changes beneath the riverbed, and provided a fully automatic advanced drilling and grouting system to address construction risks in the riverbed fracture zone, laying a solid foundation for the smooth progress of the project.

When the "Chuangxin" shield machine bored to a point where the tunnel crown was 55 meters below the river surface, it encountered a fracture zone. The faults and fractures in the zone were extremely developed, with high clay content and strong stickiness, and the moderately weathered dolomite fragments had a strength of up to 147.6 MPa.

Faced with these difficulties, Li Longhui, shield manager of China Railway 14th Bureau Group, and his team sought breakthroughs in cutter type and cutter-housing type, improving the cutter body type and cutter materials to enhance cutter durability and make the cutters better suited to the fracture zone with uneven soft and hard strata, solving the problem of cutter rings breaking easily due to uneven stress. In addition, they developed a new type of sealing plate to plug the reserved holes for cutter-edge rotation on the cutter seat—roughly equivalent to fitting wheel-arch covers and mudguards around a car tire—effectively solving the problem of debris accumulation inside the cutter housing and reducing the rate of uneven cutter wear.

In this way, Li Longhui and his team battled the fracture zone for 165 days and nights. The "Chuangxin" shield machine changed cutters 40 times, replacing a total of 391 cutters, at an average frequency of one cutter change every 3.9 rings and nearly 10 cutters per change, successfully crossing the fracture-zone strata.

Protecting the "giant panda in the water"

The river nurtures the people on both banks and also sustains the countless lives within its waters. The Yangtze finless porpoise is an indicator species for Yangtze ecology, and the Nanjing Yangtze Finless Porpoise Nature Reserve, with a total area of 86.92 square kilometers, was established precisely for them.

"The Jianning West Road river-crossing passage, which crosses the Yangtze in the form of a shield tunnel, is located in the buffer zone and experimental zone of the porpoise reserve," Mu Zheng said. Since construction began, they have been committed to creating a safe living environment for the porpoises and doing their utmost to protect the "giant panda in the water."

"We uphold the concept of building an 'eco-environment-friendly' green project, fully implement the Yangtze River protection policies, focus on protecting wild species in the Yangtze, and help restore Yangtze ecology," Mu Zheng said. They built a full set of shield slurry circulation and treatment facilities and installed isolation beneath the slurry storage tank to prevent slurry from seeping outward. Domestic sewage is treated by treatment equipment to meet standards and then discharged at designated points, with regular water quality testing.

When the "Chuangxin" shield machine crossed the levee, in order to minimize the impact of construction on porpoise activity, they used the all-round high-pressure jet grouting method. Compared with traditional techniques, this not only provides a better reinforcement effect with less construction noise and vibration, but also uses internal pipes to extract the soil and excess grout cut by the jet grouting, so that slurry does not overflow.

At the same time, around the construction site, the project installed porpoise deterrent devices and acoustic recorders to record porpoise activity characteristics, along with alarm devices. When porpoises are found in or tending to approach the construction waters, the construction team suspends work to let the porpoises pass safely, or uses ultrasonic deterrent devices to drive them away from the construction area.

The mighty Yangtze flows on without cease. The successful breakthrough of the left-line shield tunnel of the Nanjing Jianning West Road river-crossing passage has laid a solid foundation for the passage's dual-line breakthrough by the end of this year. Nanjing, a city born by the river, will gain new momentum for development.