An interior view of the Jintang Subsea Tunnel. [Photo/www.wifizs.cn]

The Jintang Subsea Tunnel of the Ningbo-Zhoushan Railway reached a milestone on Sept 22, as the domestically built shield machine "Yongzhou" completed 2,470 meters of excavation at a depth of 65 meters under the water, marking the halfway point on the Ningbo section. The tunnel will become the world's longest subsea high-speed rail tunnel once completed.

Stretching 16.18 kilometers between Ningbo and Zhoushan in Zhejiang province, the tunnel includes an 11.21-kilometer shield section. The "Yongzhou" shield machine is moving eastward from Ningbo, while the "Dinghai" shield machine advances from the Zhoushan side.

The construction of the Ningbo section, 4,940 meters long, must navigate challenges including high water pressure, petroleum pipelines, sea walls, docks, and navigation channels. The tunnel reaches a maximum depth of 78 meters with water pressure up to 8.4 bars. Nearly 70 percent of the ground is uneven rock and mixed strata, with some hard rock as strong as 200 megapascals.

To address these conditions, engineers upgraded the "Yongzhou" shield machine with heavy-duty cutterheads, denser disc cutters, and a unique monitoring system to track cutter wear. Precision guidance and pre-grouting systems enhance safety in complex strata. The design also uses a double-shell structure, allowing dismantling in narrow seabed spaces while supporting the ground.

So far, the machine has completed 12 stratum transitions and replaced more than 2,400 cutters, averaging one replacement per meter. The project also features China's first domestically developed pressurized chamber entry technology, improving efficiency and safety in high-pressure environments.