Deep beneath China’s seabed, workers just spent 21 days underwater while building the world’s longest undersea high-speed rail tunnel, linking the Zhoushan islands with China's rail network

China has completed a difficult stage of work on a massive railway tunnel being built beneath the seabed off Zhejiang province, with workers spending 21 days carrying out maintenance on a giant tunnel boring machine under extreme underwater pressure.

The operation involved replacing 104 cutting tools at the front of the machine. The tools help the machine break through rock and soil as it digs its way through the seabed.

The work forms part of the construction of the 16.18-km Jintang undersea tunnel, a key section of the Ningbo-Zhoushan high-speed railway. Once complete, the tunnel will connect the Zhoushan islands with China's high-speed rail network and is set to become the world's longest undersea high-speed rail tunnel.

Workers replaced 104 cutting tools deep beneath the seabed

The cutting tools sit on the front of the giant shield machine and break through rock and soil as it digs the tunnel. They face heavy wear because the machine has to pass through alternating layers of soft and hard rock.

One section of the tunnel runs through 24 different rock layers, with rock strength reaching 191 megapascals.

At its deepest point, the tunnel lies around 92 metres below sea level, where combined water and soil pressure can reach about 8.5 bar.

These conditions make cutter replacement far more difficult than routine maintenance on a tunnel boring machine. Workers have to reach the front of the machine, inspect the equipment and replace worn cutters while operating under extreme pressure.

China adapted deep-sea diving technology for the tunnel

To make the operation possible, engineers spent three years adapting technology used in deep-sea saturation diving.

The resulting system, which the project team calls the “Deep-Sea Space Station”, allows workers to remain in a controlled pressurised environment while carrying out maintenance.

The system can support up to nine workers, with each worker able to spend as long as eight hours at the worksite. That is significantly longer than conventional compressed-air methods, which can give workers only around 40 minutes of effective working time per day under similar conditions.

The system also reduces repeated pressure changes. Instead of repeatedly moving between normal atmospheric pressure and the high-pressure work area, workers can remain pressurised for their assignment and undergo controlled decompression afterwards.

The 21-day cutter replacement operation provided a major test of the technology under actual tunnel construction conditions.

The tunnel will link Zhoushan to China's high-speed rail network

The Jintang tunnel is part of the Ningbo-Zhoushan high-speed railway, which will provide a faster connection between the mainland city of Ningbo and the Zhoushan islands.

The undersea section stretches 16.18 km, including an 11.21-km shield-bored section. Two large shield machines are excavating the tunnel from opposite sides.

The machine involved in the latest cutter replacement, Yongzhou, has already advanced nearly 80% of its assigned route, covering about 3,936 metres.

The completed railway is expected to make travel between Ningbo and Zhoushan much faster while bringing Zhoushan more directly into China's high-speed rail network.

The project also highlights the engineering challenges involved in extending high-speed rail through underwater environments, where extreme pressure, difficult geology and machine maintenance all have to be managed at the same time.

(With TOI inputs)